@kentcdodds/audible

/**
 * Minimal AES-128 CBC decrypt (zero / no padding) for Audible voucher blobs.
 * Pure JS so Cloudflare Workers / WebCrypto PKCS#7 limits do not apply.
 */

const SBOX = new Uint8Array([
	0x63, 0x7c, 0x77, 0x7b, 0xf2, 0x6b, 0x6f, 0xc5, 0x30, 0x01, 0x67, 0x2b, 0xfe,
	0xd7, 0xab, 0x76, 0xca, 0x82, 0xc9, 0x7d, 0xfa, 0x59, 0x47, 0xf0, 0xad, 0xd4,
	0xa2, 0xaf, 0x9c, 0xa4, 0x72, 0xc0, 0xb7, 0xfd, 0x93, 0x26, 0x36, 0x3f, 0xf7,
	0xcc, 0x34, 0xa5, 0xe5, 0xf1, 0x71, 0xd8, 0x31, 0x15, 0x04, 0xc7, 0x23, 0xc3,
	0x18, 0x96, 0x05, 0x9a, 0x07, 0x12, 0x80, 0xe2, 0xeb, 0x27, 0xb2, 0x75, 0x09,
	0x83, 0x2c, 0x1a, 0x1b, 0x6e, 0x5a, 0xa0, 0x52, 0x3b, 0xd6, 0xb3, 0x29, 0xe3,
	0x2f, 0x84, 0x53, 0xd1, 0x00, 0xed, 0x20, 0xfc, 0xb1, 0x5b, 0x6a, 0xcb, 0xbe,
	0x39, 0x4a, 0x4c, 0x58, 0xcf, 0xd0, 0xef, 0xaa, 0xfb, 0x43, 0x4d, 0x33, 0x85,
	0x45, 0xf9, 0x02, 0x7f, 0x50, 0x3c, 0x9f, 0xa8, 0x51, 0xa3, 0x40, 0x8f, 0x92,
	0x9d, 0x38, 0xf5, 0xbc, 0xb6, 0xda, 0x21, 0x10, 0xff, 0xf3, 0xd2, 0xcd, 0x0c,
	0x13, 0xec, 0x5f, 0x97, 0x44, 0x17, 0xc4, 0xa7, 0x7e, 0x3d, 0x64, 0x5d, 0x19,
	0x73, 0x60, 0x81, 0x4f, 0xdc, 0x22, 0x2a, 0x90, 0x88, 0x46, 0xee, 0xb8, 0x14,
	0xde, 0x5e, 0x0b, 0xdb, 0xe0, 0x32, 0x3a, 0x0a, 0x49, 0x06, 0x24, 0x5c, 0xc2,
	0xd3, 0xac, 0x62, 0x91, 0x95, 0xe4, 0x79, 0xe7, 0xc8, 0x37, 0x6d, 0x8d, 0xd5,
	0x4e, 0xa9, 0x6c, 0x56, 0xf4, 0xea, 0x65, 0x7a, 0xae, 0x08, 0xba, 0x78, 0x25,
	0x2e, 0x1c, 0xa6, 0xb4, 0xc6, 0xe8, 0xdd, 0x74, 0x1f, 0x4b, 0xbd, 0x8b, 0x8a,
	0x70, 0x3e, 0xb5, 0x66, 0x48, 0x03, 0xf6, 0x0e, 0x61, 0x35, 0x57, 0xb9, 0x86,
	0xc1, 0x1d, 0x9e, 0xe1, 0xf8, 0x98, 0x11, 0x69, 0xd9, 0x8e, 0x94, 0x9b, 0x1e,
	0x87, 0xe9, 0xce, 0x55, 0x28, 0xdf, 0x8c, 0xa1, 0x89, 0x0d, 0xbf, 0xe6, 0x42,
	0x68, 0x41, 0x99, 0x2d, 0x0f, 0xb0, 0x54, 0xbb, 0x16,
])

const INV_SBOX = new Uint8Array(256)
for (let i = 0; i < 256; i++) INV_SBOX[SBOX[i]!] = i

const RCON = new Uint8Array([
	0x00, 0x01, 0x02, 0x04, 0x08, 0x10, 0x20, 0x40, 0x80, 0x1b, 0x36,
])

function xtime(a: number): number {
	return ((a << 1) ^ ((a & 0x80) !== 0 ? 0x1b : 0)) & 0xff
}

function mul(a: number, b: number): number {
	let result = 0
	let aa = a
	let bb = b
	while (bb) {
		if (bb & 1) result ^= aa
		aa = xtime(aa)
		bb >>= 1
	}
	return result & 0xff
}

function expandKey(key: Uint8Array): Uint8Array {
	const w = new Uint8Array(176)
	w.set(key.subarray(0, 16))
	let bytesGenerated = 16
	let rconIter = 1
	const temp = new Uint8Array(4)
	while (bytesGenerated < 176) {
		temp.set(w.subarray(bytesGenerated - 4, bytesGenerated))
		if (bytesGenerated % 16 === 0) {
			const t = temp[0]!
			temp[0] = SBOX[temp[1]!]! ^ RCON[rconIter++]!
			temp[1] = SBOX[temp[2]!]!
			temp[2] = SBOX[temp[3]!]!
			temp[3] = SBOX[t]!
		}
		for (let i = 0; i < 4; i++) {
			w[bytesGenerated] = w[bytesGenerated - 16]! ^ temp[i]!
			bytesGenerated++
		}
	}
	return w
}

function addRoundKey(state: Uint8Array, roundKey: Uint8Array, round: number) {
	const offset = round * 16
	for (let i = 0; i < 16; i++) state[i]! ^= roundKey[offset + i]!
}

function invSubBytes(state: Uint8Array) {
	for (let i = 0; i < 16; i++) state[i] = INV_SBOX[state[i]!]!
}

function invShiftRows(state: Uint8Array) {
	let t = state[13]!
	state[13] = state[9]!
	state[9] = state[5]!
	state[5] = state[1]!
	state[1] = t
	t = state[2]!
	state[2] = state[10]!
	state[10] = t
	t = state[6]!
	state[6] = state[14]!
	state[14] = t
	t = state[3]!
	state[3] = state[7]!
	state[7] = state[11]!
	state[11] = state[15]!
	state[15] = t
}

function invMixColumns(state: Uint8Array) {
	for (let c = 0; c < 4; c++) {
		const i = c * 4
		const a0 = state[i]!
		const a1 = state[i + 1]!
		const a2 = state[i + 2]!
		const a3 = state[i + 3]!
		state[i] = mul(a0, 0x0e) ^ mul(a1, 0x0b) ^ mul(a2, 0x0d) ^ mul(a3, 0x09)
		state[i + 1] = mul(a0, 0x09) ^ mul(a1, 0x0e) ^ mul(a2, 0x0b) ^ mul(a3, 0x0d)
		state[i + 2] = mul(a0, 0x0d) ^ mul(a1, 0x09) ^ mul(a2, 0x0e) ^ mul(a3, 0x0b)
		state[i + 3] = mul(a0, 0x0b) ^ mul(a1, 0x0d) ^ mul(a2, 0x09) ^ mul(a3, 0x0e)
	}
}

function decryptBlock(roundKey: Uint8Array, block: Uint8Array): Uint8Array {
	const state = block.slice()
	addRoundKey(state, roundKey, 10)
	for (let round = 9; round >= 1; round--) {
		invShiftRows(state)
		invSubBytes(state)
		addRoundKey(state, roundKey, round)
		invMixColumns(state)
	}
	invShiftRows(state)
	invSubBytes(state)
	addRoundKey(state, roundKey, 0)
	return state
}

/** AES-128 CBC decrypt with zero padding stripped. */
export function aes128CbcDecrypt(
	key: Uint8Array,
	iv: Uint8Array,
	ciphertext: Uint8Array,
): Uint8Array {
	if (key.length !== 16) throw new Error('AES-128 key must be 16 bytes.')
	if (iv.length !== 16) throw new Error('IV must be 16 bytes.')
	if (ciphertext.length % 16 !== 0) {
		throw new Error('Ciphertext length must be a multiple of 16.')
	}
	const roundKey = expandKey(key)
	const out = new Uint8Array(ciphertext.length)
	let prev = iv
	for (let offset = 0; offset < ciphertext.length; offset += 16) {
		const block = ciphertext.subarray(offset, offset + 16)
		const decrypted = decryptBlock(roundKey, block)
		for (let i = 0; i < 16; i++) out[offset + i] = decrypted[i]! ^ prev[i]!
		prev = block
	}
	let end = out.length
	while (end > 0 && out[end - 1] === 0) end--
	return out.subarray(0, end)
}