Merge pull request #61 from matter-labs/vb-fix-oz-keccak256-audit-n-02

Add comments for keccak256 wrapper

contracts/precompiles/Keccak256.yul

@@ -11,12 +11,21 @@ object "Keccak256" {
             //                      CONSTANTS
             ////////////////////////////////////////////////////////////////
 
-            /// @dev The size of the processing keccak256 block in bytes.
+            /// @dev Returns the block size used by the keccak256 hashing function.
+            /// The value 136 bytes corresponds to the size of the input data block that the keccak256 
+            /// algorithm processes in each round, as defined in the keccak256 specification. This is derived 
+            /// from the formula (1600 - 2 * bit length of the digest) / 8, where the bit length for keccak256
+            /// is 256 bits. For more details, refer to the Keccak specification at
+            /// https://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.202.pdf#page=30
             function BLOCK_SIZE() -> ret {
                 ret := 136
             }
 
             /// @dev The gas cost of processing one keccak256 round.
+            /// @dev This constant is made equal to the corresponding constant in
+            /// https://github.com/matter-labs/era-zkevm_opcode_defs/blob/v1.4.1/src/circuit_prices.rs,
+            /// which was automatically generated depending on the capacity of rounds for a 
+            /// single Keccak256 circuit.
             function KECCAK_ROUND_GAS_COST() -> ret {
                 ret := 40
             }