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kem.go
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kem.go
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/* SPDX-FileCopyrightText: © 2020-2024 Nadim Kobeissi <[email protected]>
* SPDX-License-Identifier: MIT */
// Package kyberk2so provides a clean implementation of the Kyber IND-CCA2-secure
// key encapsulation mechanism (KEM), whose security is based on the hardness of
// solving the learning-with-errors (LWE) problem over module lattices.
package kyberk2so
import (
"crypto/rand"
"crypto/subtle"
"golang.org/x/crypto/sha3"
)
// KemKeypair512 returns a Kyber-512 private key
// and a corresponding Kyber-512 public key.
// An accompanying error is returned if no sufficient
// randomness could be obtained from the system.
func KemKeypair512() ([Kyber512SKBytes]byte, [Kyber512PKBytes]byte, error) {
const paramsK = 2
var privateKeyFixedLength [Kyber512SKBytes]byte
var publicKeyFixedLength [Kyber512PKBytes]byte
indcpaPrivateKey, indcpaPublicKey, err := indcpaKeypair(paramsK)
if err != nil {
return privateKeyFixedLength, publicKeyFixedLength, err
}
pkh := sha3.Sum256(indcpaPublicKey)
rnd := make([]byte, paramsSymBytes)
_, err = rand.Read(rnd)
if err != nil {
return privateKeyFixedLength, publicKeyFixedLength, err
}
privateKey := append(indcpaPrivateKey, indcpaPublicKey...)
privateKey = append(privateKey, pkh[:]...)
privateKey = append(privateKey, rnd...)
copy(privateKeyFixedLength[:], privateKey)
copy(publicKeyFixedLength[:], indcpaPublicKey)
return privateKeyFixedLength, publicKeyFixedLength, nil
}
// KemKeypair768 returns a Kyber-768 private key
// and a corresponding Kyber-768 public key.
// An accompanying error is returned if no sufficient
// randomness could be obtained from the system.
func KemKeypair768() ([Kyber768SKBytes]byte, [Kyber768PKBytes]byte, error) {
const paramsK int = 3
var privateKeyFixedLength [Kyber768SKBytes]byte
var publicKeyFixedLength [Kyber768PKBytes]byte
indcpaPrivateKey, indcpaPublicKey, err := indcpaKeypair(paramsK)
if err != nil {
return privateKeyFixedLength, publicKeyFixedLength, err
}
pkh := sha3.Sum256(indcpaPublicKey)
rnd := make([]byte, paramsSymBytes)
_, err = rand.Read(rnd)
if err != nil {
return privateKeyFixedLength, publicKeyFixedLength, err
}
privateKey := append(indcpaPrivateKey, indcpaPublicKey...)
privateKey = append(privateKey, pkh[:]...)
privateKey = append(privateKey, rnd...)
copy(privateKeyFixedLength[:], privateKey)
copy(publicKeyFixedLength[:], indcpaPublicKey)
return privateKeyFixedLength, publicKeyFixedLength, nil
}
// KemKeypair1024 returns a Kyber-1024 private key
// and a corresponding Kyber-1024 public key.
// An accompanying error is returned if no sufficient
// randomness could be obtained from the system.
func KemKeypair1024() ([Kyber1024SKBytes]byte, [Kyber1024PKBytes]byte, error) {
const paramsK int = 4
var privateKeyFixedLength [Kyber1024SKBytes]byte
var publicKeyFixedLength [Kyber1024PKBytes]byte
indcpaPrivateKey, indcpaPublicKey, err := indcpaKeypair(paramsK)
if err != nil {
return privateKeyFixedLength, publicKeyFixedLength, err
}
pkh := sha3.Sum256(indcpaPublicKey)
rnd := make([]byte, paramsSymBytes)
_, err = rand.Read(rnd)
if err != nil {
return privateKeyFixedLength, publicKeyFixedLength, err
}
privateKey := append(indcpaPrivateKey, indcpaPublicKey...)
privateKey = append(privateKey, pkh[:]...)
privateKey = append(privateKey, rnd...)
copy(privateKeyFixedLength[:], privateKey)
copy(publicKeyFixedLength[:], indcpaPublicKey)
return privateKeyFixedLength, publicKeyFixedLength, nil
}
// KemEncrypt512 takes a public key (from KemKeypair512) as input
// and returns a ciphertext and a 32-byte shared secret.
// An accompanying error is returned if no sufficient
// randomness could be obtained from the system.
func KemEncrypt512(publicKey [Kyber512PKBytes]byte) (
[Kyber512CTBytes]byte, [KyberSSBytes]byte, error,
) {
const paramsK int = 2
var ciphertextFixedLength [Kyber512CTBytes]byte
var sharedSecretFixedLength [KyberSSBytes]byte
sharedSecret := make([]byte, paramsSymBytes)
buf := make([]byte, 2*paramsSymBytes)
_, err := rand.Read(buf[:paramsSymBytes])
if err != nil {
return ciphertextFixedLength, sharedSecretFixedLength, err
}
buf1 := sha3.Sum256(buf[:paramsSymBytes])
buf2 := sha3.Sum256(publicKey[:])
kr := sha3.Sum512(append(buf1[:], buf2[:]...))
ciphertext, err := indcpaEncrypt(buf1[:], publicKey[:], kr[paramsSymBytes:], paramsK)
krc := sha3.Sum256(ciphertext)
sha3.ShakeSum256(sharedSecret, append(kr[:paramsSymBytes], krc[:]...))
copy(ciphertextFixedLength[:], ciphertext)
copy(sharedSecretFixedLength[:], sharedSecret)
return ciphertextFixedLength, sharedSecretFixedLength, err
}
// KemEncrypt768 takes a public key (from KemKeypair768) as input and
// returns a ciphertext and a 32-byte shared secret.
// An accompanying error is returned if no sufficient
// randomness could be obtained from the system.
func KemEncrypt768(publicKey [Kyber768PKBytes]byte) (
[Kyber768CTBytes]byte, [KyberSSBytes]byte, error,
) {
const paramsK int = 3
var ciphertextFixedLength [Kyber768CTBytes]byte
var sharedSecretFixedLength [KyberSSBytes]byte
sharedSecret := make([]byte, paramsSymBytes)
buf := make([]byte, 2*paramsSymBytes)
_, err := rand.Read(buf[:paramsSymBytes])
if err != nil {
return ciphertextFixedLength, sharedSecretFixedLength, err
}
buf1 := sha3.Sum256(buf[:paramsSymBytes])
buf2 := sha3.Sum256(publicKey[:])
kr := sha3.Sum512(append(buf1[:], buf2[:]...))
ciphertext, err := indcpaEncrypt(buf1[:], publicKey[:], kr[paramsSymBytes:], paramsK)
krc := sha3.Sum256(ciphertext)
sha3.ShakeSum256(sharedSecret, append(kr[:paramsSymBytes], krc[:]...))
copy(ciphertextFixedLength[:], ciphertext)
copy(sharedSecretFixedLength[:], sharedSecret)
return ciphertextFixedLength, sharedSecretFixedLength, err
}
// KemEncrypt1024 takes a public key (from KemKeypair1024) as input
// and returns a ciphertext and a 32-byte shared secret.
// An accompanying error is returned if no sufficient
// randomness could be obtained from the system.
func KemEncrypt1024(publicKey [Kyber1024PKBytes]byte) (
[Kyber1024CTBytes]byte, [KyberSSBytes]byte, error,
) {
const paramsK int = 4
var ciphertextFixedLength [Kyber1024CTBytes]byte
var sharedSecretFixedLength [KyberSSBytes]byte
sharedSecret := make([]byte, paramsSymBytes)
buf := make([]byte, 2*paramsSymBytes)
_, err := rand.Read(buf[:paramsSymBytes])
if err != nil {
return ciphertextFixedLength, sharedSecretFixedLength, err
}
buf1 := sha3.Sum256(buf[:paramsSymBytes])
buf2 := sha3.Sum256(publicKey[:])
kr := sha3.Sum512(append(buf1[:], buf2[:]...))
ciphertext, err := indcpaEncrypt(buf1[:], publicKey[:], kr[paramsSymBytes:], paramsK)
krc := sha3.Sum256(ciphertext)
sha3.ShakeSum256(sharedSecret, append(kr[:paramsSymBytes], krc[:]...))
copy(ciphertextFixedLength[:], ciphertext)
copy(sharedSecretFixedLength[:], sharedSecret)
return ciphertextFixedLength, sharedSecretFixedLength, err
}
// KemDecrypt512 takes a ciphertext (from KeyEncrypt512),
// a private key (from KemKeypair512) and returns a 32-byte shared secret.
// An accompanying error is returned if no sufficient
// randomness could be obtained from the system.
func KemDecrypt512(
ciphertext [Kyber512CTBytes]byte,
privateKey [Kyber512SKBytes]byte,
) ([KyberSSBytes]byte, error) {
const paramsK int = 2
var sharedSecretFixedLength [KyberSSBytes]byte
sharedSecret := make([]byte, KyberSSBytes)
indcpaPrivateKey := privateKey[:paramsIndcpaSecretKeyBytesK512]
pki := paramsIndcpaSecretKeyBytesK512 + paramsIndcpaPublicKeyBytesK512
publicKey := privateKey[paramsIndcpaSecretKeyBytesK512:pki]
buf := indcpaDecrypt(ciphertext[:], indcpaPrivateKey, paramsK)
ski := Kyber512SKBytes - 2*paramsSymBytes
kr := sha3.Sum512(append(buf, privateKey[ski:ski+paramsSymBytes]...))
cmp, err := indcpaEncrypt(buf, publicKey, kr[paramsSymBytes:], paramsK)
fail := byte(subtle.ConstantTimeCompare(ciphertext[:], cmp) - 1)
krh := sha3.Sum256(ciphertext[:])
for i := 0; i < paramsSymBytes; i++ {
skx := privateKey[:Kyber512SKBytes-paramsSymBytes+i]
kr[i] = kr[i] ^ (fail & (kr[i] ^ skx[i]))
}
sha3.ShakeSum256(sharedSecret, append(kr[:paramsSymBytes], krh[:]...))
copy(sharedSecretFixedLength[:], sharedSecret)
return sharedSecretFixedLength, err
}
// KemDecrypt768 takes a ciphertext (from KeyEncrypt768),
// a private key (from KemKeypair768) and returns a 32-byte shared secret.
// An accompanying error is returned if no sufficient
// randomness could be obtained from the system.
func KemDecrypt768(
ciphertext [Kyber768CTBytes]byte,
privateKey [Kyber768SKBytes]byte,
) ([KyberSSBytes]byte, error) {
const paramsK int = 3
var sharedSecretFixedLength [KyberSSBytes]byte
sharedSecret := make([]byte, KyberSSBytes)
indcpaPrivateKey := privateKey[:paramsIndcpaSecretKeyBytesK768]
pki := paramsIndcpaSecretKeyBytesK768 + paramsIndcpaPublicKeyBytesK768
publicKey := privateKey[paramsIndcpaSecretKeyBytesK768:pki]
buf := indcpaDecrypt(ciphertext[:], indcpaPrivateKey, paramsK)
ski := Kyber768SKBytes - 2*paramsSymBytes
kr := sha3.Sum512(append(buf, privateKey[ski:ski+paramsSymBytes]...))
cmp, err := indcpaEncrypt(buf, publicKey, kr[paramsSymBytes:], paramsK)
fail := byte(subtle.ConstantTimeCompare(ciphertext[:], cmp) - 1)
krh := sha3.Sum256(ciphertext[:])
for i := 0; i < paramsSymBytes; i++ {
skx := privateKey[:Kyber768SKBytes-paramsSymBytes+i]
kr[i] = kr[i] ^ (fail & (kr[i] ^ skx[i]))
}
sha3.ShakeSum256(sharedSecret, append(kr[:paramsSymBytes], krh[:]...))
copy(sharedSecretFixedLength[:], sharedSecret)
return sharedSecretFixedLength, err
}
// KemDecrypt1024 takes a ciphertext (from KeyEncrypt1024),
// a private key (from KemKeypair1024) and returns a 32-byte shared secret.
// An accompanying error is returned if no sufficient
// randomness could be obtained from the system.
func KemDecrypt1024(
ciphertext [Kyber1024CTBytes]byte,
privateKey [Kyber1024SKBytes]byte,
) ([KyberSSBytes]byte, error) {
const paramsK int = 4
var sharedSecretFixedLength [KyberSSBytes]byte
sharedSecret := make([]byte, KyberSSBytes)
indcpaPrivateKey := privateKey[:paramsIndcpaSecretKeyBytesK1024]
pki := paramsIndcpaSecretKeyBytesK1024 + paramsIndcpaPublicKeyBytesK1024
publicKey := privateKey[paramsIndcpaSecretKeyBytesK1024:pki]
buf := indcpaDecrypt(ciphertext[:], indcpaPrivateKey, paramsK)
ski := Kyber1024SKBytes - 2*paramsSymBytes
kr := sha3.Sum512(append(buf, privateKey[ski:ski+paramsSymBytes]...))
cmp, err := indcpaEncrypt(buf, publicKey, kr[paramsSymBytes:], paramsK)
fail := byte(subtle.ConstantTimeCompare(ciphertext[:], cmp) - 1)
krh := sha3.Sum256(ciphertext[:])
for i := 0; i < paramsSymBytes; i++ {
skx := privateKey[:Kyber1024SKBytes-paramsSymBytes+i]
kr[i] = kr[i] ^ (fail & (kr[i] ^ skx[i]))
}
sha3.ShakeSum256(sharedSecret, append(kr[:paramsSymBytes], krh[:]...))
copy(sharedSecretFixedLength[:], sharedSecret)
return sharedSecretFixedLength, err
}