// SPDX-License-Identifier: MIT
// Copyright (c) 2026 Thibault Ducray
//
// This file is part of MyPwdTool's open-source sync/encryption core — see
// LICENSE-SYNC-CRYPTO.md at the repo root and https://tducray.fr/mypwdtool/open-source/.
// The rest of this application is proprietary and NOT covered by this license.

package fr.tducray.mypwdtool.relay

import javax.crypto.Mac
import javax.crypto.spec.SecretKeySpec

/**
 * Port of `RelayAPIClient.computeChallengeResponse(challenge:platform:clientVersion:)` in
 * `Sync/RelayAPIClient.swift` — the two-level HMAC registration-challenge response (§3.3 of the
 * E2EE Relay API spec). Kept separate from the network client so it's testable without a live
 * server or HTTP mocking; the master key itself is passed in rather than read from a constant
 * here, deliberately — see `ChallengeResponseTest` for why the test vector uses a dummy key
 * rather than the real production one.
 */
object ChallengeResponse {
    /**
     * `daily_key = HMAC-SHA256(master_key, version + ":" + server_date)`
     * `response  = HMAC-SHA256(daily_key, nonce + ":" + version + ":" + platform + ":" + client_version)`
     * Returns the response as base64url without padding.
     */
    fun compute(
        masterKeyB64Url: String,
        nonce: String,
        version: String,
        serverDate: String,
        platform: String,
        clientVersion: String,
    ): String {
        val masterKeyBytes = Base64Url.decode(masterKeyB64Url)
        require(masterKeyBytes != null && masterKeyBytes.isNotEmpty()) { "invalid master key" }

        val dailyMessage = "$version:$serverDate".encodeToByteArray()
        val dailyKeyBytes = hmacSha256(masterKeyBytes, dailyMessage)

        val responseMessage = "$nonce:$version:$platform:$clientVersion".encodeToByteArray()
        val responseBytes = hmacSha256(dailyKeyBytes, responseMessage)

        return Base64Url.encode(responseBytes)
    }

    private fun hmacSha256(key: ByteArray, message: ByteArray): ByteArray {
        val mac = Mac.getInstance("HmacSHA256")
        mac.init(SecretKeySpec(key, "HmacSHA256"))
        return mac.doFinal(message)
    }
}