Java: convert between ASN.1 and P1363 Encoding of Signature

convert.Java

import org.bouncycastle.asn1.ASN1EncodableVector;
import org.bouncycastle.asn1.ASN1Integer;
import org.bouncycastle.asn1.ASN1Sequence;
import org.bouncycastle.asn1.DERSequence;
import java.math.BigInteger;

  private static byte[] toP1363(byte[] asn1EncodedSignature) throws Exception {
    ASN1Sequence seq = ASN1Sequence.getInstance(asn1EncodedSignature);
    BigInteger r = ((ASN1Integer) seq.getObjectAt(0)).getValue();
    BigInteger s = ((ASN1Integer) seq.getObjectAt(1)).getValue();
    int n = (r.bitLength() + 7) / 8;
    // round up to nearest even integer
    n = (int) Math.round((n+1)/2) * 2;
    byte[] out = new byte[2 * n];
    toFixed(r, out, 0, n);
    toFixed(s, out, n, n);
    return out;
  }

  private static byte[] toASN1(byte[] p1363EncodedSignature) throws IOException {
    int n = p1363EncodedSignature.length / 2;
    BigInteger r = new BigInteger(+1, Arrays.copyOfRange(p1363EncodedSignature, 0, n));
    BigInteger s = new BigInteger(+1, Arrays.copyOfRange(p1363EncodedSignature, n, n * 2));
    ASN1EncodableVector v = new ASN1EncodableVector();
    v.add(new ASN1Integer(r));
    v.add(new ASN1Integer(s));
    return new DERSequence(v).getEncoded();
  }

Hi @DinoChiesa

If I use toASN1 and pass the result to ContentInfo:

https://github.com/bcgit/bc-java/blob/1cae5431b0f315e1c7dc40392bcd60cf785831b4/core/src/main/java/org/bouncycastle/asn1/pkcs/ContentInfo.java#L45

That line expects the first entry in the sequence to be an ASN1ObjectIdentifier type but of course it is of type ASN1Integer, which results in a class cast exception.

How does one incorporate the expected ASN1ObjectIdentifier ? I assume the value is something like:

1.2.840.10045.4.1 ecdsaWithSHA1 (ANSI X9.62 ECDSA algorithm with SHA2)
https://stackoverflow.com/questions/72097765/retrieving-signature-r-and-s-element-from-asn-1-cms-signature-bouncy-castle

Hi there, I found a better solution for toP1363 that relies 100% on the BouncyCastle library and works on Android (you don't need to add BouncyCastle as a global provider, you can add the dependency and use the code below as is):

import org.bouncycastle.asn1.ASN1Integer;
import org.bouncycastle.asn1.ASN1Sequence;
import org.bouncycastle.crypto.signers.PlainDSAEncoding;
import org.bouncycastle.math.ec.custom.sec.SecP256R1Curve;
    
// the asn1EncodedSignature param is typically generated by
// Signature signature = Signature.getInstance("SHA256withECDSA");
private byte[] toP1363(byte[] asn1EncodedSignature) {
    ASN1Sequence seq = ASN1Sequence.getInstance(asn1EncodedSignature);
    BigInteger r = ((ASN1Integer) seq.getObjectAt(0)).getValue();
    BigInteger s = ((ASN1Integer) seq.getObjectAt(1)).getValue();
    BigInteger n = new SecP256R1Curve().getOrder();
    return PlainDSAEncoding.INSTANCE.encode(n, r, s);
}

SecP256R1Curve

awesome simplification. God bless you, @nicobao.