# Signing and verifying data using pycrypto (RSA)

Posted on

### Question :

Signing and verifying data using pycrypto (RSA)

I am trying to familiarize myself with the pycrypto module, but the lack of clear documentation makes things difficult.

To start with, I would like to understand signing and verifying data. Could someone please provide an example for how this would be written?

This is a fleshed-out version of the example in the old PyCrypto documentation:

Ensure you are using `pycryptodome` and not `pycrypto` (which is unmaintained!)

pycryptodome can be installed with `pip install pycryptodome`

``````import Crypto.Hash.MD5 as MD5
import Crypto.PublicKey.RSA as RSA
import Crypto.PublicKey.DSA as DSA
import Crypto.PublicKey.ElGamal as ElGamal
import Crypto.Util.number as CUN
import os

plaintext = 'The rain in Spain falls mainly on the Plain'

# Here is a hash of the message
hash = MD5.new(plaintext).digest()
print(repr(hash))
# 'xb1./Jxa883x974xa4xacx1ex1b!xc8x11'

for alg in (RSA, DSA, ElGamal):
# Generates a fresh public/private key pair
key = alg.generate(384, os.urandom)

if alg == DSA:
K = CUN.getRandomNumber(128, os.urandom)
elif alg == ElGamal:
K = CUN.getPrime(128, os.urandom)
while CUN.GCD(K, key.p - 1) != 1:
print('K not relatively prime with {n}'.format(n=key.p - 1))
K = CUN.getPrime(128, os.urandom)
# print('GCD({K},{n})=1'.format(K=K,n=key.p-1))
else:
K = ''

# You sign the hash
signature = key.sign(hash, K)
print(len(signature), alg.__name__)
# (1, 'Crypto.PublicKey.RSA')
# (2, 'Crypto.PublicKey.DSA')
# (2, 'Crypto.PublicKey.ElGamal')

# You share pubkey with Friend
pubkey = key.publickey()

# You send message (plaintext) and signature to Friend.
# Friend knows how to compute hash.
# Friend verifies the message came from you this way:
assert pubkey.verify(hash, signature)

# A different hash should not pass the test.
assert not pubkey.verify(hash[:-1], signature)
``````

Below is the helper class I created to perform all necessary RSA functions (encryption, decryption, signing, verifying signature & generating new keys)

rsa.py

``````from Crypto.PublicKey import RSA
from Crypto.Cipher import PKCS1_OAEP
from Crypto.Signature import PKCS1_v1_5
from Crypto.Hash import SHA512, SHA384, SHA256, SHA, MD5
from Crypto import Random
from base64 import b64encode, b64decode

hash = "SHA-256"

def newkeys(keysize):
key = RSA.generate(keysize, random_generator)
private, public = key, key.publickey()
return public, private

def importKey(externKey):
return RSA.importKey(externKey)

def getpublickey(priv_key):
return priv_key.publickey()

def encrypt(message, pub_key):
#RSA encryption protocol according to PKCS#1 OAEP
cipher = PKCS1_OAEP.new(pub_key)
return cipher.encrypt(message)

def decrypt(ciphertext, priv_key):
#RSA encryption protocol according to PKCS#1 OAEP
cipher = PKCS1_OAEP.new(priv_key)
return cipher.decrypt(ciphertext)

def sign(message, priv_key, hashAlg="SHA-256"):
global hash
hash = hashAlg
signer = PKCS1_v1_5.new(priv_key)
if (hash == "SHA-512"):
digest = SHA512.new()
elif (hash == "SHA-384"):
digest = SHA384.new()
elif (hash == "SHA-256"):
digest = SHA256.new()
elif (hash == "SHA-1"):
digest = SHA.new()
else:
digest = MD5.new()
digest.update(message)
return signer.sign(digest)

def verify(message, signature, pub_key):
signer = PKCS1_v1_5.new(pub_key)
if (hash == "SHA-512"):
digest = SHA512.new()
elif (hash == "SHA-384"):
digest = SHA384.new()
elif (hash == "SHA-256"):
digest = SHA256.new()
elif (hash == "SHA-1"):
digest = SHA.new()
else:
digest = MD5.new()
digest.update(message)
return signer.verify(digest, signature)
``````

Sample Usage

``````import rsa
from base64 import b64encode, b64decode

msg1 = "Hello Tony, I am Jarvis!"
msg2 = "Hello Toni, I am Jarvis!"
keysize = 2048
(public, private) = rsa.newkeys(keysize)
encrypted = b64encode(rsa.encrypt(msg1, public))
decrypted = rsa.decrypt(b64decode(encrypted), private)
signature = b64encode(rsa.sign(msg1, private, "SHA-512"))
verify = rsa.verify(msg1, b64decode(signature), public)

print(private.exportKey('PEM'))
print(public.exportKey('PEM'))
print("Encrypted: " + encrypted)
print("Decrypted: '%s'" % decrypted)
print("Signature: " + signature)
print("Verify: %s" % verify)
rsa.verify(msg2, b64decode(signature), public)
``````