Add support for UMAC

[fpoirotte]

The objective behind this issue is to add support for UMAC in Pssht. UMAC is known to be supported by at least OpenSSH.

See the following websites for more information:

• http://en.wikipedia.org/wiki/UMAC (generic information)
• http://fastcrypto.org/umac/ (several resources: code samples, security analysis, etc.)
• https://tools.ietf.org/html/draft-miller-secsh-umac-01 (OpenSSH draft)
• https://tools.ietf.org/html/rfc4418 (RFC defining the UHASH & UMAC concepts)

[fpoirotte]

OpenSSH uses the following MAC names for UMAC:

• umac-64-etm@openssh.com
• umac-128-etm@openssh.com
• umac-64@openssh.com
• umac-128@openssh.com

while the draft defines the following MAC methods:

• umac-32
• umac-64
• umac-96
• umac-128

Not really sure what the differences are... guess I'll just have to find out along the way. The EtM variants are pretty obvious. As for the "@openssh.com" suffix, maybe they added it later as an indicator of a custom vendor extension with no other change to the implementation proposed in the draft.

[fpoirotte]

Good news, the umac-64 vs. umac-64@openssh.com topic is now closed based on http://cpansearch.perl.org/src/SALVA/Net-SFTP-Foreign-1.77/rfc/OpenSSH-protocol-deviations.txt

They both implement the same algorithm.