P256 signatures don't verify?

[paulehoffman]

Greetings. I'm doing some simple signing and verifying with P256 keys and am failing even in trivial cases.

from Crypto.Hash import SHA256
from Crypto.PublicKey import ECC
from Crypto.Signature import DSS
msg = SHA256.new(b"abc")
a_priv = ECC.import_key(open("private/k-p256-private.der", mode="rb").read())
a_pub =  ECC.import_key(open("private/k-p256-public.der", mode="rb").read())
a_signer = DSS.new(a_priv, "fips-186-3")
a_signature = a_signer.sign(msg)
a_verifier = DSS.new(a_pub, "fips-186-3")
a_verifier.verify(msg, a_signature)

In fact, an even more trivial case fails:

a_signer.verify(msg, a_signer.sign(msg))

Given the lack of exception text for .verify, I can't tell if there is something simple I'm missing, or if the verification just doesn't work. Any clues would be appreciated, and I'll feed anything I get back into the documentation.

[Legrandin]

Can you clarify what error you get? Are you sure that private and public keys really match?
I tried that code and it works (no exceptions, as signatures are valid).

[paulehoffman]

My apologies. I was testing this in the REPL and was misinterpreting the results. a_verifier.verify(msg, a_signature) returns False, even if it doesn't cause an exception. I was looking at the return, not the exception state.

So, I'll turn this issue into a doc request: say that verify() returns False regardless of whether the verification worked. Or, better, explain why "False" was chosen for the return.

[Legrandin]

Returning a boolean as the signature verification result was actually the old behavior and it led to security issues because it is not always clear what the right value should be (is True an indication of error or success? One needs to always check the documentation each time).
Having the result not documented is actually done on purpose.

[paulehoffman]

The first part makes a bit of sense, but the verb "verify" is pretty clear what the result would mean.

The second part is antithetical to the purpose of documentation. Please consider adding "The result is undefined; verification failure is only represented by an exception."