Verify signature on X509 certificate in Python

I have an RSA public key and a signed X509 certificate. How can I check that the key signed the certificate? (My example happens to be a self-signed certificate.)

Here's what I'm doing now:

Generate self-signed cert and convert to DER encoding with openssl cli:

(I'm expecting DER in my real application)

openssl req -x509 -newkey rsa:2048 -keyout selfsigned.key -nodes -out selfsigned.cert -sha256 -days 1000
openssl x509 -outform der -in selfsigned.cert -out self.der

Decode it to a Crypto.Util.asn1.DerSequence instance.

>>> from Crypto.Util.asn1 import DerSequence
>>> from Crypto.PublicKey import RSA
>>> der = open('self.der').read()
>>> cert = DerSequence()
>>> cert.decode(der)
905L
>>> # according to RFC5280, this is a 3-length sequence:
>>> # tbsCertificate, signatureAlgorithm, signatureValue
>>> # "tbs" == "to be signed"
>>> len(cert)
3

Then I pull the RSA public key out:

>>> tbscert = DerSequence()
>>> tbscert.decode(cert[0])
>>> subjectPublicKeyInfo = tbscert[6]
>>> rsa_key = RSA.importKey(subjectPublicKeyInfo)
>>> >>> rsa_key
<_RSAobj @0x7fb27287d128 n(2048),e>

Then I pull the signature out

This is very annoying. I'm using another library to decode the DER again because this one gives me a slightly more convenient representation of the "bit string" encoding of the signature value. For now, I copy and paste the base-2 string representation of the value into int() to get a long (what the RSA.verify() method expects).

>>> from pyasn1_modules import  rfc2437,rfc2459
>>> from pyasn1.codec.der import decoder
>>> cert2,rest = decoder.decode(der, asn1Spec=rfc2459.Certificate())
>>> sig_bits = cert2.getComponentByName("signatureValue")
>>> sig_bits
BitString("'10101110101100111010100000111001000110111111101001110000100111110111011111111110011010110101001001110110111011011001110001010000111001001001110111001110011101000000100000001100001101000111000110010100110101000110111101110001011011001001100011110011010101011100000001010111101110111001110011010011110101101001110101110100011111011111110001110001110100000110110100010000001010111010000100101110101001110000111001100010011110110100101001010000101001110101101111100111001111000010111001110000000101000000011110011110000110000101101101110110101101110101011110101111111000101011001000010000110100111111001011111100110011011011001001111110000000110100000001101000011010010100001100110100001001000111001011111100000011000101001100001010101001111000001010100101010101000100001000100011101111100101010010001111001110100001011110101100010111001010100010001011100000100101110001011101100010010111100010111110010010110111111100100100101010010000000010001011111110011000011000101000001000000000000011111101110010101100100000010111111110101000110010101111100101011101000010010110101000101101110001001101101000110101110011101011111100010000101001111011100100100010101001011011110001110000000000010011011111011110100000111001100010100000100001101111110010000111100110110110000001010010110011010111100101110101000001111001010011101001101101101101011000100010011110000101010110111011100010100011110101100110101011010111000011111000001111111111000101101101011110010111100101011100010000111011101101010101001011101101111011001001110000010011001111010001011110000011001011000110100011100000100100111000111100000010000001001001010001100000010011110100000111010010100001101001111111001111110111010001101010110100001100111010101000010000101000000111100001001001000100011100110010110101001110111101000101101011011100000010010000100111001100001110010101000100000010010111110001100011110010000100001000101100011000011000110010110011100010100010111011011111111010000001001100000100011010000000110111100111010101001110101000011111011000100010111101100110100010101111000110111110'B")
>>> bit_string = '10101110101100111010100000111001000110111111101001110000100111110111011111111110011010110101001001110110111011011001110001010000111001001001110111001110011101000000100000001100001101000111000110010100110101000110111101110001011011001001100011110011010101011100000001010111101110111001110011010011110101101001110101110100011111011111110001110001110100000110110100010000001010111010000100101110101001110000111001100010011110110100101001010000101001110101101111100111001111000010111001110000000101000000011110011110000110000101101101110110101101110101011110101111111000101011001000010000110100111111001011111100110011011011001001111110000000110100000001101000011010010100001100110100001001000111001011111100000011000101001100001010101001111000001010100101010101000100001000100011101111100101010010001111001110100001011110101100010111001010100010001011100000100101110001011101100010010111100010111110010010110111111100100100101010010000000010001011111110011000011000101000001000000000000011111101110010101100100000010111111110101000110010101111100101011101000010010110101000101101110001001101101000110101110011101011111100010000101001111011100100100010101001011011110001110000000000010011011111011110100000111001100010100000100001101111110010000111100110110110000001010010110011010111100101110101000001111001010011101001101101101101011000100010011110000101010110111011100010100011110101100110101011010111000011111000001111111111000101101101011110010111100101011100010000111011101101010101001011101101111011001001110000010011001111010001011110000011001011000110100011100000100100111000111100000010000001001001010001100000010011110100000111010010100001101001111111001111110111010001101010110100001100111010101000010000101000000111100001001001000100011100110010110101001110111101000101101011011100000010010000100111001100001110010101000100000010010111110001100011110010000100001000101100011000011000110010110011100010100010111011011111111010000001001100000100011010000000110111100111010101001110101000011111011000100010111101100110100010101111000110111110'
>>> len(bit_string)
2048
>>> sig_long = int(bit_string, 2)
22054057292543290008991218833668878365914778519473463062473060546762899555976103489048033910135613221569150796460758806399269198735780309519101363051388009338597879536630494212385605300708879019160215628821483902624509955250980351374010304684207884550324020859785789812498991361733361061223150200173076263554090698006436248180914014712709890577579243572383188197634606581121383593473899061397708617253275982314075801792358481980896751043809539358665686019958496887281091997170247998458556812030465091755579654010246474389968142047627934047174316731806191431717418170761689395728146445291177267566370799362894264463806L

Then I compute the SHA256 hash of the "to be signed certificate":

>>> import Crypto.Hash.SHA256
>>> comp_hash = Crypto.Hash.SHA256.new(cert[0]).digest()
>>> comp_hash
'xa3tx84xd6xf5xfex16xb9xdb(&x12xb3m^+x94xa7bZxf9sxf7xbayxa1jxa3Yxeaxa8x7f'

Then the verify() method tells me the signature doesn't match.

>>> rsa_key.verify(comp_hash, (sig_long, None))
False

I hope there's a better way (this doesn't even work), but I've spent hours looking at PyCrypto and PyOpenSSL and haven't found it.