peridot/vendor/github.com/ProtonMail/go-crypto/ocb/ocb.go
2022-07-07 22:13:21 +02:00

318 lines
9.4 KiB
Go

// Copyright (C) 2019 ProtonTech AG
// Package ocb provides an implementation of the OCB (offset codebook) mode of
// operation, as described in RFC-7253 of the IRTF and in Rogaway, Bellare,
// Black and Krovetz - OCB: A BLOCK-CIPHER MODE OF OPERATION FOR EFFICIENT
// AUTHENTICATED ENCRYPTION (2003).
// Security considerations (from RFC-7253): A private key MUST NOT be used to
// encrypt more than 2^48 blocks. Tag length should be at least 12 bytes (a
// brute-force forging adversary succeeds after 2^{tag length} attempts). A
// single key SHOULD NOT be used to decrypt ciphertext with different tag
// lengths. Nonces need not be secret, but MUST NOT be reused.
// This package only supports underlying block ciphers with 128-bit blocks,
// such as AES-{128, 192, 256}, but may be extended to other sizes.
package ocb
import (
"bytes"
"crypto/cipher"
"crypto/subtle"
"errors"
"github.com/ProtonMail/go-crypto/internal/byteutil"
"math/bits"
)
type ocb struct {
block cipher.Block
tagSize int
nonceSize int
mask mask
// Optimized en/decrypt: For each nonce N used to en/decrypt, the 'Ktop'
// internal variable can be reused for en/decrypting with nonces sharing
// all but the last 6 bits with N. The prefix of the first nonce used to
// compute the new Ktop, and the Ktop value itself, are stored in
// reusableKtop. If using incremental nonces, this saves one block cipher
// call every 63 out of 64 OCB encryptions, and stores one nonce and one
// output of the block cipher in memory only.
reusableKtop reusableKtop
}
type mask struct {
// L_*, L_$, (L_i)_{i ∈ N}
lAst []byte
lDol []byte
L [][]byte
}
type reusableKtop struct {
noncePrefix []byte
Ktop []byte
}
const (
defaultTagSize = 16
defaultNonceSize = 15
)
const (
enc = iota
dec
)
func (o *ocb) NonceSize() int {
return o.nonceSize
}
func (o *ocb) Overhead() int {
return o.tagSize
}
// NewOCB returns an OCB instance with the given block cipher and default
// tag and nonce sizes.
func NewOCB(block cipher.Block) (cipher.AEAD, error) {
return NewOCBWithNonceAndTagSize(block, defaultNonceSize, defaultTagSize)
}
// NewOCBWithNonceAndTagSize returns an OCB instance with the given block
// cipher, nonce length, and tag length. Panics on zero nonceSize and
// exceedingly long tag size.
//
// It is recommended to use at least 12 bytes as tag length.
func NewOCBWithNonceAndTagSize(
block cipher.Block, nonceSize, tagSize int) (cipher.AEAD, error) {
if block.BlockSize() != 16 {
return nil, ocbError("Block cipher must have 128-bit blocks")
}
if nonceSize < 1 {
return nil, ocbError("Incorrect nonce length")
}
if nonceSize >= block.BlockSize() {
return nil, ocbError("Nonce length exceeds blocksize - 1")
}
if tagSize > block.BlockSize() {
return nil, ocbError("Custom tag length exceeds blocksize")
}
return &ocb{
block: block,
tagSize: tagSize,
nonceSize: nonceSize,
mask: initializeMaskTable(block),
reusableKtop: reusableKtop{
noncePrefix: nil,
Ktop: nil,
},
}, nil
}
func (o *ocb) Seal(dst, nonce, plaintext, adata []byte) []byte {
if len(nonce) > o.nonceSize {
panic("crypto/ocb: Incorrect nonce length given to OCB")
}
ret, out := byteutil.SliceForAppend(dst, len(plaintext)+o.tagSize)
o.crypt(enc, out, nonce, adata, plaintext)
return ret
}
func (o *ocb) Open(dst, nonce, ciphertext, adata []byte) ([]byte, error) {
if len(nonce) > o.nonceSize {
panic("Nonce too long for this instance")
}
if len(ciphertext) < o.tagSize {
return nil, ocbError("Ciphertext shorter than tag length")
}
sep := len(ciphertext) - o.tagSize
ret, out := byteutil.SliceForAppend(dst, len(ciphertext))
ciphertextData := ciphertext[:sep]
tag := ciphertext[sep:]
o.crypt(dec, out, nonce, adata, ciphertextData)
if subtle.ConstantTimeCompare(ret[sep:], tag) == 1 {
ret = ret[:sep]
return ret, nil
}
for i := range out {
out[i] = 0
}
return nil, ocbError("Tag authentication failed")
}
// On instruction enc (resp. dec), crypt is the encrypt (resp. decrypt)
// function. It returns the resulting plain/ciphertext with the tag appended.
func (o *ocb) crypt(instruction int, Y, nonce, adata, X []byte) []byte {
//
// Consider X as a sequence of 128-bit blocks
//
// Note: For encryption (resp. decryption), X is the plaintext (resp., the
// ciphertext without the tag).
blockSize := o.block.BlockSize()
//
// Nonce-dependent and per-encryption variables
//
// Zero out the last 6 bits of the nonce into truncatedNonce to see if Ktop
// is already computed.
truncatedNonce := make([]byte, len(nonce))
copy(truncatedNonce, nonce)
truncatedNonce[len(truncatedNonce)-1] &= 192
Ktop := make([]byte, blockSize)
if bytes.Equal(truncatedNonce, o.reusableKtop.noncePrefix) {
Ktop = o.reusableKtop.Ktop
} else {
// Nonce = num2str(TAGLEN mod 128, 7) || zeros(120 - bitlen(N)) || 1 || N
paddedNonce := append(make([]byte, blockSize-1-len(nonce)), 1)
paddedNonce = append(paddedNonce, truncatedNonce...)
paddedNonce[0] |= byte(((8 * o.tagSize) % (8 * blockSize)) << 1)
// Last 6 bits of paddedNonce are already zero. Encrypt into Ktop
paddedNonce[blockSize-1] &= 192
Ktop = paddedNonce
o.block.Encrypt(Ktop, Ktop)
o.reusableKtop.noncePrefix = truncatedNonce
o.reusableKtop.Ktop = Ktop
}
// Stretch = Ktop || ((lower half of Ktop) XOR (lower half of Ktop << 8))
xorHalves := make([]byte, blockSize/2)
byteutil.XorBytes(xorHalves, Ktop[:blockSize/2], Ktop[1:1+blockSize/2])
stretch := append(Ktop, xorHalves...)
bottom := int(nonce[len(nonce)-1] & 63)
offset := make([]byte, len(stretch))
byteutil.ShiftNBytesLeft(offset, stretch, bottom)
offset = offset[:blockSize]
//
// Process any whole blocks
//
// Note: For encryption Y is ciphertext || tag, for decryption Y is
// plaintext || tag.
checksum := make([]byte, blockSize)
m := len(X) / blockSize
for i := 0; i < m; i++ {
index := bits.TrailingZeros(uint(i + 1))
if len(o.mask.L)-1 < index {
o.mask.extendTable(index)
}
byteutil.XorBytesMut(offset, o.mask.L[bits.TrailingZeros(uint(i+1))])
blockX := X[i*blockSize : (i+1)*blockSize]
blockY := Y[i*blockSize : (i+1)*blockSize]
byteutil.XorBytes(blockY, blockX, offset)
switch instruction {
case enc:
o.block.Encrypt(blockY, blockY)
byteutil.XorBytesMut(blockY, offset)
byteutil.XorBytesMut(checksum, blockX)
case dec:
o.block.Decrypt(blockY, blockY)
byteutil.XorBytesMut(blockY, offset)
byteutil.XorBytesMut(checksum, blockY)
}
}
//
// Process any final partial block and compute raw tag
//
tag := make([]byte, blockSize)
if len(X)%blockSize != 0 {
byteutil.XorBytesMut(offset, o.mask.lAst)
pad := make([]byte, blockSize)
o.block.Encrypt(pad, offset)
chunkX := X[blockSize*m:]
chunkY := Y[blockSize*m : len(X)]
byteutil.XorBytes(chunkY, chunkX, pad[:len(chunkX)])
// P_* || bit(1) || zeroes(127) - len(P_*)
switch instruction {
case enc:
paddedY := append(chunkX, byte(128))
paddedY = append(paddedY, make([]byte, blockSize-len(chunkX)-1)...)
byteutil.XorBytesMut(checksum, paddedY)
case dec:
paddedX := append(chunkY, byte(128))
paddedX = append(paddedX, make([]byte, blockSize-len(chunkY)-1)...)
byteutil.XorBytesMut(checksum, paddedX)
}
byteutil.XorBytes(tag, checksum, offset)
byteutil.XorBytesMut(tag, o.mask.lDol)
o.block.Encrypt(tag, tag)
byteutil.XorBytesMut(tag, o.hash(adata))
copy(Y[blockSize*m+len(chunkY):], tag[:o.tagSize])
} else {
byteutil.XorBytes(tag, checksum, offset)
byteutil.XorBytesMut(tag, o.mask.lDol)
o.block.Encrypt(tag, tag)
byteutil.XorBytesMut(tag, o.hash(adata))
copy(Y[blockSize*m:], tag[:o.tagSize])
}
return Y
}
// This hash function is used to compute the tag. Per design, on empty input it
// returns a slice of zeros, of the same length as the underlying block cipher
// block size.
func (o *ocb) hash(adata []byte) []byte {
//
// Consider A as a sequence of 128-bit blocks
//
A := make([]byte, len(adata))
copy(A, adata)
blockSize := o.block.BlockSize()
//
// Process any whole blocks
//
sum := make([]byte, blockSize)
offset := make([]byte, blockSize)
m := len(A) / blockSize
for i := 0; i < m; i++ {
chunk := A[blockSize*i : blockSize*(i+1)]
index := bits.TrailingZeros(uint(i + 1))
// If the mask table is too short
if len(o.mask.L)-1 < index {
o.mask.extendTable(index)
}
byteutil.XorBytesMut(offset, o.mask.L[index])
byteutil.XorBytesMut(chunk, offset)
o.block.Encrypt(chunk, chunk)
byteutil.XorBytesMut(sum, chunk)
}
//
// Process any final partial block; compute final hash value
//
if len(A)%blockSize != 0 {
byteutil.XorBytesMut(offset, o.mask.lAst)
// Pad block with 1 || 0 ^ 127 - bitlength(a)
ending := make([]byte, blockSize-len(A)%blockSize)
ending[0] = 0x80
encrypted := append(A[blockSize*m:], ending...)
byteutil.XorBytesMut(encrypted, offset)
o.block.Encrypt(encrypted, encrypted)
byteutil.XorBytesMut(sum, encrypted)
}
return sum
}
func initializeMaskTable(block cipher.Block) mask {
//
// Key-dependent variables
//
lAst := make([]byte, block.BlockSize())
block.Encrypt(lAst, lAst)
lDol := byteutil.GfnDouble(lAst)
L := make([][]byte, 1)
L[0] = byteutil.GfnDouble(lDol)
return mask{
lAst: lAst,
lDol: lDol,
L: L,
}
}
// Extends the L array of mask m up to L[limit], with L[i] = GfnDouble(L[i-1])
func (m *mask) extendTable(limit int) {
for i := len(m.L); i <= limit; i++ {
m.L = append(m.L, byteutil.GfnDouble(m.L[i-1]))
}
}
func ocbError(err string) error {
return errors.New("crypto/ocb: " + err)
}