<p align="center">
  <b>no_std-first Ethereum protocol building blocks for Rust.</b><br>
  Explicit domains, bounded decode policy, constant-time primitives, and security-gated release evidence.
</p>

<div align="center">
  <a href="https://docs.rs/eth">Docs.rs</a>
  |
  <a href="https://github.com/valkyoth/eth/blob/main/docs/RELEASE_PLAN.md">Release Plan</a>
  |
  <a href="https://github.com/valkyoth/eth/blob/main/docs/threat-model.md">Threat Model</a>
  |
  <a href="https://github.com/valkyoth/eth/blob/main/SECURITY.md">Security</a>
</div>

<br>

<p align="center">
  <a href="https://github.com/valkyoth/eth">
    <img src="https://raw.githubusercontent.com/valkyoth/eth/main/.github/images/eth.webp" alt="eth Rust crate overview">
  </a>
</p>

# eth

`eth` is the public facade crate for a `no_std`-first Ethereum
execution-layer protocol workspace.

The crate is intentionally conservative at `0.31.0`: it provides explicit
Ethereum primitive domains, bounded decode-budget policy, stable error
categories, primitive RLP bridge helpers, a caller-provided Keccak-256 boundary,
RLP fuzz-harness evidence, a transaction envelope shell, unvalidated legacy
transaction field decoding, unvalidated EIP-2930 access-list transaction field
decoding, unvalidated EIP-1559 dynamic-fee transaction field decoding,
unvalidated EIP-4844 blob transaction field decoding, unvalidated EIP-7702
set-code transaction field decoding, no-allocation canonical transaction
envelope encoding for admitted decoded domains, explicit chain and fork
activation context, proof-gated transaction typestate transitions, replay-domain
validation for transaction chain binding, transaction signing-hash helpers,
EIP-7702 authorization signing and signer recovery helpers, an EIP-7702
set-code transaction validity gate, decoded transaction signature validation
helpers, public RLP derive support, digest-level secp256k1 sender recovery,
EIP-712 domain-safety checks, EIP-712 typed-data hashing over borrowed
descriptors, an opt-in bounded JSON typed-data parser, syntactic block header
decoding and hashing, syntactic receipt and withdrawal-list decoding, an
opt-in reviewed
`tiny-keccak` software backend, small first-party crate boundaries, optional
sanitization support, and release evidence before RPC, signer, EVM, Reth, or
P2P integrations become real dependencies.

## Current Status

The current release candidate is `0.31.0`; MPT node decoding has passed pentest
and is waiting for final GitHub checks before tagging.

Implemented now:

- `no_std` facade with small first-party support crates.
- Ethereum domain newtypes for chain, block, gas, nonce, timestamp, address,
  hash, wei, and transaction type values.
- Constant-time equality composition for fixed-width hash and wei values.
- Bounded decode limits plus stateful cumulative allocation, item, and proof-node
  accounting.
- Canonical RLP scalar, list, and integer decoding plus no-allocation canonical
  encoding helpers.
- No-allocation primitive RLP encode and exact-decode helpers for chain, block,
  gas, nonce, timestamp, address, hash, and wei values.
- EIP-2718 transaction envelope shell classification for typed and legacy
  transaction bytes.
- Unvalidated legacy transaction field decoding for nonce, gas price, gas
  limit, to/create, value, input, and signature words.
- Unvalidated EIP-2930 access-list transaction field decoding, including
  bounded borrowed access-list entry and storage-key iteration.
- Unvalidated EIP-1559 dynamic-fee transaction field decoding for max priority
  fee, max fee, gas limit, destination/create, value, calldata, access list, and
  signature words.
- Unvalidated EIP-4844 blob transaction field decoding for blob fee, required
  call target, blob versioned hash list, calldata, access list, and signature
  words.
- Unvalidated EIP-7702 set-code transaction field decoding for destination,
  calldata, access list, authorization list, and signature words.
- No-allocation canonical transaction envelope encoding for admitted
  unvalidated legacy, EIP-2930, EIP-1559, EIP-4844, and EIP-7702 transaction
  domains.
- Explicit caller-provided `ChainSpec`, `ForkSpec`, `Hardfork`, and
  `ValidationContext` APIs for fork activation selection, including
  fail-closed checks for duplicate forks, wrong-chain entries, and
  non-monotonic fork or activation ordering.
- Unvalidated execution block header decoding for legacy, London, Shanghai,
  Cancun, and Prague field sets, plus block header hashing through the Keccak
  trait boundary and a distinct `BlockHash` domain newtype.
- Unvalidated legacy and EIP-2718 typed receipt decoding, including
  status/root policy, 256-byte logs bloom, borrowed log entries, topics, and
  data.
- Unvalidated EIP-4895 withdrawal-list decoding, including global withdrawal
  indexes, validator indexes, recipient addresses, and nonzero Gwei amounts.
- Bounded syntactic MPT node decoding for branch, extension, and leaf nodes,
  including compact-path validation, eager inline child shape checks, and
  cumulative proof-node byte/count accounting.
- Proof-gated transaction typestate transitions for decoded, canonical,
  fork-validated, and sender-recovered state tokens.
- Replay-domain validation for legacy EIP-155 and typed transaction chain IDs
  before sender recovery results are accepted.
- Canonical transaction signing-preimage encoding and signing-hash helpers for
  legacy EIP-155, EIP-2930, EIP-1559, EIP-4844, and EIP-7702 decoded
  transaction domains.
- EIP-7702 authorization tuple signing-hash and signer recovery helpers, kept
  separate from transaction signing hashes with explicit domain newtypes.
- EIP-7702 set-code transaction validity gate for Prague/Pectra fork context,
  non-empty authorization lists, fee order, caller-computed gas policy, and
  caller-provided authority account-state checks. Per-authorization failures
  are counted as skipped tuples instead of rejecting the whole transaction.
- Digest-level secp256k1 sender recovery with low-s rejection, Ethereum
  y-parity policy, and caller-provided Keccak-256 public-key hashing.
- Decoded transaction signature validation helpers that combine replay-domain
  checks, signing hashes, low-s/y-parity policy, sender recovery, and optional
  expected-sender comparison.
- EIP-712 domain-safety checks for required `chainId` and
  `verifyingContract` fields, plus a domain-gated sender recovery helper.
- No-allocation EIP-712 typed-data encoder for caller-provided schemas and
  borrowed values, including `encodeType`, `encodeData`, `hashStruct`, domain
  separator construction, and typed-data signing digest construction.
- Optional `eip712-json` parser boundary for JSON-RPC typed-data payloads with
  duplicate-key rejection, explicit parser limits, and no default dependency
  impact.
- Optional `keccak-tiny` software backend using reviewed `tiny-keccak`,
  disabled by default and covered by Keccak-256 KATs.
- Public `RlpEncode`/`RlpDecode` traits and derive macros for reviewed simple
  structs, with bounded decode and trybuild compile-fail coverage.
- Caller-provided Keccak-256 trait boundary with no default hash
  implementation dependency.
- RLP fuzz harness with committed hex seed corpus and crash reproduction docs.
- Stable error codes, messages, categories, and formatting for codec,
  protocol, fork, feature, resource, and verification failures.
- Optional sanitization bridge and derive macros outside the default feature
  set.
- Release gates for formatting, clippy, tests, packaging, MSRV compatibility,
  dependency policy, audit, SBOM, and pentest evidence.

Not implemented yet:

- No RPC transport.
- No signer or local key storage.
- No EVM execution adapter.
- No Reth or P2P integration.
- No block parser yet.
- No ABI/contract helper surface yet; scheduled for `v0.47.0` through
  `v0.55.0`.
- No trie-root proof verification yet; MPT node decoding is scheduled to feed
  inclusion proof verification in `v0.32.0` and account/storage proofs in
  `v0.33.0`.
- No consensus/Engine API support yet; scheduled for `v0.56.0` through
  `v0.62.0`.
- No P2P, txpool, sync, mining, builder, or validator-adjacent boundary yet;
  scheduled for `v0.63.0` through `v0.69.0`.

## Trust Dashboard

| Area | Status |
| --- | --- |
| License | `MIT OR Apache-2.0` |
| MSRV | Rust `1.90.0` |
| Latest verified stable | Rust `1.96.1` |
| Default target | `no_std` |
| Default features | protocol-core only |
| Default networking/signing | none |
| Unsafe policy | first-party crates use `#![forbid(unsafe_code)]` |
| Release evidence | local gates, cargo-deny, cargo-audit, SBOM, pentest report |
| Formal verification | Kani harness planned for `v0.71.0` as extra assurance |
| Crate versions | tracked in the [version matrix](https://github.com/valkyoth/eth/blob/main/docs/CRATE_VERSION_MATRIX.md) |

## Install

```toml
[dependencies]
eth = "0.31"
```

Disable defaults explicitly for embedded or freestanding builds:

```toml
[dependencies]
eth = { version = "0.31", default-features = false }
```

Optional sanitization support:

```toml
[dependencies]
eth = { version = "0.31", features = ["sanitization"] }
```

## Features

| Feature | Default | Purpose |
| --- | --- | --- |
| `std` | no | Enables `std` support in admitted core crates. |
| `evm` | no | Future explicit EVM adapter boundary. |
| `rpc` | no | Future explicit RPC trust-policy boundary. |
| `eip712-json` | no | Enables the optional `std` JSON-RPC EIP-712 typed-data parser boundary. |
| `keccak-tiny` | no | Enables the optional reviewed `tiny-keccak` software backend. |
| `sanitization` | no | Re-exports optional secret sanitization bridge APIs. |
| `signer` | no | Future signer isolation boundary. |
| `reth` | no | Future Reth integration boundary. |
| `testkit` | no | Test fixtures, conformance helpers, and adversarial inputs. |

Default builds do not enable networking, signing, local key storage, Reth, P2P,
or EVM execution.

Optional reviewed software Keccak backend:

```toml
[dependencies]
eth = { version = "0.31", features = ["keccak-tiny"] }
```

```rust
use eth::hash::{KECCAK256_ABC, TinyKeccak256, hash_one};

let digest = hash_one(TinyKeccak256::default(), b"abc");
assert_eq!(<[u8; 32]>::from(digest), KECCAK256_ABC);
```

## Primitive Domains

Use explicit Ethereum domains instead of unqualified integers and byte arrays:

```rust
use eth::primitives::{
    Address, B256, BlockNumber, ChainId, Gas, Nonce, TransactionType, Wei,
};

let chain = ChainId::new(1);
let block = BlockNumber::new(19_000_000);
let gas = Gas::new(21_000);
let nonce = Nonce::new(7);
let address = Address::from([0x11_u8; 20]);
let hash = B256::from([0x22_u8; 32]);
let value = Wei::from_u128(1_000_000_000_000_000_000);
let tx_type = TransactionType::try_new_typed(2);

assert_eq!(u64::from(chain), 1);
assert_eq!(u64::from(block), 19_000_000);
assert_eq!(u64::from(gas), 21_000);
assert_eq!(u64::from(nonce), 7);
assert_eq!(<[u8; 20]>::from(address), [0x11_u8; 20]);
assert_eq!(<[u8; 32]>::from(hash), [0x22_u8; 32]);
assert_eq!(value.to_be_bytes()[31], 0);
assert_eq!(tx_type.map(u8::from), Ok(2));
```

Legacy transactions are not typed EIP-2718 envelopes. Use
`TransactionType::LEGACY` for APIs that need a legacy domain value, and
`try_new_typed` for type bytes that will be encoded as typed envelopes.

Primitive domains bridge directly to the bounded codec without allocation:

```rust
use eth::codec::DecodeLimits;
use eth::primitives::{Address, ChainId, Wei};

let limits = DecodeLimits {
    max_input_bytes: 64,
    max_list_items: 4,
    max_nesting_depth: 4,
    max_total_allocation: 64,
    max_proof_nodes: 4,
    max_total_items: 4,
};

let chain = ChainId::new(1);
let mut encoded_chain = [0_u8; 8];
let written = chain.encode_rlp(&mut encoded_chain)?;
assert_eq!(encoded_chain.get(..written), Some([0x01].as_slice()));
assert_eq!(ChainId::try_from_rlp(&[0x01], limits)?, chain);

let value = Wei::from_u128(1024);
let mut encoded_value = [0_u8; 8];
let written = value.encode_rlp(&mut encoded_value)?;
assert_eq!(encoded_value.get(..written), Some([0x82, 0x04, 0x00].as_slice()));
assert_eq!(Wei::try_from_rlp(&[0x82, 0x04, 0x00], limits)?, value);

let address = Address::from([0x11_u8; 20]);
let mut encoded_address = [0_u8; 21];
let written = address.encode_rlp(&mut encoded_address)?;
assert_eq!(written, 21);
assert_eq!(Address::try_from_rlp(&encoded_address, limits)?, address);
# Ok::<(), eth::primitives::PrimitiveRlpError>(())
```

## Transaction Decode

Transaction decoders return explicitly unvalidated borrowed field models. They
classify and bound wire data, but do not validate signatures from the full
transaction, check account state, or prove fork validity:

```rust
use eth::codec::DecodeLimits;
use eth::primitives::{Gas, Nonce, Wei};
use eth::protocol::{
    DynamicFeeTransactionTo, SignatureYParity, decode_dynamic_fee_transaction,
    encode_dynamic_fee_transaction,
};

let dynamic_fee_tx = [
    0x02, 0xce, 0x01, 0x02, 0x03, 0x04, 0x82, 0x52, 0x08, 0x80, 0x05, 0x80,
    0xc0, 0x01, 0x01, 0x02,
];

let limits = DecodeLimits {
    max_input_bytes: 64,
    max_list_items: 16,
    max_nesting_depth: 8,
    max_total_allocation: 64,
    max_proof_nodes: 4,
    max_total_items: 32,
};
let tx = decode_dynamic_fee_transaction(&dynamic_fee_tx, limits)?;

assert_eq!(tx.chain_id.get(), 1);
assert_eq!(tx.nonce, Nonce::new(2));
assert_eq!(tx.max_priority_fee_per_gas, Wei::from_u128(3));
assert_eq!(tx.max_fee_per_gas, Wei::from_u128(4));
assert_eq!(tx.gas_limit, Gas::new(21_000));
assert_eq!(tx.to, DynamicFeeTransactionTo::Create);
assert_eq!(tx.value, Wei::from_u128(5));
assert_eq!(tx.access_list.address_count(), 0);
assert_eq!(tx.access_list.storage_key_count(), 0);
assert_eq!(tx.y_parity, SignatureYParity::Odd);

let mut encoded = [0_u8; 32];
let written = encode_dynamic_fee_transaction(&tx, &mut encoded)?;
assert_eq!(encoded.get(..written), Some(dynamic_fee_tx.as_slice()));
# Ok::<(), Box<dyn std::error::Error>>(())
```

## Replay Domain Checks

Replay-domain helpers reject wrong-chain transactions before sender recovery
results are trusted:

```rust
use eth::codec::DecodeLimits;
use eth::primitives::ChainId;
use eth::protocol::decode_dynamic_fee_transaction;
use eth::verify::{VerifyError, require_dynamic_fee_replay_domain};

let dynamic_fee_tx = [
    0x02, 0xce, 0x01, 0x02, 0x03, 0x04, 0x82, 0x52, 0x08, 0x80, 0x05, 0x80,
    0xc0, 0x01, 0x01, 0x02,
];

let limits = DecodeLimits {
    max_input_bytes: 64,
    max_list_items: 16,
    max_nesting_depth: 8,
    max_total_allocation: 64,
    max_proof_nodes: 4,
    max_total_items: 32,
};
let tx = decode_dynamic_fee_transaction(&dynamic_fee_tx, limits)?;

require_dynamic_fee_replay_domain(ChainId::new(1), &tx)?;
assert_eq!(
    require_dynamic_fee_replay_domain(ChainId::new(5), &tx),
    Err(VerifyError::WrongChain)
);
# Ok::<(), Box<dyn std::error::Error>>(())
```

## Transaction Signing Hashes

Decoded transaction domains can be converted into canonical signing hashes
without admitting a default hash backend:

```rust
use eth::hash::Keccak256;
use eth::primitives::B256;
use eth::protocol::decode_dynamic_fee_transaction;
use eth::verify::dynamic_fee_transaction_signing_hash;
use eth::codec::DecodeLimits;

struct PlatformKeccak {
    output: B256,
}

impl Keccak256 for PlatformKeccak {
    fn update(&mut self, input: &[u8]) {
        let _ = input;
    }

    fn finalize(self) -> B256 {
        self.output
    }
}

let dynamic_fee_tx = [
    0x02, 0xce, 0x01, 0x02, 0x03, 0x04, 0x82, 0x52, 0x08, 0x80, 0x05, 0x80,
    0xc0, 0x01, 0x01, 0x02,
];
let limits = DecodeLimits {
    max_input_bytes: 64,
    max_list_items: 16,
    max_nesting_depth: 8,
    max_total_allocation: 64,
    max_proof_nodes: 4,
    max_total_items: 32,
};
let tx = decode_dynamic_fee_transaction(&dynamic_fee_tx, limits)?;
let mut scratch = [0_u8; 64];
let signing_hash = dynamic_fee_transaction_signing_hash(
    &tx,
    &mut scratch,
    PlatformKeccak {
        output: B256::from([0x44_u8; 32]),
    },
)?;

assert_eq!(signing_hash.to_b256(), B256::from([0x44_u8; 32]));
# Ok::<(), Box<dyn std::error::Error>>(())
```

The example hasher is illustrative only. Production hashers must compute
Ethereum Keccak-256. For full decoded transaction signature validation, use
`validate_transaction_signature` or the type-specific validation helpers so
replay-domain checks, signing-hash construction, low-s/y-parity policy, sender
recovery, and optional expected-sender comparison are applied together. Callers
that reuse the scratch buffer across multiple in-flight transactions should
zero it after hashing before reusing or releasing it.

EIP-7702 authorization tuples use a separate signing-hash domain:

```rust
use eth::hash::Keccak256;
use eth::primitives::{Address, B256, Nonce};
use eth::protocol::{SetCodeAuthorization, SetCodeAuthorizationChainId, SignatureYParity};
use eth::verify::set_code_authorization_signing_hash;

struct PlatformKeccak {
    output: B256,
}

impl Keccak256 for PlatformKeccak {
    fn update(&mut self, input: &[u8]) {
        let _ = input;
    }

    fn finalize(self) -> B256 {
        self.output
    }
}

let mut chain_id = [0_u8; 32];
if let Some(last) = chain_id.last_mut() {
    *last = 1;
}
let authorization = SetCodeAuthorization {
    chain_id: SetCodeAuthorizationChainId::from_be_bytes(chain_id),
    address: Address::from([0x11_u8; 20]),
    nonce: Nonce::new(7),
    y_parity: SignatureYParity::Even,
    r: [0_u8; 32],
    s: [0_u8; 32],
};
let mut scratch = [0_u8; 128];
let authorization_hash = set_code_authorization_signing_hash(
    authorization,
    &mut scratch,
    PlatformKeccak {
        output: B256::from([0x55_u8; 32]),
    },
)?;

assert_eq!(authorization_hash.to_b256(), B256::from([0x55_u8; 32]));
# Ok::<(), Box<dyn std::error::Error>>(())
```

## EIP-712 Typed Data

EIP-712 signing paths can build the structured-data digest from reviewed
borrowed type descriptors and values without adding a concrete Keccak backend
to the default graph:

```rust
use eth::hash::Keccak256;
use eth::primitives::{Address, B256, ChainId};
use eth::verify::{
    Eip712DomainData, Eip712Field, Eip712StructType, Eip712Value,
    Eip712ValueKind, eip712_typed_data_signing_digest,
};

let types = [Eip712StructType {
    name: "Permit",
    fields: &[
        Eip712Field { name: "owner", type_name: "address" },
        Eip712Field { name: "spender", type_name: "address" },
        Eip712Field { name: "value", type_name: "uint256" },
    ],
}];
let values = [
    Eip712Value {
        name: "owner",
        value: Eip712ValueKind::Address(Address::from([0x11_u8; 20])),
    },
    Eip712Value {
        name: "spender",
        value: Eip712ValueKind::Address(Address::from([0x22_u8; 20])),
    },
    Eip712Value {
        name: "value",
        value: Eip712ValueKind::Uint64(10),
    },
];
let domain = Eip712DomainData {
    name: Some("Example"),
    version: Some("1"),
    chain_id: Some(ChainId::new(1)),
    verifying_contract: Some(Address::from([0xcc_u8; 20])),
    salt: None,
};
let mut scratch = [0_u8; 256];
let _digest = eip712_typed_data_signing_digest::<ExampleKeccak>(
    domain,
    &types,
    "Permit",
    &values,
    &mut scratch,
)?;
# #[derive(Default)]
# struct ExampleKeccak;
# impl eth::hash::Keccak256 for ExampleKeccak {
#     fn update(&mut self, input: &[u8]) { let _ = input; }
#     fn finalize(self) -> B256 { B256::from([0x33_u8; 32]) }
# }
# Ok::<(), Box<dyn std::error::Error>>(())
```

JSON-RPC typed-data parsing is available only through the opt-in
`eip712-json` feature. It uses explicit parser limits, rejects duplicate JSON
object keys, and still relies on a caller-provided Keccak backend.

```rust,ignore
use eth::verify::{Eip712JsonLimits, eip712_json_typed_data_signing_digest};

let json = r#"{
  "types": {"Permit": [{"name": "owner", "type": "address"}]},
  "primaryType": "Permit",
  "domain": {"chainId": 1},
  "message": {"owner": "0x1111111111111111111111111111111111111111"}
}"#;
let mut scratch = [0_u8; 512];
let _digest = eip712_json_typed_data_signing_digest::<ExampleKeccak>(
    json,
    Eip712JsonLimits::DEFAULT,
    &mut scratch,
)?;
# Ok::<(), Box<dyn std::error::Error>>(())
```

## Sender Recovery

Sender recovery operates on an already constructed Ethereum signing digest.
Transaction callers should prefer the signing-hash helpers above over
hand-built transaction digests, then recover the sender with an admitted
Keccak-256 backend:

```rust
use eth::hash::Keccak256;
use eth::primitives::B256;
use eth::protocol::SignatureYParity;
use eth::verify::{EthereumSignature, recover_sender_from_digest};

struct PlatformKeccak {
    output: B256,
}

impl Keccak256 for PlatformKeccak {
    fn update(&mut self, input: &[u8]) {
        let _ = input;
    }

    fn finalize(self) -> B256 {
        self.output
    }
}

let digest = B256::from([0x44_u8; 32]);
let signature = EthereumSignature::from_parts(
    [0x11_u8; 32],
    [0x22_u8; 32],
    SignatureYParity::Even,
);

let _result = recover_sender_from_digest(
    digest,
    signature,
    PlatformKeccak {
        output: B256::from([0x33_u8; 32]),
    },
);
```

The recovery layer rejects malformed scalar values, high-s signatures, and
non-Ethereum recovery IDs. The example hasher above is illustrative only and
does not compute a real digest. Production hashers must implement Ethereum
Keccak-256, not FIPS SHA3-256, and should be checked with
`eth::hash::verify_empty_digest_with` before being wired into
`recover_sender_from_digest`. A wrong backend produces a wrong sender address
silently; there is no runtime cross-check. A successful recovered address is
still not a full transaction-validity proof.

## Constant-Time Composition

`B256::ct_eq` and `Wei::ct_eq` return `subtle::Choice` so compound checks can
use `&` and `|` without short-circuiting:

```rust
use eth::primitives::B256;

let block_hash = B256::from([1_u8; 32]);
let expected_block_hash = B256::from([1_u8; 32]);
let receipts_root = B256::from([2_u8; 32]);
let expected_receipts_root = B256::from([2_u8; 32]);

let valid = block_hash.ct_eq(&expected_block_hash)
    & receipts_root.ct_eq(&expected_receipts_root);

assert!(bool::from(valid));
```

Convert `Choice` to `bool` only at the final trust boundary.

## Keccak Boundary

`eth` defines a `no_std` Keccak-256 trait boundary and intentionally does not
ship a default hashing backend yet:

```rust
use eth::hash::{Keccak256, hash_one};
use eth::primitives::B256;

struct PlatformKeccak {
    output: B256,
}

impl Keccak256 for PlatformKeccak {
    fn update(&mut self, input: &[u8]) {
        let _ = input;
    }

    fn finalize(self) -> B256 {
        self.output
    }
}

let digest = hash_one(
    PlatformKeccak {
        output: B256::from([0x44_u8; 32]),
    },
    b"ethereum",
);

assert_eq!(<[u8; 32]>::from(digest), [0x44_u8; 32]);
```

Implementations must compute Ethereum Keccak-256, not FIPS SHA3-256. See the
[Keccak boundary document](https://github.com/valkyoth/eth/blob/main/docs/keccak-boundary.md)
for the dependency decision and future backend admission checklist.

## Stable Errors

Error values expose stable codes, messages, and categories. They do not carry
input bytes, keys, signatures, or other secret-bearing payloads:

```rust
use eth::error::{DecodeError, DecodeErrorCategory, ResourceError};

let error = DecodeError::AllocationExceeded;

assert_eq!(error.code(), "ETH_CODEC_ALLOCATION_EXCEEDED");
assert_eq!(error.category(), DecodeErrorCategory::ResourceExhaustion);
assert_eq!(error.resource(), Some(ResourceError::AllocationBytes));
assert_eq!(error.to_string(), "decoder exceeded the active allocation limit");
```

## Decode Budgets

Every future untrusted decoder is required to use explicit limits. Use
`DecodeAccumulator` when more than one allocation can occur:

```rust
use eth::codec::{DecodeError, DecodeLimits};

let limits = DecodeLimits {
    max_input_bytes: 1024,
    max_list_items: 16,
    max_nesting_depth: 4,
    max_total_allocation: 64,
    max_proof_nodes: 8,
    max_total_items: 32,
};

assert_eq!(limits.check_input_len(512), Ok(()));

let mut budget = limits.accumulator();
assert_eq!(budget.check_allocation(32), Ok(()));
assert_eq!(budget.check_allocation(32), Ok(()));
assert_eq!(budget.check_allocation(1), Err(DecodeError::AllocationExceeded));
assert_eq!(budget.account_items(33), Err(DecodeError::ItemCountExceeded));
```

## RLP Codec

The RLP codec admits canonical byte-string scalars, lists, and Ethereum
integers with exact consumption. Decoders require explicit limits; encoders are
buffer-based and do not allocate:

```rust
use eth::codec::{
    DecodeLimits, RlpListForm, RlpScalarForm, decode_rlp_list, decode_rlp_scalar, decode_rlp_u64,
    encode_decoded_scalar, encode_rlp_list_payload, encode_rlp_scalar,
};

let limits = DecodeLimits {
    max_input_bytes: 32,
    max_list_items: 4,
    max_nesting_depth: 4,
    max_total_allocation: 32,
    max_proof_nodes: 4,
    max_total_items: 4,
};
let scalar = decode_rlp_scalar(&[0x83, b'd', b'o', b'g'], limits)?;

assert_eq!(scalar.payload(), b"dog");
assert_eq!(scalar.encoded_len(), 4);
assert_eq!(scalar.header_len(), 1);
assert_eq!(scalar.form(), RlpScalarForm::ShortString);

let mut encoded = [0_u8; 8];
let written = encode_decoded_scalar(scalar, &mut encoded)?;
assert_eq!(written, 4);
assert_eq!(encoded.get(..written), Some([0x83, b'd', b'o', b'g'].as_slice()));

assert_eq!(decode_rlp_u64(&[0x82, 0x04, 0x00], limits)?, 1024);
assert!(decode_rlp_u64(&[0x82, 0x00, 0x01], limits).is_err());

let list = decode_rlp_list(&[0xc8, 0x83, b'c', b'a', b't', 0x83, b'd', b'o', b'g'], limits)?;

assert_eq!(list.item_count(), 2);
assert_eq!(list.form(), RlpListForm::ShortList);
let mut items = list.items();
let first = items.next().transpose()?.and_then(|item| item.as_scalar());
let second = items.next().transpose()?.and_then(|item| item.as_scalar());
assert!(matches!(first, Some(item) if item.payload() == b"cat"));
assert!(matches!(second, Some(item) if item.payload() == b"dog"));

let mut scalar_output = [0_u8; 8];
assert_eq!(encode_rlp_scalar(b"cat", &mut scalar_output)?, 4);
assert_eq!(scalar_output.get(..4), Some([0x83, b'c', b'a', b't'].as_slice()));

let list_payload = [0x83, b'c', b'a', b't', 0x83, b'd', b'o', b'g'];
let mut list_output = [0_u8; 16];
assert_eq!(encode_rlp_list_payload(&list_payload, limits, &mut list_output)?, 9);
assert_eq!(list_output.get(..9), Some([0xc8, 0x83, b'c', b'a', b't', 0x83, b'd', b'o', b'g'].as_slice()));
# Ok::<(), eth::error::DecodeError>(())
```

The RLP parser surface has cargo-fuzz targets and committed seed fixtures. See
the [fuzzing guide](https://github.com/valkyoth/eth/blob/main/docs/fuzzing.md)
for seed materialization, target scope, and crash reproduction.

## Withdrawals

EIP-4895 withdrawal lists decode into an explicitly unvalidated borrowed model.
The decoder checks canonical RLP shape, `uint64` indexes, 20-byte recipient
addresses, and nonzero Gwei amounts, but it does not prove header
`withdrawals_root` membership or state-balance application:

```rust
use eth::codec::DecodeLimits;
use eth::protocol::decode_withdrawals;

let limits = DecodeLimits {
    max_input_bytes: 64,
    max_list_items: 8,
    max_nesting_depth: 4,
    max_total_allocation: 64,
    max_proof_nodes: 4,
    max_total_items: 16,
};
let raw = [
    0xd9, 0xd8, 0x01, 0x02, 0x94, 0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36,
    0x37, 0x38, 0x39, 0x3a, 0x3b, 0x3c, 0x3d, 0x3e, 0x3f, 0x40, 0x41, 0x42,
    0x43, 0x03,
];

let withdrawals = decode_withdrawals(&raw, limits)?;
let mut entries = withdrawals.entries();
let first = entries.next().transpose()?.ok_or("missing withdrawal")?;

assert_eq!(withdrawals.len(), 1);
assert_eq!(first.index.get(), 1);
assert_eq!(first.validator_index.get(), 2);
assert_eq!(first.amount.get(), 3);
assert!(entries.next().is_none());
# Ok::<(), Box<dyn std::error::Error>>(())
```

## MPT Nodes

The verifier crate decodes Merkle Patricia Trie node shape without computing a
root. Branch nodes must contain sixteen child references plus one scalar value;
extension and leaf nodes must contain a compact hex-prefix path plus a child
reference or scalar value:

```rust
use eth::codec::DecodeLimits;
use eth::verify::{MptNode, MptNodeReference, decode_mpt_node};

let limits = DecodeLimits {
    max_input_bytes: 64,
    max_list_items: 32,
    max_nesting_depth: 8,
    max_total_allocation: 64,
    max_proof_nodes: 4,
    max_total_items: 64,
};
let raw_leaf = [0xc5, 0x20, 0x83, b'd', b'o', b'g'];

let node = decode_mpt_node(&raw_leaf, limits)?;

if let MptNode::Leaf(leaf) = node {
    assert!(leaf.path.is_leaf());
    assert_eq!(leaf.path.nibble_count()?, 0);
    assert_eq!(leaf.value, b"dog");
} else {
    assert!(false);
}

let branch = [0xd1, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80,
    0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80];
let branch = decode_mpt_node(&branch, limits)?;
if let MptNode::Branch(branch) = branch {
    assert!(branch
        .children()
        .all(|child| matches!(child, Ok(MptNodeReference::Empty))));
} else {
    assert!(false);
}
# Ok::<(), Box<dyn std::error::Error>>(())
```

This is not inclusion proof verification. Transaction, receipt, account,
storage, and withdrawal root checks remain separate verification milestones.
See [`docs/mpt-nodes.md`](../../docs/mpt-nodes.md).

## Transaction Envelopes

The protocol crate can classify the outer transaction envelope without decoding
or validating transaction fields:

```rust
use eth::codec::DecodeLimits;
use eth::protocol::{TransactionEnvelope, decode_transaction_envelope};

let limits = DecodeLimits {
    max_input_bytes: 32,
    max_list_items: 4,
    max_nesting_depth: 4,
    max_total_allocation: 32,
    max_proof_nodes: 4,
    max_total_items: 4,
};

let envelope = decode_transaction_envelope(&[0x02, 0xc0], limits)?;

assert!(matches!(envelope, TransactionEnvelope::Typed(_)));
if let TransactionEnvelope::Typed(typed) = envelope {
    assert_eq!(u8::from(typed.transaction_type), 2);
    assert_eq!(typed.payload, &[0xc0]);
}
# Ok::<(), eth::error::TransactionEnvelopeError>(())
```

Typed payloads can be classified first, then decoded with the matching
transaction decoder. Legacy transactions can also be decoded into an explicitly
unvalidated field model:

```rust
use eth::codec::DecodeLimits;
use eth::protocol::{LegacyTransactionTo, decode_legacy_transaction};

let limits = DecodeLimits {
    max_input_bytes: 64,
    max_list_items: 16,
    max_nesting_depth: 4,
    max_total_allocation: 64,
    max_proof_nodes: 4,
    max_total_items: 32,
};
let raw = [0xcb, 0x01, 0x02, 0x82, 0x52, 0x08, 0x80, 0x80, 0x80, 0x1b, 0x01, 0x02];

let tx = decode_legacy_transaction(&raw, limits)?;

assert_eq!(tx.nonce.get(), 1);
assert_eq!(tx.gas_limit.get(), 21_000);
assert_eq!(tx.to, LegacyTransactionTo::Create);
assert_eq!(tx.input, &[] as &[u8]);
assert_eq!(tx.eip155_chain_id(), None);
# Ok::<(), eth::error::LegacyTransactionDecodeError>(())
```

The decoded value is not chain-valid, signature-valid, sender-recovered, or
fork-valid. It is only a bounded, canonical field parse. Use
`eip155_chain_id` instead of subtracting directly from the raw `v` signature
word; reserved `ChainId(0)` maps to `None`.

## Optional Sanitization

The main facade stays small by default. Applications that handle local secret
material can opt into the sanitization bridge:

```rust,ignore
use eth::sanitization::{SecretBytes32, SecureSanitize};

let mut key = SecretBytes32::from_array([0x42_u8; 32]);
key.secure_sanitize();
assert!(key.constant_time_eq(&[0_u8; 32]));
```

For derive macros, depend on the support crate directly:

```toml
[dependencies]
eth-valkyoth-sanitization = { version = "0.7", features = ["derive"] }
```

Public RLP encode/decode derives live in `eth-valkyoth-derive`:

```toml
[dependencies]
eth-valkyoth-derive = "0.17"
eth-valkyoth-codec = "0.17"
```

The derive surface is intentionally conservative. It supports reviewed structs
only, rejects generics/enums/unions, requires `DecodeLimits` for decode, and
keeps skipped fields explicit with `#[eth_rlp(skip, default, reason = "...")]`.

## Support Crates

Most users should depend on `eth`. The `eth-valkyoth-*` crates are published so
the workspace can keep small, auditable boundaries:

| Crate | Default | Purpose |
| --- | --- | --- |
| `eth-valkyoth-primitives` | yes | Chain, block, gas, nonce, address, hash, wei, and transaction type domains. |
| `eth-valkyoth-codec` | yes | Bounded exact-consumption wire codec policy. |
| `eth-valkyoth-hash` | yes | Keccak-256 trait boundary for caller-provided hash implementations. |
| `eth-valkyoth-protocol` | yes | Fork-aware validation states and protocol context. |
| `eth-valkyoth-verify` | yes | Verification boundaries for signatures, proofs, replay domains, and EIP-712 typed-data hashing. |
| `eth-valkyoth-sanitization` | no | Optional bridge to the `sanitization` crate. |
| `eth-valkyoth-derive` | no | Optional sanitization and RLP derive macros. |
| `eth-valkyoth-evm` | no | Future EVM adapter boundary. |
| `eth-valkyoth-rpc` | no | Future RPC trust-policy boundary. |
| `eth-valkyoth-signer` | no | Future signer isolation boundary. |
| `eth-valkyoth-reth` | no | Future Reth integration boundary. |
| `eth-valkyoth-testkit` | no | Future fixtures and conformance helpers. |

## Rust Version Support

The minimum supported Rust version is Rust `1.90.0`. New deployments should use
the latest stable Rust verified by the release gates.

Compatibility evidence for `0.31.0`:

| Rust | Local Evidence |
| --- | --- |
| `1.90.0` | `cargo check --workspace --all-features` |
| `1.91.0` | `cargo check --workspace --all-features` |
| `1.92.0` | `cargo check --workspace --all-features` |
| `1.93.0` | `cargo check --workspace --all-features` |
| `1.94.0` | `cargo check --workspace --all-features` |
| `1.95.0` | `cargo check --workspace --all-features` |
| `1.96.0` | `cargo check --workspace --all-features` |
| `1.96.1` | full release gate |

## License

Licensed under either of Apache License, Version 2.0 or MIT license at your
option.
