Generate MDC packets for better integrity protection

[wyllys66]

The Mailvelope plugin for Chrome and Firefox rejects messages without an MDC, which apparently is what 1.4.7 generates. I haven't tested with 1.5.0 yet. gpg just shows a warning when the MDC is missing. It's easy to test to see if the MDC is generated, just use "gpg --list-packets" on the encrypted message to see if it is generating an encrypted data packet with the mdc:

:encrypted data packet:
    length: unknown
    mdc_method: 2

The messages generate by php-pgp do not have the MDC method:

:encrypted data packet:
    length: 546

See: mailvelope/mailvelope#341

[DanielRuf]

https://tools.ietf.org/html/rfc4880#section-5.13

We have to add this packet in a future version.

https://github.com/singpolyma/openpgp-php/blob/a1fe3a6e58c83a7c84a954397ac0bbc373a12fc7/lib/openpgp_crypt_symmetric.php#L25

https://github.com/singpolyma/openpgp-php/blob/d37e91efdae960582b7932cf2be7d59c1894c3b1/lib/openpgp.php#L1820

Contributors and pull requests are very welcome.

Structure of the packet (body) [\xD3] [\x14] [bytes of the sha1 hash of the plaintext message]

[DanielRuf]

php-gpg 1.6.0 is now available and will add a MDC packet to encrypted messages. But old messages will still throw a warning about the missing MDC packet.