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stupidgcm: revamp package documentation

Maybe interesting for people following
https://github.com/rfjakob/gocryptfs/issues/452
master
Jakob Unterwurzacher 10 months ago
parent
commit
f47e287c20
  1. 2
      internal/stupidgcm/chacha.go
  2. 59
      internal/stupidgcm/doc.go
  3. 9
      internal/stupidgcm/gcm.go
  4. 5
      internal/stupidgcm/xchacha.go

2
internal/stupidgcm/chacha.go

@ -35,6 +35,8 @@ func init() {
// gocryptfs only uses ChaCha20-Poly1305 as a building block for OpenSSL
// XChaCha20-Poly1305. This function is hot because it gets called once for each
// block by XChaCha20-Poly1305.
//
// Only 32-bytes keys and 12-byte IVs are supported.
func NewChacha20poly1305(key []byte) *stupidChacha20poly1305 {
if len(key) != chacha20poly1305.KeySize {
log.Panicf("Only %d-byte keys are supported, you passed %d bytes", chacha20poly1305.KeySize, len(key))

59
internal/stupidgcm/doc.go

@ -0,0 +1,59 @@
// 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

9
internal/stupidgcm/gcm.go

@ -1,7 +1,5 @@
// +build !without_openssl
// Package stupidgcm is a thin wrapper for OpenSSL's GCM encryption and
// decryption functions. It only support 32-byte keys and 16-bit IVs.
package stupidgcm
// #include <openssl/evp.h>
@ -25,10 +23,9 @@ type stupidGCM struct {
stupidAEADCommon
}
// Verify that we satisfy the interface
var _ cipher.AEAD = &stupidGCM{}
// New returns a new cipher.AEAD implementation..
// New returns a new AES-GCM-256 cipher that satisfies the cipher.AEAD interface.
//
// Only 32-bytes keys and 16-byte IVs are supported.
func New(keyIn []byte, forceDecode bool) cipher.AEAD {
if len(keyIn) != keyLen {
log.Panicf("Only %d-byte keys are supported", keyLen)

5
internal/stupidgcm/xchacha.go

@ -26,12 +26,15 @@ type stupidXchacha20poly1305 struct {
wiped bool
}
// NewXchacha20poly1305 returns a XChaCha20-Poly1305 AEAD that uses the given 256-bit key.
// NewXchacha20poly1305 returns a XChaCha20-Poly1305 cipher that satisfied the
// cipher.AEAD interface.
//
// XChaCha20-Poly1305 is a ChaCha20-Poly1305 variant that takes a longer nonce,
// suitable to be generated randomly without risk of collisions. It should be
// preferred when nonce uniqueness cannot be trivially ensured, or whenever
// nonces are randomly generated.
//
// Only 32-bytes keys and 24-byte IVs are supported.
func NewXchacha20poly1305(key []byte) cipher.AEAD {
if len(key) != chacha20poly1305.KeySize {
log.Panic("bad key length")

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