/
ccm.go
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/
ccm.go
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// Package ccm implements a CCM, Counter with CBC-MAC
// as per RFC 3610.
//
// See https://tools.ietf.org/html/rfc3610
package ccm
import (
"crypto/cipher"
"crypto/subtle"
"encoding/binary"
"errors"
"fmt"
"math"
)
// ccm represents a Counter with CBC-MAC with a specific key.
type ccm struct {
b cipher.Block
M uint8
L uint8
}
const ccmBlockSize = 16
// CCM is a block cipher in Counter with CBC-MAC mode.
// Providing authenticated encryption with associated data via the cipher.AEAD interface.
type CCM interface {
cipher.AEAD
// MaxLength returns the maxium length of plaintext in calls to Seal.
// The maximum length of ciphertext in calls to Open is MaxLength()+Overhead().
// The maximum length is related to CCM's `L` parameter (15-noncesize) and
// is 1<<(8*L) - 1 (but also limited by the maxium size of an int).
MaxLength() int
}
// NewCCM returns the given 128-bit block cipher wrapped in CCM.
// The tagsize must be an even integer between 4 and 16 inclusive
// and is used as CCM's `M` parameter.
// The noncesize must be an integer between 7 and 13 inclusive,
// 15-noncesize is used as CCM's `L` parameter.
func NewCCM(b cipher.Block, tagsize, noncesize int) (CCM, error) {
if b.BlockSize() != ccmBlockSize {
return nil, errors.New("ccm: NewCCM requires 128-bit block cipher")
}
if tagsize < 4 || tagsize > 16 || tagsize&1 != 0 {
return nil, errors.New("ccm: tagsize must be 4, 6, 8, 10, 12, 14, or 16")
}
lensize := 15 - noncesize
if lensize < 2 || lensize > 8 {
return nil, errors.New("ccm: invalid noncesize")
}
c := &ccm{b: b, M: uint8(tagsize), L: uint8(lensize)}
return c, nil
}
func (c *ccm) NonceSize() int { return 15 - int(c.L) }
func (c *ccm) Overhead() int { return int(c.M) }
func (c *ccm) MaxLength() int { return maxlen(c.L, c.Overhead()) }
func maxlen(L uint8, tagsize int) int {
max := (uint64(1) << (8 * L)) - 1
if m64 := uint64(math.MaxInt64) - uint64(tagsize); L > 8 || max > m64 {
max = m64 // The maximum lentgh on a 64bit arch
}
if max != uint64(int(max)) {
return math.MaxInt32 - tagsize // We have only 32bit int's
}
return int(max)
}
// MaxNonceLength returns the maximum nonce length for a given plaintext length.
// A return value <= 0 indicates that plaintext length is too large for
// any nonce length.
func MaxNonceLength(pdatalen int) int {
const tagsize = 16
for L := 2; L <= 8; L++ {
if maxlen(uint8(L), tagsize) >= pdatalen {
return 15 - L
}
}
return 0
}
func (c *ccm) cbcRound(mac, data []byte) {
for i := 0; i < ccmBlockSize; i++ {
mac[i] ^= data[i]
}
c.b.Encrypt(mac, mac)
}
func (c *ccm) cbcData(mac, data []byte) {
for len(data) >= ccmBlockSize {
c.cbcRound(mac, data[:ccmBlockSize])
data = data[ccmBlockSize:]
}
if len(data) > 0 {
var block [ccmBlockSize]byte
copy(block[:], data)
c.cbcRound(mac, block[:])
}
}
func (c *ccm) tag(nonce, plaintext, adata []byte) ([]byte, error) {
var mac [ccmBlockSize]byte
if len(adata) > 0 {
mac[0] |= 1 << 6
}
mac[0] |= (c.M - 2) << 2
mac[0] |= c.L - 1
if len(nonce) != c.NonceSize() {
return nil, errors.New("ccm: Invalid nonce size")
}
if len(plaintext) > c.MaxLength() {
return nil, errors.New("ccm: plaintext too large")
}
binary.BigEndian.PutUint64(mac[ccmBlockSize-8:], uint64(len(plaintext)))
copy(mac[1:ccmBlockSize-c.L], nonce)
c.b.Encrypt(mac[:], mac[:])
var block [ccmBlockSize]byte
if n := uint64(len(adata)); n > 0 {
// First adata block includes adata length
i := 2
if n <= 0xfeff {
binary.BigEndian.PutUint16(block[:i], uint16(n))
} else {
block[0] = 0xfe
block[1] = 0xff
if n < uint64(1<<32) {
i = 2 + 4
binary.BigEndian.PutUint32(block[2:i], uint32(n))
} else {
i = 2 + 8
binary.BigEndian.PutUint64(block[2:i], uint64(n))
}
}
i = copy(block[i:], adata)
c.cbcRound(mac[:], block[:])
c.cbcData(mac[:], adata[i:])
}
if len(plaintext) > 0 {
c.cbcData(mac[:], plaintext)
}
return mac[:c.M], nil
}
// 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.
// From crypto/cipher/gcm.go
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, authenticates the
// additional data and appends the result to dst, returning the updated
// slice. The nonce must be NonceSize() bytes long and unique for all
// time, for a given key.
// The plaintext must be no longer than MaxLength() bytes long.
//
// The plaintext and dst may alias exactly or not at all.
func (c *ccm) Seal(dst, nonce, plaintext, adata []byte) []byte {
tag, err := c.tag(nonce, plaintext, adata)
if err != nil {
// The cipher.AEAD interface doesn't allow for an error return.
panic(err)
}
var iv, s0 [ccmBlockSize]byte
iv[0] = c.L - 1
copy(iv[1:ccmBlockSize-c.L], nonce)
c.b.Encrypt(s0[:], iv[:])
for i := 0; i < int(c.M); i++ {
tag[i] ^= s0[i]
}
iv[len(iv)-1] |= 1
stream := cipher.NewCTR(c.b, iv[:])
ret, out := sliceForAppend(dst, len(plaintext)+int(c.M))
stream.XORKeyStream(out, plaintext)
copy(out[len(plaintext):], tag)
return ret
}
var errOpen = errors.New("ccm: message authentication failed")
func (c *ccm) Open(dst, nonce, ciphertext, adata []byte) ([]byte, error) {
if len(ciphertext) < int(c.M) {
return nil, errors.New("ccm: ciphertext too short")
}
if len(ciphertext) > c.MaxLength()+c.Overhead() {
return nil, errors.New("ccm: ciphertext too long")
}
var tag = make([]byte, int(c.M), int(c.M))
copy(tag, ciphertext[len(ciphertext)-int(c.M):])
ciphertextWithoutTag := ciphertext[:len(ciphertext)-int(c.M)]
var iv, s0 [ccmBlockSize]byte
iv[0] = c.L - 1
copy(iv[1:ccmBlockSize-c.L], nonce)
c.b.Encrypt(s0[:], iv[:])
for i := 0; i < int(c.M); i++ {
tag[i] ^= s0[i]
}
iv[len(iv)-1] |= 1
stream := cipher.NewCTR(c.b, iv[:])
// Cannot decrypt directly to dst since we're not supposed to
// reveal the plaintext to the caller if authentication fails.
plaintext := make([]byte, len(ciphertextWithoutTag))
stream.XORKeyStream(plaintext, ciphertextWithoutTag)
expectedTag, err := c.tag(nonce, plaintext, adata)
if err != nil {
return nil, err
}
if subtle.ConstantTimeCompare(tag, expectedTag) != 1 {
return nil, errors.New(fmt.Sprintf("ccm: t[%X] != et[%X]", tag, expectedTag))
//return nil, errOpen
}
return append(dst, plaintext...), nil
}