Binary MIME Messages

This document registers a tag for binary MIME messages in Concise Binary Object Representation (CBOR) (ref. 1).


The CBOR specification already defines a tag for messages according to the Multipurpose Internet Mail Extensions (MIME). But unfortunately, its choice of major type, a UTF-8 string, leaves much to be desired; namely, it becomes undefined how to interpret a string representing a MIME message if the message uses a Content-Transfer-Encoding of 8bit or Binary. The tag defined here serves to identify these and other kinds of MIME messages using their binary encoding rather than a UTF-8 encoding.

Detailed Semantics

Tag 257 can be applied to a byte string (major type 2) to indicate that the byte string is a MIME message under RFC 2045 (ref. 2). Note that headers are included in MIME messages.

A CBOR decoder can treat data items with tag 257 that are not MIME messages as an error, but this specification doesn't define how a CBOR implementation ought to behave in this case. Section 3.4 of RFC 7049 (ref. 1) details this kind of error-handling behavior.

Security Considerations

If a CBOR implementation interprets a media type encoded in a MIME message, the security considerations of that media type apply. In particular, MIME messages can use character encodings other than ASCII and Unicode, and some features of other character encodings can cause security issues when converting to Unicode due to misinterpretation of the encoded bytes (see section 3.6 of Unicode Technical Report 36 for more information, ref. 3).

MIME is based on the Internet Message Format (RFC 5322, ref. 4); see section 5 of that RFC for additional security considerations.


Ref. 1. Bormann, C. and Hoffman, P. "Concise Binary Object Representation (CBOR)". RFC 7049, October 2013.

Ref. 2. Freed, N. and Borenstein, N. "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies". RFC 2045, November 1996.

Ref. 3 (informative). Davis, M. and Suignard, M. "Unicode Security Considerations". Unicode Technical Report 36, 12 Nov. 2013.

Ref. 4 (informative). Resnick, P., ed. "Internet Message Format". RFC 5322, October 2008.


Peter Occil (poccil14 at gmail dot com)

My CBOR home page.

Any copyright to this specification is released to the Public Domain.