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draft-ietf-822ext-md5-00.txt
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draft Content-MD5 Header Apr 3
The Content-MD5 Header
Sat Apr 3 17:10:53 1993
Marshall T. Rose
Dover Beach Consulting, Inc.
mrose@dbc.mtview.ca.us
Status of this Memo
This document is an Internet Draft. Internet Drafts are
working documents of the Internet Engineering Task Force
(IETF), its Areas, and its Working Groups. Note that other
groups may also distribute working documents as Internet
Drafts.
Internet Drafts are valid for a maximum of six months and may
be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet Drafts as reference
material or to cite them other than as a "work in progress".
Abstract
This memo specifies an optional header field, Content-MD5, for
use with MIME-conformant messages.
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1. Introduction
Despite all of the mechanisms MIME [1] provides to 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 the application of that algorithm can be
optionally used as an integrity check for MIME mail.
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2. Computation of the Content-MD5 Header
The MD5 algorithm is computed on the canonical form of the
data. In particular, this means that the sender computes the
MIC on the raw data, before applying any content-transfer-
encoding, and that the receiver also computes the MIC on the
raw data, after undoing any content-transfer-encoding. For
textual data, the algorithm must be computed on data in which
the canonical form for newlines applies, that is, in which
each newline is represented by CRLF.
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 must
then be encoded according to the base64 algorithm in order to
obtain a value that can legally be placed in a message header
field. Thus, if the data in a MIME entity has an MD5 MIC that
consists of the (unlikely) 16 octets "Check Integrity!", then
that MIME entity's header could contain the field
Content-MD5: Q2hlY2sgSW50ZWdyaXR5IQ==
3. Use of the Content-MD5 header
Use of the Content-MD5 field is completely optional, but its
use is recommended wherever complete data integrity is
desired, but Privacy-Enhanced Mail services [3] are not
available.
If the Content-MD5 field is present, a MIME-conformant reader
may choose to use it to verify that the contents of a MIME
entity have not been modified during transport.
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 header is used.
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4. References
[1] N. Borenstein, N. Freed. MIME: Mechanisms for Specifying
and Describing the Format of Internet Message Bodies.
Request for Comments 1341, (June, 1992).
[2] R. Rivest, The MD5 Message-Digest Algorithm. Request for
Comments 1321, (April, 1992).
[3] J. Linn, Privacy Enhancement for Internet Electronic
Mail, Part I: Message Encryption and Authentication
Procedures. Request for Comments 1421, (February, 1993).
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Table of Contents
1 Introduction .......................................... 2
2 Computation of the Content-MD5 Header ................. 3
3 Use of the Content-MD5 header ......................... 3
4 References ............................................ 4
Expires October 3, 1993 [Page 5]