stupidgcm: introduce stupidAEADCommon and use for both chacha & gcm

Nice deduplication and brings the GCM decrypt speed up to par.

internal/speed$ benchstat old new
name                old time/op   new time/op   delta
StupidGCM-4          4.71µs ± 0%   4.66µs ± 0%   -0.99%  (p=0.008 n=5+5)
StupidGCMDecrypt-4   5.77µs ± 1%   4.51µs ± 0%  -21.80%  (p=0.008 n=5+5)

name                old speed     new speed     delta
StupidGCM-4         870MB/s ± 0%  879MB/s ± 0%   +1.01%  (p=0.008 n=5+5)
StupidGCMDecrypt-4  710MB/s ± 1%  908MB/s ± 0%  +27.87%  (p=0.008 n=5+5)

1 files changed, 0 insertions, 111 deletions

diff --git a/internal/stupidgcm/stupidxchacha.go b/internal/stupidgcm/stupidxchacha.go
deleted file mode 100644
index 9f2ac2f..0000000
--- a/internal/stupidgcm/stupidxchacha.go
+++ /dev/null
@@ -1,111 +0,0 @@
-// Copyright 2018 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-//
-// Copied from
-// https://github.com/golang/crypto/blob/32db794688a5a24a23a43f2a984cecd5b3d8da58/chacha20poly1305/xchacha20poly1305.go
-// and adapted for stupidgcm by @rfjakob.
-
-package stupidgcm
-
-import (
-	"crypto/cipher"
-	"errors"
-
-	"golang.org/x/crypto/chacha20"
-	"golang.org/x/crypto/chacha20poly1305"
-)
-
-type stupidXchacha20poly1305 struct {
-	key   [chacha20poly1305.KeySize]byte
-	wiped bool
-}
-
-// NewXchacha20poly1305 returns a XChaCha20-Poly1305 AEAD that uses the given 256-bit key.
-//
-// 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.
-func NewXchacha20poly1305(key []byte) cipher.AEAD {
-	if len(key) != chacha20poly1305.KeySize {
-		panic("bad key length")
-	}
-	ret := new(stupidXchacha20poly1305)
-	copy(ret.key[:], key)
-	return ret
-}
-
-func (*stupidXchacha20poly1305) NonceSize() int {
-	return chacha20poly1305.NonceSizeX
-}
-
-func (*stupidXchacha20poly1305) Overhead() int {
-	return 16
-}
-
-func (x *stupidXchacha20poly1305) Seal(dst, nonce, plaintext, additionalData []byte) []byte {
-	if x.wiped {
-		panic("BUG: tried to use wiped key")
-	}
-	if len(nonce) != chacha20poly1305.NonceSizeX {
-		panic("bad nonce length passed to Seal")
-	}
-
-	// XChaCha20-Poly1305 technically supports a 64-bit counter, so there is no
-	// size limit. However, since we reuse the ChaCha20-Poly1305 implementation,
-	// the second half of the counter is not available. This is unlikely to be
-	// an issue because the cipher.AEAD API requires the entire message to be in
-	// memory, and the counter overflows at 256 GB.
-	if uint64(len(plaintext)) > (1<<38)-64 {
-		panic("plaintext too large")
-	}
-
-	c := new(stupidChacha20poly1305)
-	hKey, _ := chacha20.HChaCha20(x.key[:], nonce[0:16])
-	copy(c.key[:], hKey)
-	defer c.Wipe()
-
-	// The first 4 bytes of the final nonce are unused counter space.
-	cNonce := make([]byte, chacha20poly1305.NonceSize)
-	copy(cNonce[4:12], nonce[16:24])
-
-	return c.Seal(dst, cNonce[:], plaintext, additionalData)
-}
-
-func (x *stupidXchacha20poly1305) Open(dst, nonce, ciphertext, additionalData []byte) ([]byte, error) {
-	if x.wiped {
-		panic("BUG: tried to use wiped key")
-	}
-	if len(nonce) != chacha20poly1305.NonceSizeX {
-		panic("bad nonce length passed to Open")
-	}
-	if len(ciphertext) < 16 {
-		return nil, errors.New("message too short")
-	}
-	if uint64(len(ciphertext)) > (1<<38)-48 {
-		panic("ciphertext too large")
-	}
-
-	c := new(stupidChacha20poly1305)
-	hKey, _ := chacha20.HChaCha20(x.key[:], nonce[0:16])
-	copy(c.key[:], hKey)
-	defer c.Wipe()
-
-	// The first 4 bytes of the final nonce are unused counter space.
-	cNonce := make([]byte, chacha20poly1305.NonceSize)
-	copy(cNonce[4:12], nonce[16:24])
-
-	return c.Open(dst, cNonce[:], ciphertext, additionalData)
-}
-
-// Wipe tries to wipe the key from memory by overwriting it with zeros.
-//
-// This is not bulletproof due to possible GC copies, but
-// still raises the bar for extracting the key.
-func (g *stupidXchacha20poly1305) Wipe() {
-	g.wiped = true
-	for i := range g.key {
-		g.key[i] = 0
-	}
-}