/**
 * Derive a key from an ECDH shared secret.
 *
 * @param {Buffer} z  The raw ECDH shared secret.
 * @param {number} keyBitLength  Desired output length in bits (e.g. 128, 256, 512).
 * @param {string} algorithmId  The `enc` value for ECDH-ES direct, or the
 *   `alg` value for ECDH-ES+A*KW (RFC 7518 §4.6.2).
 * @param {Buffer} [apu]  Agreement PartyUInfo (decoded `apu`), default empty.
 * @param {Buffer} [apv]  Agreement PartyVInfo (decoded `apv`), default empty.
 * @returns {Buffer} The derived key, `keyBitLength / 8` bytes.
 */
export function concatKdf(z: Buffer, keyBitLength: number, algorithmId: string, apu?: Buffer, apv?: Buffer): Buffer;