chore: code enhancements

- Update code to use BigInteger from `ionspin` lib instead of the `JVM` one
- Added the required code for signature bytes modification in the other platforms
- Moved unit tests from `jvmTest` to `commonTest`
- Added documentation to the two newly added methods
- Replace `JVM` method `System.arraycopy` and use `Kotlin` method instead

Signed-off-by: Ahmed Moussa <ahmed.moussa@iohk.io>

apollo/src/androidMain/kotlin/io/iohk/atala/prism/apollo/secp256k1/Secp256k1Lib.kt

@@ -1,5 +1,6 @@
 package io.iohk.atala.prism.apollo.secp256k1
 
+import com.ionspin.kotlin.bignum.integer.Sign
 import fr.acinq.secp256k1.Secp256k1
 import org.kotlincrypto.hash.sha2.SHA256
 
@@ -75,7 +76,50 @@ actual class Secp256k1Lib {
         if (Secp256k1.verify(normalisedSignature, sha, publicKey)) {
             return true
         }
-        return Secp256k1.verify(signature, sha, publicKey)
+        return Secp256k1.verify(transcodeSignatureToDERBitcoin(normalisedSignature), sha, publicKey)
+    }
+
+    /**
+     * Reverses the order of bytes in the given byte array.
+     *
+     * @param inputBytes The byte array to be reversed.
+     * @return A new byte array with the reversed order of bytes.
+     */
+    private fun reverseB32(inputBytes: ByteArray): ByteArray {
+        val reversedBytes = ByteArray(inputBytes.size)
+        for (i in inputBytes.indices) {
+            reversedBytes[inputBytes.size - i - 1] = inputBytes[i]
+        }
+        return reversedBytes
+    }
+
+    /**
+     * Transcodes a signature to DER format for use in Bitcoin transactions.
+     *
+     * @param signature The signature to be transcoded, in byte array format.
+     * @return A byte array representing the signature in DER format.
+     */
+    private fun transcodeSignatureToDERBitcoin(signature: ByteArray): ByteArray {
+        val rawLen = signature.size / 2
+        val bigR = com.ionspin.kotlin.bignum.integer.BigInteger.fromByteArray(signature.copyOfRange(0, rawLen), Sign.POSITIVE)
+        val bigS = com.ionspin.kotlin.bignum.integer.BigInteger.fromByteArray(signature.copyOfRange(rawLen, signature.size), Sign.POSITIVE)
+        var r = bigR.toByteArray()
+        var s = bigS.toByteArray()
+        r = reverseB32(r)
+        s = reverseB32(s)
+        val lenR = r.size
+        val lenS = s.size
+        val derLength = 6 + lenR + lenS
+        val derSignature = ByteArray(derLength)
+        derSignature[0] = 0x30
+        derSignature[1] = (4 + lenR + lenS).toByte()
+        derSignature[2] = 0x02
+        derSignature[3] = lenR.toByte()
+        r.copyInto(derSignature, 4, 0, lenR)
+        derSignature[4 + lenR] = 0x02
+        derSignature[5 + lenR] = lenS.toByte()
+        s.copyInto(derSignature, 6 + lenR, 0, lenS)
+        return derSignature
     }
 
     /**

apollo/src/appleMain/kotlin/io/iohk/atala/prism/apollo/secp256k1/Secp256k1Lib.kt

@@ -1,5 +1,7 @@
 package io.iohk.atala.prism.apollo.secp256k1
 
+import com.ionspin.kotlin.bignum.integer.BigInteger
+import com.ionspin.kotlin.bignum.integer.Sign
 import fr.acinq.secp256k1.Secp256k1
 import fr.acinq.secp256k1.Secp256k1Native
 import org.kotlincrypto.hash.sha2.SHA256
@@ -73,7 +75,53 @@ actual class Secp256k1Lib {
             return true
         }
         val normalisedSignature = Secp256k1.signatureNormalize(signature).first
-        return Secp256k1.verify(normalisedSignature, sha, publicKey)
+        if (Secp256k1.verify(normalisedSignature, sha, publicKey)) {
+            return true
+        }
+        return Secp256k1.verify(transcodeSignatureToDERBitcoin(normalisedSignature), sha, publicKey)
+    }
+
+    /**
+     * Reverses the order of bytes in the given byte array.
+     *
+     * @param inputBytes The byte array to be reversed.
+     * @return A new byte array with the reversed order of bytes.
+     */
+    private fun reverseB32(inputBytes: ByteArray): ByteArray {
+        val reversedBytes = ByteArray(inputBytes.size)
+        for (i in inputBytes.indices) {
+            reversedBytes[inputBytes.size - i - 1] = inputBytes[i]
+        }
+        return reversedBytes
+    }
+
+    /**
+     * Transcodes a signature to DER format for use in Bitcoin transactions.
+     *
+     * @param signature The signature to be transcoded, in byte array format.
+     * @return A byte array representing the signature in DER format.
+     */
+    private fun transcodeSignatureToDERBitcoin(signature: ByteArray): ByteArray {
+        val rawLen = signature.size / 2
+        val bigR = BigInteger.fromByteArray(signature.copyOfRange(0, rawLen), Sign.POSITIVE)
+        val bigS = BigInteger.fromByteArray(signature.copyOfRange(rawLen, signature.size), Sign.POSITIVE)
+        var r = bigR.toByteArray()
+        var s = bigS.toByteArray()
+        r = reverseB32(r)
+        s = reverseB32(s)
+        val lenR = r.size
+        val lenS = s.size
+        val derLength = 6 + lenR + lenS
+        val derSignature = ByteArray(derLength)
+        derSignature[0] = 0x30
+        derSignature[1] = (4 + lenR + lenS).toByte()
+        derSignature[2] = 0x02
+        derSignature[3] = lenR.toByte()
+        r.copyInto(derSignature, 4, 0, lenR)
+        derSignature[4 + lenR] = 0x02
+        derSignature[5 + lenR] = lenS.toByte()
+        s.copyInto(derSignature, 6 + lenR, 0, lenS)
+        return derSignature
     }
 
     /**