Sign string with private key using RSA in Python

Programming
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1

Issue

I am unable to sign a string using my private key with the following conditions:

  1. Use SHA 256 algorithm to calculate the hash of the string
  2. Use the private key and RSA (PKCS1_PADDING) algorithm to sign the Hash Value
  3. Base64 encode the encrypted Hash Value

The code I am using is below, yet the signature does not match on the server side.

from Crypto.Signature import PKCS1_v1_5
from Crypto.Hash import SHA256
from Crypto.PublicKey import RSA
import base64


pkey = RSA.importKey(keystring)

message = "Hello world"

h = SHA256.new(message.encode())
signature = PKCS1_v1_5.new(pkey).sign(h)
result = base64.b64encode(signature).decode()

Can anyone help me troubleshoot why my signature is not matching?

2

Answer

The issue could be caused by the encoding of the string before hashing. Try encoding the string using UTF-8 before hashing it. Here’s the updated code:

from Crypto.Signature import PKCS1_v1_5
from Crypto.Hash import SHA256
from Crypto.PublicKey import RSA
import base64

pkey = RSA.importKey(keystring)

message = "Hello world"

h = SHA256.new(message.encode('utf-8')) # encode the message using 'utf-8'
signature = PKCS1_v1_5.new(pkey).sign(h)
result = base64.b64encode(signature).decode()

Additionally, ensure that the keystring variable is properly defined and contains the private key.