How do you verify an RSA SHA1 signature in Python?

Question

I've got a string, a signature, and a public key, and I want to verify the signature on the string. The key looks like this:

-----BEGIN PUBLIC KEY-----
MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDfG4IuFO2h/LdDNmonwGNw5srW
nUEWzoBrPRF1NM8LqpOMD45FAPtZ1NmPtHGo0BAS1UsyJEGXx0NPJ8Gw1z+huLrl
XnAVX5B4ec6cJfKKmpL/l94WhP2v8F3OGWrnaEX1mLMoxe124Pcfamt0SPCGkeal
VvXw13PLINE/YptjkQIDAQAB
-----END PUBLIC KEY-----

I've been reading the pycrypto docs for a while, but I can't figure out how to make an RSAobj with this kind of key. If you know PHP, I'm trying to do the following:

openssl_verify($data, $signature, $public_key, OPENSSL_ALGO_SHA1);

Also, if I'm confused about any terminology, please let me know.

Answer 1

A public key contains both a modulus(very long number, can be 1024bit, 2058bit, 4096bit) and a public key exponent(much smaller number, usually equals one more than a two to some power). You need to find out how to split up that public key into the two components before you can do anything with it.

I don't know much about pycrypto but to verify a signature, take the hash of the string. Now we must decrypt the signature. Read up on modular exponentiation; the formula to decrypt a signature is message^public exponent % modulus. The last step is to check if the hash you made and the decrypted signature you got are the same.

Answer 2

I think ezPyCrypto might make this a little easier. The high-level methods of the key class includes these two methods which I hope will solve your problem:

• verifyString - verify a string against a signature
• importKey - import public key (and possibly private key too)

Rasmus points out in the comments that verifyString is hard-coded to use MD5, in which case ezPyCryto can't help Andrew unless he wades into its code. I defer to joeforker's answer: consider M2Crypto.

Answer 3

More on the DER decoding.

DER encoding always follows a TLV triplet format: (Tag, Length, Value)

• Tag specifies the type (i.e. data structure) of the value
• Length specifies the number of byte this value field occupies
• Value is the actual value which could be another triplet

Tag basically tells how to interpret the bytes data in the Value field. ANS.1 does have a type system, e.g. 0x02 means integer, 0x30 means sequence (an ordered collection of one or more other type instances)

Length presentation has a special logic:

• If the length < 127, the L field only uses one byte and coded as the length number value directly
• If the length > 127, then in the first byte of L field, the first bit must be 1, and the rest 7 bits represents the number of following bytes used to specifies the length of the Value field. Value, the actually bytes of the value itself.

For example, say I want to encode a number of 256 bytes long, then it would be like this

02  82  01  00  1F  2F  3F  4F  …   DE  AD  BE  EF

• Tag, 0x02 means it's a number
• Length, 0x82, bit presentation of it is 1000 0010, meaning the following two bytes specifies the actually length of the value, which his 0x0100 meaning the value field is 256 bytes long
• Value, from 1F to EF, the actual 256 bytes.

Now looking at your example

30819f300d06092a864886f70d010101050003818d0030818902818100df1b822e14eda1fcb74336
6a27c06370e6cad69d4116ce806b3d117534cf0baa938c0f8e4500fb59d4d98fb471a8d01012d54b
32244197c7434f27c1b0d73fa1b8bae55e70155f907879ce9c25f28a9a92ff97de1684fdaff05dce
196ae76845f598b328c5ed76e0f71f6a6b7448f08691e6a556f5f0d773cb20d13f629b6391020301
0001

It interprets as just what Rasmus Faber put in his reply

Answer 4

I try the code given by joeforker but it does not work. Here is my example code and it works fine.

from Crypto.Signature import PKCS1_v1_5
from Crypto.PublicKey import RSA 
from Crypto.Hash import SHA

pem = """-----BEGIN PUBLIC KEY-----
MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDfG4IuFO2h/LdDNmonwGNw5srW
nUEWzoBrPRF1NM8LqpOMD45FAPtZ1NmPtHGo0BAS1UsyJEGXx0NPJ8Gw1z+huLrl
XnAVX5B4ec6cJfKKmpL/l94WhP2v8F3OGWrnaEX1mLMoxe124Pcfamt0SPCGkeal
VvXw13PLINE/YptjkQIDAQAB
-----END PUBLIC KEY-----""" # your example key

key = RSA.importKey(pem)
h = SHA.new(self.populateSignStr(params))
verifier = PKCS1_v1_5.new(key)
if verifier.verify(h, signature):
  print "verified"
else:
  print "not verified"

Answer 5

Maybe this isn't the answer you're looking for, but if all you need is to turn the key into bits, it looks like it's Base64 encoded. Look at the codecs module (I think) in the standard Python library.

Answer 6

Using M2Crypto, the above answers does not work. Here is a tested example.

import base64
import hashlib
import M2Crypto as m2

# detach the signature from the message if it's required in it (useful for url encoded data)
message_without_sign = message.split("&SIGN=")[0]
# decode base64 the signature
binary_signature = base64.b64decode(signature)
# create a pubkey object with the public key stored in a separate file
pubkey = m2.RSA.load_pub_key(os.path.join(os.path.dirname(__file__), 'pubkey.pem'))
# verify the key
assert pubkey.check_key(), 'Key Verification Failed'
# digest the message
sha1_hash = hashlib.sha1(message_without_sign).digest()
# and verify the signature
assert pubkey.verify(data=sha1_hash, signature=binary_signature), 'Certificate Verification Failed'

And that's about it