Example of digital signature signing and verification

digital_signature.py

from M2Crypto import EVP, RSA, X509
import binascii

class C(object):

    def __init__(self):
        pass

    def sign(self, message_to_encrypt):
        key = EVP.load_key("mycert-private.pem")
        key.reset_context(md='sha256')
        key.sign_init()
        key.sign_update(message_to_encrypt)
        
        signature = key.sign_final()
        
        hex_signature = binascii.b2a_hex(signature)
        
        return hex_signature

    def verify(self, message, encrypted_message):

        java_sig = binascii.a2b_hex(encrypted_message)

        cert = X509.load_cert("mycert.pem")
        
        pubkey = cert.get_pubkey()
        pubkey.reset_context(md="sha256")
        pubkey.verify_init()
        pubkey.verify_update(message)
        
        assert pubkey.verify_final(java_sig) == 1

message = "something"

c = C()
encrypted_message = c.sign(message)

c.verify(message, encrypted_message)

message = " something esle"

try:
    c.verify(message, encrypted_message)
except AssertionError:
    has_error = True
else:
    has_error = False
    
assert has_error