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diff --git a/doc/rfc/rfc3961.txt b/doc/rfc/rfc3961.txt new file mode 100644 index 0000000..0ac50b9 --- /dev/null +++ b/doc/rfc/rfc3961.txt @@ -0,0 +1,2803 @@ + + + + + + +Network Working Group K. Raeburn +Request for Comments: 3961 MIT +Category: Standards Track February 2005 + + + Encryption and Checksum Specifications + for Kerberos 5 + +Status of This Memo + + This document specifies an Internet standards track protocol for the + Internet community, and requests discussion and suggestions for + improvements. Please refer to the current edition of the "Internet + Official Protocol Standards" (STD 1) for the standardization state + and status of this protocol. Distribution of this memo is unlimited. + +Copyright Notice + + Copyright (C) The Internet Society (2005). + +Abstract + + This document describes a framework for defining encryption and + checksum mechanisms for use with the Kerberos protocol, defining an + abstraction layer between the Kerberos protocol and related + protocols, and the actual mechanisms themselves. The document also + defines several mechanisms. Some are taken from RFC 1510, modified + in form to fit this new framework and occasionally modified in + content when the old specification was incorrect. New mechanisms are + presented here as well. This document does NOT indicate which + mechanisms may be considered "required to implement". + +Table of Contents + + 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 + 2. Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . 2 + 3. Encryption Algorithm Profile . . . . . . . . . . . . . . . . 4 + 4. Checksum Algorithm Profile . . . . . . . . . . . . . . . . . 9 + 5. Simplified Profile for CBC Ciphers with Key Derivation . . . 10 + 5.1. A Key Derivation Function . . . . . . . . . . . . . . . 10 + 5.2. Simplified Profile Parameters . . . . . . . . . . . . . 12 + 5.3. Cryptosystem Profile Based on Simplified Profile . . . 13 + 5.4. Checksum Profiles Based on Simplified Profile . . . . . 16 + 6. Profiles for Kerberos Encryption and Checksum Algorithms . . 16 + 6.1. Unkeyed Checksums . . . . . . . . . . . . . . . . . . . 17 + 6.2. DES-based Encryption and Checksum Types . . . . . . . . 18 + 6.3. Triple-DES Based Encryption and Checksum Types . . . . 28 + 7. Use of Kerberos Encryption Outside This Specification . . . . 30 + + + +Raeburn Standards Track [Page 1] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + 8. Assigned Numbers . . . . . . . . . . . . . . . . . . . . . . 31 + 9. Implementation Notes . . . . . . . . . . . . . . . . . . . . 32 + 10. Security Considerations . . . . . . . . . . . . . . . . . . . 33 + 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35 + 12. Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . 36 + A. Test vectors . . . . . . . . . . . . . . . . . . . . . . . . 38 + A.1. n-fold . . . . . . . . . . . . . . . . . . . . . . . . 38 + A.2. mit_des_string_to_key . . . . . . . . . . . . . . . . . 39 + A.3. DES3 DR and DK . . . . . . . . . . . . . . . . . . . . 43 + A.4. DES3string_to_key . . . . . . . . . . . . . . . . . . . 44 + A.5. Modified CRC-32 . . . . . . . . . . . . . . . . . . . . 44 + B. Significant Changes from RFC 1510 . . . . . . . . . . . . . . 45 + Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 + Normative References. . . . . . . . . . . . . . . . . . . . . . . 47 + Informative References. . . . . . . . . . . . . . . . . . . . . . 48 + Editor's Address. . . . . . . . . . . . . . . . . . . . . . . . . 49 + Full Copyright Statement. . . . . . . . . . . . . . . . . . . . . 50 + +1. Introduction + + The Kerberos protocols [Kerb] are designed to encrypt messages of + arbitrary sizes, using block encryption ciphers or, less commonly, + stream encryption ciphers. Encryption is used to prove the + identities of the network entities participating in message + exchanges. However, nothing in the Kerberos protocol requires that + any specific encryption algorithm be used, as long as the algorithm + includes certain operations. + + The following sections specify the encryption and checksum mechanisms + currently defined for Kerberos, as well as a framework for defining + future mechanisms. The encoding, chaining, padding, and other + requirements for each are described. Appendix A gives test vectors + for several functions. + +2. Concepts + + Both encryption and checksum mechanisms are profiled in later + sections. Each profile specifies a collection of operations and + attributes that must be defined for a mechanism. A Kerberos + encryption or checksum mechanism specification is not complete if it + does not define all of these operations and attributes. + + An encryption mechanism must provide for confidentiality and + integrity of the original plaintext. (Incorporating a checksum may + permit integrity checking, if the encryption mode does not provide an + integrity check itself.) It must also provide non-malleability + + + + + +Raeburn Standards Track [Page 2] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + [Bellare98] [Dolev91]. Use of a random confounder prepended to the + plaintext is recommended. It should not be possible to determine if + two ciphertexts correspond to the same plaintext without the key. + + A checksum mechanism [1] must provide proof of the integrity of the + associated message and must preserve the confidentiality of the + message in case it is not sent in the clear. Finding two plaintexts + with the same checksum should be infeasible. It is NOT required that + an eavesdropper be unable to determine whether two checksums are for + the same message, as the messages themselves would presumably be + visible to any such eavesdropper. + + Due to advances in cryptography, some cryptographers consider using + the same key for multiple purposes unwise. Since keys are used in + performing a number of different functions in Kerberos, it is + desirable to use different keys for each of these purposes, even + though we start with a single long-term or session key. + + We do this by enumerating the different uses of keys within Kerberos + and by making the "usage number" an input to the encryption or + checksum mechanisms; such enumeration is outside the scope of this + document. Later sections define simplified profile templates for + encryption and checksum mechanisms that use a key derivation function + applied to a CBC mode (or similar) cipher and a checksum or hash + algorithm. + + We distinguish the "base key" specified by other documents from the + "specific key" for a specific encryption or checksum operation. It + is expected but not required that the specific key be one or more + separate keys derived from the original protocol key and the key + usage number. The specific key should not be explicitly referenced + outside of this document. The typical language used in other + documents should be something like, "encrypt this octet string using + this key and this usage number"; generation of the specific key and + cipher state (described in the next section) are implicit. The + creation of a new cipher-state object, or the re-use of one from a + previous encryption operation, may also be explicit. + + New protocols defined in terms of the Kerberos encryption and + checksum types should use their own key usage values. Key usages are + unsigned 32-bit integers; zero is not permitted. + + All data is assumed to be in the form of strings of octets or eight- + bit bytes. Environments with other byte sizes will have to emulate + this behavior in order to get correct results. + + + + + + +Raeburn Standards Track [Page 3] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + Each algorithm is assigned an encryption type (or "etype") or + checksum type number, for algorithm identification within the + Kerberos protocol. The full list of current type number assignments + is given in section 8. + +3. Encryption Algorithm Profile + + An encryption mechanism profile must define the following attributes + and operations. The operations must be defined as functions in the + mathematical sense. No additional or implicit inputs (such as + Kerberos principal names or message sequence numbers) are permitted. + + protocol key format + This describes which octet string values represent valid keys. + For encryption mechanisms that don't have perfectly dense key + spaces, this will describe the representation used for encoding + keys. It need not describe invalid specific values; all key + generation routines should avoid such values. + + specific key structure + This is not a protocol format at all, but a description of the + keying material derived from the chosen key and used to encrypt or + decrypt data or compute or verify a checksum. It may, for + example, be a single key, a set of keys, or a combination of the + original key with additional data. The authors recommend using + one or more keys derived from the original key via one-way key + derivation functions. + + required checksum mechanism + This indicates a checksum mechanism that must be available when + this encryption mechanism is used. Since Kerberos has no built in + mechanism for negotiating checksum mechanisms, once an encryption + mechanism is decided, the corresponding checksum mechanism can be + used. + + key-generation seed length, K + This is the length of the random bitstring needed to generate a + key with the encryption scheme's random-to-key function (described + below). This must be a fixed value so that various techniques for + producing a random bitstring of a given length may be used with + key generation functions. + + key generation functions + Keys must be generated in a number of cases, from different types + of inputs. All function specifications must indicate how to + generate keys in the proper wire format and must avoid generating + keys that significantly compromise the confidentiality of + encrypted data, if the cryptosystem has such. Entropy from each + + + +Raeburn Standards Track [Page 4] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + source should be preserved as much as possible. Many of the + inputs, although unknown, may be at least partly predictable + (e.g., a password string is likely to be entirely in the ASCII + subset and of fairly short length in many environments; a semi- + random string may include time stamps). The benefit of such + predictability to an attacker must be minimized. + + string-to-key (UTF-8 string, UTF-8 string, opaque)->(protocol-key) + This function generates a key from two UTF-8 strings and an opaque + octet string. One of the strings is usually the principal's pass + phrase, but generally it is merely a secret string. The other + string is a "salt" string intended to produce different keys from + the same password for different users or realms. Although the + strings provided will use UTF-8 encoding, no specific version of + Unicode should be assumed; all valid UTF-8 strings should be + allowed. Strings provided in other encodings MUST first be + converted to UTF-8 before applying this function. + + The third argument, the octet string, may be used to pass + mechanism-specific parameters into this function. Since doing so + implies knowledge of the specific encryption system, generating + non-default parameter values should be an uncommon operation, and + normal Kerberos applications should be able to treat this + parameter block as an opaque object supplied by the Key + Distribution Center or defaulted to some mechanism-specific + constant value. + + The string-to-key function should be a one-way function so that + compromising a user's key in one realm does not compromise it in + another, even if the same password (but a different salt) is used. + + random-to-key (bitstring[K])->(protocol-key) + This function generates a key from a random bitstring of a + specific size. All the bits of the input string are assumed to be + equally random, even though the entropy present in the random + source may be limited. + + key-derivation (protocol-key, integer)->(specific-key) + In this function, the integer input is the key usage value, as + described above. An attacker is assumed to know the usage values. + The specific-key output value was described in section 2. + + string-to-key parameter format + This describes the format of the block of data that can be passed + to the string-to-key function above to configure additional + parameters for that function. Along with the mechanism of + encoding parameter values, bounds on the allowed parameters should + also be described to avoid allowing a spoofed KDC to compromise + + + +Raeburn Standards Track [Page 5] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + the user's password. If practical it may be desirable to + construct the encoding so that values unacceptably weakening the + resulting key cannot be encoded. + + Local security policy might permit tighter bounds to avoid excess + resource consumption. If so, the specification should recommended + defaults for these bounds. The description should also outline + possible weaknesses if bounds checks or other validations are not + applied to a parameter string received from the network. + + As mentioned above, this should be considered opaque to most + normal applications. + + default string-to-key parameters (octet string) + This default value for the "params" argument to the string-to-key + function should be used when the application protocol (Kerberos or + other) does not explicitly set the parameter value. As indicated + above, in most cases this parameter block should be treated as an + opaque object. + + cipher state + This describes any information that can be carried over from one + encryption or decryption operation to the next, for use with a + given specific key. For example, a block cipher used in CBC mode + may put an initial vector of one block in the cipher state. Other + encryption modes may track nonces or other data. + + This state must be non-empty and must influence encryption so that + messages are decrypted in the same order they were a encrypted, if + the cipher state is carried over from one encryption to the next. + Distinguishing out-of-order or missing messages from corrupted + messages is not required. If desired, this can be done at a + higher level by including sequence numbers and not "chaining" the + cipher state between encryption operations. + + The cipher state may not be reused in multiple encryption or + decryption operations. These operations all generate a new cipher + state that may be used for following operations using the same key + and operation. + + The contents of the cipher state must be treated as opaque outside + of encryption system specifications. + + initial cipher state (specific-key, direction)->(state) + This describes the generation of the initial value for the cipher + state if it is not being carried over from a previous encryption + or decryption operation. + + + + +Raeburn Standards Track [Page 6] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + This describes any initial state setup needed before encrypting + arbitrary amounts of data with a given specific key. The specific + key and the direction of operations to be performed (encrypt + versus decrypt) must be the only input needed for this + initialization. + + This state should be treated as opaque in any uses outside of an + encryption algorithm definition. + + IMPLEMENTATION NOTE: [Kerb1510] was vague on whether and to what + degree an application protocol could exercise control over the + initial vector used in DES CBC operations. Some existing + implementations permit setting the initial vector. This framework + does not provide for application control of the cipher state + (beyond "initialize" and "carry over from previous encryption"), + as the form and content of the initial cipher state can vary + between encryption systems and may not always be a single block of + random data. + + New Kerberos application protocols should not assume control over + the initial vector, or that one even exists. However, a general- + purpose implementation may wish to provide the capability, in case + applications explicitly setting it are encountered. + + encrypt (specific-key, state, octet string)->(state, octet string) + This function takes the specific key, cipher state, and a non- + empty plaintext string as input and generates ciphertext and a new + cipher state as outputs. If the basic encryption algorithm itself + does not provide for integrity protection (e.g., DES in CBC mode), + then some form of verifiable MAC or checksum must be included. + Some random factor such as a confounder should be included so that + an observer cannot know if two messages contain the same + plaintext, even if the cipher state and specific keys are the + same. The exact length of the plaintext need not be encoded, but + if it is not and if padding is required, the padding must be added + at the end of the string so that the decrypted version may be + parsed from the beginning. + + The specification of the encryption function must indicate not + only the precise contents of the output octet string, but also the + output cipher state. The application protocol may carry the + output cipher state forward from one encryption with a given + specific key to another; the effect of this "chaining" must be + defined [2]. + + Assuming that values for the specific key and cipher state are + correctly-produced, no input octet string may result in an error + indication. + + + +Raeburn Standards Track [Page 7] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + decrypt (specific-key, state, octet string)->(state, octet string) + This function takes the specific key, cipher state, and ciphertext + as inputs and verifies the integrity of the supplied ciphertext. + If the ciphertext's integrity is intact, this function produces + the plaintext and a new cipher state as outputs; otherwise, an + error indication must be returned, and the data discarded. + + The result of the decryption may be longer than the original + plaintext, as, for example, when the encryption mode adds padding + to reach a multiple of a block size. If this is the case, any + extra octets must come after the decoded plaintext. An + application protocol that needs to know the exact length of the + message must encode a length or recognizable "end of message" + marker within the plaintext [3]. + + As with the encryption function, a correct specification for this + function must indicate not only the contents of the output octet + string, but also the resulting cipher state. + + pseudo-random (protocol-key, octet-string)->(octet-string) + This pseudo-random function should generate an octet string of + some size that is independent of the octet string input. The PRF + output string should be suitable for use in key generation, even + if the octet string input is public. It should not reveal the + input key, even if the output is made public. + + These operations and attributes are all that is required to support + Kerberos and various proposed preauthentication schemes. + + For convenience of certain application protocols that may wish to use + the encryption profile, we add the constraint that, for any given + plaintext input size, a message size must exist between that given + size and that size plus 65,535 such that the length of the decrypted + version of the ciphertext will never have extra octets at the end. + + Expressed mathematically, for every message length L1, there exists a + message size L2 such that + + L2 >= L1 + L2 < L1 + 65,536 + for every message M with |M| = L2, decrypt(encrypt(M)) = M + + A document defining a new encryption type should also describe known + weaknesses or attacks, so that its security may be fairly assessed, + and should include test vectors or other validation procedures for + the operations defined. Specific references to information that is + readily available elsewhere are sufficient. + + + + +Raeburn Standards Track [Page 8] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + +4. Checksum Algorithm Profile + + A checksum mechanism profile must define the following attributes and + operations: + + associated encryption algorithm(s) + This indicates the types of encryption keys this checksum + mechanism can be used with. + + A keyed checksum mechanism may have more than one associated + encryption algorithm if they share the same wire-key format, + string-to-key function, default string-to-key-parameters, and key + derivation function. (This combination means that, for example, a + checksum type, key usage value, and password are adequate to get + the specific key used to compute a checksum.) + + An unkeyed checksum mechanism can be used with any encryption + type, as the key is ignored, but its use must be limited to cases + where the checksum itself is protected, to avoid trivial attacks. + + get_mic function + This function generates a MIC token for a given specific key (see + section 3) and message (represented as an octet string) that may + be used to verify the integrity of the associated message. This + function is not required to return the same deterministic result + for each use; it need only generate a token that the verify_mic + routine can check. + + The output of this function will also dictate the size of the + checksum. It must be no larger than 65,535 octets. + + verify_mic function + Given a specific key, message, and MIC token, this function + ascertains whether the message integrity has been compromised. + For a deterministic get_mic routine, the corresponding verify_mic + may simply generate another checksum and compare the two. + + The get_mic and verify_mic operations must allow inputs of arbitrary + length; if any padding is needed, the padding scheme must be + specified as part of these functions. + + These operations and attributes are all that should be required to + support Kerberos and various proposed preauthentication schemes. + + As with encryption mechanism definition documents, documents defining + new checksum mechanisms should indicate validation processes and + known weaknesses. + + + + +Raeburn Standards Track [Page 9] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + +5. Simplified Profile for CBC Ciphers with Key Derivation + + The profile outlined in sections 3 and 4 describes a large number of + operations that must be defined for encryption and checksum + algorithms to be used with Kerberos. Here we describe a simpler + profile that can generate both encryption and checksum mechanism + definitions, filling in uses of key derivation in appropriate places, + providing integrity protection, and defining multiple operations for + the cryptosystem profile based on a smaller set of operations. Not + all of the existing cryptosystems for Kerberos fit into this + simplified profile, but we recommend that future cryptosystems use it + or something based on it [4]. + + Not all the operations in the complete profiles are defined through + this mechanism; several must still be defined for each new algorithm + pair. + +5.1. A Key Derivation Function + + Rather than define some scheme by which a "protocol key" is composed + of a large number of encryption keys, we use keys derived from a base + key to perform cryptographic operations. The base key must be used + only for generating the derived keys, and this derivation must be + non-invertible and entropy preserving. Given these restrictions, + compromise of one derived key does not compromise others. Attack of + the base key is limited, as it is only used for derivation and is not + exposed to any user data. + + To generate a derived key from a base key, we generate a pseudorandom + octet string by using an algorithm DR, described below, and generate + a key from that octet string by using a function dependent on the + encryption algorithm. The input length needed for that function, + which is also dependent on the encryption algorithm, dictates the + length of the string to be generated by the DR algorithm (the value + "k" below). These procedures are based on the key derivation in + [Blumenthal96]. + + Derived Key = DK(Base Key, Well-Known Constant) + + DK(Key, Constant) = random-to-key(DR(Key, Constant)) + + DR(Key, Constant) = k-truncate(E(Key, Constant, + initial-cipher-state)) + + Here DR is the random-octet generation function described below, and + DK is the key-derivation function produced from it. In this + construction, E(Key, Plaintext, CipherState) is a cipher, Constant is + a well-known constant determined by the specific usage of this + + + +Raeburn Standards Track [Page 10] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + function, and k-truncate truncates its argument by taking the first k + bits. Here, k is the key generation seed length needed for the + encryption system. + + The output of the DR function is a string of bits; the actual key is + produced by applying the cryptosystem's random-to-key operation on + this bitstring. + + If the Constant is smaller than the cipher block size of E, then it + must be expanded with n-fold() so it can be encrypted. If the output + of E is shorter than k bits, it is fed back into the encryption as + many times as necessary. The construct is as follows (where | + indicates concatentation): + + K1 = E(Key, n-fold(Constant), initial-cipher-state) + K2 = E(Key, K1, initial-cipher-state) + K3 = E(Key, K2, initial-cipher-state) + K4 = ... + + DR(Key, Constant) = k-truncate(K1 | K2 | K3 | K4 ...) + + n-fold is an algorithm that takes m input bits and "stretches" them + to form n output bits with equal contribution from each input bit to + the output, as described in [Blumenthal96]: + + We first define a primitive called n-folding, which takes a + variable-length input block and produces a fixed-length output + sequence. The intent is to give each input bit approximately + equal weight in determining the value of each output bit. Note + that whenever we need to treat a string of octets as a number, the + assumed representation is Big-Endian -- Most Significant Byte + first. + + To n-fold a number X, replicate the input value to a length that + is the least common multiple of n and the length of X. Before + each repetition, the input is rotated to the right by 13 bit + positions. The successive n-bit chunks are added together using + 1's-complement addition (that is, with end-around carry) to yield + a n-bit result.... + + Test vectors for n-fold are supplied in appendix A [5]. + + In this section, n-fold is always used to produce c bits of output, + where c is the cipher block size of E. + + The size of the Constant must not be larger than c, because reducing + the length of the Constant by n-folding can cause collisions. + + + + +Raeburn Standards Track [Page 11] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + If the size of the Constant is smaller than c, then the Constant must + be n-folded to length c. This string is used as input to E. If the + block size of E is less than the random-to-key input size, then the + output from E is taken as input to a second invocation of E. This + process is repeated until the number of bits accumulated is greater + than or equal to the random-to-key input size. When enough bits have + been computed, the first k are taken as the random data used to + create the key with the algorithm-dependent random-to-key function. + + As the derived key is the result of one or more encryptions in the + base key, deriving the base key from the derived key is equivalent to + determining the key from a very small number of plaintext/ciphertext + pairs. Thus, this construction is as strong as the cryptosystem + itself. + +5.2. Simplified Profile Parameters + + These are the operations and attributes that must be defined: + + protocol key format + string-to-key function + default string-to-key parameters + key-generation seed length, k + random-to-key function + As above for the normal encryption mechanism profile. + + unkeyed hash algorithm, H + This should be a collision-resistant hash algorithm with fixed- + size output, suitable for use in an HMAC [HMAC]. It must support + inputs of arbitrary length. Its output must be at least the + message block size (below). + + HMAC output size, h + This indicates the size of the leading substring output by the + HMAC function that should be used in transmitted messages. It + should be at least half the output size of the hash function H, + and at least 80 bits; it need not match the output size. + + message block size, m + This is the size of the smallest units the cipher can handle in + the mode in which it is being used. Messages will be padded to a + multiple of this size. If a block cipher is used in a mode that + + + + + + + + + +Raeburn Standards Track [Page 12] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + can handle messages that are not multiples of the cipher block + size, such as CBC mode with cipher text stealing (CTS, see [RC5]), + this value would be one octet. For traditional CBC mode with + padding, it would be the underlying cipher's block size. + + This value must be a multiple of eight bits (one octet). + + encryption/decryption functions, E and D + These are basic encryption and decryption functions for messages + of sizes that are multiples of the message block size. No + integrity checking or confounder should be included here. For + inputs these functions take the IV or similar data, a protocol- + format key, and an octet string, returning a new IV and octet + string. + + The encryption function is not required to use CBC mode but is + assumed to be using something with similar properties. In + particular, prepending a cipher block-size confounder to the + plaintext should alter the entire ciphertext (comparable to + choosing and including a random initial vector for CBC mode). + + The result of encrypting one cipher block (of size c, above) must + be deterministic for the random octet generation function DR in + the previous section to work. For best security, it should also + be no larger than c. + + cipher block size, c + This is the block size of the block cipher underlying the + encryption and decryption functions indicated above, used for key + derivation and for the size of the message confounder and initial + vector. (If a block cipher is not in use, some comparable + parameter should be determined.) It must be at least 5 octets. + + This is not actually an independent parameter; rather, it is a + property of the functions E and D. It is listed here to clarify + the distinction between it and the message block size, m. + + Although there are still a number of properties to specify, they are + fewer and simpler than in the full profile. + +5.3. Cryptosystem Profile Based on Simplified Profile + + The above key derivation function is used to produce three + intermediate keys. One is used for computing checksums of + unencrypted data. The other two are used for encrypting and + checksumming plaintext to be sent encrypted. + + + + + +Raeburn Standards Track [Page 13] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + The ciphertext output is the concatenation of the output of the basic + encryption function E and a (possibly truncated) HMAC using the + specified hash function H, both applied to the plaintext with a + random confounder prefix and sufficient padding to bring it to a + multiple of the message block size. When the HMAC is computed, the + key is used in the protocol key form. + + Decryption is performed by removing the (partial) HMAC, decrypting + the remainder, and verifying the HMAC. The cipher state is an + initial vector, initialized to zero. + + The substring notation "[1..h]" in the following table should be read + as using 1-based indexing; leading substrings are used. + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Raeburn Standards Track [Page 14] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + Cryptosystem from Simplified Profile +------------------------------------------------------------------------ +protocol key format As given. + +specific key structure Three protocol-format keys: { Kc, Ke, Ki }. + +key-generation seed As given. +length + +required checksum As defined below in section 5.4. +mechanism + +cipher state Initial vector (usually of length c) + +initial cipher state All bits zero + +encryption function conf = Random string of length c + pad = Shortest string to bring confounder + and plaintext to a length that's a + multiple of m. + (C1, newIV) = E(Ke, conf | plaintext | pad, + oldstate.ivec) + H1 = HMAC(Ki, conf | plaintext | pad) + ciphertext = C1 | H1[1..h] + newstate.ivec = newIV + +decryption function (C1,H1) = ciphertext + (P1, newIV) = D(Ke, C1, oldstate.ivec) + if (H1 != HMAC(Ki, P1)[1..h]) + report error + newstate.ivec = newIV + +default string-to-key As given. +params + +pseudo-random function tmp1 = H(octet-string) + tmp2 = truncate tmp1 to multiple of m + PRF = E(DK(protocol-key, prfconstant), + tmp2, initial-cipher-state) + + The "prfconstant" used in the PRF operation is the three-octet string + "prf". + + + + + + + + + +Raeburn Standards Track [Page 15] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + Cryptosystem from Simplified Profile +------------------------------------------------------------------------ +key generation functions: + +string-to-key function As given. + +random-to-key function As given. + +key-derivation function The "well-known constant" used for the DK + function is the key usage number, expressed as + four octets in big-endian order, followed by + one octet indicated below. + + Kc = DK(base-key, usage | 0x99); + Ke = DK(base-key, usage | 0xAA); + Ki = DK(base-key, usage | 0x55); + +5.4. Checksum Profiles Based on Simplified Profile + + When an encryption system is defined with the simplified profile + given in section 5.2, a checksum algorithm may be defined for it as + follows: + + Checksum Mechanism from Simplified Profile + -------------------------------------------------- + associated cryptosystem As defined above. + + get_mic HMAC(Kc, message)[1..h] + + verify_mic get_mic and compare + + The HMAC function and key Kc are as described in section 5.3. + +6. Profiles for Kerberos Encryption and Checksum Algorithms + + These profiles describe the encryption and checksum systems defined + for Kerberos. The astute reader will notice that some of them do not + fulfill all the requirements outlined in previous sections. These + systems are defined for backward compatibility; newer implementations + should (whenever possible) attempt to utilize encryption systems that + satisfy all the profile requirements. + + The full list of current encryption and checksum type number + assignments, including values currently reserved but not defined in + this document, is given in section 8. + + + + + + +Raeburn Standards Track [Page 16] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + +6.1. Unkeyed Checksums + + These checksum types use no encryption keys and thus can be used in + combination with any encryption type, but they may only be used with + caution, in limited circumstances where the lack of a key does not + provide a window for an attack, preferably as part of an encrypted + message [6]. Keyed checksum algorithms are recommended. + +6.1.1. The RSA MD5 Checksum + + The RSA-MD5 checksum calculates a checksum by using the RSA MD5 + algorithm [MD5-92]. The algorithm takes as input an input message of + arbitrary length and produces as output a 128-bit (sixteen octet) + checksum. + + rsa-md5 + ---------------------------------------------- + associated cryptosystem any + + get_mic rsa-md5(msg) + + verify_mic get_mic and compare + + The rsa-md5 checksum algorithm is assigned a checksum type number of + seven (7). + +6.1.2. The RSA MD4 Checksum + + The RSA-MD4 checksum calculates a checksum using the RSA MD4 + algorithm [MD4-92]. The algorithm takes as input an input message of + arbitrary length and produces as output a 128-bit (sixteen octet) + checksum. + + rsa-md4 + ---------------------------------------------- + associated cryptosystem any + + get_mic md4(msg) + + verify_mic get_mic and compare + + The rsa-md4 checksum algorithm is assigned a checksum type number of + two (2). + + + + + + + + +Raeburn Standards Track [Page 17] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + +6.1.3. CRC-32 Checksum + + This CRC-32 checksum calculates a checksum based on a cyclic + redundancy check as described in ISO 3309 [CRC] but modified as + described below. The resulting checksum is four (4) octets in + length. The CRC-32 is neither keyed nor collision-proof; thus, the + use of this checksum is not recommended. An attacker using a + probabilistic chosen-plaintext attack as described in [SG92] might be + able to generate an alternative message that satisfies the checksum. + + The CRC-32 checksum used in the des-cbc-crc encryption mode is + identical to the 32-bit FCS described in ISO 3309 with two + exceptions: The sum with the all-ones polynomial times x**k is + omitted, and the final remainder is not ones-complemented. ISO 3309 + describes the FCS in terms of bits, whereas this document describes + the Kerberos protocol in terms of octets. To clarify the ISO 3309 + definition for the purpose of computing the CRC-32 in the des-cbc-crc + encryption mode, the ordering of bits in each octet shall be assumed + to be LSB first. Given this assumed ordering of bits within an + octet, the mapping of bits to polynomial coefficients shall be + identical to that specified in ISO 3309. + + Test values for this modified CRC function are included in appendix + A.5. + + crc32 + ---------------------------------------------- + associated cryptosystem any + + get_mic crc32(msg) + + verify_mic get_mic and compare + + The crc32 checksum algorithm is assigned a checksum type number of + one (1). + +6.2. DES-Based Encryption and Checksum Types + + These encryption systems encrypt information under the Data + Encryption Standard [DES77] by using the cipher block chaining mode + [DESM80]. A checksum is computed as described below and placed in + the cksum field. DES blocks are eight bytes. As a result, the data + to be encrypted (the concatenation of confounder, checksum, and + message) must be padded to an eight byte boundary before encryption. + The values of the padding bytes are unspecified. + + + + + + +Raeburn Standards Track [Page 18] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + Plaintext and DES ciphertext are encoded as blocks of eight octets, + which are concatenated to make the 64-bit inputs for the DES + algorithms. The first octet supplies the eight most significant bits + (with the octet's MSB used as the DES input block's MSB, etc.), the + second octet the next eight bits, and so on. The eighth octet + supplies the 8 least significant bits. + + Encryption under DES using cipher block chaining requires an + additional input in the form of an initialization vector; this vector + is specified below for each encryption system. + + The DES specifications [DESI81] identify four 'weak' and twelve + 'semi-weak' keys; these keys SHALL NOT be used for encrypting + messages for use in Kerberos. The "variant keys" generated for the + RSA-MD5-DES, RSA-MD4-DES, and DES-MAC checksum types by an + eXclusive-OR of a DES key with a constant are not checked for this + property. + + A DES key is eight octets of data. This consists of 56 bits of + actual key data, and eight parity bits, one per octet. The key is + encoded as a series of eight octets written in MSB-first order. The + bits within the key are also encoded in MSB order. For example, if + the encryption key is + (B1,B2,...,B7,P1,B8,...,B14,P2,B15,...,B49,P7,B50,...,B56,P8), where + B1,B2,...,B56 are the key bits in MSB order, and P1,P2,...,P8 are the + parity bits, the first octet of the key would be B1,B2,...,B7,P1 + (with B1 as the most significant bit). See the [DESM80] introduction + for reference. + + Encryption Data Format + + The format for the data to be encrypted includes a one-block + confounder, a checksum, the encoded plaintext, and any necessary + padding, as described in the following diagram. The msg-seq field + contains the part of the protocol message to be encrypted. + + +-----------+----------+---------+-----+ + |confounder | checksum | msg-seq | pad | + +-----------+----------+---------+-----+ + + One generates a random confounder of one block, placing it in + 'confounder'; zeros out the 'checksum' field (of length appropriate + to exactly hold the checksum to be computed); adds the necessary + padding; calculates the appropriate checksum over the whole sequence, + placing the result in 'checksum'; and then encrypts using the + specified encryption type and the appropriate key. + + + + + +Raeburn Standards Track [Page 19] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + String or Random-Data to Key Transformation + + To generate a DES key from two UTF-8 text strings (password and + salt), the two strings are concatenated, password first, and the + result is then padded with zero-valued octets to a multiple of eight + octets. + + The top bit of each octet (always zero if the password is plain + ASCII, as was assumed when the original specification was written) is + discarded, and the remaining seven bits of each octet form a + bitstring. This is then fan-folded and eXclusive-ORed with itself to + produce a 56-bit string. An eight-octet key is formed from this + string, each octet using seven bits from the bitstring, leaving the + least significant bit unassigned. The key is then "corrected" by + correcting the parity on the key, and if the key matches a 'weak' or + 'semi-weak' key as described in the DES specification, it is + eXclusive-ORed with the constant 0x00000000000000F0. This key is + then used to generate a DES CBC checksum on the initial string with + the salt appended. The result of the CBC checksum is then + "corrected" as described above to form the result, which is returned + as the key. + + For purposes of the string-to-key function, the DES CBC checksum is + calculated by CBC encrypting a string using the key as IV and the + final eight byte block as the checksum. + + Pseudocode follows: + + removeMSBits(8byteblock) { + /* Treats a 64 bit block as 8 octets and removes the MSB in + each octet (in big endian mode) and concatenates the + result. E.g., the input octet string: + 01110000 01100001 11110011 01110011 11110111 01101111 + 11110010 01100100 + results in the output bitstring: + 1110000 1100001 1110011 1110011 1110111 1101111 + 1110010 1100100 */ + } + + reverse(56bitblock) { + /* Treats a 56-bit block as a binary string and reverses it. + E.g., the input string: + 1000001 1010100 1001000 1000101 1001110 1000001 + 0101110 1001101 + results in the output string: + 1011001 0111010 1000001 0111001 1010001 0001001 + 0010101 1000001 */ + } + + + +Raeburn Standards Track [Page 20] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + add_parity_bits(56bitblock) { + /* Copies a 56-bit block into a 64-bit block, left shifts + content in each octet, and add DES parity bit. + E.g., the input string: + 1100000 0001111 0011100 0110100 1000101 1100100 + 0110110 0010111 + results in the output string: + 11000001 00011111 00111000 01101000 10001010 11001000 + 01101101 00101111 */ + } + + key_correction(key) { + fixparity(key); + if (is_weak_key(key)) + key = key XOR 0xF0; + return(key); + } + + mit_des_string_to_key(string,salt) { + odd = 1; + s = string | salt; + tempstring = 0; /* 56-bit string */ + pad(s); /* with nulls to 8 byte boundary */ + for (8byteblock in s) { + 56bitstring = removeMSBits(8byteblock); + if (odd == 0) reverse(56bitstring); + odd = ! odd; + tempstring = tempstring XOR 56bitstring; + } + tempkey = key_correction(add_parity_bits(tempstring)); + key = key_correction(DES-CBC-check(s,tempkey)); + return(key); + } + + des_string_to_key(string,salt,params) { + if (length(params) == 0) + type = 0; + else if (length(params) == 1) + type = params[0]; + else + error("invalid params"); + if (type == 0) + mit_des_string_to_key(string,salt); + else + error("invalid params"); + } + + + + + +Raeburn Standards Track [Page 21] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + One common extension is to support the "AFS string-to-key" algorithm, + which is not defined here, if the type value above is one (1). + + For generation of a key from a random bitstring, we start with a 56- + bit string and, as with the string-to-key operation above, insert + parity bits. If the result is a weak or semi-weak key, we modify it + by eXclusive-OR with the constant 0x00000000000000F0: + + des_random_to_key(bitstring) { + return key_correction(add_parity_bits(bitstring)); + } + +6.2.1. DES with MD5 + + The des-cbc-md5 encryption mode encrypts information under DES in CBC + mode with an all-zero initial vector and with an MD5 checksum + (described in [MD5-92]) computed and placed in the checksum field. + + The encryption system parameters for des-cbc-md5 are as follows: + + des-cbc-md5 + -------------------------------------------------------------------- + protocol key format 8 bytes, parity in low bit of each + + specific key structure copy of original key + + required checksum rsa-md5-des + mechanism + + key-generation seed 8 bytes + length + + cipher state 8 bytes (CBC initial vector) + + initial cipher state all-zero + + encryption function des-cbc(confounder | checksum | msg | pad, + ivec=oldstate) + where + checksum = md5(confounder | 0000... + | msg | pad) + + newstate = last block of des-cbc output + + decryption function decrypt encrypted text and verify checksum + + newstate = last block of ciphertext + + + + +Raeburn Standards Track [Page 22] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + des-cbc-md5 + -------------------------------------------------------------------- + default string-to-key empty string + params + + pseudo-random function des-cbc(md5(input-string), ivec=0) + + key generation functions: + + string-to-key des_string_to_key + + random-to-key des_random_to_key + + key-derivation identity + + The des-cbc-md5 encryption type is assigned the etype value three + (3). + +6.2.2. DES with MD4 + + The des-cbc-md4 encryption mode also encrypts information under DES + in CBC mode, with an all-zero initial vector. An MD4 checksum + (described in [MD4-92]) is computed and placed in the checksum field. + + des-cbc-md4 + -------------------------------------------------------------------- + protocol key format 8 bytes, parity in low bit of each + + specific key structure copy of original key + + required checksum rsa-md4-des + mechanism + + key-generation seed 8 bytes + length + + cipher state 8 bytes (CBC initial vector) + + initial cipher state all-zero + + encryption function des-cbc(confounder | checksum | msg | pad, + ivec=oldstate) + where + checksum = md4(confounder | 0000... + | msg | pad) + + newstate = last block of des-cbc output + + + + +Raeburn Standards Track [Page 23] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + des-cbc-md4 + -------------------------------------------------------------------- + + decryption function decrypt encrypted text and verify checksum + + newstate = last block of ciphertext + + default string-to-key empty string + params + + pseudo-random function des-cbc(md5(input-string), ivec=0) + + key generation functions: + + string-to-key des_string_to_key + + random-to-key copy input, then fix parity bits + + key-derivation identity + + Note that des-cbc-md4 uses md5, not md4, in the PRF definition. + + The des-cbc-md4 encryption algorithm is assigned the etype value two + (2). + +6.2.3. DES with CRC + + The des-cbc-crc encryption type uses DES in CBC mode with the key + used as the initialization vector, with a four-octet CRC-based + checksum computed as described in section 6.1.3. Note that this is + not a standard CRC-32 checksum, but a slightly modified one. + + des-cbc-crc + -------------------------------------------------------------------- + protocol key format 8 bytes, parity in low bit of each + + specific key structure copy of original key + + required checksum rsa-md5-des + mechanism + + key-generation seed 8 bytes + length + + cipher state 8 bytes (CBC initial vector) + + + + + + +Raeburn Standards Track [Page 24] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + des-cbc-crc + -------------------------------------------------------------------- + initial cipher state copy of original key + + encryption function des-cbc(confounder | checksum | msg | pad, + ivec=oldstate) + where + checksum = crc(confounder | 00000000 + | msg | pad) + + newstate = last block of des-cbc output + + decryption function decrypt encrypted text and verify checksum + + newstate = last block of ciphertext + + default string-to-key empty string + params + + pseudo-random function des-cbc(md5(input-string), ivec=0) + + key generation functions: + + string-to-key des_string_to_key + + random-to-key copy input, then fix parity bits + + key-derivation identity + + The des-cbc-crc encryption algorithm is assigned the etype value one + (1). + +6.2.4. RSA MD5 Cryptographic Checksum Using DES + + The RSA-MD5-DES checksum calculates a keyed collision-proof checksum + by prepending an eight octet confounder before the text, applying the + RSA MD5 checksum algorithm, and encrypting the confounder and the + checksum by using DES in cipher-block-chaining (CBC) mode with a + variant of the key, where the variant is computed by eXclusive-ORing + the key with the hexadecimal constant 0xF0F0F0F0F0F0F0F0. The + initialization vector should be zero. The resulting checksum is 24 + octets long. + + + + + + + + + +Raeburn Standards Track [Page 25] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + rsa-md5-des + ---------------------------------------------------------------- + associated cryptosystem des-cbc-md5, des-cbc-md4, des-cbc-crc + + get_mic des-cbc(key XOR 0xF0F0F0F0F0F0F0F0, + conf | rsa-md5(conf | msg)) + + verify_mic decrypt and verify rsa-md5 checksum + + The rsa-md5-des checksum algorithm is assigned a checksum type number + of eight (8). + +6.2.5. RSA MD4 Cryptographic Checksum Using DES + + The RSA-MD4-DES checksum calculates a keyed collision-proof checksum + by prepending an eight octet confounder before the text, applying the + RSA MD4 checksum algorithm [MD4-92], and encrypting the confounder + and the checksum using DES in cipher-block-chaining (CBC) mode with a + variant of the key, where the variant is computed by eXclusive-ORing + the key with the constant 0xF0F0F0F0F0F0F0F0 [7]. The initialization + vector should be zero. The resulting checksum is 24 octets long. + + rsa-md4-des + ---------------------------------------------------------------- + associated cryptosystem des-cbc-md5, des-cbc-md4, des-cbc-crc + + get_mic des-cbc(key XOR 0xF0F0F0F0F0F0F0F0, + conf | rsa-md4(conf | msg), + ivec=0) + + verify_mic decrypt and verify rsa-md4 checksum + + The rsa-md4-des checksum algorithm is assigned a checksum type number + of three (3). + +6.2.6. RSA MD4 Cryptographic Checksum Using DES Alternative + + The RSA-MD4-DES-K checksum calculates a keyed collision-proof + checksum by applying the RSA MD4 checksum algorithm and encrypting + the results by using DES in cipher block chaining (CBC) mode with a + DES key as both key and initialization vector. The resulting + checksum is 16 octets long. This checksum is tamper-proof and + believed to be collision-proof. Note that this checksum type is the + old method for encoding the RSA-MD4-DES checksum; it is no longer + recommended. + + + + + + +Raeburn Standards Track [Page 26] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + rsa-md4-des-k + ---------------------------------------------------------------- + associated cryptosystem des-cbc-md5, des-cbc-md4, des-cbc-crc + + get_mic des-cbc(key, md4(msg), ivec=key) + + verify_mic decrypt, compute checksum and compare + + The rsa-md4-des-k checksum algorithm is assigned a checksum type + number of six (6). + +6.2.7. DES CBC Checksum + + The DES-MAC checksum is computed by prepending an eight octet + confounder to the plaintext, padding with zero-valued octets if + necessary to bring the length to a multiple of eight octets, + performing a DES CBC-mode encryption on the result by using the key + and an initialization vector of zero, taking the last block of the + ciphertext, prepending the same confounder, and encrypting the pair + by using DES in cipher-block-chaining (CBC) mode with a variant of + the key, where the variant is computed by eXclusive-ORing the key + with the constant 0xF0F0F0F0F0F0F0F0. The initialization vector + should be zero. The resulting checksum is 128 bits (sixteen octets) + long, 64 bits of which are redundant. This checksum is tamper-proof + and collision-proof. + + des-mac + --------------------------------------------------------------------- + associated des-cbc-md5, des-cbc-md4, des-cbc-crc + cryptosystem + + get_mic des-cbc(key XOR 0xF0F0F0F0F0F0F0F0, + conf | des-mac(key, conf | msg | pad, ivec=0), + ivec=0) + + verify_mic decrypt, compute DES MAC using confounder, compare + + The des-mac checksum algorithm is assigned a checksum type number of + four (4). + +6.2.8. DES CBC Checksum Alternative + + The DES-MAC-K checksum is computed by performing a DES CBC-mode + encryption of the plaintext, with zero-valued padding bytes if + necessary to bring the length to a multiple of eight octets, and by + using the last block of the ciphertext as the checksum value. It is + keyed with an encryption key that is also used as the initialization + vector. The resulting checksum is 64 bits (eight octets) long. This + + + +Raeburn Standards Track [Page 27] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + checksum is tamper-proof and collision-proof. Note that this + checksum type is the old method for encoding the DESMAC checksum; it + is no longer recommended. + + des-mac-k + ---------------------------------------------------------------- + associated cryptosystem des-cbc-md5, des-cbc-md4, des-cbc-crc + + get_mic des-mac(key, msg | pad, ivec=key) + + verify_mic compute MAC and compare + + The des-mac-k checksum algorithm is assigned a checksum type number + of five (5). + +6.3. Triple-DES Based Encryption and Checksum Types + + This encryption and checksum type pair is based on the Triple DES + cryptosystem in Outer-CBC mode and on the HMAC-SHA1 message + authentication algorithm. + + A Triple DES key is the concatenation of three DES keys as described + above for des-cbc-md5. A Triple DES key is generated from random + data by creating three DES keys from separate sequences of random + data. + + Encrypted data using this type must be generated as described in + section 5.3. If the length of the input data is not a multiple of + the block size, zero-valued octets must be used to pad the plaintext + to the next eight-octet boundary. The confounder must be eight + random octets (one block). + + The simplified profile for Triple DES, with key derivation as defined + in section 5, is as follows: + + des3-cbc-hmac-sha1-kd, hmac-sha1-des3-kd + ------------------------------------------------ + protocol key format 24 bytes, parity in low + bit of each + + key-generation seed 21 bytes + length + + + + + + + + + +Raeburn Standards Track [Page 28] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + des3-cbc-hmac-sha1-kd, hmac-sha1-des3-kd + ------------------------------------------------ + hash function SHA-1 + + HMAC output size 160 bits + + message block size 8 bytes + + default string-to-key empty string + params + + encryption and triple-DES encrypt and + decryption functions decrypt, in outer-CBC + mode (cipher block size + 8 octets) + + key generation functions: + + random-to-key DES3random-to-key (see + below) + + string-to-key DES3string-to-key (see + below) + + The des3-cbc-hmac-sha1-kd encryption type is assigned the value + sixteen (16). The hmac-sha1-des3-kd checksum algorithm is assigned a + checksum type number of twelve (12). + +6.3.1. Triple DES Key Production (random-to-key, string-to-key) + + The 168 bits of random key data are converted to a protocol key value + as follows. First, the 168 bits are divided into three groups of 56 + bits, which are expanded individually into 64 bits as follows: + + DES3random-to-key: + 1 2 3 4 5 6 7 p + 9 10 11 12 13 14 15 p + 17 18 19 20 21 22 23 p + 25 26 27 28 29 30 31 p + 33 34 35 36 37 38 39 p + 41 42 43 44 45 46 47 p + 49 50 51 52 53 54 55 p + 56 48 40 32 24 16 8 p + + The "p" bits are parity bits computed over the data bits. The output + of the three expansions, each corrected to avoid "weak" and "semi- + weak" keys as in section 6.2, are concatenated to form the protocol + key value. + + + +Raeburn Standards Track [Page 29] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + The string-to-key function is used to transform UTF-8 passwords into + DES3 keys. The DES3 string-to-key function relies on the "N-fold" + algorithm and DK function, described in section 5. + + The n-fold algorithm is applied to the password string concatenated + with a salt value. For 3-key triple DES, the operation will involve + a 168-fold of the input password string, to generate an intermediate + key, from which the user's long-term key will be derived with the DK + function. The DES3 string-to-key function is shown here in + pseudocode: + + DES3string-to-key(passwordString, salt, params) + if (params != emptyString) + error("invalid params"); + s = passwordString + salt + tmpKey = random-to-key(168-fold(s)) + key = DK (tmpKey, KerberosConstant) + + Weak key checking is performed in the random-to-key and DK + operations. The KerberosConstant value is the byte string {0x6b 0x65 + 0x72 0x62 0x65 0x72 0x6f 0x73}. These values correspond to the ASCII + encoding for the string "kerberos". + +7. Use of Kerberos Encryption Outside This Specification + + Several Kerberos-based application protocols and preauthentication + systems have been designed and deployed that perform encryption and + message integrity checks in various ways. Although in some cases + there may be good reason for specifying these protocols in terms of + specific encryption or checksum algorithms, we anticipate that in + many cases this will not be true, and more generic approaches + independent of particular algorithms will be desirable. Rather than + have each protocol designer reinvent schemes for protecting data, + using multiple keys, etc., we have attempted to present in this + section a general framework that should be sufficient not only for + the Kerberos protocol itself but also for many preauthentication + systems and application protocols, while trying to avoid some of the + assumptions that can work their way into such protocol designs. + + Some problematic assumptions we've seen (and sometimes made) include + the following: a random bitstring is always valid as a key (not true + for DES keys with parity); the basic block encryption chaining mode + provides no integrity checking, or can easily be separated from such + checking (not true for many modes in development that do both + simultaneously); a checksum for a message always results in the same + value (not true if a confounder is incorporated); an initial vector + is used (may not be true if a block cipher in CBC mode is not in + use). + + + +Raeburn Standards Track [Page 30] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + Although such assumptions the may hold for any given set of + encryption and checksum algorithms, they may not be true of the next + algorithms to be defined, leaving the application protocol unable to + make use of those algorithms without updates to its specification. + + The Kerberos protocol uses only the attributes and operations + described in sections 3 and 4. Preauthentication systems and + application protocols making use of Kerberos are encouraged to use + them as well. The specific key and string-to-key parameters should + generally be treated as opaque. Although the string-to-key + parameters are manipulated as an octet string, the representation for + the specific key structure is implementation defined; it may not even + be a single object. + + We don't recommend doing so, but some application protocols will + undoubtedly continue to use the key data directly, even if only in + some of the currently existing protocol specifications. An + implementation intended to support general Kerberos applications may + therefore need to make the key data available, as well as the + attributes and operations described in sections 3 and 4 [8]. + +8. Assigned Numbers + + The following encryption-type numbers are already assigned or + reserved for use in Kerberos and related protocols. + + encryption type etype section or comment + ----------------------------------------------------------------- + des-cbc-crc 1 6.2.3 + des-cbc-md4 2 6.2.2 + des-cbc-md5 3 6.2.1 + [reserved] 4 + des3-cbc-md5 5 + [reserved] 6 + des3-cbc-sha1 7 + dsaWithSHA1-CmsOID 9 (pkinit) + md5WithRSAEncryption-CmsOID 10 (pkinit) + sha1WithRSAEncryption-CmsOID 11 (pkinit) + rc2CBC-EnvOID 12 (pkinit) + rsaEncryption-EnvOID 13 (pkinit from PKCS#1 v1.5) + rsaES-OAEP-ENV-OID 14 (pkinit from PKCS#1 v2.0) + des-ede3-cbc-Env-OID 15 (pkinit) + des3-cbc-sha1-kd 16 6.3 + aes128-cts-hmac-sha1-96 17 [KRB5-AES] + aes256-cts-hmac-sha1-96 18 [KRB5-AES] + rc4-hmac 23 (Microsoft) + rc4-hmac-exp 24 (Microsoft) + subkey-keymaterial 65 (opaque; PacketCable) + + + +Raeburn Standards Track [Page 31] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + (The "des3-cbc-sha1" assignment is a deprecated version using no key + derivation. It should not be confused with des3-cbc-sha1-kd.) + + Several numbers have been reserved for use in encryption systems not + defined here. Encryption-type numbers have unfortunately been + overloaded on occasion in Kerberos-related protocols, so some of the + reserved numbers do not and will not correspond to encryption systems + fitting the profile presented here. + + The following checksum-type numbers are assigned or reserved. As + with encryption-type numbers, some overloading of checksum numbers + has occurred. + + Checksum type sumtype checksum section or + value size reference + --------------------------------------------------------------------- + CRC32 1 4 6.1.3 + rsa-md4 2 16 6.1.2 + rsa-md4-des 3 24 6.2.5 + des-mac 4 16 6.2.7 + des-mac-k 5 8 6.2.8 + rsa-md4-des-k 6 16 6.2.6 + rsa-md5 7 16 6.1.1 + rsa-md5-des 8 24 6.2.4 + rsa-md5-des3 9 24 ?? + sha1 (unkeyed) 10 20 ?? + hmac-sha1-des3-kd 12 20 6.3 + hmac-sha1-des3 13 20 ?? + sha1 (unkeyed) 14 20 ?? + hmac-sha1-96-aes128 15 20 [KRB5-AES] + hmac-sha1-96-aes256 16 20 [KRB5-AES] + [reserved] 0x8003 ? [GSS-KRB5] + + Encryption and checksum-type numbers are signed 32-bit values. Zero + is invalid, and negative numbers are reserved for local use. All + standardized values must be positive. + +9. Implementation Notes + + The "interface" described here is the minimal information that must + be defined to make a cryptosystem useful within Kerberos in an + interoperable fashion. The use of functional notation used in some + places is not an attempt to define an API for cryptographic + functionality within Kerberos. Actual implementations providing + clean APIs will probably make additional information available, that + could be derived from a specification written to the framework given + here. For example, an application designer may wish to determine the + largest number of bytes that can be encrypted without overflowing a + + + +Raeburn Standards Track [Page 32] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + certain size output buffer or conversely, the maximum number of bytes + that might be obtained by decrypting a ciphertext message of a given + size. (In fact, an implementation of the GSS-API Kerberos mechanism + [GSS-KRB5] will require some of these.) + + The presence of a mechanism in this document should not be taken to + indicate that it must be implemented for compliance with any + specification; required mechanisms will be specified elsewhere. + Indeed, some of the mechanisms described here for backward + compatibility are now considered rather weak for protecting critical + data. + +10. Security Considerations + + Recent years have brought so many advancements in large-scale attacks + capability against DES that it is no longer considered a strong + encryption mechanism. Triple-DES is generally preferred in its + place, despite its poorer performance. See [ESP-DES] for a summary + of some of the potential attacks and [EFF-DES] for a detailed + discussion of the implementation of particular attacks. However, + most Kerberos implementations still have DES as their primary + interoperable encryption type. + + DES has four 'weak' keys and twelve 'semi-weak' keys, and the use of + single-DES here avoids them. However, DES also has 48 'possibly- + weak' keys [Schneier96] (note that the tables in many editions of the + reference contains errors) that are not avoided. + + DES weak keys have the property that E1(E1(P)) = P (where E1 denotes + encryption of a single block with key 1). DES semi-weak keys, or + "dual" keys, are pairs of keys with the property that E1(P) = D2(P), + and thus E2(E1(P)) = P. Because of the use of CBC mode and the + leading random confounder, however, these properties are unlikely to + present a security problem. + + Many of the choices concerning when to perform weak-key corrections + relate more to compatibility with existing implementations than to + any risk analysis. + + Although checks are also done for the component DES keys in a + triple-DES key, the nature of the weak keys make it extremely + unlikely that they will weaken the triple-DES encryption. It is only + slightly more likely than having the middle of the three sub-keys + match one of the other two, which effectively converts the encryption + to single-DES - a case we make no effort to avoid. + + + + + + +Raeburn Standards Track [Page 33] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + The true CRC-32 checksum is not collision-proof; an attacker could + use a probabilistic chosen-plaintext attack to generate a valid + message even if a confounder is used [SG92]. The use of collision- + proof checksums is of course recommended for environments where such + attacks represent a significant threat. The "simplifications" (read: + bugs) introduced when CRC-32 was implemented for Kerberos cause + leading zeros effectively to be ignored, so messages differing only + in leading zero bits will have the same checksum. + + [HMAC] and [IPSEC-HMAC] discuss weaknesses of the HMAC algorithm. + Unlike [IPSEC-HMAC], the triple-DES specification here does not use + the suggested truncation of the HMAC output. As pointed out in + [IPSEC-HMAC], SHA-1 was not developed for use as a keyed hash + function, which is a criterion of HMAC. [HMAC-TEST] contains test + vectors for HMAC-SHA-1. + + The mit_des_string_to_key function was originally constructed with + the assumption that all input would be ASCII; it ignores the top bit + of each input byte. Folding with XOR is also not an especially good + mixing mechanism for preserving randomness. + + The n-fold function used in the string-to-key operation for des3- + cbc-hmac-sha1-kd was designed to cause each bit of input to + contribute equally to the output. It was not designed to maximize or + equally distribute randomness in the input, and conceivably + randomness may be lost in cases of partially structured input. This + should only be an issue for highly structured passwords, however. + + [RFC1851] discusses the relative strength of triple-DES encryption. + The relatively slow speed of triple-DES encryption may also be an + issue for some applications. + + [Bellovin91] suggests that analyses of encryption schemes include a + model of an attacker capable of submitting known plaintexts to be + encrypted with an unknown key, as well as be able to perform many + types of operations on known protocol messages. Recent experiences + with the chosen-plaintext attacks on Kerberos version 4 bear out the + value of this suggestion. + + The use of unkeyed encrypted checksums, such as those used in the + single-DES cryptosystems specified in [Kerb1510], allows for cut- + and-paste attacks, especially if a confounder is not used. In + addition, unkeyed encrypted checksums are vulnerable to chosen- + plaintext attacks: An attacker with access to an encryption oracle + can easily encrypt the required unkeyed checksum along with the + + + + + + +Raeburn Standards Track [Page 34] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + chosen plaintext. [Bellovin99] These weaknesses, combined with a + common implementation design choice described below, allow for a + cross-protocol attack from version 4 to version 5. + + The use of a random confounder is an important means to prevent an + attacker from making effective use of protocol exchanges as an + encryption oracle. In Kerberos version 4, the encryption of constant + plaintext to constant ciphertext makes an effective encryption oracle + for an attacker. The use of random confounders in [Kerb1510] + frustrates this sort of chosen-plaintext attack. + + Using the same key for multiple purposes can enable or increase the + scope of chosen-plaintext attacks. Some software that implements + both versions 4 and 5 of the Kerberos protocol uses the same keys for + both versions. This enables the encryption oracle of version 4 to be + used to attack version 5. Vulnerabilities to attacks such as this + cross-protocol attack make it unwise to use a key for multiple + purposes. + + This document, like the Kerberos protocol, does not address limiting + the amount of data a key may be used with to a quantity based on the + robustness of the algorithm or size of the key. It is assumed that + any defined algorithms and key sizes will be strong enough to support + very large amounts of data, or they will be deprecated once + significant attacks are known. + + This document also places no bounds on the amount of data that can be + handled in various operations. To avoid denial of service attacks, + implementations will probably seek to restrict message sizes at some + higher level. + +11. IANA Considerations + + Two registries for numeric values have been created: Kerberos + Encryption Type Numbers and Kerberos Checksum Type Numbers. These + are signed values ranging from -2147483648 to 2147483647. Positive + values should be assigned only for algorithms specified in accordance + with this specification for use with Kerberos or related protocols. + Negative values are for private use; local and experimental + algorithms should use these values. Zero is reserved and may not be + assigned. + + Positive encryption- and checksum-type numbers may be assigned + following either of two policies described in [BCP26]. + + Standards-track specifications may be assigned values under the + Standards Action policy. + + + + +Raeburn Standards Track [Page 35] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + Specifications in non-standards track RFCs may be assigned values + after Expert Review. A non-IETF specification may be assigned values + by publishing an Informational or standards-track RFC referencing the + external specification; that specification must be public and + published in some permanent record, much like the IETF RFCs. It is + highly desirable, though not required, that the full specification be + published as an IETF RFC. + + Smaller encryption type values should be used for IETF standards- + track mechanisms, and much higher values (16777216 and above) for + other mechanisms. (Rationale: In the Kerberos ASN.1 encoding, + smaller numbers encode to smaller octet sequences, so this favors + standards-track mechanisms with slightly smaller messages.) Aside + from that guideline, IANA may choose numbers as it sees fit. + + Internet-Draft specifications should not include values for + encryption- and checksum-type numbers. Instead, they should indicate + that values would be assigned by IANA when the document is approved + as an RFC. For development and interoperability testing, values in + the private-use range (negative values) may be used but should not be + included in the draft specification. + + Each registered value should have an associated unique reference + name. The lists given in section 8 were used to create the initial + registry; they include reservations for specifications in progress in + parallel with this document, and certain other values believed to + already be in use. + +12. Acknowledgements + + This document is an extension of the encryption specification + included in [Kerb1510] by B. Clifford Neuman and John Kohl, and much + of the text of the background, concepts, and DES specifications is + drawn directly from that document. + + The abstract framework presented in this document was put together by + Jeff Altman, Sam Hartman, Jeff Hutzelman, Cliff Neuman, Ken Raeburn, + and Tom Yu, and the details were refined several times based on + comments from John Brezak and others. + + Marc Horowitz wrote the original specification of triple-DES and key + derivation in a pair of Internet-Drafts (under the names draft- + horowitz-key-derivation and draft-horowitz-kerb-key-derivation) that + were later folded into a draft revision of [Kerb1510], from which + this document was later split off. + + + + + + +Raeburn Standards Track [Page 36] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + Tom Yu provided the text describing the modifications to the standard + CRC algorithm as Kerberos implementations actually use it, and some + of the text in the Security Considerations section. + + Miroslav Jurisic provided information for one of the UTF-8 test cases + for the string-to-key functions. + + Marcus Watts noticed some errors in earlier versions and pointed out + that the simplified profile could easily be modified to support + cipher text stealing modes. + + Simon Josefsson contributed some clarifications to the DES "CBC + checksum" and string-to-key and weak key descriptions, and some test + vectors. + + Simon Josefsson, Louis LeVay, and others also caught some errors in + earlier versions of this document. + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Raeburn Standards Track [Page 37] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + +A. Test Vectors + + This section provides test vectors for various functions defined or + described in this document. For convenience, most inputs are ASCII + strings, though some UTF-8 samples are provided for string-to-key + functions. Keys and other binary data are specified as hexadecimal + strings. + +A.1. n-fold + + The n-fold function is defined in section 5.1. As noted there, the + sample vector in the original paper defining the algorithm appears to + be incorrect. Here are some test cases provided by Marc Horowitz and + Simon Josefsson: + + 64-fold("012345") = + 64-fold(303132333435) = be072631276b1955 + + 56-fold("password") = + 56-fold(70617373776f7264) = 78a07b6caf85fa + + 64-fold("Rough Consensus, and Running Code") = + 64-fold(526f75676820436f6e73656e7375732c20616e642052756e + 6e696e6720436f6465) = bb6ed30870b7f0e0 + + 168-fold("password") = + 168-fold(70617373776f7264) = + 59e4a8ca7c0385c3c37b3f6d2000247cb6e6bd5b3e + + 192-fold("MASSACHVSETTS INSTITVTE OF TECHNOLOGY") + 192-fold(4d41535341434856534554545320494e5354495456544520 + 4f4620544543484e4f4c4f4759) = + db3b0d8f0b061e603282b308a50841229ad798fab9540c1b + + 168-fold("Q") = + 168-fold(51) = + 518a54a2 15a8452a 518a54a2 15a8452a + 518a54a2 15 + + 168-fold("ba") = + 168-fold(6261) = + fb25d531 ae897449 9f52fd92 ea9857c4 + ba24cf29 7e + + Here are some additional values corresponding to folded values of the + string "kerberos"; the 64-bit form is used in the des3 string-to-key + (section 6.3.1). + + + + +Raeburn Standards Track [Page 38] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + 64-fold("kerberos") = + 6b657262 65726f73 + 128-fold("kerberos") = + 6b657262 65726f73 7b9b5b2b 93132b93 + 168-fold("kerberos") = + 8372c236 344e5f15 50cd0747 e15d62ca + 7a5a3bce a4 + 256-fold("kerberos") = + 6b657262 65726f73 7b9b5b2b 93132b93 + 5c9bdcda d95c9899 c4cae4de e6d6cae4 + + Note that the initial octets exactly match the input string when the + output length is a multiple of the input length. + +A.2. mit_des_string_to_key + + The function mit_des_string_to_key is defined in section 6.2. We + present here several test values, with some of the intermediate + results. The fourth test demonstrates the use of UTF-8 with three + characters. The last two tests are specifically constructed so as to + trigger the weak-key fixups for the intermediate key produced by + fan-folding; we have no test cases that cause such fixups for the + final key. + +UTF-8 encodings used in test vector: +eszett U+00DF C3 9F s-caron U+0161 C5 A1 +c-acute U+0107 C4 87 g-clef U+1011E F0 9D 84 9E + +Test vector: + +salt: "ATHENA.MIT.EDUraeburn" + 415448454e412e4d49542e4544557261656275726e +password: "password" 70617373776f7264 +fan-fold result: c01e38688ac86c2e +intermediate key: c11f38688ac86d2f +DES key: cbc22fae235298e3 + +salt: "WHITEHOUSE.GOVdanny" + 5748495445484f5553452e474f5664616e6e79 +password: "potatoe" 706f7461746f65 +fan-fold result: a028944ee63c0416 +intermediate key: a129944fe63d0416 +DES key: df3d32a74fd92a01 + +salt: "EXAMPLE.COMpianist" 4558414D504C452E434F4D7069616E697374 +password: g-clef (U+1011E) f09d849e +fan-fold result: 3c4a262c18fab090 +intermediate key: 3d4a262c19fbb091 + + + +Raeburn Standards Track [Page 39] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + +DES key: 4ffb26bab0cd9413 + +salt: "ATHENA.MIT.EDUJuri" + s-caron(U+0161) + "i" + c-acute(U+0107) + 415448454e412e4d49542e4544554a757269c5a169c487 +password: eszett(U+00DF) + c39f +fan-fold result:b8f6c40e305afc9e +intermediate key: b9f7c40e315bfd9e +DES key: 62c81a5232b5e69d + +salt: "AAAAAAAA" 4141414141414141 +password: "11119999" 3131313139393939 +fan-fold result: e0e0e0e0f0f0f0f0 +intermediate key: e0e0e0e0f1f1f101 +DES key: 984054d0f1a73e31 + +salt: "FFFFAAAA" 4646464641414141 +password: "NNNN6666" 4e4e4e4e36363636 +fan-fold result: 1e1e1e1e0e0e0e0e +intermediate key: 1f1f1f1f0e0e0efe +DES key: c4bf6b25adf7a4f8 + + This trace provided by Simon Josefsson shows the intermediate + processing stages of one of the test inputs: + + string_to_key (des-cbc-md5, string, salt) + ;; string: + ;; `password' (length 8 bytes) + ;; 70 61 73 73 77 6f 72 64 + ;; salt: + ;; `ATHENA.MIT.EDUraeburn' (length 21 bytes) + ;; 41 54 48 45 4e 41 2e 4d 49 54 2e 45 44 55 72 61 + ;; 65 62 75 72 6e + des_string_to_key (string, salt) + ;; String: + ;; `password' (length 8 bytes) + ;; 70 61 73 73 77 6f 72 64 + ;; Salt: + ;; `ATHENA.MIT.EDUraeburn' (length 21 bytes) + ;; 41 54 48 45 4e 41 2e 4d 49 54 2e 45 44 55 72 61 + ;; 65 62 75 72 6e + odd = 1; + s = string | salt; + tempstring = 0; /* 56-bit string */ + pad(s); /* with nulls to 8 byte boundary */ + ;; s = pad(string|salt): + ;; `passwordATHENA.MIT.EDUraeburn\x00\x00\x00' + ;; (length 32 bytes) + + + +Raeburn Standards Track [Page 40] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + ;; 70 61 73 73 77 6f 72 64 41 54 48 45 4e 41 2e 4d + ;; 49 54 2e 45 44 55 72 61 65 62 75 72 6e 00 00 00 + for (8byteblock in s) { + ;; loop iteration 0 + ;; 8byteblock: + ;; `password' (length 8 bytes) + ;; 70 61 73 73 77 6f 72 64 + ;; 01110000 01100001 01110011 01110011 01110111 01101111 + ;; 01110010 01100100 + 56bitstring = removeMSBits(8byteblock); + ;; 56bitstring: + ;; 1110000 1100001 1110011 1110011 1110111 1101111 + ;; 1110010 1100100 + if (odd == 0) reverse(56bitstring); ;; odd=1 + odd = ! odd + tempstring = tempstring XOR 56bitstring; + ;; tempstring + ;; 1110000 1100001 1110011 1110011 1110111 1101111 + ;; 1110010 1100100 + + for (8byteblock in s) { + ;; loop iteration 1 + ;; 8byteblock: + ;; `ATHENA.M' (length 8 bytes) + ;; 41 54 48 45 4e 41 2e 4d + ;; 01000001 01010100 01001000 01000101 01001110 01000001 + ;; 00101110 01001101 + 56bitstring = removeMSBits(8byteblock); + ;; 56bitstring: + ;; 1000001 1010100 1001000 1000101 1001110 1000001 + ;; 0101110 1001101 + if (odd == 0) reverse(56bitstring); ;; odd=0 + reverse(56bitstring) + ;; 56bitstring after reverse + ;; 1011001 0111010 1000001 0111001 1010001 0001001 + ;; 0010101 1000001 + odd = ! odd + tempstring = tempstring XOR 56bitstring; + ;; tempstring + ;; 0101001 1011011 0110010 1001010 0100110 1100110 + ;; 1100111 0100101 + + for (8byteblock in s) { + ;; loop iteration 2 + ;; 8byteblock: + ;; `IT.EDUra' (length 8 bytes) + ;; 49 54 2e 45 44 55 72 61 + ;; 01001001 01010100 00101110 01000101 01000100 01010101 + + + +Raeburn Standards Track [Page 41] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + ;; 01110010 01100001 + 56bitstring = removeMSBits(8byteblock); + ;; 56bitstring: + ;; 1001001 1010100 0101110 1000101 1000100 1010101 + ;; 1110010 1100001 + if (odd == 0) reverse(56bitstring); ;; odd=1 + odd = ! odd + tempstring = tempstring XOR 56bitstring; + ;; tempstring + ;; 1100000 0001111 0011100 0001111 1100010 0110011 + ;; 0010101 1000100 + + for (8byteblock in s) { + ;; loop iteration 3 + ;; 8byteblock: + ;; `eburn\x00\x00\x00' (length 8 bytes) + ;; 65 62 75 72 6e 00 00 00 + ;; 01100101 01100010 01110101 01110010 01101110 00000000 + ;; 00000000 00000000 + 56bitstring = removeMSBits(8byteblock); + ;; 56bitstring: + ;; 1100101 1100010 1110101 1110010 1101110 0000000 + ;; 0000000 0000000 + if (odd == 0) reverse(56bitstring); ;; odd=0 + reverse(56bitstring) + ;; 56bitstring after reverse + ;; 0000000 0000000 0000000 0111011 0100111 1010111 + ;; 0100011 1010011 + odd = ! odd + tempstring = tempstring XOR 56bitstring; + ;; tempstring + ;; 1100000 0001111 0011100 0110100 1000101 1100100 + ;; 0110110 0010111 + + for (8byteblock in s) { + } + ;; for loop terminated + + tempkey = key_correction(add_parity_bits(tempstring)); + ;; tempkey + ;; `\xc1\x1f8h\x8a\xc8m\x2f' (length 8 bytes) + ;; c1 1f 38 68 8a c8 6d 2f + ;; 11000001 00011111 00111000 01101000 10001010 11001000 + ;; 01101101 00101111 + + key = key_correction(DES-CBC-check(s,tempkey)); + ;; key + ;; `\xcb\xc2\x2f\xae\x23R\x98\xe3' (length 8 bytes) + + + +Raeburn Standards Track [Page 42] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + ;; cb c2 2f ae 23 52 98 e3 + ;; 11001011 11000010 00101111 10101110 00100011 01010010 + ;; 10011000 11100011 + + ;; string_to_key key: + ;; `\xcb\xc2\x2f\xae\x23R\x98\xe3' (length 8 bytes) + ;; cb c2 2f ae 23 52 98 e3 + +A.3. DES3 DR and DK + + These tests show the derived-random and derived-key values for the + des3-hmac-sha1-kd encryption scheme, using the DR and DK functions + defined in section 6.3.1. The input keys were randomly generated; + the usage values are from this specification. + + key: dce06b1f64c857a11c3db57c51899b2cc1791008ce973b92 + usage: 0000000155 + DR: 935079d14490a75c3093c4a6e8c3b049c71e6ee705 + DK: 925179d04591a79b5d3192c4a7e9c289b049c71f6ee604cd + + key: 5e13d31c70ef765746578531cb51c15bf11ca82c97cee9f2 + usage: 00000001aa + DR: 9f58e5a047d894101c469845d67ae3c5249ed812f2 + DK: 9e58e5a146d9942a101c469845d67a20e3c4259ed913f207 + + key: 98e6fd8a04a4b6859b75a176540b9752bad3ecd610a252bc + usage: 0000000155 + DR: 12fff90c773f956d13fc2ca0d0840349dbd39908eb + DK: 13fef80d763e94ec6d13fd2ca1d085070249dad39808eabf + + key: 622aec25a2fe2cad7094680b7c64940280084c1a7cec92b5 + usage: 00000001aa + DR: f8debf05b097e7dc0603686aca35d91fd9a5516a70 + DK: f8dfbf04b097e6d9dc0702686bcb3489d91fd9a4516b703e + + key: d3f8298ccb166438dcb9b93ee5a7629286a491f838f802fb + usage: 6b65726265726f73 ("kerberos") + DR: 2270db565d2a3d64cfbfdc5305d4f778a6de42d9da + DK: 2370da575d2a3da864cebfdc5204d56df779a7df43d9da43 + + key: c1081649ada74362e6a1459d01dfd30d67c2234c940704da + usage: 0000000155 + DR: 348056ec98fcc517171d2b4d7a9493af482d999175 + DK: 348057ec98fdc48016161c2a4c7a943e92ae492c989175f7 + + key: 5d154af238f46713155719d55e2f1f790dd661f279a7917c + usage: 00000001aa + DR: a8818bc367dadacbe9a6c84627fb60c294b01215e5 + + + +Raeburn Standards Track [Page 43] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + DK: a8808ac267dada3dcbe9a7c84626fbc761c294b01315e5c1 + + key: 798562e049852f57dc8c343ba17f2ca1d97394efc8adc443 + usage: 0000000155 + DR: c813f88b3be2b2f75424ce9175fbc8483b88c8713a + DK: c813f88a3be3b334f75425ce9175fbe3c8493b89c8703b49 + + key: 26dce334b545292f2feab9a8701a89a4b99eb9942cecd016 + usage: 00000001aa + DR: f58efc6f83f93e55e695fd252cf8fe59f7d5ba37ec + DK: f48ffd6e83f83e7354e694fd252cf83bfe58f7d5ba37ec5d + +A.4. DES3string_to_key + + These are the keys generated for some of the above input strings for + triple-DES with key derivation as defined in section 6.3.1. + + salt: "ATHENA.MIT.EDUraeburn" + passwd: "password" + key: 850bb51358548cd05e86768c313e3bfef7511937dcf72c3e + + salt: "WHITEHOUSE.GOVdanny" + passwd: "potatoe" + key: dfcd233dd0a43204ea6dc437fb15e061b02979c1f74f377a + + salt: "EXAMPLE.COMbuckaroo" + passwd: "penny" + key: 6d2fcdf2d6fbbc3ddcadb5da5710a23489b0d3b69d5d9d4a + + salt: "ATHENA.MIT.EDUJuri" + s-caron(U+0161) + "i" + + c-acute(U+0107) + passwd: eszett(U+00DF) + key: 16d5a40e1ce3bacb61b9dce00470324c831973a7b952feb0 + + salt: "EXAMPLE.COMpianist" + passwd: g-clef(U+1011E) + key: 85763726585dbc1cce6ec43e1f751f07f1c4cbb098f40b19 + +A.5. Modified CRC-32 + + Below are modified-CRC32 values for various ASCII and octet strings. + Only the printable ASCII characters are checksummed, without a C- + style trailing zero-valued octet. The 32-bit modified CRC and the + sequence of output bytes as used in Kerberos are shown. (The octet + values are separated here to emphasize that they are octet values and + not 32-bit numbers, which will be the most convenient form for + manipulation in some implementations. The bit and byte order used + + + + +Raeburn Standards Track [Page 44] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + internally for such a number is irrelevant; the octet sequence + generated is what is important.) + + mod-crc-32("foo") = 33 bc 32 73 + mod-crc-32("test0123456789") = d6 88 3e b8 + mod-crc-32("MASSACHVSETTS INSTITVTE OF TECHNOLOGY") = f7 80 41 e3 + mod-crc-32(8000) = 4b 98 83 3b + mod-crc-32(0008) = 32 88 db 0e + mod-crc-32(0080) = 20 83 b8 ed + mod-crc-32(80) = 20 83 b8 ed + mod-crc-32(80000000) = 3b b6 59 ed + mod-crc-32(00000001) = 96 30 07 77 + +B. Significant Changes from RFC 1510 + + The encryption and checksum mechanism profiles are new. The old + specification defined a few operations for various mechanisms but + didn't outline what abstract properties should be required of new + mechanisms, or how to ensure that a mechanism specification is + complete enough for interoperability between implementations. The + new profiles differ from the old specification in a few ways: + + Some message definitions in [Kerb1510] could be read as permitting + the initial vector to be specified by the application; the text + was too vague. It is explicitly not permitted in this + specification. Some encryption algorithms may not use + initialization vectors, so relying on chosen, secret + initialization vectors for security is unwise. Also, the + prepended confounder in the existing algorithms is roughly + equivalent to a per-message initialization vector that is revealed + in encrypted form. However, carrying state across from one + encryption to another is explicitly permitted through the opaque + "cipher state" object. + + The use of key derivation is new. + + Several new methods are introduced, including generation of a key + in wire-protocol format from random input data. + + The means for influencing the string-to-key algorithm are laid out + more clearly. + + Triple-DES support is new. + + The pseudo-random function is new. + + The des-cbc-crc, DES string-to-key and CRC descriptions have been + updated to align them with existing implementations. + + + +Raeburn Standards Track [Page 45] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + [Kerb1510] did not indicate what character set or encoding might be + used for pass phrases and salts. + + In [Kerb1510], key types, encryption algorithms, and checksum + algorithms were only loosely associated, and the association was not + well described. In this specification, key types and encryption + algorithms have a one-to-one correspondence, and associations between + encryption and checksum algorithms are described so that checksums + can be computed given negotiated keys, without requiring further + negotiation for checksum types. + +Notes + + [1] Although Message Authentication Code (MAC) or Message Integrity + Check (MIC) would be more appropriate terms for many of the uses + in this document, we continue to use the term checksum for + historical reasons. + + [2] Extending CBC mode across messages would be one obvious example + of this chaining. Another might be the use of counter mode, with + a counter randomly initialized and attached to the ciphertext; a + second message could continue incrementing the counter when + chaining the cipher state, thus avoiding having to transmit + another counter value. However, this chaining is only useful for + uninterrupted, ordered sequences of messages. + + [3] In the case of Kerberos, the encrypted objects will generally be + ASN.1 DER encodings, which contain indications of their length in + the first few octets. + + [4] As of the time of this writing, new modes of operation have been + proposed, some of which may permit encryption and integrity + protection simultaneously. After some of these proposals have + been subjected to adequate analysis, we may wish to formulate a + new simplified profile based on one of them. + + [5] It should be noted that the sample vector in appendix B.2 of the + original paper appears to be incorrect. Two independent + implementations from the specification (one in C by Marc + Horowitz, and another in Scheme by Bill Sommerfeld) agree on a + value different from that in [Blumenthal96]. + + [6] For example, in MIT's implementation of [Kerb1510], the rsa-md5 + unkeyed checksum of application data may be included in an + authenticator encrypted in a service's key. + + [7] Using a variant of the key limits the use of a key to a + particular function, separating the functions of generating a + + + +Raeburn Standards Track [Page 46] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + checksum from other encryption performed using the session key. + The constant 0xF0F0F0F0F0F0F0F0 was chosen because it maintains + key parity. The properties of DES precluded the use of the + complement. The same constant is used for similar purpose in the + Message Integrity Check in the Privacy Enhanced Mail standard. + + [8] Perhaps one of the more common reasons for directly performing + encryption is direct control over the negotiation and to select a + "sufficiently strong" encryption algorithm (whatever that means + in the context of a given application). Although Kerberos + directly provides no direct facility for negotiating encryption + types between the application client and server, there are other + means to accomplish similar goals (for example, requesting only + "strong" session key types from the KDC, and assuming that the + type actually returned by the KDC will be understood and + supported by the application server). + +Normative References + + [BCP26] Narten, T. and H. Alvestrand, "Guidelines for Writing + an IANA Considerations Section in RFCs", BCP 26, RFC + 2434, October 1998. + + [Bellare98] Bellare, M., Desai, A., Pointcheval, D., and P. + Rogaway, "Relations Among Notions of Security for + Public-Key Encryption Schemes". Extended abstract + published in Advances in Cryptology-Crypto 98 + Proceedings, Lecture Notes in Computer Science Vol. + 1462, H. Krawcyzk ed., Springer-Verlag, 1998. + + [Blumenthal96] Blumenthal, U. and S. Bellovin, "A Better Key Schedule + for DES-Like Ciphers", Proceedings of PRAGOCRYPT '96, + 1996. + + [CRC] International Organization for Standardization, "ISO + Information Processing Systems - Data Communication - + High-Level Data Link Control Procedure - Frame + Structure," IS 3309, 3rd Edition, October 1984. + + [DES77] National Bureau of Standards, U.S. Department of + Commerce, "Data Encryption Standard," Federal + Information Processing Standards Publication 46, + Washington, DC, 1977. + + + + + + + + +Raeburn Standards Track [Page 47] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + [DESI81] National Bureau of Standards, U.S. Department of + Commerce, "Guidelines for implementing and using NBS + Data Encryption Standard," Federal Information + Processing Standards Publication 74, Washington, DC, + 1981. + + [DESM80] National Bureau of Standards, U.S. Department of + Commerce, "DES Modes of Operation," Federal + Information Processing Standards Publication 81, + Springfield, VA, December 1980. + + [Dolev91] Dolev, D., Dwork, C., and M. Naor, "Non-malleable + cryptography", Proceedings of the 23rd Annual + Symposium on Theory of Computing, ACM, 1991. + + [HMAC] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: + Keyed-Hashing for Message Authentication", RFC 2104, + February 1997. + + [KRB5-AES] Raeburn, K., "Advanced Encryption Standard (AES) + Encryption for Kerberos 5", RFC 3962, February 2005. + + [MD4-92] Rivest, R., "The MD4 Message-Digest Algorithm", RFC + 1320, April 1992. + + [MD5-92] Rivest, R., "The MD5 Message-Digest Algorithm ", RFC + 1321, April 1992. + + [SG92] Stubblebine, S. and V. D. Gligor, "On Message + Integrity in Cryptographic Protocols," in Proceedings + of the IEEE Symposium on Research in Security and + Privacy, Oakland, California, May 1992. + +Informative References + + [Bellovin91] Bellovin, S. M. and M. Merrit, "Limitations of the + Kerberos Authentication System", in Proceedings of the + Winter 1991 Usenix Security Conference, January, 1991. + + [Bellovin99] Bellovin, S. M. and D. Atkins, private communications, + 1999. + + [EFF-DES] Electronic Frontier Foundation, "Cracking DES: Secrets + of Encryption Research, Wiretap Politics, and Chip + Design", O'Reilly & Associates, Inc., May 1998. + + [ESP-DES] Madson, C. and N. Doraswamy, "The ESP DES-CBC Cipher + Algorithm With Explicit IV", RFC 2405, November 1998. + + + +Raeburn Standards Track [Page 48] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + + [GSS-KRB5] Linn, J., "The Kerberos Version 5 GSS-API Mechanism", + RFC 1964, June 1996. + + [HMAC-TEST] Cheng, P. and R. Glenn, "Test Cases for HMAC-MD5 and + HMAC-SHA-1", RFC 2202, September 1997. + + [IPSEC-HMAC] Madson, C. and R. Glenn, "The Use of HMAC-SHA-1-96 + within ESP and AH", RFC 2404, November 1998. + + [Kerb] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The + Kerberos Network Authentication Service (V5)", Work in + Progress, September 2004. + + [Kerb1510] Kohl, J. and C. Neuman, "The Kerberos Network + Authentication Service (V5)", RFC 1510, September + 1993. + + [RC5] Baldwin, R. and R. Rivest, "The RC5, RC5-CBC, RC5- + CBC-Pad, and RC5-CTS Algorithms", RFC 2040, October + 1996. + + [RFC1851] Karn, P., Metzger, P., and W. Simpson, "The ESP Triple + DES Transform", RFC 1851, September 1995. + + [Schneier96] Schneier, B., "Applied Cryptography Second Edition", + John Wiley & Sons, New York, NY, 1996. ISBN 0-471- + 12845-7. + +Editor's Address + + Kenneth Raeburn + Massachusetts Institute of Technology + 77 Massachusetts Avenue + Cambridge, MA 02139 + + EMail: raeburn@mit.edu + + + + + + + + + + + + + + + +Raeburn Standards Track [Page 49] + +RFC 3961 Encryption and Checksum Specifications February 2005 + + +Full Copyright Statement + + Copyright (C) The Internet Society (2005). + + This document is subject to the rights, licenses and restrictions + contained in BCP 78, and except as set forth therein, the authors + retain all their rights. + + This document and the information contained herein are provided on an + "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS + OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET + ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, + INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE + INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED + WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. + +Intellectual Property + + The IETF takes no position regarding the validity or scope of any + Intellectual Property Rights or other rights that might be claimed to + pertain to the implementation or use of the technology described in + this document or the extent to which any license under such rights + might or might not be available; nor does it represent that it has + made any independent effort to identify any such rights. 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