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/*
* crypto.go - Cryptographic algorithms used by the rest of fscrypt.
*
* Copyright 2017 Google Inc.
* Author: Joe Richey (joerichey@google.com)
*
* Licensed under the Apache License, Version 2.0 (the "License"); you may not
* use this file except in compliance with the License. You may obtain a copy of
* the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
* WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
* License for the specific language governing permissions and limitations under
* the License.
*/
// Package crypto manages all the cryptography for fscrypt. This includes:
// - Key management (key.go)
// - Securely holding keys in memory
// - Inserting keys into the keyring
// - Randomness (rand.go)
// - Cryptographic algorithms (crypto.go)
// - encryption (AES256-CTR)
// - authentication (SHA256-based HMAC)
// - key stretching (SHA256-based HKDF)
// - key wrapping/unwrapping (Encrypt then MAC)
package crypto
import (
"crypto/aes"
"crypto/cipher"
"crypto/hmac"
"crypto/sha256"
"fmt"
"io"
"golang.org/x/crypto/hkdf"
"golang.org/x/sys/unix"
"fscrypt/metadata"
"fscrypt/util"
)
// Lengths for our keys and buffers used for crypto.
const (
// We always use 256-bit keys internally (compared to 512-bit policy keys).
InternalKeyLen = 32
IVLen = 16
SaltLen = 16
// PolicyKeyLen is the length of all keys passed directly to the Keyring
PolicyKeyLen = unix.FS_MAX_KEY_SIZE
)
// "name" has invalid length if expected != actual
func checkInputLength(name string, expected, actual int) {
if expected != actual {
util.NeverError(util.InvalidLengthError(name, expected, actual))
}
}
// stretchKey stretches a key of length KeyLen using unsalted HKDF to make two
// keys of length KeyLen.
func stretchKey(key *Key) (encKey, authKey *Key) {
checkInputLength("hkdf key", InternalKeyLen, key.Len())
// The new hkdf function uses the hash and key to create a reader that
// can be used to securely initialize multiple keys. This means that
// reads on the hkdf give independent cryptographic keys. The hkdf will
// also always have enough entropy to read two keys.
hkdf := hkdf.New(sha256.New, key.data, nil, nil)
encKey, err := NewFixedLengthKeyFromReader(hkdf, InternalKeyLen)
util.NeverError(err)
authKey, err = NewFixedLengthKeyFromReader(hkdf, InternalKeyLen)
util.NeverError(err)
return
}
// Runs AES256-CTR on the input using the provided key and iv. This function can
// be used to either encrypt or decrypt input of any size. Note that input and
// output must be the same size.
func aesCTR(key *Key, iv, input, output []byte) {
checkInputLength("aesCTR key", InternalKeyLen, key.Len())
checkInputLength("aesCTR iv", IVLen, len(iv))
checkInputLength("aesCTR output", len(input), len(output))
blockCipher, err := aes.NewCipher(key.data)
util.NeverError(err) // Key is checked to have correct length
stream := cipher.NewCTR(blockCipher, iv)
stream.XORKeyStream(output, input)
}
// Get a HMAC (with a SHA256-based hash) of some data using the provided key.
func getHMAC(key *Key, data ...[]byte) []byte {
checkInputLength("hmac key", InternalKeyLen, key.Len())
mac := hmac.New(sha256.New, key.data)
for _, buffer := range data {
// SHA256 HMAC should never be unable to write the data
_, err := mac.Write(buffer)
util.NeverError(err)
}
return mac.Sum(nil)
}
// Wrap takes a wrapping Key of length InternalKeyLen, and uses it to wrap a
// secret Key of any length. This wrapping uses a random IV, the encrypted data,
// and an HMAC to verify the wrapping key was correct. All of this is included
// in the returned WrappedKeyData structure.
func Wrap(wrappingKey, secretKey *Key) (*metadata.WrappedKeyData, error) {
if wrappingKey.Len() != InternalKeyLen {
return nil, util.InvalidLengthError("wrapping key", InternalKeyLen, wrappingKey.Len())
}
data := &metadata.WrappedKeyData{
IV: make([]byte, IVLen),
EncryptedKey: make([]byte, secretKey.Len()),
}
// Get random IV
if _, err := io.ReadFull(RandReader, data.IV); err != nil {
return nil, err
}
// Stretch key for encryption and authentication (unsalted).
encKey, authKey := stretchKey(wrappingKey)
defer encKey.Wipe()
defer authKey.Wipe()
// Encrypt the secret and include the HMAC of the output ("Encrypt-then-MAC").
aesCTR(encKey, data.IV, secretKey.data, data.EncryptedKey)
data.Hmac = getHMAC(authKey, data.IV, data.EncryptedKey)
return data, nil
}
// Unwrap takes a wrapping Key of length KeyLen, and uses it to unwrap the
// WrappedKeyData to get the unwrapped secret Key. The Wrapped Key data includes
// an authentication check, so an error will be returned if that check fails.
func Unwrap(wrappingKey *Key, data *metadata.WrappedKeyData) (*Key, error) {
if wrappingKey.Len() != InternalKeyLen {
return nil, util.InvalidLengthError("wrapping key", InternalKeyLen, wrappingKey.Len())
}
// Stretch key for encryption and authentication (unsalted).
encKey, authKey := stretchKey(wrappingKey)
defer encKey.Wipe()
defer authKey.Wipe()
// Check validity of the HMAC
if !hmac.Equal(getHMAC(authKey, data.IV, data.EncryptedKey), data.Hmac) {
return nil, fmt.Errorf("key authentication check failed")
}
secretKey, err := newBlankKey(len(data.EncryptedKey))
if err != nil {
return nil, err
}
aesCTR(encKey, data.IV, data.EncryptedKey, secretKey.data)
return secretKey, nil
}
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