The Next-Generation Multi-Factor Key Derivation Function (MFKDF2) is a function that takes multiple inputs and outputs a string of bytes that can be used as a cryptographic key. It serves the same purpose as a password-based key derivation function (PBKDF), but is stronger than password-based key derivation due to its support for multiple authentication factors, including HOTP, TOTP, and hardware tokens like YubiKey. MFKDF also enables self-service account recovery via K-of-N (secret-sharing style) key derivation, eliminating the need for central recovery keys, and supports arbitrarily complex key derivation policies.