Dover Beach Consulting, Inc.
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.
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 .......................... 3 4. Security Considerations ................................... 3 5. Acknowledgements .......................................... 3 6. References ................................................ 3 7. Authors' Addresses ........................................ 4
Despite all of the mechanisms provided by MIME  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. The Content-MD5 header may also be placed in the encapsulated headers of an object of type message/external-body, to be used to verify that the retreived and decoded data are the same data that were initially referenced.
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 . 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  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 .
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
Finally, as discussed in Appendix B of , 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 their 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 , is conjectured to protect data from all modifications.
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.
 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.  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, MIT Laboratory for Computer Science and RSA Data Security, Inc., April 1992.  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. Authors' Addresses
John G. Myers
Carnegie Mellon University
Marshall T. Rose
Dover Beach Consulting, Inc.