DER to P1363 in Java using BouncyCastle only

[nicobao]

Hi there,

I came across this repository upon a Google search on how to convert DER encoded signature to P1363 in Java.
Thanks for your work!

After lots of tinkering, I discovered it is possible to convert to P1363 by relying on BouncyCastle entirely, without any custom arithmetic:

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");
// with AndroidKeyStore as provider
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);
}

Note that it works on Android, and you don't need to add BouncyCastle as a global Provider or anything. You can just use the classes as shown above.

Just wanted to add this here it case it can help somebody - as for production, it is safer to rely on a well-known library than using custom code.