Rocket.Chat/apps/meteor/client/lib/e2ee/prefixed.ts

import { Base64 } from '@rocket.chat/base64';

import type { ICodec } from './codec';

// A 256-byte array always encodes to 344 characters in Base64.
const DECODED_LENGTH = 256;
// ((4 * 256 / 3) + 3) & ~3 = 344
const ENCODED_LENGTH = 344;

/**
 * A codec for strings formatted as "prefix + base64(data)", where:
 * - `prefix` is an arbitrary-length string
 * - `data` is a 256-byte Uint8Array encoded in Base64
 * - the total length of the Base64-encoded data is always 344 characters
 * This is used for encoding/decoding E2EE keys with a key ID prefix.
 */
export const PrefixedBase64: ICodec<string, [prefix: string, data: Uint8Array<ArrayBuffer>]> = {
	decode: (input) => {
		// 1. Validate the input string length
		if (input.length < ENCODED_LENGTH) {
			throw new RangeError('Invalid input length.');
		}

		// 2. Split the string into its two parts
		const prefix = input.slice(0, -ENCODED_LENGTH);
		const base64Data = input.slice(-ENCODED_LENGTH);

		// 3. Decode the Base64 string. atob() decodes to a "binary string".
		const bytes = Base64.decode(base64Data);

		if (bytes.length !== DECODED_LENGTH) {
			// This is a sanity check in case the Base64 string was valid but didn't decode to 256 bytes.
			throw new RangeError('Decoded data length is too short.');
		}

		return [prefix, bytes];
	},
	encode: ([prefix, data]) => {
		// 1. Validate the input data length
		if (data.length !== DECODED_LENGTH) {
			throw new RangeError(`Input data length is ${data.length}, but expected ${DECODED_LENGTH} bytes.`);
		}

		// 2. Convert the byte array (Uint8Array) into a Base64 string
		const base64 = Base64.encode(data);

		if (base64.length !== ENCODED_LENGTH) {
			// This is a sanity check in case something went wrong during encoding.
			throw new RangeError(`Encoded Base64 length is ${base64.length}, but expected ${ENCODED_LENGTH} characters.`);
		}

		// 3. Concatenate the prefix and the Base64 string
		return prefix + base64;
	},
};