RFC 1864 (rfc1864) - Page 2 of 4

The Content-MD5 Header Field

RFC 1864                Content-MD5 Header Field            October 1995


   algorithm itself is defined in [2]. This memo specifies how the
   algorithm may be used as an integrity check for MIME mail.

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 MIME entity's object.  In
   particular, this means that the sender applies the MD5 algorithm on
   the data immediately after conversion to canonical form, before
   applying any content-transfer-encoding, and that the receiver also
   applies the MD5 algorithm on the canonical form, after undoing any
   content-transfer-encoding.  For textual data, this means 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 canonical encoding model of MIME is described in
   Appendix G of [1].

   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.







Myers & Rose                Standards Track