RFC1544 - The Content-MD5 Header Field

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　　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.

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.

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", RFC1521, Bellcore,
Innosoft, September 1993.

[2] Rivest, R., "The MD5 Message-Digest Algorithm", RFC1321, 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", RFC1421,
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

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