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ascon.go
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ascon.go
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// Package ascon implements the ASCON AEAD cipher.
//
// References:
//
// [ascon]: https://ascon.iaik.tugraz.at
//
package ascon
import (
"crypto/cipher"
"encoding/binary"
"errors"
"runtime"
"strconv"
"github.com/ericlagergren/subtle"
)
//go:generate go run github.com/ericlagergren/lwcrypto/ascon/internal/cmd/pgen
var errOpen = errors.New("ascon: message authentication failed")
const (
// BlockSize128a is the size in bytes of an ASCON-128a block.
BlockSize128a = 16
// BlockSize128 is the size in bytes of an ASCON-128 block.
BlockSize128 = 8
// KeySize is the size in bytes of ASCON-128 and ASCON-128a
// keys.
KeySize = 16
// NonceSize is the size in bytes of ASCON-128 and ASCON-128a
// nonces.
NonceSize = 16
// TagSize is the size in bytes of ASCON-128 and ASCON-128a
// authenticators.
TagSize = 16
)
type ascon struct {
k0, k1 uint64
iv uint64
}
var _ cipher.AEAD = (*ascon)(nil)
// New128 creates a 128-bit ASCON-128 AEAD.
//
// ASCON-128 provides lower throughput but increased robustness
// against partial or full state recovery compared to ASCON-128a.
//
// Each unique key can encrypt a maximum 2^68 bytes (i.e., 2^64
// plaintext and associated data blocks). Nonces must never be
// reused with the same key. Violating either of these
// constraints compromises the security of the algorithm.
//
// There are no other constraints on the composition of the
// nonce. For example, the nonce can be a counter.
//
// Refer to ASCON's documentation for more information.
func New128(key []byte) (cipher.AEAD, error) {
if len(key) != KeySize {
return nil, errors.New("ascon: bad key length")
}
return &ascon{
k0: binary.BigEndian.Uint64(key[0:8]),
k1: binary.BigEndian.Uint64(key[8:16]),
iv: iv128,
}, nil
}
// New128a creates a 128-bit ASCON-128a AEAD.
//
// ASCON-128a provides higher throughput but reduced robustness
// against partial or full state recovery compared to ASCON-128.
//
// Each unique key can encrypt a maximum 2^68 bytes (i.e., 2^64
// plaintext and associated data blocks). Nonces must never be
// reused with the same key. Violating either of these
// constraints compromises the security of the algorithm.
//
// There are no other constraints on the composition of the
// nonce. For example, the nonce can be a counter.
//
// Refer to ASCON's documentation for more information.
func New128a(key []byte) (cipher.AEAD, error) {
if len(key) != KeySize {
return nil, errors.New("ascon: bad key length")
}
return &ascon{
k0: binary.BigEndian.Uint64(key[0:8]),
k1: binary.BigEndian.Uint64(key[8:16]),
iv: iv128a,
}, nil
}
func (a *ascon) NonceSize() int {
return NonceSize
}
func (a *ascon) Overhead() int {
return TagSize
}
func (a *ascon) Seal(dst, nonce, plaintext, additionalData []byte) []byte {
if len(nonce) != NonceSize {
panic("ascon: incorrect nonce length: " + strconv.Itoa(len(nonce)))
}
// TODO(eric): ciphertext max length?
n0 := binary.BigEndian.Uint64(nonce[0:8])
n1 := binary.BigEndian.Uint64(nonce[8:16])
var s state
s.init(a.iv, a.k0, a.k1, n0, n1)
if a.iv == iv128a {
s.additionalData128a(additionalData)
} else {
s.additionalData128(additionalData)
}
ret, out := subtle.SliceForAppend(dst, len(plaintext)+TagSize)
if subtle.InexactOverlap(out, plaintext) {
panic("ascon: invalid buffer overlap")
}
if a.iv == iv128a {
s.encrypt128a(out[:len(plaintext)], plaintext)
} else {
s.encrypt128(out[:len(plaintext)], plaintext)
}
if a.iv == iv128a {
s.finalize128a(a.k0, a.k1)
} else {
s.finalize128(a.k0, a.k1)
}
s.tag(out[len(out)-TagSize:])
return ret
}
func (a *ascon) Open(dst, nonce, ciphertext, additionalData []byte) ([]byte, error) {
if len(nonce) != NonceSize {
panic("ascon: incorrect nonce length: " + strconv.Itoa(len(nonce)))
}
if len(ciphertext) < TagSize {
return nil, errOpen
}
// TODO(eric): ciphertext max length?
tag := ciphertext[len(ciphertext)-TagSize:]
ciphertext = ciphertext[:len(ciphertext)-TagSize]
n0 := binary.BigEndian.Uint64(nonce[0:8])
n1 := binary.BigEndian.Uint64(nonce[8:16])
var s state
s.init(a.iv, a.k0, a.k1, n0, n1)
if a.iv == iv128a {
s.additionalData128a(additionalData)
} else {
s.additionalData128(additionalData)
}
ret, out := subtle.SliceForAppend(dst, len(ciphertext))
if subtle.InexactOverlap(out, ciphertext) {
panic("ascon: invalid buffer overlap")
}
if a.iv == iv128a {
s.decrypt128a(out, ciphertext)
} else {
s.decrypt128(out, ciphertext)
}
if a.iv == iv128a {
s.finalize128a(a.k0, a.k1)
} else {
s.finalize128(a.k0, a.k1)
}
expectedTag := make([]byte, TagSize)
s.tag(expectedTag)
if subtle.ConstantTimeCompare(expectedTag, tag) != 1 {
for i := range out {
out[i] = 0
}
runtime.KeepAlive(out)
return nil, errOpen
}
return ret, nil
}
const (
iv128 uint64 = 0x80400c0600000000 // Ascon-128
iv128a uint64 = 0x80800c0800000000 // Ascon-128a
)
type state struct {
x0, x1, x2, x3, x4 uint64
}
func (s *state) init(iv, k0, k1, n0, n1 uint64) {
s.x0 = iv
s.x1 = k0
s.x2 = k1
s.x3 = n0
s.x4 = n1
p12(s)
s.x3 ^= k0
s.x4 ^= k1
}
func (s *state) finalize128a(k0, k1 uint64) {
s.x2 ^= k0
s.x3 ^= k1
p12(s)
s.x3 ^= k0
s.x4 ^= k1
}
func (s *state) additionalData128a(ad []byte) {
if len(ad) > 0 {
n := len(ad) &^ (BlockSize128a - 1)
if n > 0 {
additionalData128a(s, ad[:n])
ad = ad[n:]
}
if len(ad) >= 8 {
s.x0 ^= binary.BigEndian.Uint64(ad[0:8])
s.x1 ^= be64n(ad[8:])
s.x1 ^= pad(len(ad) - 8)
} else {
s.x0 ^= be64n(ad)
s.x0 ^= pad(len(ad))
}
p8(s)
}
s.x4 ^= 1
}
func (s *state) encrypt128a(dst, src []byte) {
n := len(src) &^ (BlockSize128a - 1)
if n > 0 {
encryptBlocks128a(s, dst[:n], src[:n])
src = src[n:]
dst = dst[n:]
}
if len(src) >= 8 {
s.x0 ^= binary.BigEndian.Uint64(src[0:8])
s.x1 ^= be64n(src[8:])
s.x1 ^= pad(len(src) - 8)
binary.BigEndian.PutUint64(dst[0:8], s.x0)
put64n(dst[8:], s.x1)
} else {
s.x0 ^= be64n(src)
put64n(dst, s.x0)
s.x0 ^= pad(len(src))
}
}
func (s *state) decrypt128a(dst, src []byte) {
n := len(src) &^ (BlockSize128a - 1)
if n > 0 {
decryptBlocks128a(s, dst[:n], src[:n])
src = src[n:]
dst = dst[n:]
}
if len(src) >= 8 {
c0 := binary.BigEndian.Uint64(src[0:8])
c1 := be64n(src[8:])
binary.BigEndian.PutUint64(dst[0:8], s.x0^c0)
put64n(dst[8:], s.x1^c1)
s.x0 = c0
s.x1 = mask(s.x1, len(src)-8)
s.x1 |= c1
s.x1 ^= pad(len(src) - 8)
} else {
c0 := be64n(src)
put64n(dst, s.x0^c0)
s.x0 = mask(s.x0, len(src))
s.x0 |= c0
s.x0 ^= pad(len(src))
}
}
func (s *state) finalize128(k0, k1 uint64) {
s.x1 ^= k0
s.x2 ^= k1
p12(s)
s.x3 ^= k0
s.x4 ^= k1
}
func (s *state) additionalData128(ad []byte) {
if len(ad) > 0 {
for len(ad) >= BlockSize128 {
s.x0 ^= binary.BigEndian.Uint64(ad[0:8])
p6(s)
ad = ad[BlockSize128:]
}
s.x0 ^= be64n(ad)
s.x0 ^= pad(len(ad))
p6(s)
}
s.x4 ^= 1
}
func (s *state) encrypt128(dst, src []byte) {
for len(src) >= BlockSize128 && len(dst) >= BlockSize128 {
s.x0 ^= binary.BigEndian.Uint64(src[0:8])
binary.BigEndian.PutUint64(dst[0:8], s.x0)
p6(s)
src = src[BlockSize128:]
dst = dst[BlockSize128:]
}
s.x0 ^= be64n(src)
put64n(dst, s.x0)
s.x0 ^= pad(len(src))
}
func (s *state) decrypt128(dst, src []byte) {
for len(src) >= BlockSize128 && len(dst) >= BlockSize128 {
c := binary.BigEndian.Uint64(src[0:8])
binary.BigEndian.PutUint64(dst[0:8], s.x0^c)
s.x0 = c
p6(s)
src = src[BlockSize128:]
dst = dst[BlockSize128:]
}
c := be64n(src)
put64n(dst, s.x0^c)
s.x0 = mask(s.x0, len(src))
s.x0 |= c
s.x0 ^= pad(len(src))
}
func (s *state) tag(dst []byte) {
binary.BigEndian.PutUint64(dst[0:8], s.x3)
binary.BigEndian.PutUint64(dst[8:16], s.x4)
}
func pad(n int) uint64 {
return 0x80 << (56 - 8*n)
}
func be64n(b []byte) uint64 {
var x uint64
for i := len(b) - 1; i >= 0; i-- {
x |= uint64(b[i]) << (56 - i*8)
}
return x
}
func put64n(b []byte, x uint64) {
for i := len(b) - 1; i >= 0; i-- {
b[i] = byte(x >> (56 - 8*i))
}
}
func mask(x uint64, n int) uint64 {
for i := 0; i < n; i++ {
x &^= 255 << (56 - 8*i)
}
return x
}