From 8128b35375dfc4846dd1573dda55ef232ffd2d66 Mon Sep 17 00:00:00 2001 From: Joe Richey Date: Thu, 2 Mar 2017 11:58:07 -0800 Subject: crypto: secure key wrapping/unwrapping This commit adds in the ability to use the WrappedKeyData from the metadata package to wrap and unwrap cryptographic keys of any length. This makes use of several cryptographic primitives: - Unsalted, SHA256-based HKDF for key stretching - AES256 in CTR mode for encryption - SHA256-based HMAC for authentication Note that the key wrapping/unwrapping uses an "Encrypt then MAC" scheme for doing authenticated unwrapping. This means we can detect if bogus metadata has been given. This package also standardizes the length for fscrypt's internal keys. This CL is the first to add benchmarks, which can be run with: go test -bench=. ./... Change-Id: I2e5fc23a8a8cc36b17ccb3f26f03edcaccc517e1 --- crypto/crypto.go | 168 +++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 168 insertions(+) create mode 100644 crypto/crypto.go (limited to 'crypto/crypto.go') diff --git a/crypto/crypto.go b/crypto/crypto.go new file mode 100644 index 0000000..5eeff50 --- /dev/null +++ b/crypto/crypto.go @@ -0,0 +1,168 @@ +/* + * 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 +} -- cgit v1.2.3