RFC 3156 (rfc3156) - Page 2 of 15
MIME Security with OpenPGP
Alternative Format: Original Text Document
RFC 3156 MIME Security with OpenPGP August 2001 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119. 2. OpenPGP data formats OpenPGP implementations can generate either ASCII armor (described in [1]) or 8-bit binary output when encrypting data, generating a digital signature, or extracting public key data. The ASCII armor output is the REQUIRED method for data transfer. This allows those users who do not have the means to interpret the formats described in this document to be able to extract and use the OpenPGP information in the message. When the amount of data to be transmitted requires that it be sent in many parts, the MIME message/partial mechanism SHOULD be used rather than the multi-part ASCII armor OpenPGP format. 3. Content-Transfer-Encoding restrictions Multipart/signed and multipart/encrypted are to be treated by agents as opaque, meaning that the data is not to be altered in any way [2], [7]. However, many existing mail gateways will detect if the next hop does not support MIME or 8-bit data and perform conversion to either Quoted-Printable or Base64. This presents serious problems for multipart/signed, in particular, where the signature is invalidated when such an operation occurs. For this reason all data signed according to this protocol MUST be constrained to 7 bits (8- bit data MUST be encoded using either Quoted-Printable or Base64). Note that this also includes the case where a signed object is also encrypted (see section 6). This restriction will increase the likelihood that the signature will be valid upon receipt. Additionally, implementations MUST make sure that no trailing whitespace is present after the MIME encoding has been applied. Note: In most cases, trailing whitespace can either be removed, or protected by applying an appropriate content-transfer-encoding. However, special care must be taken when any header lines - either in MIME entity headers, or in embedded RFC 822 headers - are present which only consist of whitespace: Such lines must be removed entirely, since replacing them by empty lines would turn them into header delimiters, and change the semantics of the message. The restrictions on whitespace are necessary in order to make the hash calculated invariant under the text and binary mode signature mechanisms provided by OpenPGP [1]. Also, they help to avoid compatibility problems with PGP implementations which predate the OpenPGP specification. Elkins, et al. Standards Track
