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+Network Working Group                                            M. Rose
+Request for Comments: 1544                  Dover Beach Consulting, Inc.
+Category: Standards Track                                  November 1993
+
+
+                      The Content-MD5 Header Field
+
+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.
+
+Abstract
+
+   This memo specifies an optional header field, Content-MD5, for use
+   with MIME-conformant messages.
+
+Table of Contents
+
+   1. Introduction ..........................................    1
+   2. Generation of the Content-MD5 Field ...................    2
+   3. Processing the Content-MD5 field ......................    2
+   4. Security Considerations ...............................    3
+   5. Acknowledgements ......................................    3
+   6. References ............................................    3
+   7. Author's Address ......................................    3
+
+1.  Introduction
+
+   Despite all of the mechanisms provided by MIME [1] which attempt to
+   protect data from being damaged in the course of email transport, it
+   is still desirable to have a mechanism for verifying that the data,
+   once decoded, are intact.  For this reason, this memo defines the use
+   of an optional header field, Content-MD5, which may be used as a
+   message integrity check (MIC), to verify that the decoded data are
+   the same data that were initially sent.
+
+   MD5 is an algorithm for computing a 128 bit "digest" of arbitrary-
+   length data, with a high degree of confidence that any alterations in
+   the data will be reflected in alterations in the digest.  The MD5
+   algorithm itself is defined in [2].  This memo specifies how the
+   algorithm may be used as an integrity check for MIME mail.
+
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+Rose                                                            [Page 1]
+
+RFC 1544                Content-MD5 Header Field           November 1993
+
+
+2.  Generation of the Content-MD5 Field
+
+   The Content-MD5 field is generated by only an originating user agent.
+   Message relays and gateways are expressly forbidden from generating a
+   Content-MD5 field.
+
+   Use of the Content-MD5 field is completely optional, but its use is
+   recommended whenever data integrity is desired, but Privacy-Enhanced
+   Mail services [3] are not available.  (Consult Section 4 for further
+   details.) The Content-MD5 field may only be added to MIME entities of
+   a `leaf' nature, i.e., the Content-MD5 field may be used with any
+   content type other than multipart or message/rfc822.
+
+   To generate the value of the Content-MD5 field, the MD5 algorithm is
+   computed on the canonical form of the data.  In particular, this
+   means that the sender applies the MD5 algorithm on the raw data,
+   before applying any content-transfer-encoding, and that the receiver
+   also applies the MD5 algorithm on the raw data, after undoing any
+   content-transfer-encoding.  For textual data, the MD5 algorithm must
+   be computed on data in which the canonical form for newlines applies,
+   that is, in which each newline is represented by a CR-LF pair.
+
+   The output of the MD5 algorithm is a 128 bit digest.  When viewed in
+   network byte order (big-endian order), this yields a sequence of 16
+   octets of binary data.  These 16 octets are then encoded according to
+   the base64 algorithm in order to obtain the value that is placed in
+   the Content-MD5 field.  Thus, if the application of the MD5 algorithm
+   over the raw data of a MIME entity results in a digest having the
+   (unlikely) value of "Check Integrity!", then that MIME entity's
+   header could contain the field
+
+               Content-MD5:  Q2hlY2sgSW50ZWdyaXR5IQ==
+
+   Finally, as discussed in Appendix B of [1], textual data is regularly
+   altered in the normal delivery of mail.  Because the addition or
+   deletion of trailing white space will result in a different digest,
+   either the quoted-printable or base64 algorithm should be employed as
+   a content-transfer-encoding when the Content-MD5 field is used.
+
+3.  Processing the Content-MD5 field
+
+   If the Content-MD5 field is present, a recipient user agent may
+   choose to use it to verify that the contents of a MIME entity have
+   not been modified during transport.  Message relays and gateways are
+   expressly forbidden to alter its processing based on the presence of
+   the Content-MD5 field.  However, a message gateway is allowed to
+   remove the Content-MD5 field if the corresponding MIME entity is
+   translated into a different content-type.
+
+
+
+Rose                                                            [Page 2]
+
+RFC 1544                Content-MD5 Header Field           November 1993
+
+
+4.  Security Considerations
+
+   This document specifies a data integrity service that protects data
+   from accidental modification while in transit from the sender to the
+   recipient.  A secure data integrity service, such as that provided by
+   Privacy Enhanced Mail [3], is conjectured to protect data from all
+   modifications.
+
+5.  Acknowledgements
+
+   This memo is based almost entirely on text originally written by
+   Nathaniel Borenstein of Bellcore.  In addition, several improvements
+   were suggested by Keith Moore of the University of Tennessee,
+   Knoxville.
+
+6.  References
+
+   [1] Borenstein, N., and N. Freed, "MIME (Multipurpose Internet Mail
+       Extensions) Part One: Mechanisms for Specifying and Describing
+       the Format of Internet Message Bodies", RFC 1521, Bellcore,
+       Innosoft, September 1993.
+
+   [2] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, MIT
+       Laboratory for Computer Science and RSA Data Security, Inc.,
+       April 1992.
+
+   [3] Linn, J., "Privacy Enhancement for Internet Electronic Mail, Part
+       I: Message Encryption and Authentication Procedures", RFC 1421,
+       IAB IRTF PSRG, IETF PEM WG, February 1993.
+
+7.  Author's Address
+
+   Marshall T. Rose
+   Dover Beach Consulting, Inc.
+   420 Whisman Court
+   Mountain View, CA 94043-2112
+
+   Phone: (415) 968-1052
+   EMail: mrose@dbc.mtview.ca.us
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+Rose                                                            [Page 3]
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