enc(1): document that AEAD is not and will not be supported

ones provided by configured engines.

The B<enc> program does not support authenticated encryption modes

like CCM and GCM. The utility does not store or retrieve the

authentication tag.

like CCM and GCM, and will not support such modes in the future.

The B<enc> interface by necessity must begin streaming output (e.g.,

to standard output when B<-out> is not used before the authentication

tag could be validated, leading to the usage of B<enc> in pipelines

that begin processing untrusted data and are not capable of rolling

back upon authentication failure.  The AEAD modes currently in common

use also suffer from catastrophic failure of confidentiality and/or

integrity upon reuse of key/iv/nonce, and since B<enc> places the

entire burden of key/iv/nonce management upon the user, the risk of

exposing AEAD modes is too great to allow.  These key/iv/nonce

management issues also affect other modes currently exposed in B<enc>,

but the failure modes are less extreme in these cases, and the

functionality cannot be removed with a stable release branch.

For bulk encryption of data, whether using authenticated encryption

modes or other modes, L<cms(1)> is recommended, as it provides a

standard data format and performs the needed key/iv/nonce management.

 base64             Base 64