AES (Advanced Encryption Standard) is a block cipher developed by Joan Daemen and Vincent Rijmen. AES is a variant of Rijndael which has a fixed block size of 128 bits, and a key size of 128, 192 or 256 bits. AES has 10 rounds for 128-bit keys, 12 rounds for 192-bit keys, and 14 rounds for 256-bit keys.

GCM (Galois Counter Mode) is a mode of operation for symmetric key cryptographic block ciphers. GCM is ideal for protecting packets of data because it has low latency and a minimum operation overhead.

In 2015, Gueron and Lindell described AES-GCM as one of the most popular authenticated encryption schemes today due to its impressive speed. They presented a new fully nonce misuse-resistant authenticated encryption scheme based on carefully combining the GCM building blocks into the SIV paradigm of Rogaway and Shrimpton.

Firefox Send uses end-to-end encryption and relies on the Web Crypto JavaScript API with the 128-bit AES-GCM algorithm to encrypt files and file metadata locally before sending to the cloud. Request authentication is handled with an HMAC SHA-256 signing key.