Sign and Verify Using RSA via cryptography (python)

sign.py

"""
RSA Sign a Message using a private key

Just turns this example into a script:
    https://cryptography.io/en/latest/hazmat/primitives/asymmetric/rsa/#signing
"""

import sys
import hashlib
import base64
from cryptography.hazmat.primitives import serialization, hashes
from cryptography.hazmat.primitives.asymmetric import padding
from cryptography.hazmat.backends import default_backend

if len(sys.argv) != 3:
    print('USAGE: sign.py privkey file')
    sys.exit(2)

privkeyfile = sys.argv[1]
messagefile = sys.argv[2]

with open(privkeyfile) as f:
    privkey = serialization.load_pem_private_key(
        f.read(), password=None, backend=default_backend())

with open(messagefile) as m:
    message = m.read()
    prehashed = hashlib.sha256(message).hexdigest()

sig = privkey.sign(
    prehashed,
    padding.PSS(
        mgf=padding.MGF1(hashes.SHA256()),
        salt_length=padding.PSS.MAX_LENGTH),
    hashes.SHA256())

print(base64.b64encode(sig))

verify.py

"""
RSA Verify a Message against a pubkey

Just turns this example into a script:
    https://cryptography.io/en/latest/hazmat/primitives/asymmetric/rsa/#signing
    
timeit tests reveal that verification takes 0.1ms on a modern CPU
"""

import sys
import hashlib
import base64
from cryptography.exceptions import InvalidSignature
from cryptography.hazmat.primitives import serialization, hashes
from cryptography.hazmat.primitives.asymmetric import padding
from cryptography.hazmat.backends import default_backend


if len(sys.argv) != 4:
    print('USAGE: verify.py pubkey messagefile sigfile')
    sys.exit(2)

pubkeyfile = sys.argv[1]
messagefile = sys.argv[2]
sigfile = sys.argv[3]

with open(pubkeyfile) as f:
    pubkey = serialization.load_pem_public_key(
        f.read(), backend=default_backend())

with open(messagefile) as m:
    message = m.read()
    prehashed_msg = hashlib.sha256(message).hexdigest()

with open(sigfile) as s:
    sig = s.read()
    decoded_sig = base64.b64decode(sig)


try:
    pubkey.verify(
        decoded_sig,
        prehashed_msg,
        padding.PSS(
            mgf=padding.MGF1(hashes.SHA256()),
            salt_length=padding.PSS.MAX_LENGTH),
        hashes.SHA256())
    print('valid!')
    sys.exit(0)
except InvalidSignature:
    print('invalid!')
    sys.exit(1)

Thank you for sharing.

I had to do 3 modifications in the 'sign.py' file:

1. Line 22:
   Error: ''initializer for ctype 'char[]' must be a bytes or list or tuple, not str"
   Solution: with open(privkeyfile, 'rb') as f:

2. Line 27:
   Error: "Unicode-objects must be encoded before hashing"
   Solution: message = m.read().encode('ascii')

3. Line 31:
   Error: "data must be bytes."
   Solution: bytes(prehashed.encode('ascii')),

I am not familiar with cryptography, so forgive me if I am saying something wrong.

Thank you for sharing.

I had to do 3 modifications in the 'sign.py' file:

1. Line 22:
   Error: ''initializer for ctype 'char[]' must be a bytes or list or tuple, not str"
   Solution: with open(privkeyfile, 'rb') as f:
2. Line 27:
   Error: "Unicode-objects must be encoded before hashing"
   Solution: message = m.read().encode('ascii')
3. Line 31:
   Error: "data must be bytes."
   Solution: bytes(prehashed.encode('ascii')),

I am not familiar with cryptography, so forgive me if I am saying something wrong.

The #3 saved my life!