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// Copyright 2015 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.
// +build ignore
// -build amd64
package aes
import (
"crypto/cipher"
"crypto/subtle"
"errors"
)
// The following functions are defined in gcm_amd64.s.
func hasGCMAsm() bool
//go:noescape
func aesEncBlock(dst, src *[16]byte, ks []uint32)
//go:noescape
func gcmAesInit(productTable *[256]byte, ks []uint32)
//go:noescape
func gcmAesData(productTable *[256]byte, data []byte, T *[16]byte)
//go:noescape
func gcmAesEnc(productTable *[256]byte, dst, src []byte, ctr, T *[16]byte, ks []uint32)
//go:noescape
func gcmAesDec(productTable *[256]byte, dst, src []byte, ctr, T *[16]byte, ks []uint32)
//go:noescape
func gcmAesFinish(productTable *[256]byte, tagMask, T *[16]byte, pLen, dLen uint64)
const (
gcmBlockSize = 16
gcmTagSize = 16
gcmStandardNonceSize = 12
)
var errOpen = errors.New("cipher: message authentication failed")
// aesCipherGCM implements crypto/cipher.gcmAble so that crypto/cipher.NewGCM
// will use the optimised implementation in this file when possible. Instances
// of this type only exist when hasGCMAsm returns true.
type aesCipherGCM struct {
aesCipherAsm
}
// Assert that aesCipherGCM implements the gcmAble interface.
var _ gcmAble = (*aesCipherGCM)(nil)
// NewGCM returns the AES cipher wrapped in Galois Counter Mode. This is only
// called by crypto/cipher.NewGCM via the gcmAble interface.
func (c *aesCipherGCM) NewGCM(nonceSize int) (cipher.AEAD, error) {
g := &gcmAsm{ks: c.enc, nonceSize: nonceSize}
gcmAesInit(&g.productTable, g.ks)
return g, nil
}
type gcmAsm struct {
// ks is the key schedule, the length of which depends on the size of
// the AES key.
ks []uint32
// productTable contains pre-computed multiples of the binary-field
// element used in GHASH.
productTable [256]byte
// nonceSize contains the expected size of the nonce, in bytes.
nonceSize int
}
func (g *gcmAsm) NonceSize() int {
return g.nonceSize
}
func (*gcmAsm) Overhead() int {
return gcmTagSize
}
// sliceForAppend takes a slice and a requested number of bytes. It returns a
// slice with the contents of the given slice followed by that many bytes and a
// second slice that aliases into it and contains only the extra bytes. If the
// original slice has sufficient capacity then no allocation is performed.
func sliceForAppend(in []byte, n int) (head, tail []byte) {
if total := len(in) + n; cap(in) >= total {
head = in[:total]
} else {
head = make([]byte, total)
copy(head, in)
}
tail = head[len(in):]
return
}
// Seal encrypts and authenticates plaintext. See the cipher.AEAD interface for
// details.
func (g *gcmAsm) Seal(dst, nonce, plaintext, data []byte) []byte {
if len(nonce) != g.nonceSize {
panic("cipher: incorrect nonce length given to GCM")
}
var counter, tagMask [gcmBlockSize]byte
if len(nonce) == gcmStandardNonceSize {
// Init counter to nonce||1
copy(counter[:], nonce)
counter[gcmBlockSize-1] = 1
} else {
// Otherwise counter = GHASH(nonce)
gcmAesData(&g.productTable, nonce, &counter)
gcmAesFinish(&g.productTable, &tagMask, &counter, uint64(len(nonce)), uint64(0))
}
aesEncBlock(&tagMask, &counter, g.ks)
var tagOut [gcmTagSize]byte
gcmAesData(&g.productTable, data, &tagOut)
ret, out := sliceForAppend(dst, len(plaintext)+gcmTagSize)
if len(plaintext) > 0 {
gcmAesEnc(&g.productTable, out, plaintext, &counter, &tagOut, g.ks)
}
gcmAesFinish(&g.productTable, &tagMask, &tagOut, uint64(len(plaintext)), uint64(len(data)))
copy(out[len(plaintext):], tagOut[:])
return ret
}
// Open authenticates and decrypts ciphertext. See the cipher.AEAD interface
// for details.
func (g *gcmAsm) Open(dst, nonce, ciphertext, data []byte) ([]byte, error) {
if len(nonce) != g.nonceSize {
panic("cipher: incorrect nonce length given to GCM")
}
if len(ciphertext) < gcmTagSize {
return nil, errOpen
}
tag := ciphertext[len(ciphertext)-gcmTagSize:]
ciphertext = ciphertext[:len(ciphertext)-gcmTagSize]
// See GCM spec, section 7.1.
var counter, tagMask [gcmBlockSize]byte
if len(nonce) == gcmStandardNonceSize {
// Init counter to nonce||1
copy(counter[:], nonce)
counter[gcmBlockSize-1] = 1
} else {
// Otherwise counter = GHASH(nonce)
gcmAesData(&g.productTable, nonce, &counter)
gcmAesFinish(&g.productTable, &tagMask, &counter, uint64(len(nonce)), uint64(0))
}
aesEncBlock(&tagMask, &counter, g.ks)
var expectedTag [gcmTagSize]byte
gcmAesData(&g.productTable, data, &expectedTag)
ret, out := sliceForAppend(dst, len(ciphertext))
if len(ciphertext) > 0 {
gcmAesDec(&g.productTable, out, ciphertext, &counter, &expectedTag, g.ks)
}
gcmAesFinish(&g.productTable, &tagMask, &expectedTag, uint64(len(ciphertext)), uint64(len(data)))
if subtle.ConstantTimeCompare(expectedTag[:], tag) != 1 {
for i := range out {
out[i] = 0
}
return nil, errOpen
}
return ret, nil
}
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