4.1.2.1 Processing Steps

A four-phase transformation procedure is employed in order to represent encrypted message text in a universally transmissible form and to enable messages encrypted on one type of host computer to be decrypted on a different type of host computer. A plaintext message is accepted in local form, using the host's native character set and line representation. The local form is converted to a canonical message text representation, defined as equivalent to the inter-SMTP representation of message text. This canonical representation forms the input to the MIC computation step (applicable to ENCRYPTED, MIC- ONLY, and MIC-CLEAR messages) and the encryption process (applicable to ENCRYPTED messages only). For ENCRYPTED PEM messages, the canonical representation is padded as required by the encryption algorithm, and this padded canonical representation is encrypted. The encrypted text (for an ENCRYPTED message) or the unpadded canonical form (for a MIC-ONLY message) is then encoded into a printable form. The printable form is composed of a restricted character set which is chosen to be universally representable across sites, and which will not be disrupted by processing within and between MTS entities. MIC-CLEAR PEM messages omit the printable encoding step.

The output of the previous processing steps is combined with a set of header fields carrying cryptographic control information. The resulting PEM message is passed to the electronic mail system to be included within the text portion of a transmitted message. There is no requirement that a PEM message comprise the entirety of an MTS message's text portion; this allows PEM-protected information to be accompanied by (unprotected) annotations. It is also permissible for multiple PEM messages (and associated unprotected text, outside the PEM message boundaries) to be represented within the encapsulated text of a higher-level PEM message. PEM message signatures are forwardable when asymmetric key management is employed; an authorized recipient of a PEM message with confidentiality applied can reduce that message to a signed but unencrypted form for forwarding purposes or can re-encrypt that message for subsequent transmission.

When a PEM message is received, the cryptographic control fields within its encapsulated header provide the information required for each authorized recipient to perform MIC validation and decryption of the received message text. For ENCRYPTED and MIC-ONLY messages, the printable encoding is converted to a bitstring. Encrypted portions of the transmitted message are decrypted. The MIC is validated. Then, the recipient PEM process converts the canonical representation to its appropriate local form.