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

Use M2Crypto. Here's how to verify for RSA and any other algorithm supported by OpenSSL:

pem = """-----BEGIN PUBLIC KEY----- MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDfG4IuFO2h/LdDNmonwGNw5srW nUEWzoBrPRF1NM8LqpOMD45FAPtZ1NmPtHGo0BAS1UsyJEGXx0NPJ8Gw1z+huLrl XnAVX5B4ec6cJfKKmpL/l94WhP2v8F3OGWrnaEX1mLMoxe124Pcfamt0SPCGkeal VvXw13PLINE/YptjkQIDAQAB -----END PUBLIC KEY-----""" # your example key  from M2Crypto import BIO, RSA, EVP bio = BIO.MemoryBuffer(pem) rsa = RSA.load_pub_key_bio(bio) pubkey = EVP.PKey() pubkey.assign_rsa(rsa)  # if you need a different digest than the default 'sha1': pubkey.reset_context(md='sha1') pubkey.verify_init() pubkey.verify_update('test  message') assert pubkey.verify_final(signature) == 1 

Answer 2

The data between the markers is the base64 encoding of the ASN.1 DER-encoding of a PKCS#8 PublicKeyInfo containing an PKCS#1 RSAPublicKey.

That is a lot of standards, and you will be best served with using a crypto-library to decode it (such as M2Crypto as suggested by joeforker). Treat the following as some fun info about the format:

If you want to, you can decode it like this:

Base64-decode the string:

30819f300d06092a864886f70d010101050003818d0030818902818100df1b822e14eda1fcb74336 6a27c06370e6cad69d4116ce806b3d117534cf0baa938c0f8e4500fb59d4d98fb471a8d01012d54b 32244197c7434f27c1b0d73fa1b8bae55e70155f907879ce9c25f28a9a92ff97de1684fdaff05dce 196ae76845f598b328c5ed76e0f71f6a6b7448f08691e6a556f5f0d773cb20d13f629b6391020301 0001 

This is the DER-encoding of:

   0 30  159: SEQUENCE {    3 30   13:   SEQUENCE {    5 06    9:     OBJECT IDENTIFIER rsaEncryption (1 2 840 113549 1 1 1)   16 05    0:     NULL             :     }   18 03  141:   BIT STRING 0 unused bits, encapsulates {   22 30  137:       SEQUENCE {   25 02  129:         INTEGER             :           00 DF 1B 82 2E 14 ED A1 FC B7 43 36 6A 27 C0 63             :           70 E6 CA D6 9D 41 16 CE 80 6B 3D 11 75 34 CF 0B             :           AA 93 8C 0F 8E 45 00 FB 59 D4 D9 8F B4 71 A8 D0             :           10 12 D5 4B 32 24 41 97 C7 43 4F 27 C1 B0 D7 3F             :           A1 B8 BA E5 5E 70 15 5F 90 78 79 CE 9C 25 F2 8A             :           9A 92 FF 97 DE 16 84 FD AF F0 5D CE 19 6A E7 68             :           45 F5 98 B3 28 C5 ED 76 E0 F7 1F 6A 6B 74 48 F0             :           86 91 E6 A5 56 F5 F0 D7 73 CB 20 D1 3F 62 9B 63             :           91  157 02    3:         INTEGER 65537             :         }             :       }             :   } 

For a 1024 bit RSA key, you can treat "30819f300d06092a864886f70d010101050003818d00308189028181" as a constant header, followed by a 00-byte, followed by the 128 bytes of the RSA modulus. After that 95% of the time you will get 0203010001, which signifies a RSA public exponent of 0x10001 = 65537.

You can use those two values as n and e in a tuple to construct a RSAobj.