This mostly adds the infrastructure for writing various forms of
packets as well as reading them. Adding symmetric encryption support
was simply an easy motivation.
There's also one brown-paper-bag fix in here. Previously I had the
conditional for the MDC hash check backwards: the code was checking
that the hash was *incorrect*. This was neatly counteracted by another
bug: it was hashing the ciphertext of the OCFB prefix, not the
plaintext.
R=bradfitz
CC=golang-dev
https://golang.org/cl/4564046
OpenPGP changed its OCFB mode for more modern packets (for example, the
MDC symmetrically encrypted packet). This change adds a bool to
determine which mode is used.
R=bradfitzgo, r, rsc
CC=golang-dev
https://golang.org/cl/4126041
cipher is intended to replace crypto/block over time. This
change only adds basic parts: CBC and CTR mode and doesn't add
the package to the top-level Makefile.
R=r, rsc
CC=golang-dev
https://golang.org/cl/3069041
