RSA signature in c# and verification in C++ with Crypto++

Question

I'm trying to sign some bytes in C# thanks to the class RSACryptoServiceProvider and to verify it in C++ with the Crypto++ library. Despite all my attempts, the validation fail although I'm sure of my key and signature.

In C# I sign as follow :

var message = "hello";
var bytes = System.Text.Encoding.UTF8.GetBytes(message);
byte[] signedHash;
using (RSACryptoServiceProvider rsa = new  RSACryptoServiceProvider())
{
    // Import the key information.
    rsa.ImportParameters(privateKey);
    // Sign the data, using SHA256 as the hashing algorithm 
    signedHash = rsa.SignData(bytes, CryptoConfig.MapNameToOID("SHA256"));
}

My keys are generated as follow :

CspParameters parameters = new CspParameters();
parameters.KeyNumber = (int)KeyNumber.Signature;
using (RSACryptoServiceProvider rsa = new RSACryptoServiceProvider(1024, parameters))
{
    privateKeyInfo = rsa.ExportParameters(true);
    publicKeyInfo = rsa.ExportParameters(false);
}

In C++, I create the public key and try to verify it as follow :

RSA::PublicKey publicKey;
byte signature[128];
signature[0]= 150;
//....fill up to 127 , corresponds to "signedHash" variable from c# code
signature[127]= 89;

string simplemessage = "hello";
string modulus = "0Z8GUI/rxlXanCCjkiP+c9HyvdlOibst2YD5XmZk4F86aLr7LbLtI7FMnr6rcQZa6RXkAykb5MIbasmkOmkLzSjhdTThnaZyuKBOBoybYB5mDecF2VMXfUIryEBFn4i6y58qhy0BnDnIhucdNXX0px10HL3uYzR2KBTC0lSFFmE=";
string exponent = "AQAB";

char modulusCharred[1024];
strncpy_s(modulusCharred, base64ToHex(modulus).c_str(), sizeof(modulusCharred));
modulusCharred[sizeof(modulusCharred) - 1] = 0;

char exponentCharred[1024];
strncpy_s(exponentCharred, base64ToHex(exponent).c_str(), sizeof(exponentCharred));
exponentCharred[sizeof(exponentCharred) - 1] = 0;

Integer n(modulusCharred);
Integer e(exponentCharred);

publicKey.Initialize(n, e);

AutoSeededRandomPool rnd;
if(!publicKey.Validate(rnd, 3))
    throw runtime_error("Rsa public key validation failed"); // no error is thrown 

RSASS<PSS, SHA256>::Verifier verifier(publicKey);
bool result = verifier.VerifyMessage((const byte*)simplemessage.c_str(),simplemessage.length(), signature,128);

if(true == result) {
    cout << "Signature on message verified" << endl;
} else {
    cout << "Message verification failed" << endl; // always fail...
}

Modulus and exponent are copy/past from the xml obtained in c# using rsa.ToXmlString(false). The function base64toHex is given by (found on another SO post):

std::string base64ToHex(std::string base64String)
{
    std::string decodedString, finalString;
    CryptoPP::StringSource river(base64String, true,
        new CryptoPP::Base64Decoder(new CryptoPP::StringSink(decodedString)));

    CryptoPP::StringSource stream(decodedString, true,
        new CryptoPP::HexEncoder(new CryptoPP::StringSink(finalString)));

    finalString.erase(std::remove(finalString.begin(), finalString.end(), '\n'), finalString.end());

    return finalString;
}

I do not want to use any external files, only bytes (or string) variable. Also I'm not sure of the way I define my verifier : RSASS<PSS, SHA256>::Verifier.

Could you help me with this ?

RSACryptoServiceProvider only does PKCS1_v1.5 signatures, it can't do PSS. So you need to use a PKCS verifier. — bartonjs, Aug 28 '17 at 03:42
thanks, I have modified my verifier to `RSASS::Verifier` but the problem is still there. — Malick, Aug 28 '17 at 10:12

Malick · Accepted Answer · 2017-08-29T13:26:14.747

This answer is for both crypto++ and the windows API.

After a lot of trials/errors I finally succeed, the problem came from the way I built the crypto++ rsa keys ( Integer type : modulus and exponent).

I have created a new function GetIntegerFromBase64String that convert directly the modulus and exponent we obtain from the xml output of rsa.ToXmlString to the Integer type to initialize key in cryptopp.

Full crypto++ Example :

        string signature_64str = "G+PQaArLByTNYF5c5BZo2X3Guf1AplyJyik6NXCJmXnZ7CD5AC/OKq+Iswcv8GboUVsMTvl8G+lCa9Od0DfytnDui7kA/c1qtH7BZzF55yA5Yf9DGOfD1RHOl3OkRvpK/mF+Sf8nJwgxsg51C3pk/oBFjA450q2zq8HfFG2KJcs=";  
        string modulus_64str = "0Z8GUI/rxlXanCCjkiP+c9HyvdlOibst2YD5XmZk4F86aLr7LbLtI7FMnr6rcQZa6RXkAykb5MIbasmkOmkLzSjhdTThnaZyuKBOBoybYB5mDecF2VMXfUIryEBFn4i6y58qhy0BnDnIhucdNXX0px10HL3uYzR2KBTC0lSFFmE=";
        string exponent_64str  = "AQAB";

        Integer mod_integer = GetIntegerFromBase64String(modulus_64str);
        Integer pub_integer = GetIntegerFromBase64String(exponent_64str);
        InvertibleRSAFunction param;
        param.SetModulus(mod_integer);
        param.SetPublicExponent(pub_integer);
        RSA::PublicKey pubkey(param);

        string decoded_sig = DecodeBase64String(signature_64str);


        if(!pubkey.Validate(rnd, 3))
                cout << "Rsa public key validation failed" << endl;
        else
               cout << " key validation success"<<  endl;


        RSASS<PKCS1v15, SHA512>::Verifier verif(pubkey);
        bool res = verif.VerifyMessage( reinterpret_cast<const byte*>(message.c_str()), message.length(), reinterpret_cast<const byte*>(decoded_sig.c_str()), decoded_sig.length() );

         if( res ) {
                    cout << "Signature on message verified " << endl;
         } else {
                    cout << "Message verification failed " << endl;
         }

with :

string DecodeBase64String(string encoded )
{
        string decoded;    
        Base64Decoder decoder;
        decoder.Attach( new StringSink( decoded ) );
        decoder.Put( (byte*)encoded.data(), encoded.size() );
        decoder.MessageEnd();
        return decoded;

}
 Integer GetIntegerFromBase64String(string encoded)
{
        string decoded = DecodeBase64String(encoded);              
        Integer integer( (byte*)decoded.c_str(),decoded.length());
        return integer;
}

Additionaly, I have reproduced the verification with the windows API, for this case I do not use the xml key but directly the blob key (encoded in 64 bit) that I obtain from rsa.ExportCspBlob(false)

Full windows api example :

In c# I got the CspBlob as follow :

  using (RSACryptoServiceProvider rsa = new RSACryptoServiceProvider())
            {
                rsa.ImportParameters(privateKey);
                var cspBlob = rsa.ExportCspBlob(false);
                var cspBlobBase_64str = Convert.ToBase64String(cspBlob);// <---- HERE

Then in c++, I load the blob and verify the signature as follow :

#include <windows.h>
...
            string ErrorMessage;
            string signature_64str = "G+PQaArLByTNYF5c5BZo2X3Guf1AplyJyik6NXCJmXnZ7CD5AC/OKq+Iswcv8GboUVsMTvl8G+lCa9Od0DfytnDui7kA/c1qtH7BZzF55yA5Yf9DGOfD1RHOl3OkRvpK/mF+Sf8nJwgxsg51C3pk/oBFjA450q2zq8HfFG2KJcs=";
            string public_key_blob_64_bit_encoded = "BgIAAACkAABSU0ExAAQAAAEAAQBhFoVU0sIUKHY0Y+69HHQdp/R1NR3nhsg5nAEthyqfy7qIn0VAyCtCfRdT2QXnDWYeYJuMBk6guHKmneE0deEozQtpOqTJahvC5BspA+QV6VoGcau+nkyxI+2yLfu6aDpf4GRmXvmA2S27iU7ZvfLRc/4jkqMgnNpVxuuPUAaf0Q==";
            string message = "hello";


            if( RSA_VerifySignature(message,   signature_64str, public_key_blob_64_bit_encoded,  ErrorMessage))
            {
                 cout << "OK : Signature on message verified " << endl;
            }
            else
            {
                 cout << "Message verification failed, Error : " << ErrorMessage << endl;
            }

with :

 bool RSA_VerifySignature(string message, string signature_64BitEncoded, string publickeyBlob_64BitEncoded, string &ErrorMessage)
{
     const size_t LENGHT_SIGNATURE = 128; // 128 bytes == 1024 RSA Key bits 
     const size_t LENGHT_BLOB_PUBLIC_KEY = 148; // 148 bytes 

     bool isSigOk = false;
     HCRYPTHASH hash;

     byte  decoded_Blob[LENGHT_BLOB_PUBLIC_KEY] ;
     size_t size_pubkey = Base64Decode(publickeyBlob_64BitEncoded, decoded_Blob, LENGHT_BLOB_PUBLIC_KEY);

     byte  decoded_signature[LENGHT_SIGNATURE] ;
     size_t size_signature =Base64Decode(signature_64BitEncoded, decoded_signature, LENGHT_SIGNATURE);

     //reverse bytes
     byte reverse_decoded_signature[LENGHT_SIGNATURE];
     for(int i=0;i<sizeof(reverse_decoded_signature);i++)
         reverse_decoded_signature[i] = decoded_signature[LENGHT_SIGNATURE-i-1];


     HCRYPTPROV cryptProvider;
     // Get a handle to the PROV_RSA_AES (for CALG_SHA_512).
     if (!CryptAcquireContext(&cryptProvider, 0, 0, PROV_RSA_AES, CRYPT_VERIFYCONTEXT)){  
        ErrorMessage = "Failure to acquire context";
        goto Exit;
     }

     HCRYPTKEY publicKeyc;
     // convert the blob to the public key
     if(!CryptImportKey(cryptProvider, decoded_Blob, LENGHT_BLOB_PUBLIC_KEY, 0, 0, &publicKeyc)){ 

        ErrorMessage = "Failure to import key";
        goto Exit;
     }

     // create the hash object
     if(!CryptCreateHash(cryptProvider, CALG_SHA_512 , 0, 0, &hash)){
        ErrorMessage = "Failure to creat Hash"  ;
        goto Exit;
     }

     //hash the message
     if(!CryptHashData(hash, (byte*) message.c_str(), message.length(), 0)){  
        ErrorMessage = "Failure to Hash Data"  ;
        goto Exit;
     }

       isSigOk = CryptVerifySignature(hash, reverse_decoded_signature, sizeof(reverse_decoded_signature), publicKeyc, nullptr, 0);

       if(!isSigOk) ErrorMessage = "Invalid Signature"  ;


     Exit:  
     // After processing, hHash and cryptProvider must be released. 
     if(hash) 
        CryptDestroyHash(hash);
     if(cryptProvider) 
        CryptReleaseContext(cryptProvider,0);
       return isSigOk;
}

where Base64Decode comes from this SO answer.

ps: note that I have switched to SHA512 in this answer .

score 2 · Answer 2 · answered Aug 29 '17 at 19:29

Regarding this:

string DecodeBase64String(string encoded )
{
        string decoded;    
        Base64Decoder decoder;
        decoder.Attach( new StringSink( decoded ) );
        decoder.Put( (byte*)encoded.data(), encoded.size() );
        decoder.MessageEnd();
        return decoded;

}
 Integer GetIntegerFromBase64String(string encoded)
{
        string decoded = DecodeBase64String(encoded);              
        Integer integer( (byte*)decoded.c_str(),decoded.length());
        return integer;
}

You might find this a little easier (or maybe not):

$ cat test.cxx
#include "cryptlib.h"
#include "integer.h"
#include "filters.h"
#include "base64.h"
#include "files.h"
#include "hex.h"

#include <iostream>
#include <string>

int main()
{
    using CryptoPP::Integer;
    using CryptoPP::StringSource;
    using CryptoPP::Base64Decoder;

    std::string signature_64str = "G+PQaArLByTNYF5c5BZo2X3Guf1AplyJyik6NXCJmXnZ7CD5AC/OKq+Iswcv8GboUVsMTvl8G+lCa9Od0DfytnDui7kA/c1qtH7BZzF55yA5Yf9DGOfD1RHOl3OkRvpK/mF+Sf8nJwgxsg51C3pk/oBFjA450q2zq8HfFG2KJcs=";
    std::string modulus_64str = "0Z8GUI/rxlXanCCjkiP+c9HyvdlOibst2YD5XmZk4F86aLr7LbLtI7FMnr6rcQZa6RXkAykb5MIbasmkOmkLzSjhdTThnaZyuKBOBoybYB5mDecF2VMXfUIryEBFn4i6y58qhy0BnDnIhucdNXX0px10HL3uYzR2KBTC0lSFFmE=";
    std::string exponent_64str  = "AQAB";

    StringSource mod64(modulus_64str, true, new Base64Decoder());
    StringSource exp64(exponent_64str, true, new Base64Decoder());

    Integer mod_integer(mod64, mod64.MaxRetrievable());
    Integer pub_integer(exp64, exp64.MaxRetrievable());

    std::cout << mod_integer << std::endl;
    std::cout << pub_integer << std::endl;

    return 0;
}

The code uses this Integer constructor:

Integer (BufferedTransformation &bt, size_t byteCount, Signedness sign=UNSIGNED, ByteOrder order=BIG_ENDIAN_ORDER)

The code produces:

$ ./test.exe
14720100561434965959279660783734794237717246172864229085441100391073349113964189
60745884288272734508848163730385929321875027666547189267336228176446737079038557
84643451276687447934542940291651491826423492370177748998568853020003645093067936
387415713280978250634089486257964777026826174628998219103122976216673.
65537.

Yes I find your way easier to create `Integer` , I will use it. Thanks — Malick, Aug 29 '17 at 20:17
@Malick - That works because sources, like `StringSource` and filters, like `Base64Decoder` all inherit from `BufferedTransformation`. Once you get accustomed to it, its easy to use sources, filter and sinks. The problem is, it sucks being a new user and the learning curve can be steep. The mechanics of [pipelines](https://www.cryptopp.com/wiki/Pipelining) are not obvious. — jww, Aug 29 '17 at 20:46

RSA signature in c# and verification in C++ with Crypto++

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