Do not remove prefix zeros when RLP encoding bytes32 values

.changeset/young-corners-help.md

@@ -0,0 +1,5 @@
+---
+'openzeppelin-solidity': minor
+---
+
+`RLP`: Encode `bytes32` as a fixed size item and not as a scalar in `encode(bytes32)`. Scalar RLP encoding remains available by casting to a `uint256` and using the `encode(uint256)` function.

contracts/utils/RLP.sol

@@ -151,7 +151,7 @@ library RLP {
     }
 
     /**
-     * @dev Encode an address as RLP.
+     * @dev Encode an address as an RLP item of fixed size (20 bytes).
      *
      * The address is encoded with its leading zeros (if it has any). If someone wants to encode the address as a scalar,
      * they can cast it to an uint256 and then call the corresponding {encode} function.
@@ -165,7 +165,11 @@ library RLP {
         }
     }
 
-    /// @dev Encode a uint256 as RLP.
+    /**
+     * @dev Encode an uint256 as an RLP scalar.
+     *
+     * Unlike {encode-bytes32-}, this function uses scalar encoding that removes the prefix zeros.
+     */
     function encode(uint256 input) internal pure returns (bytes memory result) {
         if (input < SHORT_OFFSET) {
             assembly ("memory-safe") {
@@ -186,9 +190,19 @@ library RLP {
         }
     }
 
-    /// @dev Encode a bytes32 as RLP. Type alias for {encode-uint256-}.
-    function encode(bytes32 input) internal pure returns (bytes memory) {
-        return encode(uint256(input));
+    /**
+     * @dev Encode a bytes32 as an RLP item of fixed size (32 bytes).
+     *
+     * Unlike {encode-uint256-}, this function uses array encoding that preserves the prefix zeros.
+     */
+    function encode(bytes32 input) internal pure returns (bytes memory result) {
+        assembly ("memory-safe") {
+            result := mload(0x40)
+            mstore(result, 0x21) // length of the encoded data: 1 (prefix) + 0x20
+            mstore8(add(result, 0x20), 0xa0) // prefix: SHORT_OFFSET + 0x20
+            mstore(add(result, 0x21), input)
+            mstore(0x40, add(result, 0x41)) // reserve memory
+        }
     }
 
     /// @dev Encode a bytes buffer as RLP.

test/utils/RLP.test.js

@@ -91,36 +91,33 @@ describe('RLP', function () {
   });
 
   it('encode/decode bytes32', async function () {
-    for (const { input, expected } of [
-      { input: '0x0000000000000000000000000000000000000000000000000000000000000000', expected: '0x80' },
-      { input: '0x0000000000000000000000000000000000000000000000000000000000000001', expected: '0x01' },
-      {
-        input: '0x1000000000000000000000000000000000000000000000000000000000000000',
-        expected: '0xa01000000000000000000000000000000000000000000000000000000000000000',
-      },
+    for (const input of [
+      '0x0000000000000000000000000000000000000000000000000000000000000000',
+      '0x0000000000000000000000000000000000000000000000000000000000000001',
+      '0x1000000000000000000000000000000000000000000000000000000000000000',
+      generators.bytes32(),
     ]) {
-      await expect(this.mock.$encode_bytes32(input)).to.eventually.equal(expected);
-      await expect(this.mock.$decodeBytes32(expected)).to.eventually.equal(input);
+      const encoded = ethers.encodeRlp(input);
+      await expect(this.mock.$encode_bytes32(input)).to.eventually.equal(encoded);
+      await expect(this.mock.$decodeBytes32(encoded)).to.eventually.equal(input);
     }
 
+    // Compact encoding for 1234
     await expect(this.mock.$decodeBytes32('0x8204d2')).to.eventually.equal(
       '0x00000000000000000000000000000000000000000000000000000000000004d2',
-    ); // Canonical encoding for 1234
+    );
+    // Encoding with one leading zero
     await expect(this.mock.$decodeBytes32('0x830004d2')).to.eventually.equal(
       '0x00000000000000000000000000000000000000000000000000000000000004d2',
-    ); // Non-canonical encoding with leading zero
+    );
+    // Encoding with two leading zeros
     await expect(this.mock.$decodeBytes32('0x84000004d2')).to.eventually.equal(
       '0x00000000000000000000000000000000000000000000000000000000000004d2',
-    ); // Non-canonical encoding with two leading zeros
-
-    // Canonical encoding for zero and non-canonical encodings with leading zeros
-    await expect(this.mock.$decodeBytes32('0x80')).to.eventually.equal(
-      '0x0000000000000000000000000000000000000000000000000000000000000000',
     );
-    // 1 leading zero is not allowed for single bytes less than 0x80, they must be encoded as themselves
-    await expect(this.mock.$decodeBytes32('0x820000')).to.eventually.equal(
-      '0x0000000000000000000000000000000000000000000000000000000000000000',
-    ); // Non-canonical encoding with two leading zeros
+    // Encoding for the value
+    await expect(this.mock.$decodeBytes32('0x80')).to.eventually.equal(ethers.ZeroHash);
+    // Encoding for two zeros (and nothing else)
+    await expect(this.mock.$decodeBytes32('0x820000')).to.eventually.equal(ethers.ZeroHash);
   });
 
   it('encode/decode empty byte', async function () {