Improve gas efficiency in ECDSA (#3853)

Signed-off-by: Pascal Marco Caversaccio <pascal.caversaccio@hotmail.ch>
Co-authored-by: Hadrien Croubois <hadrien.croubois@gmail.com>
Co-authored-by: Pascal Marco Caversaccio <pcaversaccio@users.noreply.github.com>

.changeset/thin-dragons-report.md

@@ -0,0 +1,5 @@
+---
+'openzeppelin-solidity': minor
+---
+
+`ECDSA`: optimize bytes32 computation by using assembly instead of `abi.encodePacked`.

contracts/utils/cryptography/ECDSA.sol

@@ -162,10 +162,15 @@ library ECDSA {
      *
      * See {recover}.
      */
-    function toEthSignedMessageHash(bytes32 hash) internal pure returns (bytes32) {
+    function toEthSignedMessageHash(bytes32 hash) internal pure returns (bytes32 message) {
         // 32 is the length in bytes of hash,
         // enforced by the type signature above
-        return keccak256(abi.encodePacked("\x19Ethereum Signed Message:\n32", hash));
+        /// @solidity memory-safe-assembly
+        assembly {
+            mstore(0x00, "\x19Ethereum Signed Message:\n32")
+            mstore(0x1c, hash)
+            message := keccak256(0x00, 0x3c)
+        }
     }
 
     /**
@@ -189,7 +194,14 @@ library ECDSA {
      *
      * See {recover}.
      */
-    function toTypedDataHash(bytes32 domainSeparator, bytes32 structHash) internal pure returns (bytes32) {
-        return keccak256(abi.encodePacked("\x19\x01", domainSeparator, structHash));
+    function toTypedDataHash(bytes32 domainSeparator, bytes32 structHash) internal pure returns (bytes32 data) {
+        /// @solidity memory-safe-assembly
+        assembly {
+            let ptr := mload(0x40)
+            mstore(ptr, "\x19\x01")
+            mstore(add(ptr, 0x02), domainSeparator)
+            mstore(add(ptr, 0x22), structHash)
+            data := keccak256(ptr, 0x42)
+        }
     }
 }