dex: Add libsecp256k1 c library.

Add a c library that has some primitive cryptographic functions needed
for working with adaptor signatures.

.gitignore

@@ -30,6 +30,7 @@ client/cmd/simnet-trade-tests/simnet-trade-tests
 client/cmd/mmbot/mmbot
 docs/examples/rpcclient/rpcclient
 dex/testing/loadbot/loadbot
+dex/libsecp256k1/secp256k1
 bin/
 bin-v*/
 client/webserver/site/template-builder/template-builder

dex/libsecp256k1/build.sh

@@ -0,0 +1,10 @@
+#!/usr/bin/env bash
+
+rm -fr secp256k1
+git clone https://github.com/tecnovert/secp256k1 -b anonswap_v0.2
+
+cd secp256k1
+./autogen.sh
+./configure --enable-module-dleag --enable-experimental --enable-module-generator --enable-module-ed25519 --enable-module-recovery
+make
+cd ..

dex/libsecp256k1/libsecp256k1.go

@@ -0,0 +1,60 @@
+// This code is available on the terms of the project LICENSE.md file,
+// also available online at https://blueoakcouncil.org/license/1.0.0.
+
+package libsecp256k1
+
+/*
+#cgo CFLAGS: -g -Wall
+#cgo LDFLAGS: -L. -l:secp256k1/.libs/libsecp256k1.a
+#include "secp256k1/include/secp256k1_dleag.h"
+#include <stdlib.h>
+
+secp256k1_context* _ctx() {
+	return secp256k1_context_create(SECP256K1_CONTEXT_SIGN | SECP256K1_CONTEXT_VERIFY);
+}
+*/
+import "C"
+import (
+	"errors"
+	"unsafe"
+
+	"decred.org/dcrdex/dex/encode"
+	"github.com/decred/dcrd/dcrec/edwards/v2"
+)
+
+const (
+	proofLength = 48893
+)
+
+// Ed25519DleagProve creates a proof for checking a discrete logarithm is equal
+// across the secp256k1 and ed25519 curves.
+func Ed25519DleagProve(privKey *edwards.PrivateKey) (proof [proofLength]byte, err error) {
+	secpCtx := C._ctx()
+	defer C.free(unsafe.Pointer(secpCtx))
+	nonce := [32]byte{}
+	copy(nonce[:], encode.RandomBytes(32))
+	key := [32]byte{}
+	copy(key[:], privKey.Serialize())
+	n := (*C.uchar)(unsafe.Pointer(&nonce))
+	k := (*C.uchar)(unsafe.Pointer(&key))
+	nBits := uint64(252)
+	nb := (*C.ulong)(unsafe.Pointer(&nBits))
+	plen := C.ulong(proofLength)
+	p := (*C.uchar)(unsafe.Pointer(&proof))
+	res := C.secp256k1_ed25519_dleag_prove(secpCtx, p, &plen, k, *nb, n)
+	if int(res) != 1 {
+		return [proofLength]byte{}, errors.New("C.secp256k1_ed25519_dleag_prove exited with error")
+	}
+	return proof, nil
+}
+
+// Ed25519DleagVerify verifies that a descrete logarithm is equal across the
+// secp256k1 and ed25519 curves.
+func Ed25519DleagVerify(proof [proofLength]byte) bool {
+	secpCtx := C._ctx()
+	defer C.free(unsafe.Pointer(secpCtx))
+	pl := C.ulong(proofLength)
+	p := (*C.uchar)(unsafe.Pointer(&proof))
+	res := C.secp256k1_ed25519_dleag_verify(secpCtx, p, pl)
+	return res == 1
+}

dex/libsecp256k1/libsecp256k1_test.go

@@ -0,0 +1,64 @@
+//go:build libsecp256k1
+
+package libsecp256k1
+
+import (
+	"testing"
+
+	"github.com/decred/dcrd/dcrec/edwards/v2"
+)
+
+func TestEd25519DleagProve(t *testing.T) {
+	tests := []struct {
+		name string
+	}{{
+		name: "ok",
+	}}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			pk, err := edwards.GeneratePrivateKey()
+			if err != nil {
+				t.Fatal(err)
+			}
+			_, err = Ed25519DleagProve(pk)
+			if err != nil {
+				t.Fatal(err)
+			}
+		})
+	}
+}
+
+func TestEd25519DleagVerify(t *testing.T) {
+	pk, err := edwards.GeneratePrivateKey()
+	if err != nil {
+		panic(err)
+	}
+	proof, err := Ed25519DleagProve(pk)
+	if err != nil {
+		panic(err)
+	}
+	tests := []struct {
+		name  string
+		proof [proofLength]byte
+		ok    bool
+	}{{
+		name:  "ok",
+		proof: proof,
+		ok:    true,
+	}, {
+		name: "bad proof",
+		proof: func() (p [proofLength]byte) {
+			copy(p[:], proof[:])
+			p[0] ^= p[0]
+			return p
+		}(),
+	}}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			ok := Ed25519DleagVerify(test.proof)
+			if ok != test.ok {
+				t.Fatalf("want %v but got %v", test.ok, ok)
+			}
+		})
+	}
+}

run_tests.sh

@@ -9,6 +9,12 @@ echo "Go version: $GV"
 # Ensure html templates pass localization.
 go generate -x ./client/webserver/site # no -write
 
+cd ./dex/libsecp256k1
+./build.sh
+go test -race -tags libsecp256k1
+
+cd "$dir"
+
 # list of all modules to test
 modules=". /dex/testing/loadbot /client/cmd/bisonw-desktop"