/
bk.go
260 lines (225 loc) · 5.91 KB
/
bk.go
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package tkn
import (
"crypto/subtle"
"fmt"
"io"
pairing "github.com/cloudflare/circl/ecc/bls12381"
"golang.org/x/crypto/blake2b"
)
// This file is based on the techniques in
// https://www.iacr.org/archive/pkc2011/65710074/65710074.pdf that
// apply the Boneh-Katz transform to Attribute based encryption.
// Seed size is chosen based on the proof for BK transform
// (https://eprint.iacr.org/2004/261.pdf - page 12, theorem 2) to maintain the
// statistical hiding property. Their input is 448 bits -> 128 bits,
// whereas we require a seed size of 576 bits to ensure a 2^(-65) statistical difference
// for our output size of 256 bits.
const macKeySeedSize = 72
func blakeEncrypt(key []byte, msg []byte) ([]byte, error) {
xof, err := blake2b.NewXOF(blake2b.OutputLengthUnknown, key)
if err != nil {
return nil, err
}
keystream := make([]byte, len(msg))
_, err = io.ReadFull(xof, keystream)
if err != nil {
return nil, err
}
for i := 0; i < len(msg); i++ {
keystream[i] ^= msg[i]
}
return keystream, nil
}
func blakeDecrypt(key []byte, msg []byte) ([]byte, error) {
return blakeEncrypt(key, msg)
}
func blakeMac(key []byte, msg []byte) (tag []byte, err error) {
mac, err := blake2b.New256(key)
if err != nil {
return nil, err
}
mac.Write(msg)
tag = mac.Sum(nil)
return
}
func expandSeed(seed []byte) (id []byte, macKey []byte, err error) {
h1, err := blake2b.New256(nil)
if err != nil {
return nil, nil, err
}
h1.Write([]byte("id computation hash"))
h2, err := blake2b.New256(nil)
if err != nil {
return nil, nil, err
}
h2.Write([]byte("key computation hash"))
h1.Write(seed)
h2.Write(seed)
id = h1.Sum(nil)
macKey = h2.Sum(nil)
return
}
func DeriveAttributeKeysCCA(rand io.Reader, sp *SecretParams, attrs *Attributes) (*AttributesKey, error) {
realAttrs := transformAttrsBK(attrs)
return deriveAttributeKeys(rand, sp, realAttrs)
}
func EncryptCCA(rand io.Reader, public *PublicParams, policy *Policy, msg []byte) ([]byte, error) {
seed := make([]byte, macKeySeedSize)
_, err := io.ReadFull(rand, seed)
if err != nil {
return nil, err
}
id, macKey, err := expandSeed(seed)
if err != nil {
return nil, err
}
numid := &pairing.Scalar{}
numid.SetBytes(id)
encPolicy := policy.transformBK(numid)
header, encPoint, err := encapsulate(rand, public, encPolicy)
if err != nil {
return nil, err
}
// Send the policy that was not enhanced. The receiver will recover with the ID.
// This avoids a bug where we omit the check that the ID is correct
header.p = policy
C1, err := header.marshalBinary()
if err != nil {
return nil, err
}
env := make([]byte, len(seed)+len(msg))
copy(env[0:len(seed)], seed)
copy(env[len(seed):], msg)
encKey, err := encPoint.MarshalBinary()
if err != nil {
return nil, err
}
hashedEncKey := blake2b.Sum256(encKey)
env, err = blakeEncrypt(hashedEncKey[:], env)
if err != nil {
return nil, err
}
macData := appendLenPrefixed(nil, C1)
macData = appendLenPrefixed(macData, env)
tag, err := blakeMac(macKey, macData)
if err != nil {
return nil, err
}
ret := appendLenPrefixed(nil, id)
ret = appendLenPrefixed(ret, macData)
ret = appendLenPrefixed(ret, tag)
return ret, nil
}
func DecryptCCA(ciphertext []byte, key *AttributesKey) ([]byte, error) {
id, rest, err := removeLenPrefixed(ciphertext)
if err != nil {
return nil, err
}
macData, rest, err := removeLenPrefixed(rest)
if err != nil {
return nil, err
}
tag, _, err := removeLenPrefixed(rest)
if err != nil {
return nil, err
}
C1, envRaw, err := removeLenPrefixed(macData)
if err != nil {
return nil, err
}
env, _, err := removeLenPrefixed(envRaw)
if err != nil {
return nil, err
}
header := &ciphertextHeader{}
err = header.unmarshalBinary(C1)
if err != nil {
return nil, err
}
numid := &pairing.Scalar{}
numid.SetBytes(id)
header.p = header.p.transformBK(numid)
encPoint, err := decapsulate(header, key)
if err != nil {
return nil, fmt.Errorf("error in decryption: %w", err)
}
encKey, err := encPoint.MarshalBinary()
if err != nil {
return nil, err
}
hashedEncKey := blake2b.Sum256(encKey)
// Decrypt the envelope
decEnv, err := blakeDecrypt(hashedEncKey[:], env)
if err != nil {
return nil, err
}
if len(decEnv) < macKeySeedSize {
return nil, fmt.Errorf("envelope too short")
}
seed := decEnv[0:macKeySeedSize]
ptx := make([]byte, len(decEnv)-macKeySeedSize)
compID, macKey, err := expandSeed(seed)
if err != nil {
return nil, err
}
compTag, err := blakeMac(macKey, macData)
if err != nil {
return nil, err
}
// Now check that compTag = tag and compID = id
// We don't want to distinguish which fails.
tagMatch := subtle.ConstantTimeCompare(compTag, tag)
idMatch := subtle.ConstantTimeCompare(compID, id)
check := tagMatch & idMatch
if check == 1 {
copy(ptx, decEnv[macKeySeedSize:])
return ptx, nil
}
return nil, fmt.Errorf("failure of decryption")
}
func CouldDecrypt(ciphertext []byte, a *Attributes) bool {
id, rest, err := removeLenPrefixed(ciphertext)
if err != nil {
return false
}
macData, _, err := removeLenPrefixed(rest)
if err != nil {
return false
}
C1, _, err := removeLenPrefixed(macData)
if err != nil {
return false
}
header := &ciphertextHeader{}
err = header.unmarshalBinary(C1)
if err != nil {
return false
}
numid := &pairing.Scalar{}
numid.SetBytes(id)
header.p = header.p.transformBK(numid)
realAttrs := transformAttrsBK(a)
_, err = header.p.Satisfaction(realAttrs)
return err == nil
}
func (p *Policy) ExtractFromCiphertext(ct []byte) error {
_, rest, err := removeLenPrefixed(ct)
if err != nil {
return fmt.Errorf("invalid ciphertext")
}
macData, _, err := removeLenPrefixed(rest)
if err != nil {
return fmt.Errorf("invalid ciphetext")
}
C1, _, err := removeLenPrefixed(macData)
if err != nil {
return fmt.Errorf("invalid ciphertext")
}
header := &ciphertextHeader{}
err = header.unmarshalBinary(C1)
if err != nil {
return fmt.Errorf("invalid ciphertext")
}
*p = *header.p
return nil
}