This change adds an additional signature parsing function which
performs additional checks to verify the signature is serialized in a
valid DER (and thus, unique) format, instead of allowing the less
strict BER signatures that ParseSignature will happily accept.
Added additional tests and updated test coverage to reflect changes.
Turns out that there are some signatures in the bitcoin blockchain that have
trailing 0s, for example
12a1b29fd6c295075b6a66f5fd90f0126ceb1fda4f15e4b44d92518bd52a5cdf has a signature
length of 0x45 where there are 0x47 bytes following that length check (one is
hashtype and is supposed to be trimmed out prior to calling the function). We
relax the paranoid length check to permit traling data, but not to permit
buffers that are too short. Change the test to passing with a big comment
stating why this is now considered a valid case.
