# `Onchain.Signer.Secp256k1`
[🔗](https://github.com/ZenHive/onchain-stack/blob/onchain-v0.16.0/packages/onchain/lib/onchain/signer/secp256k1.ex#L1)

Signer backend that signs with a local secp256k1 private key.

Implements `Onchain.Signer.Backend` — its `config` is the raw private-key
binary. The behaviour's `c:Onchain.Signer.Backend.sign_payload/2` signs the
32-byte digest it is handed directly; `sign/2` is a convenience wrapper that
keccaks a raw message first.

Uses the precompiled RustCrypto k256 backend. Private keys remain in BEAM memory;
use `Onchain.Signer.CloudKMS` when keys must stay in an HSM.

# `get_address`

```elixir
@spec get_address(binary()) :: {:ok, binary()} | {:error, atom()}
```

Get the Ethereum address associated with the given private key.

## Examples

    iex> priv_key = "800509fa3e80882ad0be77c27505bdc91380f800d51ed80897d22f9fcc75f4bf" |> Base.decode16!(case: :mixed)
    iex> {:ok, address} = Onchain.Signer.Secp256k1.get_address(priv_key)
    iex> Onchain.Hex.to_address(address)
    "0x63Cc7c25e0cdb121aBb0fE477a6b9901889F99A7"

# `public_key`

```elixir
@spec public_key(binary()) :: {:ok, binary()} | {:error, atom()}
```

Get the uncompressed secp256k1 public key for the given private key.

## Examples

    iex> priv_key = "800509fa3e80882ad0be77c27505bdc91380f800d51ed80897d22f9fcc75f4bf" |> Base.decode16!(case: :mixed)
    iex> {:ok, pub} = Onchain.Signer.Secp256k1.public_key(priv_key)
    iex> Onchain.Hex.to_address(Onchain.Address.from_public_key(pub))
    "0x63Cc7c25e0cdb121aBb0fE477a6b9901889F99A7"

# `sign`

```elixir
@spec sign(String.t(), binary()) :: {:ok, Onchain.Signature.t()} | {:error, atom()}
```

Signs a raw message, keccak-digesting it first.

Back-compat convenience over `sign_payload/2` for EIP-191-style raw-message
signing; for typed data (EIP-712, Hyperliquid) the caller should compute the
digest and call `sign_payload/2` directly.

## Examples

    iex> use Onchain.Hex
    iex> priv_key = ~h[0x800509fa3e80882ad0be77c27505bdc91380f800d51ed80897d22f9fcc75f4bf]
    iex> {:ok, sig} = Onchain.Signer.Secp256k1.sign("test", priv_key)
    iex> {:ok, recid} = Onchain.Recover.find_recid("test", sig, ~h[0x63Cc7c25e0cdb121aBb0fE477a6b9901889F99A7])
    iex> Onchain.Recover.recover_eth("test", %{sig|recid: recid}) |> Onchain.Hex.to_address()
    "0x63Cc7c25e0cdb121aBb0fE477a6b9901889F99A7"

# `sign_digest`

```elixir
@spec sign_digest(String.t(), binary()) ::
  {:ok, Onchain.Signature.t()} | {:error, atom()}
```

Deprecated alias for `sign_payload/2`; signs an already-digested message.

# `sign_payload`

```elixir
@spec sign_payload(<<_::256>>, binary()) ::
  {:ok, Onchain.Signature.t()} | {:error, atom()}
```

Signs the 32-byte digest it is handed directly — the pure-payload contract.

This performs no hashing: the caller (`Onchain.Signer`) owns digest
computation, so the same backend serves plain Eth tx, EIP-712, and Hyperliquid.

## Examples

    iex> use Onchain.Hex
    iex> priv_key = ~h[0x800509fa3e80882ad0be77c27505bdc91380f800d51ed80897d22f9fcc75f4bf]
    iex> message_hash = ~h[0x9c22ff5f21f0b81b113e63f7db6da94fedef11b2119b4088b89664fb9a3cb658]
    iex> {:ok, sig} = Onchain.Signer.Secp256k1.sign_payload(message_hash, priv_key)
    iex> {:ok, recid} = Onchain.Recover.find_recid("test", sig, ~h[0x63Cc7c25e0cdb121aBb0fE477a6b9901889F99A7])
    iex> Onchain.Recover.recover_eth("test", %{sig|recid: recid}) |> Onchain.Hex.to_address()
    "0x63Cc7c25e0cdb121aBb0fE477a6b9901889F99A7"

---

*Consult [api-reference.md](api-reference.md) for complete listing*