Issue 1112 encrypted exports
Work on issue #1112.
- Use a random salt different for each export,
- the salt is embedded in the export,
- use HMAC-SHA256 instead of HMAC-SHA512 for PBKDF2's pseudo random function. Even HMAC-SHA1 is considered secure,
- use AES in GCM mode for its authentication feature,
- support AES128, AES192 and AES256. But there is no configuration setting (yet?),
- many cipher parameters are saved along the export.
- Save de KDF (PBKDF2) in the export,
- save the PBKDF2 PRF (HMAC-SHA256) in the export,
- save the IV size in the export