SAFE: Sealed, Algorithm-Flexible Envelope

SAFE defines an encryption envelope that encrypts a payload once for multiple recipients. Decryption can require multiple credentials in sequence (public keys, passphrases, or other registered methods), so that no single compromise reveals content. The format targets large, writable files: it supports streaming decryption, random-access reads at block granularity, and selective re-encryption of modified blocks without re-keying. Per-block Authenticated Encryption with Associated Data (AEAD) provides confidentiality and integrity and detects reordering, truncation, and extension. SAFE accommodates post-quantum key encapsulation without format changes, provides algorithm agility through IANA registries, supports recipient privacy, and defines application profiles for common deployment scenarios.