libgo/go/crypto/aes/gcm_s390x.go - gofrontend - Git at Google

 // Copyright 2016 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

 package aes

 import (
 	"crypto/cipher"
 	"crypto/subtle"
 	"errors"
 )

 // This file contains two implementations of AES-GCM. The first implementation
 // (gcmAsm) uses the KMCTR instruction to encrypt using AES in counter mode and
 // the KIMD instruction for GHASH. The second implementation (gcmKMA) uses the
 // newer KMA instruction which performs both operations.

 // gcmCount represents a 16-byte big-endian count value.
 type gcmCount [16]byte

 // inc increments the rightmost 32-bits of the count value by 1.
 func (x *gcmCount) inc() {
 	// The compiler should optimize this to a 32-bit addition.
 	n := uint32(x[15]) | uint32(x[14])<<8 | uint32(x[13])<<16 | uint32(x[12])<<24
 	n += 1
 	x[12] = byte(n >> 24)
 	x[13] = byte(n >> 16)
 	x[14] = byte(n >> 8)
 	x[15] = byte(n)
 }

 // gcmLengths writes len0 || len1 as big-endian values to a 16-byte array.
 func gcmLengths(len0, len1 uint64) [16]byte {
 	return [16]byte{
 		byte(len0 >> 56),
 		byte(len0 >> 48),
 		byte(len0 >> 40),
 		byte(len0 >> 32),
 		byte(len0 >> 24),
 		byte(len0 >> 16),
 		byte(len0 >> 8),
 		byte(len0),
 		byte(len1 >> 56),
 		byte(len1 >> 48),
 		byte(len1 >> 40),
 		byte(len1 >> 32),
 		byte(len1 >> 24),
 		byte(len1 >> 16),
 		byte(len1 >> 8),
 		byte(len1),
 	}
 }

 // gcmHashKey represents the 16-byte hash key required by the GHASH algorithm.
 type gcmHashKey [16]byte

 type gcmAsm struct {
 	block     *aesCipherAsm
 	hashKey   gcmHashKey
 	nonceSize int
 }

 const (
 	gcmBlockSize         = 16
 	gcmTagSize           = 16
 	gcmStandardNonceSize = 12
 )

 var errOpen = errors.New("cipher: message authentication failed")

 // Assert that aesCipherAsm implements the gcmAble interface.
 var _ gcmAble = (*aesCipherAsm)(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 *aesCipherAsm) NewGCM(nonceSize int) (cipher.AEAD, error) {
 	var hk gcmHashKey
 	c.Encrypt(hk[:], hk[:])
 	g := gcmAsm{
 		block:     c,
 		hashKey:   hk,
 		nonceSize: nonceSize,
 	}
 	if hasKMA {
 		g := gcmKMA{g}
 		return &g, nil
 	}
 	return &g, nil
 }

 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
 }

 // ghash uses the GHASH algorithm to hash data with the given key. The initial
 // hash value is given by hash which will be updated with the new hash value.
 // The length of data must be a multiple of 16-bytes.
 //go:noescape
 func ghash(key *gcmHashKey, hash *[16]byte, data []byte)

 // paddedGHASH pads data with zeroes until its length is a multiple of
 // 16-bytes. It then calculates a new value for hash using the GHASH algorithm.
 func (g *gcmAsm) paddedGHASH(hash *[16]byte, data []byte) {
 	siz := len(data) &^ 0xf // align size to 16-bytes
 	if siz > 0 {
 		ghash(&g.hashKey, hash, data[:siz])
 		data = data[siz:]
 	}
 	if len(data) > 0 {
 		var s [16]byte
 		copy(s[:], data)
 		ghash(&g.hashKey, hash, s[:])
 	}
 }

 // cryptBlocksGCM encrypts src using AES in counter mode using the given
 // function code and key. The rightmost 32-bits of the counter are incremented
 // between each block as required by the GCM spec. The initial counter value
 // is given by cnt, which is updated with the value of the next counter value
 // to use.
 //
 // The lengths of both dst and buf must be greater than or equal to the length
 // of src. buf may be partially or completely overwritten during the execution
 // of the function.
 //go:noescape
 func cryptBlocksGCM(fn code, key, dst, src, buf []byte, cnt *gcmCount)

 // counterCrypt encrypts src using AES in counter mode and places the result
 // into dst. cnt is the initial count value and will be updated with the next
 // count value. The length of dst must be greater than or equal to the length
 // of src.
 func (g *gcmAsm) counterCrypt(dst, src []byte, cnt *gcmCount) {
 	// Copying src into a buffer improves performance on some models when
 	// src and dst point to the same underlying array. We also need a
 	// buffer for counter values.
 	var ctrbuf, srcbuf [2048]byte
 	for len(src) >= 16 {
 		siz := len(src)
 		if len(src) > len(ctrbuf) {
 			siz = len(ctrbuf)
 		}
 		siz &^= 0xf // align siz to 16-bytes
 		copy(srcbuf[:], src[:siz])
 		cryptBlocksGCM(g.block.function, g.block.key, dst[:siz], srcbuf[:siz], ctrbuf[:], cnt)
 		src = src[siz:]
 		dst = dst[siz:]
 	}
 	if len(src) > 0 {
 		var x [16]byte
 		g.block.Encrypt(x[:], cnt[:])
 		for i := range src {
 			dst[i] = src[i] ^ x[i]
 		}
 		cnt.inc()
 	}
 }

 // deriveCounter computes the initial GCM counter state from the given nonce.
 // See NIST SP 800-38D, section 7.1.
 func (g *gcmAsm) deriveCounter(nonce []byte) gcmCount {
 	// GCM has two modes of operation with respect to the initial counter
 	// state: a "fast path" for 96-bit (12-byte) nonces, and a "slow path"
 	// for nonces of other lengths. For a 96-bit nonce, the nonce, along
 	// with a four-byte big-endian counter starting at one, is used
 	// directly as the starting counter. For other nonce sizes, the counter
 	// is computed by passing it through the GHASH function.
 	var counter gcmCount
 	if len(nonce) == gcmStandardNonceSize {
 		copy(counter[:], nonce)
 		counter[gcmBlockSize-1] = 1
 	} else {
 		var hash [16]byte
 		g.paddedGHASH(&hash, nonce)
 		lens := gcmLengths(0, uint64(len(nonce))*8)
 		g.paddedGHASH(&hash, lens[:])
 		copy(counter[:], hash[:])
 	}
 	return counter
 }

 // auth calculates GHASH(ciphertext, additionalData), masks the result with
 // tagMask and writes the result to out.
 func (g *gcmAsm) auth(out, ciphertext, additionalData []byte, tagMask *[gcmTagSize]byte) {
 	var hash [16]byte
 	g.paddedGHASH(&hash, additionalData)
 	g.paddedGHASH(&hash, ciphertext)
 	lens := gcmLengths(uint64(len(additionalData))*8, uint64(len(ciphertext))*8)
 	g.paddedGHASH(&hash, lens[:])

 	copy(out, hash[:])
 	for i := range out {
 		out[i] ^= tagMask[i]
 	}
 }

 // 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")
 	}
 	if uint64(len(plaintext)) > ((1<<32)-2)*BlockSize {
 		panic("cipher: message too large for GCM")
 	}

 	ret, out := sliceForAppend(dst, len(plaintext)+gcmTagSize)

 	counter := g.deriveCounter(nonce)

 	var tagMask [gcmBlockSize]byte
 	g.block.Encrypt(tagMask[:], counter[:])
 	counter.inc()

 	g.counterCrypt(out, plaintext, &counter)
 	g.auth(out[len(plaintext):], out[:len(plaintext)], data, &tagMask)

 	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
 	}
 	if uint64(len(ciphertext)) > ((1<<32)-2)*BlockSize+gcmTagSize {
 		return nil, errOpen
 	}

 	tag := ciphertext[len(ciphertext)-gcmTagSize:]
 	ciphertext = ciphertext[:len(ciphertext)-gcmTagSize]

 	counter := g.deriveCounter(nonce)

 	var tagMask [gcmBlockSize]byte
 	g.block.Encrypt(tagMask[:], counter[:])
 	counter.inc()

 	var expectedTag [gcmTagSize]byte
 	g.auth(expectedTag[:], ciphertext, data, &tagMask)

 	ret, out := sliceForAppend(dst, len(ciphertext))

 	if subtle.ConstantTimeCompare(expectedTag[:], tag) != 1 {
 		// The AESNI code decrypts and authenticates concurrently, and
 		// so overwrites dst in the event of a tag mismatch. That
 		// behavior is mimicked here in order to be consistent across
 		// platforms.
 		for i := range out {
 			out[i] = 0
 		}
 		return nil, errOpen
 	}

 	g.counterCrypt(out, ciphertext, &counter)
 	return ret, nil
 }

 // supportsKMA reports whether the message-security-assist 8 facility is available.
 // This function call may be expensive so hasKMA should be queried instead.
 func supportsKMA() bool

 // hasKMA contains the result of supportsKMA.
 var hasKMA = supportsKMA()

 // gcmKMA implements the cipher.AEAD interface using the KMA instruction. It should
 // only be used if hasKMA is true.
 type gcmKMA struct {
 	gcmAsm
 }

 // flags for the KMA instruction
 const (
 	kmaHS      = 1 << 10 // hash subkey supplied
 	kmaLAAD    = 1 << 9  // last series of additional authenticated data
 	kmaLPC     = 1 << 8  // last series of plaintext or ciphertext blocks
 	kmaDecrypt = 1 << 7  // decrypt
 )

 // kmaGCM executes the encryption or decryption operation given by fn. The tag
 // will be calculated and written to tag. cnt should contain the current
 // counter state and will be overwritten with the updated counter state.
 // TODO(mundaym): could pass in hash subkey
 //go:noescape
 func kmaGCM(fn code, key, dst, src, aad []byte, tag *[16]byte, cnt *gcmCount)

 // Seal encrypts and authenticates plaintext. See the cipher.AEAD interface for
 // details.
 func (g *gcmKMA) Seal(dst, nonce, plaintext, data []byte) []byte {
 	if len(nonce) != g.nonceSize {
 		panic("cipher: incorrect nonce length given to GCM")
 	}
 	if uint64(len(plaintext)) > ((1<<32)-2)*BlockSize {
 		panic("cipher: message too large for GCM")
 	}

 	ret, out := sliceForAppend(dst, len(plaintext)+gcmTagSize)

 	counter := g.deriveCounter(nonce)
 	fc := g.block.function | kmaLAAD | kmaLPC

 	var tag [gcmTagSize]byte
 	kmaGCM(fc, g.block.key, out[:len(plaintext)], plaintext, data, &tag, &counter)
 	copy(out[len(plaintext):], tag[:])

 	return ret
 }

 // Open authenticates and decrypts ciphertext. See the cipher.AEAD interface
 // for details.
 func (g *gcmKMA) 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
 	}
 	if uint64(len(ciphertext)) > ((1<<32)-2)*BlockSize+gcmTagSize {
 		return nil, errOpen
 	}

 	tag := ciphertext[len(ciphertext)-gcmTagSize:]
 	ciphertext = ciphertext[:len(ciphertext)-gcmTagSize]
 	ret, out := sliceForAppend(dst, len(ciphertext))

 	counter := g.deriveCounter(nonce)
 	fc := g.block.function | kmaLAAD | kmaLPC | kmaDecrypt

 	var expectedTag [gcmTagSize]byte
 	kmaGCM(fc, g.block.key, out[:len(ciphertext)], ciphertext, data, &expectedTag, &counter)

 	if subtle.ConstantTimeCompare(expectedTag[:], tag) != 1 {
 		// The AESNI code decrypts and authenticates concurrently, and
 		// so overwrites dst in the event of a tag mismatch. That
 		// behavior is mimicked here in order to be consistent across
 		// platforms.
 		for i := range out {
 			out[i] = 0
 		}
 		return nil, errOpen
 	}

 	return ret, nil
 }
	// Copyright 2016 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

	package aes

	import (
	"crypto/cipher"
	"crypto/subtle"
	"errors"
	)

	// This file contains two implementations of AES-GCM. The first implementation
	// (gcmAsm) uses the KMCTR instruction to encrypt using AES in counter mode and
	// the KIMD instruction for GHASH. The second implementation (gcmKMA) uses the
	// newer KMA instruction which performs both operations.

	// gcmCount represents a 16-byte big-endian count value.
	type gcmCount [16]byte

	// inc increments the rightmost 32-bits of the count value by 1.
	func (x *gcmCount) inc() {
	// The compiler should optimize this to a 32-bit addition.
	n := uint32(x[15]) \| uint32(x[14])<<8 \| uint32(x[13])<<16 \| uint32(x[12])<<24
	n += 1
	x[12] = byte(n >> 24)
	x[13] = byte(n >> 16)
	x[14] = byte(n >> 8)
	x[15] = byte(n)
	}

	// gcmLengths writes len0 \|\| len1 as big-endian values to a 16-byte array.
	func gcmLengths(len0, len1 uint64) [16]byte {
	return [16]byte{
	byte(len0 >> 56),
	byte(len0 >> 48),
	byte(len0 >> 40),
	byte(len0 >> 32),
	byte(len0 >> 24),
	byte(len0 >> 16),
	byte(len0 >> 8),
	byte(len0),
	byte(len1 >> 56),
	byte(len1 >> 48),
	byte(len1 >> 40),
	byte(len1 >> 32),
	byte(len1 >> 24),
	byte(len1 >> 16),
	byte(len1 >> 8),
	byte(len1),
	}
	}

	// gcmHashKey represents the 16-byte hash key required by the GHASH algorithm.
	type gcmHashKey [16]byte

	type gcmAsm struct {
	block *aesCipherAsm
	hashKey gcmHashKey
	nonceSize int
	}

	const (
	gcmBlockSize = 16
	gcmTagSize = 16
	gcmStandardNonceSize = 12
	)

	var errOpen = errors.New("cipher: message authentication failed")

	// Assert that aesCipherAsm implements the gcmAble interface.
	var _ gcmAble = (*aesCipherAsm)(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 *aesCipherAsm) NewGCM(nonceSize int) (cipher.AEAD, error) {
	var hk gcmHashKey
	c.Encrypt(hk[:], hk[:])
	g := gcmAsm{
	block: c,
	hashKey: hk,
	nonceSize: nonceSize,
	}
	if hasKMA {
	g := gcmKMA{g}
	return &g, nil
	}
	return &g, nil
	}

	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
	}

	// ghash uses the GHASH algorithm to hash data with the given key. The initial
	// hash value is given by hash which will be updated with the new hash value.
	// The length of data must be a multiple of 16-bytes.
	//go:noescape
	func ghash(key gcmHashKey, hash [16]byte, data []byte)

	// paddedGHASH pads data with zeroes until its length is a multiple of
	// 16-bytes. It then calculates a new value for hash using the GHASH algorithm.
	func (g gcmAsm) paddedGHASH(hash [16]byte, data []byte) {
	siz := len(data) &^ 0xf // align size to 16-bytes
	if siz > 0 {
	ghash(&g.hashKey, hash, data[:siz])
	data = data[siz:]
	}
	if len(data) > 0 {
	var s [16]byte
	copy(s[:], data)
	ghash(&g.hashKey, hash, s[:])
	}
	}

	// cryptBlocksGCM encrypts src using AES in counter mode using the given
	// function code and key. The rightmost 32-bits of the counter are incremented
	// between each block as required by the GCM spec. The initial counter value
	// is given by cnt, which is updated with the value of the next counter value
	// to use.
	//
	// The lengths of both dst and buf must be greater than or equal to the length
	// of src. buf may be partially or completely overwritten during the execution
	// of the function.
	//go:noescape
	func cryptBlocksGCM(fn code, key, dst, src, buf []byte, cnt *gcmCount)

	// counterCrypt encrypts src using AES in counter mode and places the result
	// into dst. cnt is the initial count value and will be updated with the next
	// count value. The length of dst must be greater than or equal to the length
	// of src.
	func (g gcmAsm) counterCrypt(dst, src []byte, cnt gcmCount) {
	// Copying src into a buffer improves performance on some models when
	// src and dst point to the same underlying array. We also need a
	// buffer for counter values.
	var ctrbuf, srcbuf [2048]byte
	for len(src) >= 16 {
	siz := len(src)
	if len(src) > len(ctrbuf) {
	siz = len(ctrbuf)
	}
	siz &^= 0xf // align siz to 16-bytes
	copy(srcbuf[:], src[:siz])
	cryptBlocksGCM(g.block.function, g.block.key, dst[:siz], srcbuf[:siz], ctrbuf[:], cnt)
	src = src[siz:]
	dst = dst[siz:]
	}
	if len(src) > 0 {
	var x [16]byte
	g.block.Encrypt(x[:], cnt[:])
	for i := range src {
	dst[i] = src[i] ^ x[i]
	}
	cnt.inc()
	}
	}

	// deriveCounter computes the initial GCM counter state from the given nonce.
	// See NIST SP 800-38D, section 7.1.
	func (g *gcmAsm) deriveCounter(nonce []byte) gcmCount {
	// GCM has two modes of operation with respect to the initial counter
	// state: a "fast path" for 96-bit (12-byte) nonces, and a "slow path"
	// for nonces of other lengths. For a 96-bit nonce, the nonce, along
	// with a four-byte big-endian counter starting at one, is used
	// directly as the starting counter. For other nonce sizes, the counter
	// is computed by passing it through the GHASH function.
	var counter gcmCount
	if len(nonce) == gcmStandardNonceSize {
	copy(counter[:], nonce)
	counter[gcmBlockSize-1] = 1
	} else {
	var hash [16]byte
	g.paddedGHASH(&hash, nonce)
	lens := gcmLengths(0, uint64(len(nonce))*8)
	g.paddedGHASH(&hash, lens[:])
	copy(counter[:], hash[:])
	}
	return counter
	}

	// auth calculates GHASH(ciphertext, additionalData), masks the result with
	// tagMask and writes the result to out.
	func (g gcmAsm) auth(out, ciphertext, additionalData []byte, tagMask [gcmTagSize]byte) {
	var hash [16]byte
	g.paddedGHASH(&hash, additionalData)
	g.paddedGHASH(&hash, ciphertext)
	lens := gcmLengths(uint64(len(additionalData))8, uint64(len(ciphertext))8)
	g.paddedGHASH(&hash, lens[:])

	copy(out, hash[:])
	for i := range out {
	out[i] ^= tagMask[i]
	}
	}

	// 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")
	}
	if uint64(len(plaintext)) > ((1<<32)-2)*BlockSize {
	panic("cipher: message too large for GCM")
	}

	ret, out := sliceForAppend(dst, len(plaintext)+gcmTagSize)

	counter := g.deriveCounter(nonce)

	var tagMask [gcmBlockSize]byte
	g.block.Encrypt(tagMask[:], counter[:])
	counter.inc()

	g.counterCrypt(out, plaintext, &counter)
	g.auth(out[len(plaintext):], out[:len(plaintext)], data, &tagMask)

	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
	}
	if uint64(len(ciphertext)) > ((1<<32)-2)*BlockSize+gcmTagSize {
	return nil, errOpen
	}

	tag := ciphertext[len(ciphertext)-gcmTagSize:]
	ciphertext = ciphertext[:len(ciphertext)-gcmTagSize]

	counter := g.deriveCounter(nonce)

	var tagMask [gcmBlockSize]byte
	g.block.Encrypt(tagMask[:], counter[:])
	counter.inc()

	var expectedTag [gcmTagSize]byte
	g.auth(expectedTag[:], ciphertext, data, &tagMask)

	ret, out := sliceForAppend(dst, len(ciphertext))

	if subtle.ConstantTimeCompare(expectedTag[:], tag) != 1 {
	// The AESNI code decrypts and authenticates concurrently, and
	// so overwrites dst in the event of a tag mismatch. That
	// behavior is mimicked here in order to be consistent across
	// platforms.
	for i := range out {
	out[i] = 0
	}
	return nil, errOpen
	}

	g.counterCrypt(out, ciphertext, &counter)
	return ret, nil
	}

	// supportsKMA reports whether the message-security-assist 8 facility is available.
	// This function call may be expensive so hasKMA should be queried instead.
	func supportsKMA() bool

	// hasKMA contains the result of supportsKMA.
	var hasKMA = supportsKMA()

	// gcmKMA implements the cipher.AEAD interface using the KMA instruction. It should
	// only be used if hasKMA is true.
	type gcmKMA struct {
	gcmAsm
	}

	// flags for the KMA instruction
	const (
	kmaHS = 1 << 10 // hash subkey supplied
	kmaLAAD = 1 << 9 // last series of additional authenticated data
	kmaLPC = 1 << 8 // last series of plaintext or ciphertext blocks
	kmaDecrypt = 1 << 7 // decrypt
	)

	// kmaGCM executes the encryption or decryption operation given by fn. The tag
	// will be calculated and written to tag. cnt should contain the current
	// counter state and will be overwritten with the updated counter state.
	// TODO(mundaym): could pass in hash subkey
	//go:noescape
	func kmaGCM(fn code, key, dst, src, aad []byte, tag [16]byte, cnt gcmCount)

	// Seal encrypts and authenticates plaintext. See the cipher.AEAD interface for
	// details.
	func (g *gcmKMA) Seal(dst, nonce, plaintext, data []byte) []byte {
	if len(nonce) != g.nonceSize {
	panic("cipher: incorrect nonce length given to GCM")
	}
	if uint64(len(plaintext)) > ((1<<32)-2)*BlockSize {
	panic("cipher: message too large for GCM")
	}

	ret, out := sliceForAppend(dst, len(plaintext)+gcmTagSize)

	counter := g.deriveCounter(nonce)
	fc := g.block.function \| kmaLAAD \| kmaLPC

	var tag [gcmTagSize]byte
	kmaGCM(fc, g.block.key, out[:len(plaintext)], plaintext, data, &tag, &counter)
	copy(out[len(plaintext):], tag[:])

	return ret
	}

	// Open authenticates and decrypts ciphertext. See the cipher.AEAD interface
	// for details.
	func (g *gcmKMA) 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
	}
	if uint64(len(ciphertext)) > ((1<<32)-2)*BlockSize+gcmTagSize {
	return nil, errOpen
	}

	tag := ciphertext[len(ciphertext)-gcmTagSize:]
	ciphertext = ciphertext[:len(ciphertext)-gcmTagSize]
	ret, out := sliceForAppend(dst, len(ciphertext))

	counter := g.deriveCounter(nonce)
	fc := g.block.function \| kmaLAAD \| kmaLPC \| kmaDecrypt

	var expectedTag [gcmTagSize]byte
	kmaGCM(fc, g.block.key, out[:len(ciphertext)], ciphertext, data, &expectedTag, &counter)

	if subtle.ConstantTimeCompare(expectedTag[:], tag) != 1 {
	// The AESNI code decrypts and authenticates concurrently, and
	// so overwrites dst in the event of a tag mismatch. That
	// behavior is mimicked here in order to be consistent across
	// platforms.
	for i := range out {
	out[i] = 0
	}
	return nil, errOpen
	}

	return ret, nil
	}