Essentially, your backups are encrypted with a key that is generated using a key derivation function (KDF), which takes in your password and outputs a "hash" th

Essentially, your backups are encrypted with a key that is generated using a key derivation function (KDF), which takes in your password and outputs a "hash" that is fed into the encryption algorithm. It's obviously much faster to brute force SHA256(dictionary_word) than a 128-bit key. Apple was previously using 10,000 rounds of SHA-1 (with PBKDF1) as a KDF. This is much slower than one round of SHA-2 they've switched to by orders of magnitude.