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Network Working Group                                            M. RoseRequest for Comments: 1544                  Dover Beach Consulting, Inc.Category: Standards Track                                  November 1993The Content-MD5 Header FieldStatus 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 Contents1. Introduction ..........................................12. Generation of the Content-MD5 Field ...................23. Processing the Content-MD5 field ......................24. Security Considerations ...............................35. Acknowledgements ......................................36. References ............................................37. Author's Address ......................................31.  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 19932.  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.  (ConsultSection 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 inAppendix 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 19934.  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.usRose                                                            [Page 3]