Merge pull request #1976 from kclowes/eip-2565

EIP-2565 ModExp Gas Cost

Author: carver

eth/precompiles/modexp.py

@@ -1,11 +1,15 @@
 from typing import (
+    Callable,
     Tuple,
 )
 
 from eth_utils import (
     big_endian_to_int,
     int_to_big_endian,
 )
+from eth_utils.toolz import (
+    curry,
+)
 
 from eth import constants
 
@@ -23,8 +27,8 @@
 )
 
 
-def _compute_adjusted_exponent_length(exponent_length: int,
-                                      first_32_exponent_bytes: bytes) -> int:
+def compute_adjusted_exponent_length(exponent_length: int,
+                                     first_32_exponent_bytes: bytes) -> int:
     exponent = big_endian_to_int(first_32_exponent_bytes)
 
     if exponent_length <= 32 and exponent == 0:
@@ -50,7 +54,7 @@ def _compute_complexity(length: int) -> int:
         return length ** 2 // 16 + 480 * length - 199680
 
 
-def _extract_lengths(data: bytes) -> Tuple[int, int, int]:
+def extract_lengths(data: bytes) -> Tuple[int, int, int]:
     # extract argument lengths
     base_length_bytes = pad32r(data[:32])
     base_length = big_endian_to_int(base_length_bytes)
@@ -64,14 +68,14 @@ def _extract_lengths(data: bytes) -> Tuple[int, int, int]:
     return base_length, exponent_length, modulus_length
 
 
-def _compute_modexp_gas_fee(data: bytes) -> int:
-    base_length, exponent_length, modulus_length = _extract_lengths(data)
+def _compute_modexp_gas_fee_eip_198(data: bytes) -> int:
+    base_length, exponent_length, modulus_length = extract_lengths(data)
 
     first_32_exponent_bytes = zpad_right(
         data[96 + base_length:96 + base_length + exponent_length],
         to_size=min(exponent_length, 32),
     )[:32]
-    adjusted_exponent_length = _compute_adjusted_exponent_length(
+    adjusted_exponent_length = compute_adjusted_exponent_length(
         exponent_length,
         first_32_exponent_bytes,
     )
@@ -86,7 +90,7 @@ def _compute_modexp_gas_fee(data: bytes) -> int:
 
 
 def _modexp(data: bytes) -> int:
-    base_length, exponent_length, modulus_length = _extract_lengths(data)
+    base_length, exponent_length, modulus_length = extract_lengths(data)
 
     if base_length == 0:
         return 0
@@ -121,18 +125,22 @@ def _modexp(data: bytes) -> int:
     return result
 
 
-def modexp(computation: BaseComputation) -> BaseComputation:
+@curry
+def modexp(
+    computation: BaseComputation,
+    gas_calculator: Callable[[bytes], int] = _compute_modexp_gas_fee_eip_198
+) -> BaseComputation:
     """
     https://github.com/ethereum/EIPs/pull/198
     """
     data = computation.msg.data_as_bytes
 
-    gas_fee = _compute_modexp_gas_fee(data)
+    gas_fee = gas_calculator(data)
     computation.consume_gas(gas_fee, reason='MODEXP Precompile')
 
     result = _modexp(data)
 
-    _, _, modulus_length = _extract_lengths(data)
+    _, _, modulus_length = extract_lengths(data)
 
     # Modulo 0 is undefined, return zero
     # https://math.stackexchange.com/questions/516251/why-is-n-mod-0-undefined

eth/vm/forks/berlin/computation.py

@@ -1,3 +1,23 @@
+import math
+
+from eth_utils.toolz import (
+    merge,
+)
+
+from eth.precompiles.modexp import (
+    compute_adjusted_exponent_length,
+    extract_lengths,
+    modexp,
+)
+from eth._utils.address import (
+    force_bytes_to_address,
+)
+from eth._utils.padding import (
+    zpad_right,
+)
+
+from eth.vm.forks.berlin import constants
+
 from eth.vm.forks.muir_glacier.computation import (
     MUIR_GLACIER_PRECOMPILES
 )
@@ -7,7 +27,35 @@
 
 from .opcodes import BERLIN_OPCODES
 
-BERLIN_PRECOMPILES = MUIR_GLACIER_PRECOMPILES
+
+def _calculate_multiplication_complexity(base_length: int, modulus_length: int) -> int:
+    max_length = max(base_length, modulus_length)
+    words = math.ceil(max_length / 8)
+    return words**2
+
+
+def _compute_modexp_gas_fee_eip_2565(data: bytes) -> int:
+    base_length, exponent_length, modulus_length = extract_lengths(data)
+
+    base_end_idx = 96 + base_length
+    exponent_end_idx = base_end_idx + exponent_length
+
+    exponent_bytes = zpad_right(
+        data[base_end_idx:exponent_end_idx],
+        to_size=exponent_length,
+    )
+
+    multiplication_complexity = _calculate_multiplication_complexity(base_length, modulus_length)
+    iteration_count = compute_adjusted_exponent_length(exponent_length, exponent_bytes)
+    return max(200,
+               multiplication_complexity * iteration_count
+               // constants.GAS_MOD_EXP_QUADRATIC_DENOMINATOR_EIP_2565)
+
+
+BERLIN_PRECOMPILES = merge(
+    MUIR_GLACIER_PRECOMPILES,
+    {force_bytes_to_address(b'\x05'): modexp(gas_calculator=_compute_modexp_gas_fee_eip_2565)},
+)
 
 
 class BerlinComputation(MuirGlacierComputation):

eth/vm/forks/berlin/constants.py

@@ -0,0 +1 @@
+GAS_MOD_EXP_QUADRATIC_DENOMINATOR_EIP_2565 = 3

newsfragments/1976.feature.rst

@@ -0,0 +1 @@
+Implement EIP-2565: Update ModExp precompile gas cost calculation