This allows easy import/export of public keys in the format
expected by OpenSSH for authorized_keys files, as well as
allowing comparisons with ServerConfig's PublickeyCallback.
Fixesgolang/go#3908.
R=agl, dave, golang-dev, bradfitz
CC=agl, golang-dev
https://golang.org/cl/6855107
These are the cosmetic cleanups from the bits of code that I
rereviewed.
1) stringLength now takes a int; the length of the string. Too many
callers were allocating with stringLength([]byte(s)) and
stringLength only needs to call len().
2) agent.go now has sendAndReceive to remove logic that was
duplicated.
3) We now reject negative DH values
4) We now reject empty packets rather than crashing.
R=dave, jonathan.mark.pittman
CC=golang-dev
https://golang.org/cl/6061052