How to create a hash object from raw bytes?

[pavel_a]

Dear experts,

I'm trying to use Crypto.Signature.pkcs1_15 with SHA256.

But I have the 32-byte digest already provided from something else.

So I need to forge a thing that can be passed to sign() and verify() from a bytes object.

I've tried this:

def make_sha(digest: bytes) :

       class x:

           pass
       x.oid = '2.16.840.1.101.3.4.2.1'
       x.digest_size = 32
       x.digest = lambda : digest

       return x

signer = Crypto.Signature.pkcs1_15.new(my_key)

h = make_sha(my_digest)

sig = signer.sign(h)

.................

and then to verify:

signer.verify(h, sig)

This looks like working, but actually verify() returns success even with invalid digest!

So I guess that my fake hash thing causes silent verify() failure in a strange way.

What is a proper way to do what I need?

[pavel_a]

Further debugging shows that this hack actually works for signing.

That's, signatures returned by sign() from a real SHA256 object and a fake objects are equal.

So the question now is why verify() succeeds with invalid digest bytes. 

Is this a bug in verify() or something caused by the fake object?

[pavel_a]

Solved. I've imported pycyptodome in a "legacy" way, it actually creates a different interface that behaves differently.

So the hack to make a SHA256 actually works, both to sign and verify.

Even much simpler:

def make_sha(digest: bytes) :

    x = SHA256.new()

    x.digest = lambda :  digest
    return x