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Simon Glass58fe7e52013-06-13 15:10:00 -07001U-Boot FIT Signature Verification
2=================================
3
4Introduction
5------------
6FIT supports hashing of images so that these hashes can be checked on
7loading. This protects against corruption of the image. However it does not
8prevent the substitution of one image for another.
9
10The signature feature allows the hash to be signed with a private key such
11that it can be verified using a public key later. Provided that the private
12key is kept secret and the public key is stored in a non-volatile place,
13any image can be verified in this way.
14
15See verified-boot.txt for more general information on verified boot.
16
17
18Concepts
19--------
20Some familiarity with public key cryptography is assumed in this section.
21
22The procedure for signing is as follows:
23
24 - hash an image in the FIT
25 - sign the hash with a private key to produce a signature
26 - store the resulting signature in the FIT
27
28The procedure for verification is:
29
30 - read the FIT
31 - obtain the public key
32 - extract the signature from the FIT
33 - hash the image from the FIT
34 - verify (with the public key) that the extracted signature matches the
35 hash
36
37The signing is generally performed by mkimage, as part of making a firmware
38image for the device. The verification is normally done in U-Boot on the
39device.
40
41
42Algorithms
43----------
44In principle any suitable algorithm can be used to sign and verify a hash.
Baruch Siach7caec832023-05-02 07:47:28 +030045U-Boot supports a few hashing and verification algorithms. See below for
46details.
Simon Glass58fe7e52013-06-13 15:10:00 -070047
48While it is acceptable to bring in large cryptographic libraries such as
49openssl on the host side (e.g. mkimage), it is not desirable for U-Boot.
50For the run-time verification side, it is important to keep code and data
51size as small as possible.
52
53For this reason the RSA image verification uses pre-processed public keys
54which can be used with a very small amount of code - just some extraction
55of data from the FDT and exponentiation mod n. Code size impact is a little
56under 5KB on Tegra Seaboard, for example.
57
58It is relatively straightforward to add new algorithms if required. If
Baruch Siach86162842023-05-02 07:47:29 +030059another RSA variant is needed, then it can be added with the
60U_BOOT_CRYPTO_ALGO() macro. If another algorithm is needed (such as DSA) then
61it can be placed in a directory alongside lib/rsa/, and its functions added
62using U_BOOT_CRYPTO_ALGO().
Simon Glass58fe7e52013-06-13 15:10:00 -070063
64
Andreas Dannenbergce53ceb2016-03-23 18:24:10 -050065Creating an RSA key pair and certificate
66----------------------------------------
67To create a new public/private key pair, size 2048 bits:
Simon Glass58fe7e52013-06-13 15:10:00 -070068
Michael van der Westhuizen89f4ed12014-07-02 10:17:26 +020069$ openssl genpkey -algorithm RSA -out keys/dev.key \
70 -pkeyopt rsa_keygen_bits:2048 -pkeyopt rsa_keygen_pubexp:65537
Simon Glass58fe7e52013-06-13 15:10:00 -070071
Andreas Dannenbergce53ceb2016-03-23 18:24:10 -050072To create a certificate for this containing the public key:
Simon Glass58fe7e52013-06-13 15:10:00 -070073
74$ openssl req -batch -new -x509 -key keys/dev.key -out keys/dev.crt
75
76If you like you can look at the public key also:
77
78$ openssl rsa -in keys/dev.key -pubout
79
80
81Device Tree Bindings
82--------------------
83The following properties are required in the FIT's signature node(s) to
Masahiro Yamada65aff482017-08-22 15:19:20 +090084allow the signer to operate. These should be added to the .its file.
Simon Glass58fe7e52013-06-13 15:10:00 -070085Signature nodes sit at the same level as hash nodes and are called
Andre Przywara9fdfb1f2017-12-04 02:05:07 +000086signature-1, signature-2, etc.
Simon Glass58fe7e52013-06-13 15:10:00 -070087
Masahiro Yamadae63e3862017-10-19 19:37:59 +090088- algo: Algorithm name (e.g. "sha1,rsa2048")
Simon Glass58fe7e52013-06-13 15:10:00 -070089
90- key-name-hint: Name of key to use for signing. The keys will normally be in
91a single directory (parameter -k to mkimage). For a given key <name>, its
92private key is stored in <name>.key and the certificate is stored in
93<name>.crt.
94
95When the image is signed, the following properties are added (mandatory):
96
97- value: The signature data (e.g. 256 bytes for 2048-bit RSA)
98
99When the image is signed, the following properties are optional:
100
101- timestamp: Time when image was signed (standard Unix time_t format)
102
103- signer-name: Name of the signer (e.g. "mkimage")
104
105- signer-version: Version string of the signer (e.g. "2013.01")
106
107- comment: Additional information about the signer or image
108
Philippe Reynesa89345e2018-11-14 13:51:02 +0100109- padding: The padding algorithm, it may be pkcs-1.5 or pss,
110 if no value is provided we assume pkcs-1.5
111
Simon Glass56ab8d62013-06-13 15:10:09 -0700112For config bindings (see Signed Configurations below), the following
113additional properties are optional:
Simon Glass58fe7e52013-06-13 15:10:00 -0700114
Simon Glass56ab8d62013-06-13 15:10:09 -0700115- sign-images: A list of images to sign, each being a property of the conf
116node that contains then. The default is "kernel,fdt" which means that these
117two images will be looked up in the config and signed if present.
Simon Glass58fe7e52013-06-13 15:10:00 -0700118
Simon Glass56ab8d62013-06-13 15:10:09 -0700119For config bindings, these properties are added by the signer:
120
121- hashed-nodes: A list of nodes which were hashed by the signer. Each is
122 a string - the full path to node. A typical value might be:
123
Andre Przywara9fdfb1f2017-12-04 02:05:07 +0000124 hashed-nodes = "/", "/configurations/conf-1", "/images/kernel",
125 "/images/kernel/hash-1", "/images/fdt-1",
126 "/images/fdt-1/hash-1";
Simon Glass56ab8d62013-06-13 15:10:09 -0700127
128- hashed-strings: The start and size of the string region of the FIT that
129 was hashed
130
131Example: See sign-images.its for an example image tree source file and
132sign-configs.its for config signing.
133
Simon Glass58fe7e52013-06-13 15:10:00 -0700134
135Public Key Storage
136------------------
137In order to verify an image that has been signed with a public key we need to
138have a trusted public key. This cannot be stored in the signed image, since
139it would be easy to alter. For this implementation we choose to store the
140public key in U-Boot's control FDT (using CONFIG_OF_CONTROL).
141
142Public keys should be stored as sub-nodes in a /signature node. Required
143properties are:
144
Alexandru Gagniucd1b3f7d2021-02-19 12:45:13 -0600145- algo: Algorithm name (e.g. "sha1,rsa2048" or "sha256,ecdsa256")
Simon Glass58fe7e52013-06-13 15:10:00 -0700146
147Optional properties are:
148
149- key-name-hint: Name of key used for signing. This is only a hint since it
150is possible for the name to be changed. Verification can proceed by checking
151all available signing keys until one matches.
152
153- required: If present this indicates that the key must be verified for the
154image / configuration to be considered valid. Only required keys are
155normally verified by the FIT image booting algorithm. Valid values are
Masahiro Yamada65aff482017-08-22 15:19:20 +0900156"image" to force verification of all images, and "conf" to force verification
Simon Glass58fe7e52013-06-13 15:10:00 -0700157of the selected configuration (which then relies on hashes in the images to
158verify those).
159
160Each signing algorithm has its own additional properties.
161
162For RSA the following are mandatory:
163
164- rsa,num-bits: Number of key bits (e.g. 2048)
165- rsa,modulus: Modulus (N) as a big-endian multi-word integer
Michael van der Westhuizen89f4ed12014-07-02 10:17:26 +0200166- rsa,exponent: Public exponent (E) as a 64 bit unsigned integer
Simon Glass58fe7e52013-06-13 15:10:00 -0700167- rsa,r-squared: (2^num-bits)^2 as a big-endian multi-word integer
168- rsa,n0-inverse: -1 / modulus[0] mod 2^32
169
Alexandru Gagniucd1b3f7d2021-02-19 12:45:13 -0600170For ECDSA the following are mandatory:
171- ecdsa,curve: Name of ECDSA curve (e.g. "prime256v1")
172- ecdsa,x-point: Public key X coordinate as a big-endian multi-word integer
173- ecdsa,y-point: Public key Y coordinate as a big-endian multi-word integer
174
Heinrich Schuchardt9cd84d22019-12-11 10:45:50 +0100175These parameters can be added to a binary device tree using parameter -K of the
176mkimage command::
177
178 tools/mkimage -f fit.its -K control.dtb -k keys -r image.fit
179
180Here is an example of a generated device tree node::
181
182 signature {
183 key-dev {
184 required = "conf";
185 algo = "sha256,rsa2048";
186 rsa,r-squared = <0xb76d1acf 0xa1763ca5 0xeb2f126
187 0x742edc80 0xd3f42177 0x9741d9d9
188 0x35bb476e 0xff41c718 0xd3801430
189 0xf22537cb 0xa7e79960 0xae32a043
190 0x7da1427a 0x341d6492 0x3c2762f5
191 0xaac04726 0x5b262d96 0xf984e86d
192 0xb99443c7 0x17080c33 0x940f6892
193 0xd57a95d1 0x6ea7b691 0xc5038fa8
194 0x6bb48a6e 0x73f1b1ea 0x37160841
195 0xe05715ce 0xa7c45bbd 0x690d82d5
196 0x99c2454c 0x6ff117b3 0xd830683b
197 0x3f81c9cf 0x1ca38a91 0x0c3392e4
198 0xd817c625 0x7b8e9a24 0x175b89ea
199 0xad79f3dc 0x4d50d7b4 0x9d4e90f8
200 0xad9e2939 0xc165d6a4 0x0ada7e1b
201 0xfb1bf495 0xfc3131c2 0xb8c6e604
202 0xc2761124 0xf63de4a6 0x0e9565f9
203 0xc8e53761 0x7e7a37a5 0xe99dcdae
204 0x9aff7e1e 0xbd44b13d 0x6b0e6aa4
205 0x038907e4 0x8e0d6850 0xef51bc20
206 0xf73c94af 0x88bea7b1 0xcbbb1b30
207 0xd024b7f3>;
208 rsa,modulus = <0xc0711d6cb 0x9e86db7f 0x45986dbe
209 0x023f1e8c9 0xe1a4c4d0 0x8a0dfdc9
210 0x023ba0c48 0x06815f6a 0x5caa0654
211 0x07078c4b7 0x3d154853 0x40729023
212 0x0b007c8fe 0x5a3647e5 0x23b41e20
213 0x024720591 0x66915305 0x0e0b29b0
214 0x0de2ad30d 0x8589430f 0xb1590325
215 0x0fb9f5d5e 0x9eba752a 0xd88e6de9
216 0x056b3dcc6 0x9a6b8e61 0x6784f61f
217 0x000f39c21 0x5eec6b33 0xd78e4f78
218 0x0921a305f 0xaa2cc27e 0x1ca917af
219 0x06e1134f4 0xd48cac77 0x4e914d07
220 0x0f707aa5a 0x0d141f41 0x84677f1d
221 0x0ad47a049 0x028aedb6 0xd5536fcf
222 0x03fef1e4f 0x133a03d2 0xfd7a750a
223 0x0f9159732 0xd207812e 0x6a807375
224 0x06434230d 0xc8e22dad 0x9f29b3d6
225 0x07c44ac2b 0xfa2aad88 0xe2429504
226 0x041febd41 0x85d0d142 0x7b194d65
227 0x06e5d55ea 0x41116961 0xf3181dde
228 0x068bf5fbc 0x3dd82047 0x00ee647e
229 0x0d7a44ab3>;
230 rsa,exponent = <0x00 0x10001>;
231 rsa,n0-inverse = <0xb3928b85>;
232 rsa,num-bits = <0x800>;
233 key-name-hint = "dev";
234 };
235 };
236
Simon Glass58fe7e52013-06-13 15:10:00 -0700237
Simon Glass56ab8d62013-06-13 15:10:09 -0700238Signed Configurations
239---------------------
240While signing images is useful, it does not provide complete protection
241against several types of attack. For example, it it possible to create a
242FIT with the same signed images, but with the configuration changed such
243that a different one is selected (mix and match attack). It is also possible
244to substitute a signed image from an older FIT version into a newer FIT
245(roll-back attack).
246
247As an example, consider this FIT:
248
249/ {
250 images {
Andre Przywara9fdfb1f2017-12-04 02:05:07 +0000251 kernel-1 {
Simon Glass56ab8d62013-06-13 15:10:09 -0700252 data = <data for kernel1>
Andre Przywara9fdfb1f2017-12-04 02:05:07 +0000253 signature-1 {
Simon Glass56ab8d62013-06-13 15:10:09 -0700254 algo = "sha1,rsa2048";
255 value = <...kernel signature 1...>
256 };
257 };
Andre Przywara9fdfb1f2017-12-04 02:05:07 +0000258 kernel-2 {
Simon Glass56ab8d62013-06-13 15:10:09 -0700259 data = <data for kernel2>
Andre Przywara9fdfb1f2017-12-04 02:05:07 +0000260 signature-1 {
Simon Glass56ab8d62013-06-13 15:10:09 -0700261 algo = "sha1,rsa2048";
262 value = <...kernel signature 2...>
263 };
264 };
Andre Przywara9fdfb1f2017-12-04 02:05:07 +0000265 fdt-1 {
Simon Glass56ab8d62013-06-13 15:10:09 -0700266 data = <data for fdt1>;
Andre Przywara9fdfb1f2017-12-04 02:05:07 +0000267 signature-1 {
Simon Glass56ab8d62013-06-13 15:10:09 -0700268 algo = "sha1,rsa2048";
Thomas Perrotec80ece2021-07-02 11:32:37 +0200269 value = <...fdt signature 1...>
Simon Glass56ab8d62013-06-13 15:10:09 -0700270 };
271 };
Andre Przywara9fdfb1f2017-12-04 02:05:07 +0000272 fdt-2 {
Simon Glass56ab8d62013-06-13 15:10:09 -0700273 data = <data for fdt2>;
Andre Przywara9fdfb1f2017-12-04 02:05:07 +0000274 signature-1 {
Simon Glass56ab8d62013-06-13 15:10:09 -0700275 algo = "sha1,rsa2048";
Thomas Perrotec80ece2021-07-02 11:32:37 +0200276 value = <...fdt signature 2...>
Simon Glass56ab8d62013-06-13 15:10:09 -0700277 };
278 };
279 };
280 configurations {
Andre Przywara9fdfb1f2017-12-04 02:05:07 +0000281 default = "conf-1";
282 conf-1 {
283 kernel = "kernel-1";
284 fdt = "fdt-1";
Simon Glass56ab8d62013-06-13 15:10:09 -0700285 };
Mickaël Tansorierb3c15b02019-07-17 17:57:16 +0200286 conf-2 {
Andre Przywara9fdfb1f2017-12-04 02:05:07 +0000287 kernel = "kernel-2";
288 fdt = "fdt-2";
Simon Glass56ab8d62013-06-13 15:10:09 -0700289 };
290 };
291};
292
293Since both kernels are signed it is easy for an attacker to add a new
294configuration 3 with kernel 1 and fdt 2:
295
296 configurations {
Andre Przywara9fdfb1f2017-12-04 02:05:07 +0000297 default = "conf-1";
298 conf-1 {
299 kernel = "kernel-1";
300 fdt = "fdt-1";
Simon Glass56ab8d62013-06-13 15:10:09 -0700301 };
Mickaël Tansorierb3c15b02019-07-17 17:57:16 +0200302 conf-2 {
Andre Przywara9fdfb1f2017-12-04 02:05:07 +0000303 kernel = "kernel-2";
304 fdt = "fdt-2";
Simon Glass56ab8d62013-06-13 15:10:09 -0700305 };
Andre Przywara9fdfb1f2017-12-04 02:05:07 +0000306 conf-3 {
307 kernel = "kernel-1";
308 fdt = "fdt-2";
Simon Glass56ab8d62013-06-13 15:10:09 -0700309 };
310 };
311
312With signed images, nothing protects against this. Whether it gains an
313advantage for the attacker is debatable, but it is not secure.
314
Masahiro Yamada65aff482017-08-22 15:19:20 +0900315To solve this problem, we support signed configurations. In this case it
Simon Glass56ab8d62013-06-13 15:10:09 -0700316is the configurations that are signed, not the image. Each image has its
317own hash, and we include the hash in the configuration signature.
318
319So the above example is adjusted to look like this:
320
321/ {
322 images {
Andre Przywara9fdfb1f2017-12-04 02:05:07 +0000323 kernel-1 {
Simon Glass56ab8d62013-06-13 15:10:09 -0700324 data = <data for kernel1>
Andre Przywara9fdfb1f2017-12-04 02:05:07 +0000325 hash-1 {
Simon Glass56ab8d62013-06-13 15:10:09 -0700326 algo = "sha1";
327 value = <...kernel hash 1...>
328 };
329 };
Andre Przywara9fdfb1f2017-12-04 02:05:07 +0000330 kernel-2 {
Simon Glass56ab8d62013-06-13 15:10:09 -0700331 data = <data for kernel2>
Andre Przywara9fdfb1f2017-12-04 02:05:07 +0000332 hash-1 {
Simon Glass56ab8d62013-06-13 15:10:09 -0700333 algo = "sha1";
334 value = <...kernel hash 2...>
335 };
336 };
Andre Przywara9fdfb1f2017-12-04 02:05:07 +0000337 fdt-1 {
Simon Glass56ab8d62013-06-13 15:10:09 -0700338 data = <data for fdt1>;
Andre Przywara9fdfb1f2017-12-04 02:05:07 +0000339 hash-1 {
Simon Glass56ab8d62013-06-13 15:10:09 -0700340 algo = "sha1";
341 value = <...fdt hash 1...>
342 };
343 };
Andre Przywara9fdfb1f2017-12-04 02:05:07 +0000344 fdt-2 {
Simon Glass56ab8d62013-06-13 15:10:09 -0700345 data = <data for fdt2>;
Andre Przywara9fdfb1f2017-12-04 02:05:07 +0000346 hash-1 {
Simon Glass56ab8d62013-06-13 15:10:09 -0700347 algo = "sha1";
348 value = <...fdt hash 2...>
349 };
350 };
351 };
352 configurations {
Andre Przywara9fdfb1f2017-12-04 02:05:07 +0000353 default = "conf-1";
354 conf-1 {
355 kernel = "kernel-1";
356 fdt = "fdt-1";
357 signature-1 {
Simon Glass56ab8d62013-06-13 15:10:09 -0700358 algo = "sha1,rsa2048";
359 value = <...conf 1 signature...>;
360 };
361 };
Andre Przywara9fdfb1f2017-12-04 02:05:07 +0000362 conf-2 {
363 kernel = "kernel-2";
364 fdt = "fdt-2";
365 signature-1 {
Simon Glass56ab8d62013-06-13 15:10:09 -0700366 algo = "sha1,rsa2048";
367 value = <...conf 1 signature...>;
368 };
369 };
370 };
371};
372
373
374You can see that we have added hashes for all images (since they are no
375longer signed), and a signature to each configuration. In the above example,
Andre Przywara9fdfb1f2017-12-04 02:05:07 +0000376mkimage will sign configurations/conf-1, the kernel and fdt that are
377pointed to by the configuration (/images/kernel-1, /images/kernel-1/hash-1,
378/images/fdt-1, /images/fdt-1/hash-1) and the root structure of the image
Simon Glass56ab8d62013-06-13 15:10:09 -0700379(so that it isn't possible to add or remove root nodes). The signature is
Andre Przywara9fdfb1f2017-12-04 02:05:07 +0000380written into /configurations/conf-1/signature-1/value. It can easily be
Simon Glass56ab8d62013-06-13 15:10:09 -0700381verified later even if the FIT has been signed with other keys in the
382meantime.
383
384
Martin Bonner5bab26b2022-07-25 08:45:59 +0100385Details
386-------
387The signature node contains a property ('hashed-nodes') which lists all the
388nodes that the signature was made over. The image is walked in order and each
389tag processed as follows:
390- DTB_BEGIN_NODE: The tag and the following name are included in the signature
391 if the node or its parent are present in 'hashed-nodes'
392- DTB_END_NODE: The tag is included in the signature if the node or its parent
393 are present in 'hashed-nodes'
394- DTB_PROPERTY: The tag, the length word, the offset in the string table, and
395 the data are all included if the current node is present in 'hashed-nodes'
396 and the property name is not 'data'.
397- DTB_END: The tag is always included in the signature.
398- DTB_NOP: The tag is included in the signature if the current node is present
399 in 'hashed-nodes'
400
401In addition, the signature contains a property 'hashed-strings' which contains
402the offset and length in the string table of the strings that are to be
403included in the signature (this is done last).
404
405IMPORTANT: To verify the signature outside u-boot, it is vital to not only
406calculate the hash of the image and verify the signature with that, but also to
407calculate the hashes of the kernel, fdt, and ramdisk images and check those
408match the hash values in the corresponding 'hash*' subnodes.
409
410
Simon Glass58fe7e52013-06-13 15:10:00 -0700411Verification
412------------
413FITs are verified when loaded. After the configuration is selected a list
414of required images is produced. If there are 'required' public keys, then
415each image must be verified against those keys. This means that every image
416that might be used by the target needs to be signed with 'required' keys.
417
418This happens automatically as part of a bootm command when FITs are used.
419
Thirupathaiah Annapureddya49efb32020-08-16 23:01:11 -0700420For Signed Configurations, the default verification behavior can be changed by
421the following optional property in /signature node in U-Boot's control FDT.
422
423- required-mode: Valid values are "any" to allow verified boot to succeed if
424the selected configuration is signed by any of the 'required' keys, and "all"
425to allow verified boot to succeed if the selected configuration is signed by
426all of the 'required' keys.
427
428This property can be added to a binary device tree using fdtput as shown in
429below examples::
430
431 fdtput -t s control.dtb /signature required-mode any
432 fdtput -t s control.dtb /signature required-mode all
433
Simon Glass58fe7e52013-06-13 15:10:00 -0700434
435Enabling FIT Verification
436-------------------------
437In addition to the options to enable FIT itself, the following CONFIGs must
438be enabled:
439
Masahiro Yamada65aff482017-08-22 15:19:20 +0900440CONFIG_FIT_SIGNATURE - enable signing and verification in FITs
Simon Glass58fe7e52013-06-13 15:10:00 -0700441CONFIG_RSA - enable RSA algorithm for signing
442
Heiko Schocher515eb122014-05-28 11:33:33 +0200443WARNING: When relying on signed FIT images with required signature check
444the legacy image format is default disabled by not defining
Tom Rinic220bd92019-05-23 07:14:07 -0400445CONFIG_LEGACY_IMAGE_FORMAT
Simon Glass58fe7e52013-06-13 15:10:00 -0700446
Mickaël Tansorier6afbde42019-07-17 17:57:29 +0200447
Simon Glass58fe7e52013-06-13 15:10:00 -0700448Testing
449-------
Masahiro Yamada65aff482017-08-22 15:19:20 +0900450An easy way to test signing and verification is to use the test script
Simon Glass58fe7e52013-06-13 15:10:00 -0700451provided in test/vboot/vboot_test.sh. This uses sandbox (a special version
452of U-Boot which runs under Linux) to show the operation of a 'bootm'
453command loading and verifying images.
454
455A sample run is show below:
456
457$ make O=sandbox sandbox_config
458$ make O=sandbox
459$ O=sandbox ./test/vboot/vboot_test.sh
Mickaël Tansorier6afbde42019-07-17 17:57:29 +0200460
461
Simon Glass58fe7e52013-06-13 15:10:00 -0700462Simple Verified Boot Test
463=========================
464
465Please see doc/uImage.FIT/verified-boot.txt for more information
466
Heiko Schocheredaf9b12014-03-03 12:19:26 +0100467/home/hs/ids/u-boot/sandbox/tools/mkimage -D -I dts -O dtb -p 2000
Simon Glass58fe7e52013-06-13 15:10:00 -0700468Build keys
Heiko Schocheredaf9b12014-03-03 12:19:26 +0100469do sha1 test
Simon Glass58fe7e52013-06-13 15:10:00 -0700470Build FIT with signed images
471Test Verified Boot Run: unsigned signatures:: OK
472Sign images
473Test Verified Boot Run: signed images: OK
474Build FIT with signed configuration
475Test Verified Boot Run: unsigned config: OK
476Sign images
477Test Verified Boot Run: signed config: OK
Heiko Schocherd7b42322014-03-03 12:19:30 +0100478check signed config on the host
Simon Glassa51991d2014-06-12 07:24:53 -0600479Signature check OK
Heiko Schocherd7b42322014-03-03 12:19:30 +0100480OK
481Test Verified Boot Run: signed config: OK
Heiko Schocheredaf9b12014-03-03 12:19:26 +0100482Test Verified Boot Run: signed config with bad hash: OK
483do sha256 test
484Build FIT with signed images
485Test Verified Boot Run: unsigned signatures:: OK
486Sign images
487Test Verified Boot Run: signed images: OK
488Build FIT with signed configuration
489Test Verified Boot Run: unsigned config: OK
490Sign images
491Test Verified Boot Run: signed config: OK
Heiko Schocherd7b42322014-03-03 12:19:30 +0100492check signed config on the host
Simon Glassa51991d2014-06-12 07:24:53 -0600493Signature check OK
Heiko Schocherd7b42322014-03-03 12:19:30 +0100494OK
495Test Verified Boot Run: signed config: OK
Heiko Schocheredaf9b12014-03-03 12:19:26 +0100496Test Verified Boot Run: signed config with bad hash: OK
Simon Glass58fe7e52013-06-13 15:10:00 -0700497
498Test passed
499
Simon Glassa51991d2014-06-12 07:24:53 -0600500
Alexandru Gagniuc5b831042021-02-19 12:45:16 -0600501Software signing: keydir vs keyfile
502-----------------------------------
503
504In the simplest case, signing is done by giving mkimage the 'keyfile'. This is
505the path to a file containing the signing key.
506
507The alternative is to pass the 'keydir' argument. In this case the filename of
508the key is derived from the 'keydir' and the "key-name-hint" property in the
509FIT. In this case the "key-name-hint" property is mandatory, and the key must
510exist in "<keydir>/<key-name-hint>.<ext>" Here the extension "ext" is
511specific to the signing algorithm.
512
513
Vesa Jääskeläinenabf8eb22019-06-16 20:53:38 +0300514Hardware Signing with PKCS#11 or with HSM
515-----------------------------------------
George McCollister23d14892017-01-06 13:14:17 -0600516
517Securely managing private signing keys can challenging, especially when the
518keys are stored on the file system of a computer that is connected to the
519Internet. If an attacker is able to steal the key, they can sign malicious FIT
520images which will appear genuine to your devices.
521
522An alternative solution is to keep your signing key securely stored on hardware
523device like a smartcard, USB token or Hardware Security Module (HSM) and have
524them perform the signing. PKCS#11 is standard for interfacing with these crypto
525device.
526
527Requirements:
Vesa Jääskeläinenabf8eb22019-06-16 20:53:38 +0300528Smartcard/USB token/HSM which can work with some openssl engine
George McCollister23d14892017-01-06 13:14:17 -0600529openssl
Vesa Jääskeläinenabf8eb22019-06-16 20:53:38 +0300530
531For pkcs11 engine usage:
George McCollister23d14892017-01-06 13:14:17 -0600532libp11 (provides pkcs11 engine)
533p11-kit (recommended to simplify setup)
534opensc (for smartcards and smartcard like USB devices)
535gnutls (recommended for key generation, p11tool)
536
Vesa Jääskeläinenabf8eb22019-06-16 20:53:38 +0300537For generic HSMs respective openssl engine must be installed and locateable by
538openssl. This may require setting up LD_LIBRARY_PATH if engine is not installed
539to openssl's default search paths.
540
541PKCS11 engine support forms "key id" based on "keydir" and with
Jan Luebbecd4eafb2020-05-13 12:26:24 +0200542"key-name-hint". "key-name-hint" is used as "object" name (if not defined in
543keydir). "keydir" (if defined) is used to define (prefix for) which PKCS11 source
544is being used for lookup up for the key.
Vesa Jääskeläinenabf8eb22019-06-16 20:53:38 +0300545
546PKCS11 engine key ids:
547 "pkcs11:<keydir>;object=<key-name-hint>;type=<public|private>"
Jan Luebbecd4eafb2020-05-13 12:26:24 +0200548or, if keydir contains "object="
549 "pkcs11:<keydir>;type=<public|private>"
Vesa Jääskeläinenabf8eb22019-06-16 20:53:38 +0300550or
551 "pkcs11:object=<key-name-hint>;type=<public|private>",
552
553Generic HSM engine support forms "key id" based on "keydir" and with
554"key-name-hint". If "keydir" is specified for mkimage it is used as a prefix in
555"key id" and is appended with "key-name-hint".
556
557Generic engine key ids:
558 "<keydir><key-name-hint>"
559or
560 "<key-name-hint>"
561
Marc Kleine-Budde88e07a72021-07-23 22:17:50 +0200562In order to set the pin in the HSM, an environment variable "MKIMAGE_SIGN_PIN"
563can be specified.
Vesa Jääskeläinenabf8eb22019-06-16 20:53:38 +0300564
565The following examples use the Nitrokey Pro using pkcs11 engine. Instructions
566for other devices may vary.
George McCollister23d14892017-01-06 13:14:17 -0600567
568Notes on pkcs11 engine setup:
569
570Make sure p11-kit, opensc are installed and that p11-kit is setup to use opensc.
571/usr/share/p11-kit/modules/opensc.module should be present on your system.
572
573
574Generating Keys On the Nitrokey:
575
576$ gpg --card-edit
577
578Reader ...........: Nitrokey Nitrokey Pro (xxxxxxxx0000000000000000) 00 00
579Application ID ...: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
580Version ..........: 2.1
581Manufacturer .....: ZeitControl
582Serial number ....: xxxxxxxx
583Name of cardholder: [not set]
584Language prefs ...: de
585Sex ..............: unspecified
586URL of public key : [not set]
587Login data .......: [not set]
588Signature PIN ....: forced
589Key attributes ...: rsa2048 rsa2048 rsa2048
590Max. PIN lengths .: 32 32 32
591PIN retry counter : 3 0 3
592Signature counter : 0
593Signature key ....: [none]
594Encryption key....: [none]
595Authentication key: [none]
596General key info..: [none]
597
598gpg/card> generate
599Make off-card backup of encryption key? (Y/n) n
600
601Please note that the factory settings of the PINs are
602 PIN = '123456' Admin PIN = '12345678'
603You should change them using the command --change-pin
604
605What keysize do you want for the Signature key? (2048) 4096
606The card will now be re-configured to generate a key of 4096 bits
607Note: There is no guarantee that the card supports the requested size.
608 If the key generation does not succeed, please check the
609 documentation of your card to see what sizes are allowed.
610What keysize do you want for the Encryption key? (2048) 4096
611The card will now be re-configured to generate a key of 4096 bits
612What keysize do you want for the Authentication key? (2048) 4096
613The card will now be re-configured to generate a key of 4096 bits
614Please specify how long the key should be valid.
615 0 = key does not expire
616 <n> = key expires in n days
617 <n>w = key expires in n weeks
618 <n>m = key expires in n months
619 <n>y = key expires in n years
620Key is valid for? (0)
621Key does not expire at all
622Is this correct? (y/N) y
623
624GnuPG needs to construct a user ID to identify your key.
625
626Real name: John Doe
627Email address: john.doe@email.com
628Comment:
629You selected this USER-ID:
630 "John Doe <john.doe@email.com>"
631
632Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? o
633
634
635Using p11tool to get the token URL:
636
637Depending on system configuration, gpg-agent may need to be killed first.
638
639$ p11tool --provider /usr/lib/opensc-pkcs11.so --list-tokens
640Token 0:
641URL: pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29
642Label: OpenPGP card (User PIN (sig))
643Type: Hardware token
644Manufacturer: ZeitControl
645Model: PKCS#15 emulated
646Serial: 000xxxxxxxxx
647Module: (null)
648
649
650Token 1:
651URL: pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%29
652Label: OpenPGP card (User PIN)
653Type: Hardware token
654Manufacturer: ZeitControl
655Model: PKCS#15 emulated
656Serial: 000xxxxxxxxx
657Module: (null)
658
659Use the portion of the signature token URL after "pkcs11:" as the keydir argument (-k) to mkimage below.
660
661
662Use the URL of the token to list the private keys:
663
664$ p11tool --login --provider /usr/lib/opensc-pkcs11.so --list-privkeys \
665"pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29"
666Token 'OpenPGP card (User PIN (sig))' with URL 'pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29' requires user PIN
667Enter PIN:
668Object 0:
669URL: pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29;id=%01;object=Signature%20key;type=private
670Type: Private key
671Label: Signature key
672Flags: CKA_PRIVATE; CKA_NEVER_EXTRACTABLE; CKA_SENSITIVE;
673ID: 01
674
675Use the label, in this case "Signature key" as the key-name-hint in your FIT.
676
677Create the fitImage:
678$ ./tools/mkimage -f fit-image.its fitImage
679
680
681Sign the fitImage with the hardware key:
682
683$ ./tools/mkimage -F -k \
684"model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29" \
685-K u-boot.dtb -N pkcs11 -r fitImage
686
687
Simon Glass58fe7e52013-06-13 15:10:00 -0700688Future Work
689-----------
690- Roll-back protection using a TPM is done using the tpm command. This can
691be scripted, but we might consider a default way of doing this, built into
692bootm.
693
694
695Possible Future Work
696--------------------
697- Add support for other RSA/SHA variants, such as rsa4096,sha512.
698- Other algorithms besides RSA
699- More sandbox tests for failure modes
700- Passwords for keys/certificates
701- Perhaps implement OAEP
702- Enhance bootm to permit scripted signature verification (so that a script
703can verify an image but not actually boot it)
704
705
706Simon Glass
707sjg@chromium.org
7081-1-13