// Vault: password-based encryption of secrets using libsodium.
// Uses Argon2id for key derivation and XSalsa20-Poly1305 for encryption.
// All crypto operations are delegated to libsodium — no raw primitives.
//
// Backend: WebAssembly, deliberately (#182).
//
// libsodium ships one file containing both a WebAssembly build and a
// wasm2js ("asm.js") translation of it. It tries WASM first and, if
// instantiation throws, silently swaps in the translation. An extension
// CSP of plain script-src 'self' refuses WASM, so every popup load used
// to take that fallback — announced by nothing but an uncaught
// CompileError in the console.
//
// Measured here, same Argon2id parameters (OPSLIMIT_INTERACTIVE,
// MEMLIMIT_INTERACTIVE = 2 passes over 64MiB), node 22 on this machine:
// WASM 141-198ms per derivation, wasm2js 3204-3660ms. The work factor is
// identical either way — it is set by the ops/mem parameters, not by wall
// time — so the fallback bought no security, it only made every password
// operation take three and a half seconds, and the wallet asks for the
// password on every signature.
//
// So both manifests declare 'wasm-unsafe-eval' for extension pages. That
// keyword permits compiling WebAssembly and nothing else: not eval() of
// strings, not inline script, not remote script. Reaching it requires
// already executing script in the extension page, which is total
// compromise on its own. 'unsafe-eval' would be a different matter and is
// not granted. tests/manifest.test.js pins both policies to exactly
// "'self' 'wasm-unsafe-eval'" so neither the grant nor the surrounding
// strictness can drift unnoticed.
//
// The fallback still exists, and a wallet that refuses to decrypt is
// worse than a slow one, so it is not disabled — it is made loud:
// cryptoBackend() reports which backend this realm can run, ensureReady()
// logs an error if it is not WASM, tests/vaultBackend.test.js asserts the
// unit tests exercise the WASM backend, and the end-to-end suite asserts
// it in the real popup under the real manifest.

const sodium = require("libsodium-wrappers-sumo");
const { log } = require("./log");

// An empty WebAssembly module: the 8-byte magic number and version header,
// no sections. Compiling it asks the cheapest possible form of the only
// question that matters here — may this realm compile WebAssembly at all —
// which is exactly what a CSP without 'wasm-unsafe-eval' refuses, and
// exactly what decides which backend libsodium ends up on.
const EMPTY_WASM_MODULE = new Uint8Array([
    0x00, 0x61, 0x73, 0x6d, 0x01, 0x00, 0x00, 0x00,
]);

// "wasm" or "asmjs": whether this realm may compile WebAssembly, which is
// what decides libsodium's backend when the CSP is the reason it cannot —
// the case this codebase guards. It probes the realm, not libsodium, so a
// fallback taken for some other reason (allocation failure, corrupt module)
// would not be caught here; tests/vaultBackend.test.js checks libsodium's
// own marker directly.
async function cryptoBackend() {
    try {
        await WebAssembly.compile(EMPTY_WASM_MODULE);
        return "wasm";
    } catch (_) {
        return "asmjs";
    }
}

let ready = false;

async function ensureReady() {
    if (!ready) {
        await sodium.ready;
        if ((await cryptoBackend()) !== "wasm") {
            log.errorf(
                "libsodium is running on the wasm2js fallback: this realm " +
                    "refuses to compile WebAssembly, so every password " +
                    "derivation costs roughly 20x what it should. See the " +
                    "backend note in src/shared/vault.js.",
            );
        }
        ready = true;
    }
}

// Returns { salt, nonce, ciphertext } (all base64-encoded strings).
async function encryptWithPassword(plaintext, password) {
    await ensureReady();
    const salt = sodium.randombytes_buf(sodium.crypto_pwhash_SALTBYTES);
    const key = sodium.crypto_pwhash(
        sodium.crypto_secretbox_KEYBYTES,
        password,
        salt,
        sodium.crypto_pwhash_OPSLIMIT_INTERACTIVE,
        sodium.crypto_pwhash_MEMLIMIT_INTERACTIVE,
        sodium.crypto_pwhash_ALG_ARGON2ID13,
    );
    const nonce = sodium.randombytes_buf(sodium.crypto_secretbox_NONCEBYTES);
    const ciphertext = sodium.crypto_secretbox_easy(
        sodium.from_string(plaintext),
        nonce,
        key,
    );
    return {
        salt: sodium.to_base64(salt),
        nonce: sodium.to_base64(nonce),
        ciphertext: sodium.to_base64(ciphertext),
    };
}

// Returns the plaintext string, or throws on wrong password.
async function decryptWithPassword(encrypted, password) {
    await ensureReady();
    const salt = sodium.from_base64(encrypted.salt);
    const nonce = sodium.from_base64(encrypted.nonce);
    const ciphertext = sodium.from_base64(encrypted.ciphertext);
    const key = sodium.crypto_pwhash(
        sodium.crypto_secretbox_KEYBYTES,
        password,
        salt,
        sodium.crypto_pwhash_OPSLIMIT_INTERACTIVE,
        sodium.crypto_pwhash_MEMLIMIT_INTERACTIVE,
        sodium.crypto_pwhash_ALG_ARGON2ID13,
    );
    const plaintext = sodium.crypto_secretbox_open_easy(ciphertext, nonce, key);
    if (!plaintext) {
        throw new Error("Decryption failed — wrong password.");
    }
    return sodium.to_string(plaintext);
}

module.exports = { cryptoBackend, decryptWithPassword, encryptWithPassword };