RFCs in HTML Format


RFC 1544

                      The Content-MD5 Header Field

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.



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