stupidgcm: revamp package documentation

Maybe interesting for people following
https://github.com/rfjakob/gocryptfs/issues/452

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))

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

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)

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")