Fixed length 64 Bytes EC P-256 Signature with JCE

Question

I need a fixed length 64 Byte ECDSA signature with the NIST P-256 Curve.

The implementation hast to use JCE.

The following code sample can generate a signature and verify it.

Provider provSign = new SunEC();
Provider provVerify = new SunEC();


    // generate EC key
    KeyPairGenerator kg = KeyPairGenerator.getInstance("EC", provSign);
    ECGenParameterSpec ecParam = new ECGenParameterSpec("secp256r1");
    kg.initialize(ecParam);      
    KeyPair keyPair = kg.generateKeyPair(); 
    PrivateKey privateKey = keyPair.getPrivate();      
    PublicKey publicKey = keyPair.getPublic();

    try
    {
      // export public key                  
      KeyFactory kf = KeyFactory.getInstance("EC", provSign);
      ECPublicKeySpec publicKeySpec = kf.getKeySpec(keyPair.getPublic(), ECPublicKeySpec.class);

      // import public key into other provider
      kf = KeyFactory.getInstance("EC", provVerify);
      publicKey = (PublicKey)kf.generatePublic(publicKeySpec);      
    }
    catch (InvalidKeySpecException ex)
    {                       
      ex.printStackTrace();
    }


      // do test        
      Signature sig = Signature.getInstance("SHA256withECDSA", provSign);
      Signature ver = Signature.getInstance("SHA256withECDSA", provVerify);

      byte[] data = new byte[64];

      // sign
      sig.initSign(privateKey);
      sig.update(data);
      byte [] sign = sig.sign();

      // Working Signature verification
      ver.initVerify(publicKey);
      ver.update(data);
      if (ver.verify(sign) == false)
      {
        throw new Exception("Signature Verification failed");
      }

The problem is that the sign is somehow encoded (I think in DER Format) and is between 70 and 72 Bytes long but I need a 64 Byte (unencoded/raw) signature.

What I have tried: Convert to fixed length 64 Byte Signature

      DerInputStream derInputStream = new DerInputStream(sign);
      DerValue[] values = derInputStream.getSequence(2);
      byte[] random = values[0].getPositiveBigInteger().toByteArray();
      byte[] signature = values[1].getPositiveBigInteger().toByteArray();


      // r and s each occupy half the array
      // Remove padding bytes
      byte[] tokenSignature = new byte[64];
      System.arraycopy(random, random.length > 32 ? 1 : 0, tokenSignature, random.length < 32 ? 1 : 0,
              random.length > 32 ? 32 : random.length);
      System.arraycopy(signature, signature.length > 32 ? 1 : 0, tokenSignature, signature.length < 32 ? 33 : 32,
              signature.length > 32 ? 32 : signature.length);

      System.out.println("Full Signature length: "+tokenSignature.length+" r length: "+random.length+" s length"+signature.length);

How to check the 64 Bytes tokenSignature now??? I don't know how to convert the 64 Byte tokenSignature back to the right format

      ver.initVerify(publicKey);
      ver.update(data);
      if (ver.verify(???) == false)
      {
        throw new Exception("Signature Verification failed");
      }

I have achieved the 64 Byte signature verification with BouncyCastle ECDSASigner. But I can't use ECDSASigner because it is not extending SignatureSpi and therefore not working with JCE complient crypto server.

Answer 1

I got it working now thanks to @MaartenBodewes

//How to Check Signature
byte[] r = Arrays.copyOfRange(tokenSignature, 0,tokenSignature.length/2);
byte[] s = Arrays.copyOfRange(tokenSignature, tokenSignature.length/2,tokenSignature.length);

ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
DEROutputStream derOutputStream = new DEROutputStream(byteArrayOutputStream);
ASN1EncodableVector v=new ASN1EncodableVector();
v.add(new ASN1Integer(new BigInteger(1,r)));
v.add(new ASN1Integer(new BigInteger(1,s)));
derOutputStream.writeObject(new DERSequence(v));
byte[] derSignature = byteArrayOutputStream.toByteArray();

ver.update(data);
if (ver.verify(derSignature) == false)
{
  throw new Exception("Signature Verification failed");
}