4.7. OTHER FIELDS

4.7.1. SUBJECT

This is intended to provide a summary, or indicate the nature, of the message.

4.7.2. COMMENTS

Permits adding text comments onto the message without disturbing the contents of the message's body.

4.7.3. ENCRYPTED

Sometimes, data encryption is used to increase the privacy of message contents. If the body of a message has been encrypted, to keep its contents private, the "Encrypted" field can be used to note the fact and to indicate the nature of the encryption. The first <word> parameter indicates the software used to encrypt the body, and the second, optional <word> is intended to aid the recipient in selecting the proper decryption key. This code word may be viewed as an index to a table of keys held by the recipient.

Note: Unfortunately, headers must contain envelope, as well as contents, information. Consequently, it is necessary that they remain unencrypted, so that mail transport services may access them. Since names, addresses, and "Subject" field contents may contain sensitive information, this requirement limits total message privacy.

Names of encryption software are registered with the Network Information Center, SRI International, Menlo Park, California.

4.7.4. EXTENSION-FIELD

A limited number of common fields have been defined in this document. As network mail requirements dictate, additional fields may be standardized. To provide user-defined fields with a measure of safety, in name selection, such extension-fields will never have names that begin with the string "X-".

Names of Extension-fields are registered with the Network Information Center, SRI International, Menlo Park, California.

4.7.5. USER-DEFINED-FIELD

Individual users of network mail are free to define and use additional header fields. Such fields must have names which are not already used in the current specification or in any definitions of extension-fields, and the overall syntax of these user-defined-fields must conform to this specification's rules for delimiting and folding fields. Due to the extension-field publishing process, the name of a user-defined-field may be pre-empted

Note: The prefatory string "X-" will never be used in the names of Extension-fields. This provides user-defined fields with a protected set of names.