/*
Most of this code was borrowed with permission from
Cliford Symack - https://github.com/clifordsymack/
*/

/* Import core modules. */
const crypto = require('crypto')
const PrivateKey = require('bitcore-lib-cash').PrivateKey
const PublicKey = require('bitcore-lib-cash').PublicKey

/**
 * AES Encrypt with IV
 */
const _aesEncryptWithIV = function (key, iv, message) {
    let cipher, crypted
    cipher = crypto.createCipheriv('aes-128-cbc', key, iv)
    cipher.setAutoPadding(true)
    crypted = cipher.update(message, 'hex', 'hex')
    crypted += cipher.final('hex')

    return Buffer.from(crypted, 'hex')
}

/**
 * AES Decrypt with IV
 */
const _aesDecryptWithIV = function (key, iv, message) {
    let cipher, crypted
    cipher = crypto.createDecipheriv('aes-128-cbc', key, iv)
    cipher.setAutoPadding(true)
    crypted = cipher.update(message, 'hex', 'hex')
    crypted += cipher.final('hex')

    return Buffer.from(crypted, 'hex')
}

/**
 * Rebuild Keypair
 *
 * TODO: Update `generateKeypair` for param and REMOVE THIS FUNCTION,
 *       as it's never used in the codebase.
 */
const rebuildKeypair = function (somePrivateKey) {
    /* Initialize keypair. */
    const keypair = {
        privateKey: new PrivateKey(somePrivateKey)
    }

    /* Set public key. */
    keypair.publicKey = keypair.privateKey.toPublicKey()

    /* Set public key (hex). */
    keypair.publicKeyHex = keypair.publicKey.toString('hex')

    /* Set private key (hex). */
    keypair.privateKeyHex = keypair.privateKey.toString('hex')

    /* Return keypair. */
    return keypair
}

/**
 * Generate Keypair
 *
 * FIXME: DRY this up, using `rebuildKeypair`.
 */
const generateKeypair = function () {
    /* Initialize keypair. */
    const keypair = {
        privateKey: new PrivateKey()
    }

    /* Set public key. */
    keypair.publicKey = keypair.privateKey.toPublicKey()

    /* Set public key (hex). */
    keypair.publicKeyHex = keypair.publicKey.toString('hex')

    /* Set private key (hex). */
    keypair.privateKeyHex = keypair.privateKey.toString('hex')

    /* Return keypair. */
    return keypair
}

/**
 * Encrypt
 */
const encrypt = function (plaintextMessage, pubkey) {
    /* Initialize public key. */
    const publicKey = PublicKey(pubkey)

    /* Initialize ephemeral (private key). */
    const ephemeral = new PrivateKey()

    /* Set ECDH key. */
    // NOTE: This is our shared secret with the pubkey holder.
    const ecdhKey = PublicKey(
        publicKey.point.mul(ephemeral.toBigNumber())).toBuffer()

    /* Set key. */
    // NOTE: We hash the shared secret as:
    //         1. First 16 bytes is the IV for AES.
    //         2. Second 16 bytes is the key for AES.
    //         3. Final 32 bytes are used for the HMAC.
    const key = crypto.createHash('sha512').update(ecdhKey).digest()

    /* Set ciphertext. */
    const ciphertext = _aesEncryptWithIV(
        key.slice(16, 32),
        key.slice(0, 16),
        Buffer.from(plaintextMessage, 'utf8')
    )

    /* Set encrypted. */
    // NOTE: We send the public key, for our ephemeral private key,
    //       as an unencrypted buffer, as part of the prefix to our
    //       encrypted package. This allows ONLY the `pubkey` holder to
    //       decrypt our (ciphertext) message.
    const encrypted = Buffer.concat([
        Buffer.from('BIE1'),
        ephemeral.publicKey.toBuffer(),
        ciphertext
    ])

    /* Set MAC. */
    const mac = crypto
        .createHmac('sha256', key.slice(32))
        .update(encrypted)
        .digest()

    /* Return encrypted. */
    return Buffer.concat([encrypted, mac]).toString('base64')
}

/**
 * Decrypt
 */
const decrypt = function (encryptedMessage, somePrivateKeyHexString) {
    /* Initialize private key. */
    const privateKey = new PrivateKey(somePrivateKeyHexString)

    /* Initialize encrypted (message). */
    const encrypted = Buffer.from(encryptedMessage, 'base64')

    /* Valiate encrypted (message). */
    if (encrypted.length < 85) {
        throw new Error('invalid ciphertext: length')
    }

    /* Set magic (bytes). */
    const magic = encrypted.slice(0, 4)

    /* Initialize ephemeral public key. */
    let ephemeralPubkey = encrypted.slice(4, 37)

    /* Set ciphertext. */
    const ciphertext = encrypted.slice(37, -32)

    /* Set MAC. */
    const mac = encrypted.slice(-32)

    /* Validate magic (bytes). */
    if (magic.toString() !== 'BIE1') {
        throw new Error('invalid ciphertext: invalid magic bytes')
    }

    try {
        /* Set ephemeral public key. */
        ephemeralPubkey = PublicKey(ephemeralPubkey)
    } catch (error) {
        throw new Error('invalid ciphertext: invalid ephemeral pubkey')
    }

    // ???
    ephemeralPubkey.point.validate()

    /* Set secret multiplier. */
    const secretMultiplier = privateKey.toBigNumber()

    /* Set ECDH key. */
    const ecdhKey = PublicKey(
        ephemeralPubkey.point.mul(secretMultiplier)).toBuffer()

    /* Set key. */
    const key = crypto.createHash('sha512').update(ecdhKey).digest()

    /* Set initialization vector. */
    const iv = key.slice(0, 16)

    /* Set key (E). */
    const keyE = key.slice(16, 32)

    /* Set key (M). */
    const keyM = key.slice(32)

    /* Calculate the "valid" MAC. */
    const validMac = crypto
        .createHmac('sha256', keyM)
        .update(encrypted.slice(0, -32))
        // .update(encrypted.slice(-32)) // THIS SHOULD WORK AS WELL
        .digest('hex')

    /* Validate MAC. */
    if (mac.toString('hex') !== validMac) {
        throw new Error('invalid password')
    }

    /* Return decrypted string. */
    return _aesDecryptWithIV(keyE, iv, ciphertext)
}

/**
 * Hash
 *
 * FIXME: DO WE STILL NEED THIS FUNCTION??
 *        IT'S NOT BEING USED ANYWHERE IN THE CODEBASE
 */
const hash = function (text, algorithm) {
    /* Initialize algorithm. */
    // algorithm = algorithm || 'sha224'
    algorithm = algorithm || 'sha256'

    try {
        return crypto
            .createHash(algorithm)
            .update(Buffer.from(text), 'utf8')
            .digest()
    } catch (error) {
        return null
    }
}

/* Export module. */
module.exports = {
    rebuildKeypair,
    generateKeypair,
    encrypt,
    decrypt,
    hash
}