    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.

    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

    Rose                                                            [Page 3]