[Cryptography] How should we encrypt external mail attachments

[Peter Gutmann]

John Levine <johnl at iecc.com> writes:

>Sounds like SHA-256 would be fine. For encryption, I guess AES CBC, so what
>IV should I use?

Just use HKDF to generate all the crypto material you need.  So the email
embeds a fixed-length secret value and then:

  AES-key = HKDF( secret, "encryption" );
  SHA-key = HKDF( secret, "authentication" );
  IV = HKDF( secret, "IV" );

This means you can later switch algorithms and modes if required without
having to mess with the details of key sizes and cryptovariable types.

Peter.