1 /*
2  * Copyright (C) 2020 The Android Open Source Project
3  *
4  * Licensed under the Apache License, Version 2.0 (the "License");
5  * you may not use this file except in compliance with the License.
6  * You may obtain a copy of the License at
7  *
8  *      http://www.apache.org/licenses/LICENSE-2.0
9  *
10  * Unless required by applicable law or agreed to in writing, software
11  * distributed under the License is distributed on an "AS IS" BASIS,
12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13  * See the License for the specific language governing permissions and
14  * limitations under the License.
15  */
16 
17 #pragma once
18 
19 #include <stdlib.h>
20 
21 namespace android {
22 namespace vold {
23 
24 // Struct representing an encryption algorithm supported by vold.
25 // "config_name" represents the name we give the algorithm in
26 // read-only properties and fstab files
27 // "kernel_name" is the name we present to the Linux kernel
28 // "keysize" is the size of the key in bytes.
29 struct CryptoType {
30     // We should only be constructing CryptoTypes as part of
31     // supported_crypto_types[].  We do it via this pseudo-builder pattern,
32     // which isn't pure or fully protected as a concession to being able to
33     // do it all at compile time.  Add new CryptoTypes in
34     // supported_crypto_types[] below.
CryptoTypeCryptoType35     constexpr CryptoType() : CryptoType(nullptr, nullptr, 0xFFFFFFFF) {}
set_keysizeCryptoType36     constexpr CryptoType set_keysize(size_t size) const {
37         return CryptoType(this->config_name, this->kernel_name, size);
38     }
set_config_nameCryptoType39     constexpr CryptoType set_config_name(const char* property) const {
40         return CryptoType(property, this->kernel_name, this->keysize);
41     }
set_kernel_nameCryptoType42     constexpr CryptoType set_kernel_name(const char* crypto) const {
43         return CryptoType(this->config_name, crypto, this->keysize);
44     }
45 
get_config_nameCryptoType46     constexpr const char* get_config_name() const { return config_name; }
get_kernel_nameCryptoType47     constexpr const char* get_kernel_name() const { return kernel_name; }
get_keysizeCryptoType48     constexpr size_t get_keysize() const { return keysize; }
49 
50   private:
51     const char* config_name;
52     const char* kernel_name;
53     size_t keysize;
54 
CryptoTypeCryptoType55     constexpr CryptoType(const char* property, const char* crypto, size_t ksize)
56         : config_name(property), kernel_name(crypto), keysize(ksize) {}
57 };
58 
59 // Use the named android property to look up a type from the table
60 // If the property is not set or matches no table entry, return the default.
61 const CryptoType& lookup_crypto_algorithm(const CryptoType table[], int table_len,
62                                           const CryptoType& default_alg, const char* property);
63 
64 // Some useful types
65 
66 constexpr CryptoType invalid_crypto_type = CryptoType();
67 
68 constexpr CryptoType aes_256_xts = CryptoType()
69                                            .set_config_name("aes-256-xts")
70                                            .set_kernel_name("aes-xts-plain64")
71                                            .set_keysize(64);
72 
73 constexpr CryptoType adiantum = CryptoType()
74                                         .set_config_name("adiantum")
75                                         .set_kernel_name("xchacha12,aes-adiantum-plain64")
76                                         .set_keysize(32);
77 
78 // Support compile-time validation of a crypto type table
79 
80 template <typename T, size_t N>
array_length(T (&)[N])81 constexpr size_t array_length(T (&)[N]) {
82     return N;
83 }
84 
isValidCryptoType(size_t max_keylen,const CryptoType & crypto_type)85 constexpr bool isValidCryptoType(size_t max_keylen, const CryptoType& crypto_type) {
86     return ((crypto_type.get_config_name() != nullptr) &&
87             (crypto_type.get_kernel_name() != nullptr) &&
88             (crypto_type.get_keysize() <= max_keylen));
89 }
90 
91 // Confirms that all supported_crypto_types have a small enough keysize and
92 // had both set_config_name() and set_kernel_name() called.
93 // Note in C++11 that constexpr functions can only have a single line.
94 // So our code is a bit convoluted (using recursion instead of a loop),
95 // but it's asserting at compile time that all of our key lengths are valid.
validateSupportedCryptoTypes(size_t max_keylen,const CryptoType types[],size_t len)96 constexpr bool validateSupportedCryptoTypes(size_t max_keylen, const CryptoType types[],
97                                             size_t len) {
98     return len == 0 || (isValidCryptoType(max_keylen, types[len - 1]) &&
99                         validateSupportedCryptoTypes(max_keylen, types, len - 1));
100 }
101 
102 }  // namespace vold
103 }  // namespace android
104