Secure ECDH key agreement and derived session key example

secure-ecdh-key-agreement.kt

override fun performEcdhKeyAgreement(serverEcPublicKeyPem: String): CryptoHelper.EcdhInfo {
    // Minimal ECDH example with P-256; expects a PEM-encoded X.509 SubjectPublicKeyInfo
    val pem = serverEcPublicKeyPem
        .replace("-----BEGIN PUBLIC KEY-----", "")
        .replace("-----END PUBLIC KEY-----", "")
        .replace("\n", "")
        .trim()
    val serverPub = Base64.decode(pem, Base64.DEFAULT)
    val kf = KeyFactory.getInstance("EC")
    val pubKey = kf.generatePublic(X509EncodedKeySpec(serverPub))

    val ka = KeyAgreement.getInstance("ECDH")
    val keyPair = java.security.KeyPairGenerator.getInstance("EC").apply {
        initialize(256, random)
    }.genKeyPair()
    ka.init(keyPair.private)
    ka.doPhase(pubKey, true)
    val shared = ka.generateSecret()

    // Derive AES key via PBKDF2 (HKDF would be better; using PBKDF2 here to avoid extra deps)
    val salt = ByteArray(16).also { random.nextBytes(it) }
    val skf = SecretKeyFactory.getInstance("PBKDF2WithHmacSHA256")
    val key = skf.generateSecret(PBEKeySpec(Base64.encodeToString(shared, Base64.NO_WRAP).toCharArray(), salt, 10_000, 256))
    val derived = SecretKeySpec(key.encoded, "AES")
    symmetricKey = derived

    return CryptoHelper.EcdhInfo(
        publicKeyPem = exportPublicKeyPem(keyPair.public.encoded),
        sharedSecretBytes = shared.size,
        derivedKeyBytes = derived.encoded.size
    )
}