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, 111 insertions, 0 deletions

diff --git a/internal/stupidgcm/xchacha.go b/internal/stupidgcm/xchacha.go
new file mode 100644
index 0000000..d8668dc
--- /dev/null
+++ b/internal/stupidgcm/xchacha.go
@@ -0,0 +1,111 @@
+// 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 {
+	// array instead of byte slice like
+	// `struct xchacha20poly1305` in x/crypto/chacha20poly1305
+	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 tagLen
+}
+
+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")
+	}
+
+	hKey, _ := chacha20.HChaCha20(x.key[:], nonce[0:16])
+	c := newChacha20poly1305(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")
+	}
+
+	hKey, _ := chacha20.HChaCha20(x.key[:], nonce[0:16])
+	c := newChacha20poly1305(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
+	}
+}