Support for outdated, weak, and dangerous parts of OpenPGP

The items to be deprecated have been reviewed and approved by our "consulting cryptographers," Payman Mohassel and Juan Garay of Yahoo Labs.

Support removed

Tag 9 packets. Yahoo and Google have both already deprecated and removed support for Tag 9 (symmetrically encrypted) packets.

These packets provide unauthenticated encryption and, if supported, can be used in a downgrade attack for senders who only use SEIPD packets. See [encrux][encrux] for details.

ASAP

V3 public keys. Yahoo and GnuPG (as of version 2.1) have both already deprecated V3 public keys for any use. We recommend that other implementations do the same.

By May 1, 2015

Yahoo has deprecated, and intends to disable support for all uses, of the following algorithms specified for use with OpenPGP v4:

Asymmetric algorithms, unless > 3070 bit key length: RSA-ES, ELG-E.
Asymmetric algorithms, generally: RSA-E, RSA-S, DSA
Symmetric cipher algorithms: IDEA, TDES, CAST5, Blowfish, Twofish
Compression algorithms: ZLIB (the format provides no benefits over DEFLATE, and is more malleable)
Hash algorithms: MD5, SHA-1, RIPEMD160, SHA-2-224

We do not, at present, support any of the CAMELLIA algorithms or Bzip2. It is unlikely that we will do so in future.

By September 1, 2015

Inconsistent combinations of primitives. In particular, it is likely that we will not support RFC 6637 keys or packets unless they conform to the 128-bit or 192-bit subprofiles specified in that document. (End-to-End does not at present support P-521, but if we add support for curves over that field, we would support an analogous "256-bit" subprofile.)

AES-128. The efficiency of multi-target attacks leaves no safety margin for cryptanalysis. The performance difference between AES-128 and AES-256 on typical messages is negligible.

Eventually

Finally, other things that may eventually result in messages or keys being treated as invalid:

A published public key that is more than 1 year old. (This is mainly taken care of by requiring > 3070 bit RSA keys...)
Signature by a public key which has ever signed a message or key using MD-5 or SHA-1
Any literal data packet tag that is unusually formatted, or contains a non-zero-length subpart
Any compressed data packet using a compression method other than algorithm 0 (uncompressed)

YahooArchive / end-to-end