I have generated an ECDSA signature in Java and I would like to get the R and S values from it. It is my understanding that the signature I have generated is DER encoded. Can someone please provide me with some Java code (maybe using Bouncy Castle) to retrieve the R and S values as BigIntegers?
Note: In case it helps, I generated the signature using a built in provider via the JCE's Signature class and the signature lengths for my P_256 EC key pair hover between 70 and 72 bytes usually.
I was able to solve this myself. In case it helps anyone here is how I did it (most exception handling has been stripped for readability):
import java.io.ByteArrayInputStream; import java.math.BigInteger; import java.security.KeyPair; import java.security.KeyPairGenerator; import java.security.Security; import java.security.Signature; import java.security.spec.ECGenParameterSpec; import org.bouncycastle.asn1.ASN1Encodable; import org.bouncycastle.asn1.ASN1InputStream; import org.bouncycastle.asn1.ASN1Integer; import org.bouncycastle.asn1.ASN1Primitive; import org.bouncycastle.asn1.ASN1Sequence; import org.bouncycastle.jce.provider.BouncyCastleProvider; public class DecodeEcdsaSignature { public static void main(String[] args) throws Exception { Security.addProvider(new BouncyCastleProvider()); byte[] signature = getSignature(); ASN1Primitive asn1 = toAsn1Primitive(signature); if (asn1 instanceof ASN1Sequence) { ASN1Sequence asn1Sequence = (ASN1Sequence) asn1; ASN1Encodable[] asn1Encodables = asn1Sequence.toArray(); for (ASN1Encodable asn1Encodable : asn1Encodables) { ASN1Primitive asn1Primitive = asn1Encodable.toASN1Primitive(); if (asn1Primitive instanceof ASN1Integer) { ASN1Integer asn1Integer = (ASN1Integer) asn1Primitive; BigInteger integer = asn1Integer.getValue(); System.out.println(integer.toString()); } } } } private static ASN1Primitive toAsn1Primitive(byte[] data) throws Exception { try (ByteArrayInputStream inStream = new ByteArrayInputStream(data); ASN1InputStream asnInputStream = new ASN1InputStream(inStream);) { return asnInputStream.readObject(); } } private static byte[] getSignature() throws Exception { KeyPairGenerator keyPairGenerator = KeyPairGenerator.getInstance("ECDSA"); ECGenParameterSpec ecParameterSpec = new ECGenParameterSpec("P-256"); keyPairGenerator.initialize(ecParameterSpec); KeyPair keyPair = keyPairGenerator.generateKeyPair(); Signature signature = Signature.getInstance("SHA256withECDSA"); signature.initSign(keyPair.getPrivate()); signature.update("message to sign".getBytes("UTF-8")); return signature.sign(); } }