add signString method

quartz/src/androidTest/java/com/vitorpamplona/quartz/crypto/CryptoUtilsTest.kt

@@ -120,4 +120,20 @@ class CryptoUtilsTest {
 
         assertEquals(msg, decrypted)
     }
+
+    @Test
+    fun signString() {
+        val random = "319cc5596fdd6cd767e5a59d976e8e059c61306af90dff1e6ee1067b3a1fdbc0".hexToByteArray()
+        val message = "8e58c8251bb406b6ded69e9eb14f55282a9a53bdab16fc49a3218c2ad3abc887".hexToByteArray()
+        val keyPair = KeyPair("a5ab474552c8f9c46c2eda5a0b68f27430ad81f96cb405e0cb4e34bf0c6494a2".hexToByteArray())
+
+        val signedMessage = CryptoUtils.sign(message, keyPair.privKey!!, random).toHexKey()
+        val expectedValue = "0f9be7e01ba53d5ee6874b9180c7956269fda7a5be424634c3d17b5cfcea6da001be89183876415ba08b7dafa6cff4555e393dc228fb8769b384344e9a27b77c"
+        assertEquals(expectedValue, signedMessage)
+
+        val message2 = "Hello"
+        val signedMessage2 = CryptoUtils.signString(message2, keyPair.privKey!!, random).toHexKey()
+        val expectedValue2 = "7ec8194a585bfb513564113b6b7bfeaafa0254c99d24eaf92280657c2291bab908b1b7bc553c83276a0254aef5041bbe6a50e93381edc4de3d859efa1c3a5a1e"
+        assertEquals(expectedValue2, signedMessage2)
+    }
 }

quartz/src/main/java/com/vitorpamplona/quartz/crypto/CryptoUtils.kt

@@ -64,10 +64,19 @@ object CryptoUtils {
         return secp256k1.secKeyVerify(il)
     }
 
+    fun signString(
+        message: String,
+        privKey: ByteArray,
+        auxrand32: ByteArray = random(32),
+    ): ByteArray {
+        return sign(sha256(message.toByteArray()), privKey, auxrand32)
+    }
+
     fun sign(
         data: ByteArray,
         privKey: ByteArray,
-    ): ByteArray = secp256k1.signSchnorr(data, privKey, null)
+        auxrand32: ByteArray? = null,
+    ): ByteArray = secp256k1.signSchnorr(data, privKey, auxrand32)
 
     fun verifySignature(
         signature: ByteArray,