How do I create a M2Crypto DSA object given parameters and key values?

Question

Using M2Crypto I'd like to create a DSA_pub object for verifying a DSA signature. I know q, p, g, and the public key, but the only way I know to instantiate a DSA object is using:

dsa = DSA.set_params(q,p,g)
dsa.gen_key()

How do I assign the known public key?

score 2 · Answer 1 · answered Jan 03 '11 at 20:17

I just ran across exactly this challenge, where I have the P, Q, G and Y parameters (in my case from an XML document), but M2Crypto does not have a way for me to create a valid public key from them.

I resorted to using pyasn1 to produce a PEM public key string, then loading that PEM public key using the M2Crypto.DSA.load_pub_key_bio factory function.

My rough code follows, in case it's useful to somebody in the future.

import sys
import M2Crypto

if sys.version_info[0] >= 3:
    bin = "{0:#0b}".format
    from functools import reduce

def _a2bits(chars):
    """Convert a string to its bits representation as a tuple of 0's and 1's"""
    return tuple(c == '1' and 1 or 0 for c in (bin(reduce(lambda x, y : (x<<8)+y, (ord(c) for c in chars), 1))[3:]))

def _make_dsa_pubkey_pem(p, q, g, y):
    from pyasn1.type import univ, namedtype
    from pyasn1.codec.der import encoder
    import base64

    class DSSParameters(univ.Sequence):
        componentType = namedtype.NamedTypes(
            namedtype.NamedType('p', univ.Integer()),
            namedtype.NamedType('q', univ.Integer()),
            namedtype.NamedType('g', univ.Integer())
        )

    class AlgorithmIdentifier(univ.Sequence):
        componentType = namedtype.NamedTypes(
            namedtype.NamedType('algorithm', univ.ObjectIdentifier()),
            namedtype.OptionalNamedType('parameters', DSSParameters())
        )

    class SubjectPublicKeyInfo(univ.Sequence):
        componentType = namedtype.NamedTypes(
            namedtype.NamedType('algorithm', AlgorithmIdentifier()),
            namedtype.NamedType('subjectPublicKey', univ.BitString()),
        )

    class DSAPublicKey(univ.Integer):
        pass


    dss_parameters = DSSParameters()
    dss_parameters.setComponentByName('p', p)
    dss_parameters.setComponentByName('q', q)
    dss_parameters.setComponentByName('g', g)

    algorithm_identifier = AlgorithmIdentifier()
    algorithm_identifier.setComponentByName('algorithm', univ.ObjectIdentifier((1, 2, 840, 10040, 4, 1)))
    algorithm_identifier.setComponentByName('parameters', dss_parameters)

    subject_public_key_info = SubjectPublicKeyInfo()
    subject_public_key_info.setComponentByName('algorithm', algorithm_identifier)
    subject_public_key_info.setComponentByName('subjectPublicKey', _a2bits(encoder.encode(DSAPublicKey(y))))

    der = encoder.encode(subject_public_key_info)
    return '-----BEGIN PUBLIC KEY-----\n' + base64.encodestring(der) + '-----END PUBLIC KEY-----\n'


p = 8652574980431835801046702501319893323628737876463029580298337449414347224525946403948627650414713523236662848134622261400464992784181209952478362597409469
q = 1102869237300951505579173947124947290564874845679
g = 4112516799587510153843416910187202701228216851472313407150913894984801048587575223178182928872781591943506026197710239402382269043796703824161282824797865
y = 2998329614411012012383616762831086330705701157164243056626309777500058049666595469116052965199021788182564677073758748878456479902088304265763443201269078
pem = _make_dsa_pubkey_pem(p, q, g, y)
bio = M2Crypto.BIO.MemoryBuffer(pem)
dsapub = M2Crypto.DSA.load_pub_key_bio(bio)

score 1 · Answer 2 · answered Jan 04 '11 at 05:06

I ended up creating a patch that adds a pub_key_from_params factory method for M2Crypto and includes regression tests in addition to the functionality. The state of the feature request is still "NEW" at the time of this post: https://bugzilla.osafoundation.org/show_bug.cgi?id=12981 . It's worked for me for the past few months. If the developers find it useful, perhaps it will be included.

score 0 · Answer 3 · answered Jun 12 '10 at 02:31

0

There are many factory functions in module DSA beyond get_params, and I think you want load_pub_key (if you have public key and params in a PEM file) or load_pub (if you have them in a BIO object). See also the BIO module for various kinds of BIO objects.

answered Jun 12 '10 at 02:31

Alex Martelli

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How might I take the raw params and key and create a BIO or PEM to in turn create a the public key using load_*? – casey Jun 12 '10 at 06:21
I don't know of Python functions to write PEM files from arbitrarily "obtained" params and keys (how did you "obtain" them in the first place BTW?) -- there may be some but if so I've never heard of any. PEM is documented at RFC 1422, http://tools.ietf.org/html/rfc1422.html , and is unfortunately a complicated format:-(. – Alex Martelli Jun 12 '10 at 16:13
Hmm, doesn't sound fun. Where did I obtain them? from DNSKEY RRs using algorithm 3 (RFC 2536) – casey Jun 12 '10 at 21:14

How do I create a M2Crypto DSA object given parameters and key values?

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