Digital Signature Verification failed using SHA256withRSA in Python

Question

I am trying to validate the digital signature with given certificate files for the offline aadhaar KYC verification application.

This instruction is given in the documentation for the verification.

Aadhaar Paperless Offline e-KYC when downloaded has the following XML :

<OKY v=""n=""r=""i=""d=""e=""m=""g=""a=""s="" />

XSD for the above xml

<?xml version="1.0" encoding="UTF-8"?>
<xs:schemaxmlns:xs="http: www.w3.org="" 2001="" xmlschema"="" attributeformdefault="unqualified" elementformdefault="qualified" targetnamespace="http://www.uidai.gov.in/offlinePaperlesseKYC/1.0">
<xs:element name="OKY">
<xs:complextype>
<xs:attribute name="v" type="xs:string"/>
<xs:attribute name="n" type="xs:string"/>
<xs:attribute name="i" type="xs:string"/>
<xs:attribute name="d" type="xs:string"/>
<xs:attribute name="e" type="xs:string"/>
<xs:attribute name="m" type="xs:string"/>
<xs:attribute name="g" type="xs:string"/>
<xs:attribute name="a" type="xs:string"/>
<xs:attribute name="r" type="xs:string"/>
<xs:attribute name="s" type="xs:string"/>
</xs:complextype>
</xs:element>
</xs:schema>

1. Read the entire XML and separate the s=”xxxx” tag from it.

2. Use a signature validation algorithm leveraging “SHA256withRSA” based hashing and encryption technique

3. Signature value present in “s” tag, remaining XML (without "s" tag) and UIDAI public key (available here.) is to be fed to the algorithm to validate the digital signature.

Sample C# code snippets provided by the organization. (PS :which is also not working)

using System;
using System.Security.Cryptography.X509Certificates;
using System.Xml;

namespace test
{
class MainClass
{
    public static void Main(string[] args)
    {
        // link -> https://drive.google.com/file/d/1aSv3HJUFf5_42Z-FqpdVHEk5b3VA3T3D/view


        string XMLFilePath = "offlineaadhaar.xml"; //Get the XML file

// link -> https://drive.google.com/file/d/1FW4ciIhZqJuelOcGF2x6VaBCSDO9J-gM/view


string KeyFilePath = "okyc-publickey.cer"; //Get the public key certificate file

        XmlDocument ObjXmlDocument = new XmlDocument();
        ObjXmlDocument.Load(XMLFilePath); //Load the XML
        XmlAttributeCollection SignatureElement = ObjXmlDocument.DocumentElement.Attributes; //Get the all XML attribute
        string SignatureValue = SignatureElement.GetNamedItem("s").InnerXml; // Get Signature value

        SignatureElement.RemoveNamedItem("s");//Remove the signature "s" attribute from XML and get the new XML to validate

        /*----------------Read and parse the public key as string-----------------------*/
        X509Certificate2 ObjX509Certificate2 = new X509Certificate2(KeyFilePath, "public"); //Initialize the public ket certificate file

        Org.BouncyCastle.X509.X509Certificate objX509Certificate;
        Org.BouncyCastle.X509.X509CertificateParser objX509CertificateParser = new Org.BouncyCastle.X509.X509CertificateParser();

        objX509Certificate = objX509CertificateParser.ReadCertificate(ObjX509Certificate2.GetRawCertData());
        /*----------------End-----------------------*/

        /* Init alg */
        Org.BouncyCastle.Crypto.ISigner signer = Org.BouncyCastle.Security.SignerUtilities.GetSigner("SHA256withRSA");

        /* Populate key */
        signer.Init(false, objX509Certificate.GetPublicKey());

        /* Get the signature into bytes */
        var expectedSig = Convert.FromBase64String(SignatureValue);

        /* Get the bytes to be signed from the string */
        var msgBytes = System.Text.Encoding.UTF8.GetBytes(ObjXmlDocument.InnerXml);


        /* Calculate the signature and see if it matches */
        signer.BlockUpdate(msgBytes, 0, msgBytes.Length);
        bool Flag = signer.VerifySignature(expectedSig);            
        if (Flag)
        {
            Console.WriteLine("XML Validate Successfully");
        }
        else
        {
            Console.WriteLine("XML Validation Failed");
        }
    }
}
}

I am trying to implement in Python and getting the XML validation failed. I am not sure if the certificate file is wrong or there is some bug on my code.

Here is my Python Code :

import xml
import xml.etree.cElementTree as etree
from xml.etree import ElementTree
import OpenSSL
from cryptography import x509
from cryptography.hazmat.backends import default_backend
from Crypto.PublicKey import RSA
from base64 import b64encode, b64decode
from M2Crypto import BIO, RSA, EVP

xmlDoc = open('adhar.xml', 'r').read()
Tr = etree.XML(xmlDoc)
Tr.keys()
# ['s', 'r', 'a', 'g', 'm', 'e', 'd', 'i', 'n', 'v']

sign = Tr.get('s')
len(sign)
# 344

del Tr.attrib['s']

from M2Crypto import X509

x509 =X509.load_cert('ekyc_public_key.cer')
#x509 =X509.load_cert(cert4)
rsa = x509.get_pubkey().get_rsa()
pubkey = EVP.PKey()
pubkey.assign_rsa(rsa)

xmlstr = etree.tostring(Tr, encoding='utf8', method='xml')
#rstr=str(xmlstr)[45:][:-1]
#rstr = rstr.encode(encoding='utf-8')


# if you need a different digest than the default 'sha1':
pubkey.reset_context(md='sha256')
pubkey.verify_init()

# hashlib.sha256(message_without_sign).digest()
pubkey.verify_update(xmlstr)
if(pubkey.verify_final(b64decode(sign)) != 1):
    print('Digital Signeture not validated')
else: 
    print('Digital Signeture validated')  

Answer 1

The description in the question is not enough to completely specify signature generation / verification. Clarification of the protocol is certainly required; it's probably best to request a formalized description. It's not for nothing that XML digsig has been specified; you need a standardized canonicalization, character set etc. In the end, the signature is calculated over bytes, not over XML / text.

"SHA256withRSA" is not a signature algorithm; it's the (rather bad) Java name for the PKCS#1 v1.5 signature scheme.

These are not good signs; you should ask if the protocol has been verified by an expert.