From a60275a159f670757d78d305fe840b72ffc05419 Mon Sep 17 00:00:00 2001
From: Jonathan Chappelow <chappjc@protonmail.com>
Date: Thu, 17 Mar 2022 15:26:27 -0500
Subject: [PATCH] client/core: recognize sigs of hashed messages

---
 client/core/core.go  | 22 ++++++++++++++++++----
 client/core/types.go |  4 +---
 2 files changed, 19 insertions(+), 7 deletions(-)

diff --git a/client/core/core.go b/client/core/core.go
index 521f4cb057..f92e045bcb 100644
--- a/client/core/core.go
+++ b/client/core/core.go
@@ -6,6 +6,7 @@ package core
 import (
 	"bytes"
 	"context"
+	"crypto/sha256"
 	"encoding/binary"
 	"encoding/hex"
 	"errors"
@@ -2906,6 +2907,9 @@ func (c *Core) register(dc *dexConnection, assetID uint32) (regRes *msgjson.Regi
 		if !bytes.Equal(dc.acct.dexPubKey.SerializeCompressed(), regRes.DEXPubKey) {
 			return nil, false, false, fmt.Errorf("different pubkeys reported by dex in 'config' and 'register' responses")
 		}
+		// Insert our pubkey back into the register result since it is excluded
+		// from the JSON serialization.
+		regRes.ClientPubKey = acctPubKey
 		// Check the DEX server's signature.
 		msg := regRes.Serialize()
 		err = checkSigS256(msg, regRes.DEXPubKey, regRes.Sig)
@@ -6277,17 +6281,27 @@ func checkSigS256(msg, pkBytes, sigBytes []byte) error {
 	if err != nil {
 		return fmt.Errorf("error decoding secp256k1 Signature from bytes: %w", err)
 	}
-	if !signature.Verify(msg, pubKey) {
-		return fmt.Errorf("secp256k1 signature verification failed")
+	hash := sha256.Sum256(msg)
+	if !signature.Verify(hash[:], pubKey) {
+		// Might be an older buggy server. (V0PURGE)
+		if !signature.Verify(msg, pubKey) {
+			return fmt.Errorf("secp256k1 signature verification failed")
+		}
 	}
 	return nil
 }
 
+// signMsg signs the message with provided private key.
+func signMsg(privKey *secp256k1.PrivateKey, msg []byte) []byte {
+	// NOTE: legacy servers will not accept this signature:
+	// hash := sha256.Sum256(msg)
+	return ecdsa.Sign(privKey, msg).Serialize()
+}
+
 // sign signs the msgjson.Signable with the provided private key.
 func sign(privKey *secp256k1.PrivateKey, payload msgjson.Signable) {
 	sigMsg := payload.Serialize()
-	sig := ecdsa.Sign(privKey, sigMsg) // should we be signing the *hash* of the payload?
-	payload.SetSig(sig.Serialize())
+	payload.SetSig(signMsg(privKey, sigMsg))
 }
 
 // stampAndSign time stamps the msgjson.Stampable, and signs it with the given
diff --git a/client/core/types.go b/client/core/types.go
index 278a802dc5..1c9ad25682 100644
--- a/client/core/types.go
+++ b/client/core/types.go
@@ -22,7 +22,6 @@ import (
 	"decred.org/dcrdex/dex/order"
 	"decred.org/dcrdex/server/account"
 	"github.com/decred/dcrd/dcrec/secp256k1/v4"
-	"github.com/decred/dcrd/dcrec/secp256k1/v4/ecdsa"
 	"github.com/decred/dcrd/hdkeychain/v3"
 )
 
@@ -769,8 +768,7 @@ func (a *dexAccount) sign(msg []byte) ([]byte, error) {
 	if a.privKey == nil {
 		return nil, fmt.Errorf("account locked")
 	}
-	sig := ecdsa.Sign(a.privKey, msg)
-	return sig.Serialize(), nil
+	return signMsg(a.privKey, msg), nil
 }
 
 // checkSig checks the signature against the message and the DEX pubkey.