1 // Copyright 2020 Google LLC
2 //
3 // Licensed under the Apache License, Version 2.0 (the "License"); you may not
4 // use this file except in compliance with the License. You may obtain a copy of
5 // the License at
6 //
7 // https://www.apache.org/licenses/LICENSE-2.0
8 //
9 // Unless required by applicable law or agreed to in writing, software
10 // distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
11 // WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
12 // License for the specific language governing permissions and limitations under
13 // the License.
14
15 // If no variable length descriptors are used in a DICE certificate, the
16 // certificate can be constructed from a template instead of using a CBOR
17 // library. This implementation includes only hashes and inline configuration in
18 // the DICE extension. For convenience this uses only the lower level curve25519
19 // implementation in boringssl. This approach may be especially useful in very
20 // low level components where simplicity is paramount.
21
22 // This is an implementation of the DiceGenerateCertificate that generates a
23 // CWT-style CBOR certificate based on a template using the ED25519-SHA512
24 // signature scheme.
25 //
26 // If no variable length descriptors are used in a DICE certificate, the
27 // certificate can be constructed from a template instead of using a CBOR /
28 // COSE library. This implementation includes only hashes and inline
29 // configuration in the certificate fields. This approach may be especially
30 // useful in very low level components where simplicity is paramount.
31 //
32 // This function will return kDiceResultInvalidInput if 'input_values' specifies
33 // any variable length descriptors. In particular:
34 // * code_descriptor_size must be zero
35 // * authority_descriptor_size must be zero
36 // * config_type must be kDiceConfigTypeInline
37
38 #include <stdint.h>
39 #include <string.h>
40
41 #include "dice/dice.h"
42 #include "dice/ops.h"
43 #include "dice/utils.h"
44
45 #if DICE_PUBLIC_KEY_SIZE != 32
46 #error "Only Ed25519 is supported; 32 bytes needed to store the public key."
47 #endif
48 #if DICE_SIGNATURE_SIZE != 64
49 #error "Only Ed25519 is supported; 64 bytes needed to store the signature."
50 #endif
51
52 // 20 bytes of header, 366 bytes of payload.
53 #define DICE_TBS_SIZE 386
54
55 // A well-formed certificate, but with zeros in all fields to be filled.
56 static const uint8_t kTemplate[441] = {
57 // Constant encoding.
58 0x84, 0x43, 0xa1, 0x01, 0x27, 0xa0, 0x59, 0x01, 0x6e,
59 // Offset 9: Payload starts here, 366 bytes.
60 0xa8, 0x01, 0x78, 0x28,
61 // Offset 13: CWT issuer, 40 bytes.
62 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
63 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
64 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
65 0x00, 0x00, 0x00, 0x00,
66 // Constant encoding.
67 0x02, 0x78, 0x28,
68 // Offset 56: CWT subject, 40 bytes.
69 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
70 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
71 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
72 0x00, 0x00, 0x00, 0x00,
73 // Constant encoding.
74 0x3a, 0x00, 0x47, 0x44, 0x50, 0x58, 0x40,
75 // Offset 103: Code hash, 64 bytes.
76 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
77 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
78 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
79 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
80 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
81 0x00, 0x00, 0x00, 0x00,
82 // Constant encoding.
83 0x3a, 0x00, 0x47, 0x44, 0x53, 0x58, 0x40,
84 // Offset 174: Configuration value, 64 bytes.
85 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
86 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
87 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
88 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
89 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
90 0x00, 0x00, 0x00, 0x00,
91 // Constant encoding.
92 0x3a, 0x00, 0x47, 0x44, 0x54, 0x58, 0x40,
93 // Offset 245: Authority hash, 64 bytes.
94 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
95 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
96 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
97 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
98 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
99 0x00, 0x00, 0x00, 0x00,
100 // Constant encoding.
101 0x3a, 0x00, 0x47, 0x44, 0x56, 0x41,
102 // Offset 315: Mode, 1 byte.
103 0x00,
104 // Constant encoding.
105 0x3a, 0x00, 0x47, 0x44, 0x57, 0x58, 0x2d, 0xa5, 0x01, 0x01, 0x03, 0x27,
106 0x04, 0x81, 0x02, 0x20, 0x06, 0x21, 0x58, 0x20,
107 // Offset 336: Public key, 32 bytes.
108 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
109 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
110 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
111 // Constant encoding (key usage).
112 0x3a, 0x00, 0x47, 0x44, 0x58, 0x41, 0x20,
113 // Offset 375: Payload ends here.
114 // Constant encoding.
115 0x58, 0x40,
116 // Offset 377: Signature, 64 bytes.
117 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
118 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
119 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
120 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
121 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
122 0x00, 0x00, 0x00, 0x00};
123
124 // The data to be signed is not the certificate, but the payload appended to
125 // this header. This is the 'Sig_structure' for COSE_Sign1, per RFC 8152.
126 static const uint8_t kTbsHeader[20] = {0x84, 0x6a, 0x53, 0x69, 0x67, 0x6e, 0x61,
127 0x74, 0x75, 0x72, 0x65, 0x31, 0x43, 0xa1,
128 0x01, 0x27, 0x40, 0x59, 0x01, 0x6e};
129
130 static const struct {
131 size_t offset;
132 size_t length;
133 } kFieldTable[] = {{13, 40}, // Issuer
134 {56, 40}, // Subject
135 {103, 64}, // Code hash
136 {174, 64}, // Config descriptor
137 {245, 64}, // Authority hash
138 {315, 1}, // Mode
139 {336, 32}, // Public key
140 {377, 64}, // Signature
141 {9, 366}}; // Payload
142
143 static const size_t kFieldIndexIssuer = 0;
144 static const size_t kFieldIndexSubject = 1;
145 static const size_t kFieldIndexCodeHash = 2;
146 static const size_t kFieldIndexConfigDescriptor = 3;
147 static const size_t kFieldIndexAuthorityHash = 4;
148 static const size_t kFieldIndexMode = 5;
149 static const size_t kFieldIndexSubjectPublicKey = 6;
150 static const size_t kFieldIndexSignature = 7;
151 static const size_t kFieldIndexPayload = 8;
152
153 // |buffer| must point to the beginning of the template buffer and |src| must
154 // point to at least <field-length> bytes.
CopyField(const uint8_t * src,size_t index,uint8_t * buffer)155 static void CopyField(const uint8_t* src, size_t index, uint8_t* buffer) {
156 memcpy(&buffer[kFieldTable[index].offset], src, kFieldTable[index].length);
157 }
158
DiceGenerateCertificate(void * context,const uint8_t subject_private_key_seed[DICE_PRIVATE_KEY_SEED_SIZE],const uint8_t authority_private_key_seed[DICE_PRIVATE_KEY_SEED_SIZE],const DiceInputValues * input_values,size_t certificate_buffer_size,uint8_t * certificate,size_t * certificate_actual_size)159 DiceResult DiceGenerateCertificate(
160 void* context,
161 const uint8_t subject_private_key_seed[DICE_PRIVATE_KEY_SEED_SIZE],
162 const uint8_t authority_private_key_seed[DICE_PRIVATE_KEY_SEED_SIZE],
163 const DiceInputValues* input_values, size_t certificate_buffer_size,
164 uint8_t* certificate, size_t* certificate_actual_size) {
165 DiceResult result = kDiceResultOk;
166
167 // Variable length descriptors are not supported.
168 if (input_values->code_descriptor_size > 0 ||
169 input_values->config_type != kDiceConfigTypeInline ||
170 input_values->authority_descriptor_size > 0 || DICE_PROFILE_NAME) {
171 return kDiceResultInvalidInput;
172 }
173
174 // We know the certificate size upfront so we can do the buffer size check.
175 *certificate_actual_size = sizeof(kTemplate);
176 if (certificate_buffer_size < sizeof(kTemplate)) {
177 return kDiceResultBufferTooSmall;
178 }
179
180 // Declare buffers which are cleared on 'goto out'.
181 uint8_t subject_private_key[DICE_PRIVATE_KEY_SIZE];
182 uint8_t authority_private_key[DICE_PRIVATE_KEY_SIZE];
183
184 // Derive keys and IDs from the private key seeds.
185 uint8_t subject_public_key[DICE_PUBLIC_KEY_SIZE];
186 result = DiceKeypairFromSeed(context, subject_private_key_seed,
187 subject_public_key, subject_private_key);
188 if (result != kDiceResultOk) {
189 goto out;
190 }
191
192 uint8_t subject_id[DICE_ID_SIZE];
193 result = DiceDeriveCdiCertificateId(context, subject_public_key,
194 DICE_PUBLIC_KEY_SIZE, subject_id);
195 if (result != kDiceResultOk) {
196 goto out;
197 }
198 uint8_t subject_id_hex[40];
199 DiceHexEncode(subject_id, sizeof(subject_id), subject_id_hex,
200 sizeof(subject_id_hex));
201
202 uint8_t authority_public_key[DICE_PUBLIC_KEY_SIZE];
203 result = DiceKeypairFromSeed(context, authority_private_key_seed,
204 authority_public_key, authority_private_key);
205 if (result != kDiceResultOk) {
206 goto out;
207 }
208
209 uint8_t authority_id[DICE_ID_SIZE];
210 result = DiceDeriveCdiCertificateId(context, authority_public_key,
211 DICE_PUBLIC_KEY_SIZE, authority_id);
212 if (result != kDiceResultOk) {
213 goto out;
214 }
215 uint8_t authority_id_hex[40];
216 DiceHexEncode(authority_id, sizeof(authority_id), authority_id_hex,
217 sizeof(authority_id_hex));
218
219 // First copy in the entire template, then fill in the fields.
220 memcpy(certificate, kTemplate, sizeof(kTemplate));
221 CopyField(authority_id_hex, kFieldIndexIssuer, certificate);
222 CopyField(subject_id_hex, kFieldIndexSubject, certificate);
223 CopyField(subject_public_key, kFieldIndexSubjectPublicKey, certificate);
224 CopyField(input_values->code_hash, kFieldIndexCodeHash, certificate);
225 CopyField(input_values->config_value, kFieldIndexConfigDescriptor,
226 certificate);
227 CopyField(input_values->authority_hash, kFieldIndexAuthorityHash,
228 certificate);
229 certificate[kFieldTable[kFieldIndexMode].offset] = input_values->mode;
230
231 // Fill the TBS structure using the payload from the certificate.
232 uint8_t tbs[DICE_TBS_SIZE];
233 memcpy(tbs, kTbsHeader, sizeof(kTbsHeader));
234 memcpy(&tbs[sizeof(kTbsHeader)],
235 &certificate[kFieldTable[kFieldIndexPayload].offset],
236 kFieldTable[kFieldIndexPayload].length);
237
238 uint8_t signature[DICE_SIGNATURE_SIZE];
239 result =
240 DiceSign(context, tbs, sizeof(tbs), authority_private_key, signature);
241 if (result != kDiceResultOk) {
242 goto out;
243 }
244 result =
245 DiceVerify(context, tbs, sizeof(tbs), signature, authority_public_key);
246 if (result != kDiceResultOk) {
247 goto out;
248 }
249 CopyField(signature, kFieldIndexSignature, certificate);
250
251 out:
252 DiceClearMemory(context, sizeof(subject_private_key), subject_private_key);
253 DiceClearMemory(context, sizeof(authority_private_key),
254 authority_private_key);
255 return result;
256 }
257