Verify musig2 secret nonces (#108)

* Verify musig2 secret nonces

Trying to generate a musig2 partial signature with a secret nonce that was generated with a public key that does not match the signing key's public key will trigger secp256k1's illegal callback (which calls abort()) and crash the application.

=> Here we verify that the secret nonce matches the signing key before we call secp256k1_musig_partial_sign().
The verification method is a bit hackish (we extract the public key from the secret nonce blob) because secp256k1 does not export the methods we need to do this cleanly.

Author: sstone

jni/src/main/kotlin/fr/acinq/secp256k1/NativeSecp256k1.kt

@@ -117,6 +117,7 @@ public object NativeSecp256k1 : Secp256k1 {
     }
 
     override fun musigPartialSign(secnonce: ByteArray, privkey: ByteArray, keyaggCache: ByteArray, session: ByteArray): ByteArray {
+        require(musigNonceValidate(secnonce, pubkeyCreate(privkey)))
         return Secp256k1CFunctions.secp256k1_musig_partial_sign(Secp256k1Context.getContext(), secnonce, privkey, keyaggCache, session)
     }
 

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

@@ -217,6 +217,26 @@ public interface Secp256k1 {
      */
     public fun musigNonceProcess(aggnonce: ByteArray, msg32: ByteArray, keyaggCache: ByteArray): ByteArray
 
+    /**
+     * Check that a secret nonce was generated with a public key that matches the private key used for signing.
+     * @param secretnonce secret nonce.
+     * @param pubkey public key for the private key that will be used, with the secret nonce, to generate a partial signature.
+     * @return false if the secret nonce does not match the public key.
+     */
+    public fun musigNonceValidate(secretnonce: ByteArray, pubkey: ByteArray): Boolean {
+        if (secretnonce.size != MUSIG2_SECRET_NONCE_SIZE) return false
+        if (pubkey.size != 33 && pubkey.size != 65) return false
+        val pk = Secp256k1.pubkeyParse(pubkey)
+        // this is a bit hackish but the secp256k1 library does not export methods to do this cleanly
+        val x = secretnonce.copyOfRange(68, 68 + 32)
+        x.reverse()
+        val y = secretnonce.copyOfRange(68 + 32, 68 + 32 + 32)
+        y.reverse()
+        val pkx = pk.copyOfRange(1, 1 + 32)
+        val pky = pk.copyOfRange(33, 33 + 32)
+        return x.contentEquals(pkx) && y.contentEquals(pky)
+    }
+
     /**
      * Create a partial signature.
      *

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

@@ -81,7 +81,7 @@ public object Secp256k1Native : Secp256k1 {
         return serialized.readBytes(Secp256k1.MUSIG2_PUBLIC_NONCE_SIZE)
     }
 
-    private fun DeferScope.toNat(bytes: ByteArray): CPointer<UByteVar>  {
+    private fun DeferScope.toNat(bytes: ByteArray): CPointer<UByteVar> {
         val ubytes = bytes.asUByteArray()
         val pinned = ubytes.pin()
         this.defer { pinned.unpin() }
@@ -112,7 +112,7 @@ public object Secp256k1Native : Secp256k1 {
     }
 
     public override fun signatureNormalize(sig: ByteArray): Pair<ByteArray, Boolean> {
-        require(sig.size >= 64){ "invalid signature ${Hex.encode(sig)}" }
+        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)
@@ -307,7 +307,16 @@ public object Secp256k1Native : Secp256k1 {
                 memcpy(n.ptr, toNat(it), Secp256k1.MUSIG2_PUBLIC_KEYAGG_CACHE_SIZE.toULong())
                 n
             }
-            secp256k1_musig_nonce_gen(ctx, secnonce.ptr, pubnonce.ptr, toNat(sessionId32), privkey?.let { toNat(it) }, nPubkey.ptr, msg32?.let { toNat(it) },nKeyAggCache?.ptr, extraInput32?.let { toNat(it) }).requireSuccess("secp256k1_musig_nonce_gen() failed")
+            secp256k1_musig_nonce_gen(
+                ctx,
+                secnonce.ptr,
+                pubnonce.ptr,
+                toNat(sessionId32),
+                privkey?.let { toNat(it) },
+                nPubkey.ptr,
+                msg32?.let { toNat(it) },
+                nKeyAggCache?.ptr,
+                extraInput32?.let { toNat(it) }).requireSuccess("secp256k1_musig_nonce_gen() failed")
             val nPubnonce = allocArray<UByteVar>(Secp256k1.MUSIG2_PUBLIC_NONCE_SIZE)
             secp256k1_musig_pubnonce_serialize(ctx, nPubnonce, pubnonce.ptr).requireSuccess("secp256k1_musig_pubnonce_serialize failed")
             secnonce.ptr.readBytes(Secp256k1.MUSIG2_SECRET_NONCE_SIZE) + nPubnonce.readBytes(Secp256k1.MUSIG2_PUBLIC_NONCE_SIZE)
@@ -339,7 +348,7 @@ public object Secp256k1Native : Secp256k1 {
                 n
             }
             secp256k1_musig_pubkey_agg(ctx, combined.ptr, nKeyAggCache?.ptr, nPubkeys.toCValues(), pubkeys.size.convert()).requireSuccess("secp256k1_musig_nonce_agg() failed")
-            val agg =  serializeXonlyPubkey(combined)
+            val agg = serializeXonlyPubkey(combined)
             keyaggCache?.let { blob -> nKeyAggCache?.let { memcpy(toNat(blob), it.ptr, Secp256k1.MUSIG2_PUBLIC_KEYAGG_CACHE_SIZE.toULong()) } }
             return agg
         }
@@ -386,13 +395,14 @@ public object Secp256k1Native : Secp256k1 {
             memcpy(toNat(session), nSession.ptr, Secp256k1.MUSIG2_PUBLIC_SESSION_SIZE.toULong())
             return session
         }
-     }
+    }
 
     override fun musigPartialSign(secnonce: ByteArray, privkey: ByteArray, keyaggCache: ByteArray, session: ByteArray): ByteArray {
         require(secnonce.size == Secp256k1.MUSIG2_SECRET_NONCE_SIZE)
         require(privkey.size == 32)
         require(keyaggCache.size == Secp256k1.MUSIG2_PUBLIC_KEYAGG_CACHE_SIZE)
         require(session.size == Secp256k1.MUSIG2_PUBLIC_SESSION_SIZE)
+        require(musigNonceValidate(secnonce, pubkeyCreate(privkey)))
 
         memScoped {
             val nSecnonce = alloc<secp256k1_musig_secnonce>()

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

@@ -5,6 +5,14 @@ import kotlin.test.*
 
 class Secp256k1Test {
 
+    val random = Random.Default
+
+    fun randomBytes(length: Int): ByteArray {
+        val buffer = ByteArray(length)
+        random.nextBytes(buffer)
+        return buffer
+    }
+
     @Test
     fun verifyValidPrivateKey() {
         val priv = Hex.decode("67E56582298859DDAE725F972992A07C6C4FB9F62A8FFF58CE3CA926A1063530".lowercase())
@@ -454,40 +462,55 @@ class Secp256k1Test {
 
     @Test
     fun testMusig2SigningSession() {
-        val privkeys = listOf(
-            "0101010101010101010101010101010101010101010101010101010101010101",
-            "0202020202020202020202020202020202020202020202020202020202020202",
-        ).map { Hex.decode(it) }.toTypedArray()
+        val privkeys = listOf(randomBytes(32), randomBytes(32))
+        val sessionId = randomBytes(32)
+        val msg32 = randomBytes(32)
         val pubkeys = privkeys.map { Secp256k1.pubkeyCreate(it) }
-
-        val sessionId = Hex.decode("0F0F0F0F0F0F0F0F0F0F0F0F0F0F0F0F0F0F0F0F0F0F0F0F0F0F0F0F0F0F0F0F")
         val nonces = pubkeys.map { Secp256k1.musigNonceGen(sessionId, null, it, null, null, null) }
+        val testData = run {
+            val builder = StringBuilder()
+            builder.append("private keys\n")
+            privkeys.forEach { builder.append(Hex.encode(it)).append("\n") }
+            builder.append("sessionId ${Hex.encode(sessionId)}\n")
+            builder.append("msg32 ${Hex.encode(msg32)}\n")
+            builder.append("nonces\n")
+            nonces.forEach { builder.append(Hex.encode(it)).append("\n") }
+            builder.toString()
+        }
         val secnonces = nonces.map { it.copyOfRange(0, 132) }
         val pubnonces = nonces.map { it.copyOfRange(132, 132 + 66) }
         val aggnonce = Secp256k1.musigNonceAgg(pubnonces.toTypedArray())
 
         val keyaggCaches = (0 until 2).map { ByteArray(Secp256k1.MUSIG2_PUBLIC_KEYAGG_CACHE_SIZE) }
         val aggpubkey = Secp256k1.musigPubkeyAgg(pubkeys.toTypedArray(), keyaggCaches[0])
-        assertContentEquals(aggpubkey, Secp256k1.musigPubkeyAgg(pubkeys.toTypedArray(), keyaggCaches[1]))
-        assertContentEquals(keyaggCaches[0], keyaggCaches[1])
+        assertContentEquals(aggpubkey, Secp256k1.musigPubkeyAgg(pubkeys.toTypedArray(), keyaggCaches[1]), testData)
+        assertContentEquals(keyaggCaches[0], keyaggCaches[1], testData)
 
-        val msg32 = Hex.decode("0303030303030303030303030303030303030303030303030303030303030303")
         val sessions = (0 until 2).map { Secp256k1.musigNonceProcess(aggnonce, msg32, keyaggCaches[it]) }
         val psigs = (0 until 2).map {
             val psig = Secp256k1.musigPartialSign(secnonces[it], privkeys[it], keyaggCaches[it], sessions[it])
-            assertEquals(1, Secp256k1.musigPartialSigVerify(psig, pubnonces[it], pubkeys[it], keyaggCaches[it], sessions[it]))
-            assertEquals(0, Secp256k1.musigPartialSigVerify(Random.nextBytes(32), pubnonces[it], pubkeys[it], keyaggCaches[it], sessions[it]))
+            assertEquals(1, Secp256k1.musigPartialSigVerify(psig, pubnonces[it], pubkeys[it], keyaggCaches[it], sessions[it]), testData)
+            assertEquals(0, Secp256k1.musigPartialSigVerify(Random.nextBytes(32), pubnonces[it], pubkeys[it], keyaggCaches[it], sessions[it]), testData)
             psig
         }
 
+        // signing fails if the secret nonce does not match the private key's public key
+        assertFails(testData) {
+            Secp256k1.musigPartialSign(secnonces[1], privkeys[0], keyaggCaches[0], sessions[0])
+        }
+
+        assertFails(testData) {
+            Secp256k1.musigPartialSign(secnonces[0], privkeys[1], keyaggCaches[1], sessions[1])
+        }
+
         val sig = Secp256k1.musigPartialSigAgg(sessions[0], psigs.toTypedArray())
-        assertContentEquals(sig, Secp256k1.musigPartialSigAgg(sessions[1], psigs.toTypedArray()))
-        assertTrue(Secp256k1.verifySchnorr(sig, msg32, aggpubkey))
+        assertContentEquals(sig, Secp256k1.musigPartialSigAgg(sessions[1], psigs.toTypedArray()), testData)
+        assertTrue(Secp256k1.verifySchnorr(sig, msg32, aggpubkey), testData)
 
         val invalidSig1 = Secp256k1.musigPartialSigAgg(sessions[0], arrayOf(psigs[0], psigs[0]))
-        assertFalse(Secp256k1.verifySchnorr(invalidSig1, msg32, aggpubkey))
+        assertFalse(Secp256k1.verifySchnorr(invalidSig1, msg32, aggpubkey), testData)
         val invalidSig2 = Secp256k1.musigPartialSigAgg(sessions[0], arrayOf(Random.nextBytes(32), Random.nextBytes(32)))
-        assertFalse(Secp256k1.verifySchnorr(invalidSig2, msg32, aggpubkey))
+        assertFalse(Secp256k1.verifySchnorr(invalidSig2, msg32, aggpubkey), testData)
     }
 
     @Test
@@ -523,15 +546,14 @@ class Secp256k1Test {
     }
 
     @Test
-    fun fuzzEcdsaSignVerify() {
-        val random = Random.Default
-
-        fun randomBytes(length: Int): ByteArray {
-            val buffer = ByteArray(length)
-            random.nextBytes(buffer)
-            return buffer
+    fun fuzzMusig2SigningSession() {
+        repeat(1000) {
+            testMusig2SigningSession()
         }
+    }
 
+    @Test
+    fun fuzzEcdsaSignVerify() {
         repeat(200) {
             val priv = randomBytes(32)
             assertTrue(Secp256k1.secKeyVerify(priv))