use two ring buffers to avoid collision attacks

EIPS/eip-4788.md

@@ -29,7 +29,7 @@ restaking constructions, smart contract bridges, MEV mitigations and more.
 |---                           |---                                           |---
 | `FORK_TIMESTAMP`             | TBD                                          |
 | `HISTORY_STORAGE_ADDRESS`    | `Bytes20(0xB)`                               |
-| `G_beacon_root`              | 2100                                         | gas
+| `G_beacon_root`              | 4200                                         | gas
 | `HISTORICAL_ROOTS_LENGTH`    | 98304                                        |
 
 ### Background
@@ -53,47 +53,65 @@ Validity is guaranteed from the consensus layer, much like how withdrawals are h
 At the start of processing any execution block where `block.timestamp >= FORK_TIMESTAMP` (i.e. before processing any transactions),
 write the parent beacon root provided in the block header into the storage of the contract at `HISTORY_STORAGE_ADDRESS`.
 
-The timestamp (a 64-bit unsigned integer value) of the header is used as a key into the contract's storage.
-To map the timestamp to the correct key, the timestamp as a number is reduced modulo `HISTORICAL_ROOTS_LENGTH` and
-this resulting 64-bit unsigned integer should be encoded as 32 bytes in big-endian format when writing to the storage.
+In order to bound the storage used by this precompile, two ring buffers are used: one to track the latest root at a given index and another to track
+the latest timestamp at a given index.
 
-The 32 bytes of the `parent_beacon_block_root` (as provided) are the
-value to write in the contract's storage.
+To derive the index `root_index` into the root ring buffer, the timestamp (a 64-bit unsigned integer value) is reduced modulo `HISTORICAL_ROOTS_LENGTH`.
+To derive the index `timestamp_index` into the timestamp ring buffer, add `HISTORICAL_ROOTS_LENGTH` to the index into the root ring buffer.
+Both resulting 64-bit unsigned integers should be encoded as 32 bytes in big-endian format when writing to the storage.
+
+The 32 bytes of the `parent_beacon_block_root` (as provided) are the value to write behind the `root_index`.
+The timestamp from the header, encoded as 32 bytes in big-endian format, is the value to write behind the `timestamp_index`.
 
 In Python pseudocode:
 
 ```python
 timestamp_reduced = block_header.timestamp % HISTORICAL_ROOTS_LENGTH
-key = to_uint256_be(timestamp_reduced)
+timestamp_extended = timestamp_reduced + HISTORICAL_ROOTS_LENGTH
+root_index = to_uint256_be(timestamp_reduced)
+timestamp_index = to_uint256_be(timestamp_extended)
 
 parent_beacon_block_root = block_header.parent_beacon_block_root
+timestamp_as_uint256 = to_uint256_be(block_header.timestamp)
 
-sstore(HISTORY_STORAGE_ADDRESS, key, parent_beacon_block_root)
+sstore(HISTORY_STORAGE_ADDRESS, root_index, parent_beacon_block_root)
+sstore(HISTORY_STORAGE_ADDRESS, timestamp_index, timestamp_as_uint256)
 ```
 
 #### New stateful precompile
 
 Beginning at the execution timestamp `FORK_TIMESTAMP`, a "stateful" precompile is deployed at `HISTORY_STORAGE_ADDRESS`.
 
 Callers of the precompile should provide the `timestamp` they are querying encoded as 32 bytes in big-endian format.
-This `timestamp` is reduced in the same way to point to a unique storage location into the ring buffer from any given block.
 
-Alongside the existing gas for calling the precompile, there is an additional gas cost of `G_beacon_root` cost to reflect the implicit `SLOAD` from
-the precompile's state.
+Given this input, the precompile reduces the `timestamp` in the same way during the write routines and first checks if
+the `timestamp` at the ring buffer index matches the one supplied by the caller.
+
+If the `timestamp` **does NOT** match, the client **MUST** return the "zero" word -- the 32-byte value where each byte is `0x00`.
 
-The parent beacon block root for the given timestamp is returned as 32 bytes in the caller's provided return buffer.
+If the `timestamp` **does** match, the client **MUST** read the root ring buffer and return the 32-byte value there in the caller's return buffer.
 
 In pseudocode:
 
 ```python
 timestamp = evm.calldata[:32]
 timestamp_reduced = to_uint64_be(timestamp) % HISTORICAL_ROOTS_LENGTH
-key = to_uint32_be(timestamp_reduced)
-root = sload(HISTORY_STORAGE_ADDRESS, key)
-evm.returndata[:32].set(root)
+timestamp_extended = timetsamp_reduced + HISTORICAL_ROOTS_LENGTH
+timestamp_index = to_uint256_be(timestamp_extended)
+
+recorded_timestamp = sload(HISTORY_STORAGE_ADDRESS, timestamp_index)
+if recorded_timestamp != timestamp:
+    evm.returndata[:32].set(0x0000000000000000000000000000000000000000000000000000000000000000)
+else:
+    root_index = to_uint256_be(timestamp_reduced)
+    root = sload(HISTORY_STORAGE_ADDRESS, root_index)
+    evm.returndata[:32].set(root)
 ```
 
-If there is no timestamp stored at the given root, the opcode follows the existing EVM semantics of `SLOAD` returning `0`.
+Alongside the existing gas for calling the precompile, there is an additional gas cost of `G_beacon_root` cost to reflect the two (2) implicit `SLOAD`s from
+the precompile's state.
+
+If there is no root stored at the given timestamp, the opcode follows the existing EVM semantics of `SLOAD` returning `0`.
 
 ## Rationale