Use local secp256k1 callbacks in kotlin native code

These callbacks are only triggered either by arguments do not match explicit rquirements of the sepc256k1 library, or by hardware failures, memory corruption
or bug in secp256k1, and not by misuse of the library.
In theory we do not need to implement them, except to find bugs in our own code, but the default callbacks print a message to stderr and call abort() which
is not nice especially on mobile apps.

=> Here we introduce 2 specific exceptions, Secp256k1ErrorCallbackException and Secp256k1IllegalCallbackException, which are thrown when the error callback or illegal callback are called.

build.gradle.kts

@@ -22,7 +22,7 @@ buildscript {
 
 allprojects {
     group = "fr.acinq.secp256k1"
-    version = "0.11.0"
+    version = "0.12.0-SNAPSHOT"
 
     repositories {
         google()

src/commonMain/kotlin/fr/acinq/secp256k1/Secp256k1.kt

@@ -166,7 +166,17 @@ public interface Secp256k1 {
 
 internal expect fun getSecpk256k1(): Secp256k1
 
-public class Secp256k1Exception : RuntimeException {
+public open class Secp256k1Exception : RuntimeException {
+    public constructor() : super()
+    public constructor(message: String?) : super(message)
+}
+
+public class Secp256k1ErrorCallbackException : Secp256k1Exception {
+    public constructor() : super()
+    public constructor(message: String?) : super(message)
+}
+
+public class Secp256k1IllegalCallbackException : Secp256k1Exception {
     public constructor() : super()
     public constructor(message: String?) : super(message)
 }

src/nativeMain/kotlin/fr/acinq/secp256k1/Secp256k1Native.kt

@@ -4,15 +4,67 @@ import kotlinx.cinterop.*
 import platform.posix.size_tVar
 import secp256k1.*
 
-@OptIn(ExperimentalUnsignedTypes::class)
+private typealias Secp256k1CallbackHandler = (String) -> Unit
+
+@OptIn(ExperimentalStdlibApi::class)
+private class CallbackHandler(ctx: CPointer<secp256k1_context>) : AutoCloseable {
+    var illegalCallBackMessage: String? = null
+    val illegalHandler: Secp256k1CallbackHandler = { x: String -> illegalCallBackMessage = x }
+    val illegalCallbackRef = StableRef.create(illegalHandler)
+    var errorCallBackMessage: String? = null
+    val errorHandler: Secp256k1CallbackHandler = { x: String -> errorCallBackMessage = x }
+    val errorCallbackRef = StableRef.create(errorHandler)
+
+    init {
+        secp256k1_context_set_error_callback(
+            ctx, staticCFunction { buffer: CPointer<ByteVar>?, data: COpaquePointer? ->
+                if (data != null) {
+                    val callback = data.asStableRef<Secp256k1CallbackHandler>().get()
+                    callback(buffer?.toKString() ?: "error callback triggered")
+                }
+            },
+            errorCallbackRef.asCPointer()
+        )
+        secp256k1_context_set_illegal_callback(
+            ctx, staticCFunction { buffer: CPointer<ByteVar>?, data: COpaquePointer? ->
+                if (data != null) {
+                    val callback = data.asStableRef<Secp256k1CallbackHandler>().get()
+                    callback(buffer?.toKString() ?: "illegal callback triggered")
+                }
+            },
+            illegalCallbackRef.asCPointer()
+        )
+    }
+
+    fun checkForErrors() {
+        errorCallBackMessage?.let { throw Secp256k1ErrorCallbackException(it) }
+        illegalCallBackMessage?.let { throw Secp256k1IllegalCallbackException(it) }
+    }
+
+    override fun close() {
+        // StableRef instances have to be disposed of manually
+        illegalCallbackRef.dispose()
+        errorCallbackRef.dispose()
+    }
+}
+
+@OptIn(ExperimentalUnsignedTypes::class, ExperimentalStdlibApi::class)
 public object Secp256k1Native : Secp256k1 {
 
     private val ctx: CPointer<secp256k1_context> by lazy {
+
         secp256k1_context_create((SECP256K1_FLAGS_TYPE_CONTEXT or SECP256K1_FLAGS_BIT_CONTEXT_SIGN or SECP256K1_FLAGS_BIT_CONTEXT_VERIFY).toUInt())
             ?: error("Could not create secp256k1 context")
     }
 
-    private fun Int.requireSuccess(message: String): Int = if (this != 1) throw Secp256k1Exception(message) else this
+    private fun Int.requireSuccess(message: String): Int {
+        return if (this != 1) throw Secp256k1Exception(message) else this
+    }
+
+    private fun Int.requireSuccess(callbackHandler: CallbackHandler, message: String): Int {
+        callbackHandler.checkForErrors()
+        return if (this != 1) throw Secp256k1Exception(message) else this
+    }
 
     private fun MemScope.allocSignature(input: ByteArray): secp256k1_ecdsa_signature {
         val sig = alloc<secp256k1_ecdsa_signature>()
@@ -58,167 +110,209 @@ public object Secp256k1Native : Secp256k1 {
     public override fun verify(signature: ByteArray, message: ByteArray, pubkey: ByteArray): Boolean {
         require(message.size == 32)
         require(pubkey.size == 33 || pubkey.size == 65)
-        memScoped {
-            val nPubkey = allocPublicKey(pubkey)
-            val nMessage = toNat(message)
-            val nSig = allocSignature(signature)
-            return secp256k1_ecdsa_verify(ctx, nSig.ptr, nMessage, nPubkey.ptr) == 1
+        CallbackHandler(ctx).use { callbackHandler ->
+            memScoped {
+                val nPubkey = allocPublicKey(pubkey)
+                val nMessage = toNat(message)
+                val nSig = allocSignature(signature)
+                val verify = secp256k1_ecdsa_verify(ctx, nSig.ptr, nMessage, nPubkey.ptr)
+                callbackHandler.checkForErrors()
+                return verify == 1
+            }
         }
     }
 
     public override fun sign(message: ByteArray, privkey: ByteArray): ByteArray {
         require(privkey.size == 32)
         require(message.size == 32)
-        memScoped {
-            val nPrivkey = toNat(privkey)
-            val nMessage = toNat(message)
-            val nSig = alloc<secp256k1_ecdsa_signature>()
-            secp256k1_ecdsa_sign(ctx, nSig.ptr, nMessage, nPrivkey, null, null).requireSuccess("secp256k1_ecdsa_sign() failed")
-            return serializeSignature(nSig)
+        CallbackHandler(ctx).use { callbackHandler ->
+            memScoped {
+                val nPrivkey = toNat(privkey)
+                val nMessage = toNat(message)
+                val nSig = alloc<secp256k1_ecdsa_signature>()
+                secp256k1_ecdsa_sign(ctx, nSig.ptr, nMessage, nPrivkey, null, null).requireSuccess(callbackHandler, "secp256k1_ecdsa_sign() failed")
+                return serializeSignature(nSig)
+            }
         }
     }
 
     public override fun signatureNormalize(sig: ByteArray): Pair<ByteArray, Boolean> {
-        require(sig.size >= 64){ "invalid signature ${Hex.encode(sig)}" }
-        memScoped {
-            val nSig = allocSignature(sig)
-            val isHighS = secp256k1_ecdsa_signature_normalize(ctx, nSig.ptr, nSig.ptr)
-            return Pair(serializeSignature(nSig), isHighS == 1)
+        require(sig.size >= 64) { "invalid signature ${Hex.encode(sig)}" }
+        CallbackHandler(ctx).use { callbackHandler ->
+            memScoped {
+                val nSig = allocSignature(sig)
+                val isHighS = secp256k1_ecdsa_signature_normalize(ctx, nSig.ptr, nSig.ptr)
+                callbackHandler.checkForErrors()
+                return Pair(serializeSignature(nSig), isHighS == 1)
+            }
         }
     }
 
     public override fun secKeyVerify(privkey: ByteArray): Boolean {
         if (privkey.size != 32) return false
-        memScoped {
-            val nPrivkey = toNat(privkey)
-            return secp256k1_ec_seckey_verify(ctx, nPrivkey) == 1
+        CallbackHandler(ctx).use { callbackHandler ->
+            memScoped {
+                val nPrivkey = toNat(privkey)
+                val result = secp256k1_ec_seckey_verify(ctx, nPrivkey) == 1
+                callbackHandler.checkForErrors()
+                return result
+            }
         }
     }
 
     public override fun pubkeyCreate(privkey: ByteArray): ByteArray {
         require(privkey.size == 32)
-        memScoped {
-            val nPrivkey = toNat(privkey)
-            val nPubkey = alloc<secp256k1_pubkey>()
-            secp256k1_ec_pubkey_create(ctx, nPubkey.ptr, nPrivkey).requireSuccess("secp256k1_ec_pubkey_create() failed")
-            return serializePubkey(nPubkey)
+        CallbackHandler(ctx).use { callbackHandler ->
+            memScoped {
+                val nPrivkey = toNat(privkey)
+                val nPubkey = alloc<secp256k1_pubkey>()
+                secp256k1_ec_pubkey_create(ctx, nPubkey.ptr, nPrivkey).requireSuccess(callbackHandler, "secp256k1_ec_pubkey_create() failed")
+                return serializePubkey(nPubkey)
+            }
         }
     }
 
     public override fun pubkeyParse(pubkey: ByteArray): ByteArray {
         require(pubkey.size == 33 || pubkey.size == 65)
-        memScoped {
-            val nPubkey = allocPublicKey(pubkey)
-            return serializePubkey(nPubkey)
+        CallbackHandler(ctx).use { callbackHandler ->
+            memScoped {
+                val nPubkey = allocPublicKey(pubkey)
+                val result = serializePubkey(nPubkey)
+                callbackHandler.checkForErrors()
+                return result
+            }
         }
     }
 
     public override fun privKeyNegate(privkey: ByteArray): ByteArray {
         require(privkey.size == 32)
-        memScoped {
-            val negated = privkey.copyOf()
-            val negPriv = toNat(negated)
-            secp256k1_ec_seckey_negate(ctx, negPriv).requireSuccess("secp256k1_ec_seckey_negate() failed")
-            return negated
+        CallbackHandler(ctx).use { callbackHandler ->
+            memScoped {
+                val negated = privkey.copyOf()
+                val negPriv = toNat(negated)
+                secp256k1_ec_seckey_negate(ctx, negPriv).requireSuccess(callbackHandler, "secp256k1_ec_seckey_negate() failed")
+                return negated
+            }
         }
     }
 
     public override fun privKeyTweakAdd(privkey: ByteArray, tweak: ByteArray): ByteArray {
         require(privkey.size == 32)
-        memScoped {
-            val added = privkey.copyOf()
-            val natAdd = toNat(added)
-            val natTweak = toNat(tweak)
-            secp256k1_ec_seckey_tweak_add(ctx, natAdd, natTweak).requireSuccess("secp256k1_ec_seckey_tweak_add() failed")
-            return added
+        CallbackHandler(ctx).use { callbackHandler ->
+            memScoped {
+                val added = privkey.copyOf()
+                val natAdd = toNat(added)
+                val natTweak = toNat(tweak)
+                secp256k1_ec_seckey_tweak_add(ctx, natAdd, natTweak).requireSuccess(callbackHandler, "secp256k1_ec_seckey_tweak_add() failed")
+                return added
+            }
         }
     }
 
     public override fun privKeyTweakMul(privkey: ByteArray, tweak: ByteArray): ByteArray {
         require(privkey.size == 32)
-        memScoped {
-            val multiplied = privkey.copyOf()
-            val natMul = toNat(multiplied)
-            val natTweak = toNat(tweak)
-            secp256k1_ec_privkey_tweak_mul(ctx, natMul, natTweak).requireSuccess("secp256k1_ec_privkey_tweak_mul() failed")
-            return multiplied
+        CallbackHandler(ctx).use { callbackHandler ->
+            memScoped {
+                val multiplied = privkey.copyOf()
+                val natMul = toNat(multiplied)
+                val natTweak = toNat(tweak)
+                secp256k1_ec_privkey_tweak_mul(ctx, natMul, natTweak).requireSuccess(callbackHandler, "secp256k1_ec_privkey_tweak_mul() failed")
+                return multiplied
+            }
         }
     }
 
     public override fun pubKeyNegate(pubkey: ByteArray): ByteArray {
         require(pubkey.size == 33 || pubkey.size == 65)
-        memScoped {
-            val nPubkey = allocPublicKey(pubkey)
-            secp256k1_ec_pubkey_negate(ctx, nPubkey.ptr).requireSuccess("secp256k1_ec_pubkey_negate() failed")
-            return serializePubkey(nPubkey)
+        CallbackHandler(ctx).use { callbackHandler ->
+            memScoped {
+                val nPubkey = allocPublicKey(pubkey)
+                secp256k1_ec_pubkey_negate(ctx, nPubkey.ptr).requireSuccess(callbackHandler, "secp256k1_ec_pubkey_negate() failed")
+                return serializePubkey(nPubkey)
+            }
         }
     }
 
     public override fun pubKeyTweakAdd(pubkey: ByteArray, tweak: ByteArray): ByteArray {
         require(pubkey.size == 33 || pubkey.size == 65)
-        memScoped {
-            val nPubkey = allocPublicKey(pubkey)
-            val nTweak = toNat(tweak)
-            secp256k1_ec_pubkey_tweak_add(ctx, nPubkey.ptr, nTweak).requireSuccess("secp256k1_ec_pubkey_tweak_add() failed")
-            return serializePubkey(nPubkey)
+        CallbackHandler(ctx).use { callbackHandler ->
+            memScoped {
+                val nPubkey = allocPublicKey(pubkey)
+                val nTweak = toNat(tweak)
+                secp256k1_ec_pubkey_tweak_add(ctx, nPubkey.ptr, nTweak).requireSuccess(callbackHandler, "secp256k1_ec_pubkey_tweak_add() failed")
+                return serializePubkey(nPubkey)
+            }
         }
     }
 
     public override fun pubKeyTweakMul(pubkey: ByteArray, tweak: ByteArray): ByteArray {
         require(pubkey.size == 33 || pubkey.size == 65)
-        memScoped {
-            val nPubkey = allocPublicKey(pubkey)
-            val nTweak = toNat(tweak)
-            secp256k1_ec_pubkey_tweak_mul(ctx, nPubkey.ptr, nTweak).requireSuccess("secp256k1_ec_pubkey_tweak_mul() failed")
-            return serializePubkey(nPubkey)
+        CallbackHandler(ctx).use { callbackHandler ->
+            memScoped {
+                val nPubkey = allocPublicKey(pubkey)
+                val nTweak = toNat(tweak)
+                secp256k1_ec_pubkey_tweak_mul(ctx, nPubkey.ptr, nTweak).requireSuccess(callbackHandler, "secp256k1_ec_pubkey_tweak_mul() failed")
+                return serializePubkey(nPubkey)
+            }
         }
     }
 
     public override fun pubKeyCombine(pubkeys: Array<ByteArray>): ByteArray {
         pubkeys.forEach { require(it.size == 33 || it.size == 65) }
-        memScoped {
-            val nPubkeys = pubkeys.map { allocPublicKey(it).ptr }
-            val combined = alloc<secp256k1_pubkey>()
-            secp256k1_ec_pubkey_combine(ctx, combined.ptr, nPubkeys.toCValues(), pubkeys.size.convert()).requireSuccess("secp256k1_ec_pubkey_combine() failed")
-            return serializePubkey(combined)
+        CallbackHandler(ctx).use { callbackHandler ->
+            memScoped {
+                val nPubkeys = pubkeys.map { allocPublicKey(it).ptr }
+                val combined = alloc<secp256k1_pubkey>()
+                secp256k1_ec_pubkey_combine(ctx, combined.ptr, nPubkeys.toCValues(), pubkeys.size.convert()).requireSuccess(callbackHandler, "secp256k1_ec_pubkey_combine() failed")
+                return serializePubkey(combined)
+            }
         }
     }
 
     public override fun ecdh(privkey: ByteArray, pubkey: ByteArray): ByteArray {
         require(privkey.size == 32)
         require(pubkey.size == 33 || pubkey.size == 65)
-        memScoped {
-            val nPubkey = allocPublicKey(pubkey)
-            val nPrivkey = toNat(privkey)
-            val output = allocArray<UByteVar>(32)
-            secp256k1_ecdh(ctx, output, nPubkey.ptr, nPrivkey, null, null).requireSuccess("secp256k1_ecdh() failed")
-            return output.readBytes(32)
+        CallbackHandler(ctx).use { callbackHandler ->
+            memScoped {
+                val nPubkey = allocPublicKey(pubkey)
+                val nPrivkey = toNat(privkey)
+                val output = allocArray<UByteVar>(32)
+                secp256k1_ecdh(ctx, output, nPubkey.ptr, nPrivkey, null, null).requireSuccess(callbackHandler, "secp256k1_ecdh() failed")
+                return output.readBytes(32)
+            }
         }
     }
 
     public override fun ecdsaRecover(sig: ByteArray, message: ByteArray, recid: Int): ByteArray {
         require(sig.size == 64)
         require(message.size == 32)
-        memScoped {
-            val nSig = toNat(sig)
-            val rSig = alloc<secp256k1_ecdsa_recoverable_signature>()
-            secp256k1_ecdsa_recoverable_signature_parse_compact(ctx, rSig.ptr, nSig, recid).requireSuccess("secp256k1_ecdsa_recoverable_signature_parse_compact() failed")
-            val nMessage = toNat(message)
-            val pubkey = alloc<secp256k1_pubkey>()
-            secp256k1_ecdsa_recover(ctx, pubkey.ptr, rSig.ptr, nMessage).requireSuccess("secp256k1_ecdsa_recover() failed")
-            return serializePubkey(pubkey)
+        // we do not check that recid is valid, which should trigger our illegal callback handler to throw a Secp256k1IllegalCallbackException
+        // require(recid in 0..3)
+
+        CallbackHandler(ctx).use { callbackHandler ->
+            memScoped {
+                val nSig = toNat(sig)
+                val rSig = alloc<secp256k1_ecdsa_recoverable_signature>()
+                secp256k1_ecdsa_recoverable_signature_parse_compact(ctx, rSig.ptr, nSig, recid).requireSuccess(callbackHandler, "secp256k1_ecdsa_recoverable_signature_parse_compact() failed")
+                val nMessage = toNat(message)
+                val pubkey = alloc<secp256k1_pubkey>()
+                secp256k1_ecdsa_recover(ctx, pubkey.ptr, rSig.ptr, nMessage).requireSuccess(callbackHandler, "secp256k1_ecdsa_recover() failed")
+                return serializePubkey(pubkey)
+            }
         }
     }
 
     public override fun compact2der(sig: ByteArray): ByteArray {
         require(sig.size == 64)
-        memScoped {
-            val nSig = allocSignature(sig)
-            val natOutput = allocArray<UByteVar>(73)
-            val len = alloc<size_tVar>()
-            len.value = 73.convert()
-            secp256k1_ecdsa_signature_serialize_der(ctx, natOutput, len.ptr, nSig.ptr).requireSuccess("secp256k1_ecdsa_signature_serialize_der() failed")
-            return natOutput.readBytes(len.value.toInt())
+        CallbackHandler(ctx).use { callbackHandler ->
+            memScoped {
+                val nSig = allocSignature(sig)
+                val natOutput = allocArray<UByteVar>(73)
+                val len = alloc<size_tVar>()
+                len.value = 73.convert()
+                secp256k1_ecdsa_signature_serialize_der(ctx, natOutput, len.ptr, nSig.ptr).requireSuccess(callbackHandler, "secp256k1_ecdsa_signature_serialize_der() failed")
+                return natOutput.readBytes(len.value.toInt())
+            }
         }
     }
 
@@ -226,13 +320,15 @@ public object Secp256k1Native : Secp256k1 {
         require(signature.size == 64)
         require(data.size == 32)
         require(pub.size == 32)
-        memScoped {
-            val nPub = toNat(pub)
-            val pubkey = alloc<secp256k1_xonly_pubkey>()
-            secp256k1_xonly_pubkey_parse(ctx, pubkey.ptr, nPub).requireSuccess("secp256k1_xonly_pubkey_parse() failed")
-            val nData = toNat(data)
-            val nSig = toNat(signature)
-            return secp256k1_schnorrsig_verify(ctx, nSig, nData, 32, pubkey.ptr) == 1
+        CallbackHandler(ctx).use { callbackHandler ->
+            memScoped {
+                val nPub = toNat(pub)
+                val pubkey = alloc<secp256k1_xonly_pubkey>()
+                secp256k1_xonly_pubkey_parse(ctx, pubkey.ptr, nPub).requireSuccess(callbackHandler, "secp256k1_xonly_pubkey_parse() failed")
+                val nData = toNat(data)
+                val nSig = toNat(signature)
+                return secp256k1_schnorrsig_verify(ctx, nSig, nData, 32u, pubkey.ptr) == 1
+            }
         }
     }
 
@@ -240,15 +336,17 @@ public object Secp256k1Native : Secp256k1 {
         require(sec.size == 32)
         require(data.size == 32)
         auxrand32?.let { require(it.size == 32) }
-        memScoped {
-            val nSec = toNat(sec)
-            val nData = toNat(data)
-            val nAuxrand32 = auxrand32?.let { toNat(it) }
-            val nSig = allocArray<UByteVar>(64)
-            val keypair = alloc<secp256k1_keypair>()
-            secp256k1_keypair_create(ctx, keypair.ptr, nSec).requireSuccess("secp256k1_keypair_create() failed")
-            secp256k1_schnorrsig_sign32(ctx, nSig, nData, keypair.ptr, nAuxrand32).requireSuccess("secp256k1_ecdsa_sign() failed")
-            return nSig.readBytes(64)
+        CallbackHandler(ctx).use { callbackHandler ->
+            memScoped {
+                val nSec = toNat(sec)
+                val nData = toNat(data)
+                val nAuxrand32 = auxrand32?.let { toNat(it) }
+                val nSig = allocArray<UByteVar>(64)
+                val keypair = alloc<secp256k1_keypair>()
+                secp256k1_keypair_create(ctx, keypair.ptr, nSec).requireSuccess(callbackHandler, "secp256k1_keypair_create() failed")
+                secp256k1_schnorrsig_sign32(ctx, nSig, nData, keypair.ptr, nAuxrand32).requireSuccess(callbackHandler, "secp256k1_ecdsa_sign() failed")
+                return nSig.readBytes(64)
+            }
         }
     }
     

tests/src/commonTest/kotlin/fr/acinq/secp256k1/Secp256k1Test.kt

@@ -275,6 +275,11 @@ class Secp256k1Test {
         val pub0 = Secp256k1.ecdsaRecover(sig, message, 0)
         val pub1 = Secp256k1.ecdsaRecover(sig, message, 1)
         assertTrue(pub.contentEquals(pub0) || pub.contentEquals(pub1))
+
+        // this is a special case, ecdsaRecover explicitly does not check that recid is valid, which triggers our illegal callback handler
+        assertFailsWith(Secp256k1IllegalCallbackException::class) {
+            Secp256k1.ecdsaRecover(sig, message, 4)
+        }
     }
 
     @Test