ECDH

Elliptic Curve Diffie-Hellman (ECDH) key agreement on the secp256k1 curve, followed by HKDF key derivation. Given a private key and a public key, produces a derived AES-256 key that both parties can independently compute. This is the foundation for encrypted communication between a contract and a user.

The output is a derived AES key, not the raw ECDH shared secret. The precompile internally runs ECDH followed by HKDF-SHA256 to produce a key suitable for AES-256-GCM encryption.

Input

Raw bytes in the following layout (65 bytes total):

Offset	Field	Type	Description
[0:32]	private key	32 bytes	secp256k1 private key
[32:65]	public key	33 bytes	Compressed secp256k1 public key

Output

Bytes	Type	Description
derived key	32 bytes	AES key derived via ECDH + HKDF

Use cases

• Key agreement between a contract and a user for encrypting event data

• Establishing shared encryption keys for private communication channels

• Enabling per-recipient encryption of on-chain data

Examples

Built-in helper

function ecdh(sbytes32 secretKey, bytes memory publicKey) view returns (bytes32);

bytes32 aesKey = ecdh(mySecretKey, recipientCompressedPubKey);

Manual usage

function deriveAESKey(
    bytes32 privateKey,
    bytes memory compressedPubKey  // 33 bytes
) internal view returns (bytes32) {
    (bool success, bytes memory result) = address(0x65).staticcall(
        abi.encodePacked(privateKey, compressedPubKey)
    );
    require(success, "ECDH precompile failed");
    return bytes32(result);
}