- I'm trying to verify a signed message using a x509 certificate holding a dsa public key. The x509 certificate was provided by an SAP System in PKCS7 encoding and after converting in PEM with openssl I'm able to read the contents (openssl x509 -in sapcert.pem -inform pem -text) It holds a public key in dsaEncryption, showing me the DSA parameters y(pub), p, q and g.
Because I did not found a DSA implementation in M2Crypto x509 class, i tried to build the DSA public key by myself. Therefor I patched MyCrypto, (see: How do I create a M2Crypto DSA object given parameters and key values?), compilied it and got a new function DSA.pub_key_from_params(p,q,g,y) to build my DSA public-key using the parameters found in the certificate. Until this point everything works fine. (The unittest although ran without errors).
in the second step I recieve the signedMessage (named seckey) as an URL parameter and after decoding it (base64) I got an proper DER string I'm able to read with openssl (openssl ans1parse -in seckey -inform der). In the output from openssl I can see the signed messageDigest, which is exactly the SHA1 encoded messagedigest I try to verify (so I can be sure the signedMessage was provided properly). Although I can see a dsaWithSHA1 signature string which seems to hold the r and s values needing for verify an dsa signed message (see: M2Crypto: verifying DSA signatures)
At this point I stuck in my attempts to verify the signedMessage for a couple of days now and I hope there is a crytographic-expert out there who can help me. I tried and googled a lot, tried the pyCrypto lib instead, all without success.
I tried to pass the SHA1 MessageDigest and the r and s values to the M2Crypto.DSA.verify function but due it fails I think I either had to pass the signedMessage or parts of it. (in a Java forum I found some posts about verifying signedMessages provided by SAP and theres somewhat about calculating the signature over "the DER encoding of the signed attributes." ?)
Here's my sample code:
# -*- coding: iso-8859-1 -*-
import M2Crypto
import urllib
import base64
from Crypto.Util import asn1
from M2Crypto import m2
import sha
# the certificate
cert = """subject=/C=DE/O=SAP Trust Community/OU=SAP Web AS/OU=I0020154766/CN=RE2
issuer=/C=DE/O=SAP Trust Community/OU=SAP Web AS/OU=I0020154766/CN=RE2
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
"""
f = open('sapcert.pem', 'w')
f.write(cert)
f.close()
# now you can see it content with openssl
# openssl x509 -in sapcert.pem -inform pem -text
# this is the signedMessage
secKey = "MIIBSwYJKoZIhvcNAQcCoIIBPDCCATgCAQExCzAJBgUrDgMCGgUAMAsGCSqGSIb3DQEHATGCARcwggETAgEBMGkwZDELMAkGA1UEBhMCREUxHDAaBgNVBAoTE1NBUCBUcnVzdCBDb21tdW5pdHkxEzARBgNVBAsTClNBUCBXZWIgQVMxFDASBgNVBAsTC0kwMDIwMTU0NzY2MQwwCgYDVQQDEwNSRTICAQAwCQYFKw4DAhoFAKBdMBgGCSqGSIb3DQEJAzELBgkqhkiG9w0BBwEwHAYJKoZIhvcNAQkFMQ8XDTExMDUyNjE1MzAyNVowIwYJKoZIhvcNAQkEMRYEFPelg4iVtaKORpuFxUvgo23Du7%2BtMAkGByqGSM44BAMELjAsAhQ46oCNmzZArb5yOFSYGY0hWu8dZwIUT35hPccJ6B9HIsOE0u8LwYZaFNk%3D"
secKey = urllib.unquote(secKey)
secKey64 = base64.b64decode(secKey)
# now you can save it as a DER encoded file
f = open('seckey.der', 'wb')
f.write(secKey64)
f.close()
# you can pass it to openssl to see the the content
# openssl asn1parse -in seckey.der -inform der
# here is the sha1 encoded messagedigest I had to verify
hashstr = "ZS4DDB9616BA031C40E1008003AC100097dCN%3DRE2,OU%3DI0020154766,OU%3DSAPWebAS,O%3DSAPTrustCommunity,C%3DDE20110526173025"
osha1=M2Crypto.EVP.MessageDigest('sha1')
osha1.update(hashstr)
sha1_md = osha1.digest()
print sha1_md.encode('hex')
# now i build a DSA key with the params found in the certificate
pub="8d5e9bfd28c3131310ff9a933e7a068bd0ed0e8b8aa52654b2f125c8c3a338c8fb9cfa22b00fb6e6f6bde2d73dd8210c77070798c72a8a93e091e9a9173eba12"
p="ffff31d41aa7e1ad3414aafd093c0f3ec931cb4cabc7b894e93e2fcdae305bddcca3677f2184d9340c45aa1ae6f9f214ac4a797f1229611989aca2f6a9109499"
q="dadcb17945beb5c7e39e6033af6f29979b85065f"
g="998cd74f31d8c2fa8a10ce3a16f417c82e4ef89227c2093b0da73b82a1809248424b568823856473d908131dc614b0f69b1e3ec89b8c43ca50e30cf81647c124"
pub1 = M2Crypto.m2.bn_to_mpi(M2Crypto.m2.hex_to_bn(pub))
p1 = M2Crypto.m2.bn_to_mpi(M2Crypto.m2.hex_to_bn(p))
q1 = M2Crypto.m2.bn_to_mpi(M2Crypto.m2.hex_to_bn(q))
g1 = M2Crypto.m2.bn_to_mpi(M2Crypto.m2.hex_to_bn(g))
# this function is available after patching und compiling M2Crypto
# https://bugzilla.osafoundation.org/attachment.cgi?id=5700
dsa1 = M2Crypto.DSA.pub_key_from_params(p1, q1, g1, pub1)
print dsa1.check_key()
# this seems to be the signature-values r and s in seckey.der
asn_hex = "302C021438EA808D9B3640ADBE72385498198D215AEF1D6702144F7E613DC709E81F4722C384D2EF0BC1865A14D9"
r = asn_hex[8:48]
s = asn_hex[52:]
r1 = M2Crypto.m2.bn_to_mpi(M2Crypto.m2.hex_to_bn(r))
s1 = M2Crypto.m2.bn_to_mpi(M2Crypto.m2.hex_to_bn(s))
# but this fails
v = dsa1.verify(sha1_md, r1, s1)
print v
# this too
sk = M2Crypto.m2.bn_to_mpi(M2Crypto.m2.bin_to_bn(secKey64))
v = dsa1.verify(sk, r1, s1)
print v
Is there anybody who knows how to do a proper dsawithsha1 signature verification? Please help! best Regards, Falko