@xmtp/content-type-remote-attachment/src/encryption/Ciphertext.ts

An XMTP content type to support sending file attachments that are stored off network

import { ciphertext } from "@xmtp/proto";

export const AESKeySize = 32; // bytes
export const KDFSaltSize = 32; // bytes
// AES-GCM defaults from https://developer.mozilla.org/en-US/docs/Web/API/AesGcmParams
export const AESGCMNonceSize = 12; // property iv
export const AESGCMTagLength = 16; // property tagLength

// Ciphertext packages the encrypted ciphertext with the salt and nonce used to produce it.
// salt and nonce are not secret, and should be transmitted/stored along with the encrypted ciphertext.
export default class Ciphertext implements ciphertext.Ciphertext {
  aes256GcmHkdfSha256: ciphertext.Ciphertext_Aes256gcmHkdfsha256 | undefined;

  constructor(obj: ciphertext.Ciphertext) {
    if (!obj.aes256GcmHkdfSha256) {
      throw new Error("invalid ciphertext");
    }
    if (obj.aes256GcmHkdfSha256.payload.length < AESGCMTagLength) {
      throw new Error(
        `invalid ciphertext ciphertext length: ${obj.aes256GcmHkdfSha256.payload.length}`,
      );
    }
    if (obj.aes256GcmHkdfSha256.hkdfSalt.length !== KDFSaltSize) {
      throw new Error(
        `invalid ciphertext salt length: ${obj.aes256GcmHkdfSha256.hkdfSalt.length}`,
      );
    }
    if (obj.aes256GcmHkdfSha256.gcmNonce.length !== AESGCMNonceSize) {
      throw new Error(
        `invalid ciphertext nonce length: ${obj.aes256GcmHkdfSha256.gcmNonce.length}`,
      );
    }
    this.aes256GcmHkdfSha256 = obj.aes256GcmHkdfSha256;
  }

  toBytes(): Uint8Array {
    return ciphertext.Ciphertext.encode(this).finish();
  }

  static fromBytes(bytes: Uint8Array): Ciphertext {
    return new Ciphertext(ciphertext.Ciphertext.decode(bytes));
  }
}