[pycrypto] Verify DSA bytestring signature

[Winston Weinert]

The signature is created using the openssl(1) command-line tool like this:

openssl dgst -sha1 -binary < "$RELEASE_ARCHIVE" \
| openssl dgst -dss1 -sign "$DSA_PRIVKEY" \
| openssl enc -base64

It verifies correctly using this command-line:

echo "$SIGNATURE” | openssl enc -base64 -d > /tmp/decoded_signature
openssl dgst -sha1 -binary < "$RELEASE_ARCHIVE" > /tmp/release_archive_sha1
openssl dgst -dss1 -verify "$DSA_PUBKEY" -signature /tmp/decoded_signature /tmp/release_archive_sha1

After I wrote my email, I dug around for awhile. After a lot of research I learned
about ASN.1 DER’s usage in Dss-Sig-Value (http://www.ietf.org/rfc/rfc2459.txt). I
wrote this code that appeared to decode my Base64 encoded signature correctly (I
checked against http://lapo.it/asn1js/):

def decode_DSA_signature(signature):
    raw_signature = base64.b64decode(signature)
    der = DerSequence()
    der.decode(raw_signature)
    return (der[0], der[1])

Unfortunately .verify() returns False on correctly verified signature and hash
pairs. I am using this new function like so:


def validate(dsa_pubkey, signature, zipfile):
    with open(dsa_pubkey, 'rb') as f:
        pubkey = DSA.importKey(f.read())
    with open(zipfile, 'rb') as f:
        h = SHA1.new()
        h.update(f.read())
        zipfile_digest = h.digest()
    signature = decode_DSA_signature(signature)

    return pubkey.verify(zipfile_digest, signature)

Maybe there is a problem with PyCrypto DSA and my environment?
>>> sys.version
'2.7.6 (default, Feb  7 2014, 12:51:34) \n[GCC 4.2.1 Compatible Apple LLVM 5.0 (clang-500.2.79)]'

For the time being I’m invoking openssl(1) for this task.

Thank you for the reply!
Winston Weinert


On Apr 6, 2014, at 4:50, Legrandin <helderijs at gmail.com> wrote:

> How was the signature created exactly?
> 
> The .verify() method of a DSA object requires two integers, and there are several ways to encode them into a bytestring. It's very hard to guess the correct one for your case.
> 
> FYI, there is a long standing pull request I created to add a saner DSA API:
> 
> https://github.com/dlitz/pycrypto/pull/53
> 
> The verification method accepts DER or big-endian encoded signatures.
> 
> 
> 
> 2014-04-05 21:03 GMT+02:00 Winston Weinert <winston at ml1.net>:
> Hello,
> 
> I noticed in Git there is a “verify” method on Crypto.PublicKey.DSA. How do
> I go about using this method? It wants a tuple, but unsure how to create
> the appropriate tuple from my bytestring (which is decoded base64 text).
> This is git revision 2d1aecd. The relevant code and error:
> 
> Code:
> 
> def validate(dsa_pubkey, signature, zipfile):
>     with open(dsa_pubkey, 'rb') as f:
>         pubkey = DSA.importKey(f.read())
>     with open(zipfile, 'rb') as f:
>         h = SHA1.new()
>         h.update(f.read())
>         zipfile_digest = h.digest()
>     decoded_signature = base64.b64decode(signature)
> 
>     return pubkey.verify(zipfile_digest, decoded_signature)
> 
> Error:
> 
> Traceback (most recent call last):
>   File "sparkle_tool.py", line 67, in <module>
>     validate_files(appcast, dsa_pubkey)
>   File "sparkle_tool.py", line 55, in validate_files
>     if validate(dsa_pubkey, signature, local_file):
>   File "sparkle_tool.py", line 33, in validate
>     return pubkey.verify(zipfile_digest, decoded_signature)
>   File "/home/winston/jobber/venv/local/lib/python2.7/site-packages/Crypto/PublicKey/DSA.py", line 222, in verify
>     return pubkey.pubkey.verify(self, M, signature)
>   File "/home/winston/jobber/venv/local/lib/python2.7/site-packages/Crypto/PublicKey/pubkey.py", line 126, in verify
>     return self._verify(M, signature)
>   File "/home/winston/jobber/venv/local/lib/python2.7/site-packages/Crypto/PublicKey/DSA.py", line 240, in _verify
>     (r, s) = sig
> ValueError: too many values to unpack
> 
> Thanks a bunch!
> —
> Winston Weinert
> winston at ml1.net
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