Re-design of the original gocryptfs code to work as a library.
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// Package stupidgcm wraps OpenSSL to provide a cipher.AEAD interface for
// authenticated encryption algorithms.
//
// The supported algorithms are:
//
// (1) AES-GCM-256 (OpenSSL EVP_aes_256_gcm)
//
// (2) ChaCha20-Poly1305 (OpenSSL EVP_chacha20_poly1305)
//
// (3) XChaCha20-Poly1305 (OpenSSL EVP_chacha20_poly1305 + Go HChaCha20)
//
// The golang.org/x/crypto libraries provides implementations for all algorithms,
// and the test suite verifies that the implementation in this package gives
// the exact same results.
//
// However, OpenSSL has optimized assembly for almost all platforms, which Go
// does not. Example for a 32-bit ARM device (Odroid XU4):
//
// $ gocrypts -speed
// gocryptfs v2.1-68-gedf9d4c.stupidchacha; go-fuse v2.1.1-0.20210825171523-3ab5d95a30ae; 2021-09-04 go1.16.7 linux/arm
// AES-GCM-256-OpenSSL 56.84 MB/s (selected in auto mode)
// AES-GCM-256-Go 16.61 MB/s
// AES-SIV-512-Go 16.49 MB/s
// XChaCha20-Poly1305-Go 39.08 MB/s (use via -xchacha flag)
// XChaCha20-Poly1305-OpenSSL 141.82 MB/s
//
// This package is "stupid" in the sense that it only supports a narrow set of
// key- and iv-lengths, and panics if it does not like what you pass it.
// See the constructor functions for which restrictions apply for each algorithm.
// Also, it is only tested for block lengths up to 5000 bytes, because this is
// what gocryptfs uses.
//
// Corrupt ciphertexts never cause a panic. Instead, ErrAuth is returned on
// decryption.
//
// XChaCha20-Poly1305
//
// The XChaCha20-Poly1305 implementation is more complicated than the others,
// because OpenSSL does not support XChaCha20-Poly1305 directly. Follow
// https://github.com/openssl/openssl/issues/5523 to get notified when it is
// accepted into OpenSSL.
//
// Fortunately, XChaCha20-Poly1305 is just ChaCha20-Poly1305 with some key+iv
// mixing using HChaCha20 in front:
//
// key (32 bytes), iv (24 bytes)
// |
// v
// HChaCha20 (provided by golang.org/x/crypto/chacha20)
// |
// v
// key2 (32 bytes), iv2 (16 bytes)
// |
// v
// ChaCha20-Poly1305 (OpenSSL EVP_chacha20_poly1305)
//
// As HChaCha20 is very fast, XChaCha20-Poly1305 gets almost the same throughput
// as ChaCha20-Poly1305 (for 4kiB blocks).
package stupidgcm