ecdh

On-chain elliptic-curve Diffie-Hellman key exchange

Compute a shared key with the ECDH precompile at 0x65.

Overview

ecdh() and async_ecdh() take a PrivateKey and a CompressedPublicKey and return a Bytes32 shared key.

The on-chain precompile performs ECDH and an HKDF derivation step, so the returned Bytes32 is ready to use as an AES key.

Signature

def ecdh(
    w3: Web3,
    *,
    sk: PrivateKey,
    pk: CompressedPublicKey,
) -> Bytes32

async def async_ecdh(
    w3: AsyncWeb3,
    *,
    sk: PrivateKey,
    pk: CompressedPublicKey,
) -> Bytes32

Parameters

Parameter	Type	Required	Description
w3	Web3 / AsyncWeb3	Yes	Connected Seismic client
sk	PrivateKey	Yes	Local 32-byte private key
pk	CompressedPublicKey	Yes	Peer compressed public key (33 bytes)

Returns

Type	Description
Bytes32	Derived shared key

Examples

Two-Party Key Exchange

import os
from seismic_web3 import PrivateKey, create_public_client
from seismic_web3.crypto.secp import private_key_to_compressed_public_key
from seismic_web3 import precompiles as sp

w3 = create_public_client("https://gcp-1.seismictest.net/rpc")

alice_sk = PrivateKey(os.urandom(32))
bob_sk = PrivateKey(os.urandom(32))

alice_pk = private_key_to_compressed_public_key(alice_sk)
bob_pk = private_key_to_compressed_public_key(bob_sk)

alice_shared = sp.ecdh(w3, sk=alice_sk, pk=bob_pk)
bob_shared = sp.ecdh(w3, sk=bob_sk, pk=alice_pk)

assert alice_shared == bob_shared

Use Shared Key for AES-GCM

from seismic_web3 import precompiles as sp

key = sp.ecdh(w3, sk=alice_sk, pk=bob_pk)
ct = sp.aes_gcm_encrypt(w3, aes_key=key, nonce=1, plaintext=b"secret")
pt = sp.aes_gcm_decrypt(w3, aes_key=key, nonce=1, ciphertext=bytes(ct))
assert bytes(pt) == b"secret"

Async Usage

import os
from seismic_web3 import create_async_public_client
from seismic_web3 import PrivateKey
from seismic_web3.crypto.secp import private_key_to_compressed_public_key
from seismic_web3 import precompiles as sp

async def main():
    w3 = create_async_public_client("https://gcp-1.seismictest.net/rpc")
    alice_sk = PrivateKey(os.urandom(32))
    bob_sk = PrivateKey(os.urandom(32))
    bob_pk = private_key_to_compressed_public_key(bob_sk)
    shared = await sp.async_ecdh(w3, sk=alice_sk, pk=bob_pk)
    print(shared.to_0x_hex())

Gas Cost

Fixed cost: 3120.

Notes

• Input layout is sk(32) || pk(33).

• Invalid keys cause the call to fail.

• Keep private keys out of logs and telemetry.

See Also

• ECDH — Precompiles Reference

• aes-gcm-encrypt

• aes-gcm-decrypt

• hkdf