signature.txt 20 KB

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  1. U-Boot FIT Signature Verification
  2. =================================
  3. Introduction
  4. ------------
  5. FIT supports hashing of images so that these hashes can be checked on
  6. loading. This protects against corruption of the image. However it does not
  7. prevent the substitution of one image for another.
  8. The signature feature allows the hash to be signed with a private key such
  9. that it can be verified using a public key later. Provided that the private
  10. key is kept secret and the public key is stored in a non-volatile place,
  11. any image can be verified in this way.
  12. See verified-boot.txt for more general information on verified boot.
  13. Concepts
  14. --------
  15. Some familiarity with public key cryptography is assumed in this section.
  16. The procedure for signing is as follows:
  17. - hash an image in the FIT
  18. - sign the hash with a private key to produce a signature
  19. - store the resulting signature in the FIT
  20. The procedure for verification is:
  21. - read the FIT
  22. - obtain the public key
  23. - extract the signature from the FIT
  24. - hash the image from the FIT
  25. - verify (with the public key) that the extracted signature matches the
  26. hash
  27. The signing is generally performed by mkimage, as part of making a firmware
  28. image for the device. The verification is normally done in U-Boot on the
  29. device.
  30. Algorithms
  31. ----------
  32. In principle any suitable algorithm can be used to sign and verify a hash.
  33. At present only one class of algorithms is supported: SHA1 hashing with RSA.
  34. This works by hashing the image to produce a 20-byte hash.
  35. While it is acceptable to bring in large cryptographic libraries such as
  36. openssl on the host side (e.g. mkimage), it is not desirable for U-Boot.
  37. For the run-time verification side, it is important to keep code and data
  38. size as small as possible.
  39. For this reason the RSA image verification uses pre-processed public keys
  40. which can be used with a very small amount of code - just some extraction
  41. of data from the FDT and exponentiation mod n. Code size impact is a little
  42. under 5KB on Tegra Seaboard, for example.
  43. It is relatively straightforward to add new algorithms if required. If
  44. another RSA variant is needed, then it can be added to the table in
  45. image-sig.c. If another algorithm is needed (such as DSA) then it can be
  46. placed alongside rsa.c, and its functions added to the table in image-sig.c
  47. also.
  48. Creating an RSA key pair and certificate
  49. ----------------------------------------
  50. To create a new public/private key pair, size 2048 bits:
  51. $ openssl genpkey -algorithm RSA -out keys/dev.key \
  52. -pkeyopt rsa_keygen_bits:2048 -pkeyopt rsa_keygen_pubexp:65537
  53. To create a certificate for this containing the public key:
  54. $ openssl req -batch -new -x509 -key keys/dev.key -out keys/dev.crt
  55. If you like you can look at the public key also:
  56. $ openssl rsa -in keys/dev.key -pubout
  57. Device Tree Bindings
  58. --------------------
  59. The following properties are required in the FIT's signature node(s) to
  60. allow the signer to operate. These should be added to the .its file.
  61. Signature nodes sit at the same level as hash nodes and are called
  62. signature-1, signature-2, etc.
  63. - algo: Algorithm name (e.g. "sha1,rsa2048")
  64. - key-name-hint: Name of key to use for signing. The keys will normally be in
  65. a single directory (parameter -k to mkimage). For a given key <name>, its
  66. private key is stored in <name>.key and the certificate is stored in
  67. <name>.crt.
  68. When the image is signed, the following properties are added (mandatory):
  69. - value: The signature data (e.g. 256 bytes for 2048-bit RSA)
  70. When the image is signed, the following properties are optional:
  71. - timestamp: Time when image was signed (standard Unix time_t format)
  72. - signer-name: Name of the signer (e.g. "mkimage")
  73. - signer-version: Version string of the signer (e.g. "2013.01")
  74. - comment: Additional information about the signer or image
  75. - padding: The padding algorithm, it may be pkcs-1.5 or pss,
  76. if no value is provided we assume pkcs-1.5
  77. For config bindings (see Signed Configurations below), the following
  78. additional properties are optional:
  79. - sign-images: A list of images to sign, each being a property of the conf
  80. node that contains then. The default is "kernel,fdt" which means that these
  81. two images will be looked up in the config and signed if present.
  82. For config bindings, these properties are added by the signer:
  83. - hashed-nodes: A list of nodes which were hashed by the signer. Each is
  84. a string - the full path to node. A typical value might be:
  85. hashed-nodes = "/", "/configurations/conf-1", "/images/kernel",
  86. "/images/kernel/hash-1", "/images/fdt-1",
  87. "/images/fdt-1/hash-1";
  88. - hashed-strings: The start and size of the string region of the FIT that
  89. was hashed
  90. Example: See sign-images.its for an example image tree source file and
  91. sign-configs.its for config signing.
  92. Public Key Storage
  93. ------------------
  94. In order to verify an image that has been signed with a public key we need to
  95. have a trusted public key. This cannot be stored in the signed image, since
  96. it would be easy to alter. For this implementation we choose to store the
  97. public key in U-Boot's control FDT (using CONFIG_OF_CONTROL).
  98. Public keys should be stored as sub-nodes in a /signature node. Required
  99. properties are:
  100. - algo: Algorithm name (e.g. "sha1,rsa2048")
  101. Optional properties are:
  102. - key-name-hint: Name of key used for signing. This is only a hint since it
  103. is possible for the name to be changed. Verification can proceed by checking
  104. all available signing keys until one matches.
  105. - required: If present this indicates that the key must be verified for the
  106. image / configuration to be considered valid. Only required keys are
  107. normally verified by the FIT image booting algorithm. Valid values are
  108. "image" to force verification of all images, and "conf" to force verification
  109. of the selected configuration (which then relies on hashes in the images to
  110. verify those).
  111. Each signing algorithm has its own additional properties.
  112. For RSA the following are mandatory:
  113. - rsa,num-bits: Number of key bits (e.g. 2048)
  114. - rsa,modulus: Modulus (N) as a big-endian multi-word integer
  115. - rsa,exponent: Public exponent (E) as a 64 bit unsigned integer
  116. - rsa,r-squared: (2^num-bits)^2 as a big-endian multi-word integer
  117. - rsa,n0-inverse: -1 / modulus[0] mod 2^32
  118. These parameters can be added to a binary device tree using parameter -K of the
  119. mkimage command::
  120. tools/mkimage -f fit.its -K control.dtb -k keys -r image.fit
  121. Here is an example of a generated device tree node::
  122. signature {
  123. key-dev {
  124. required = "conf";
  125. algo = "sha256,rsa2048";
  126. rsa,r-squared = <0xb76d1acf 0xa1763ca5 0xeb2f126
  127. 0x742edc80 0xd3f42177 0x9741d9d9
  128. 0x35bb476e 0xff41c718 0xd3801430
  129. 0xf22537cb 0xa7e79960 0xae32a043
  130. 0x7da1427a 0x341d6492 0x3c2762f5
  131. 0xaac04726 0x5b262d96 0xf984e86d
  132. 0xb99443c7 0x17080c33 0x940f6892
  133. 0xd57a95d1 0x6ea7b691 0xc5038fa8
  134. 0x6bb48a6e 0x73f1b1ea 0x37160841
  135. 0xe05715ce 0xa7c45bbd 0x690d82d5
  136. 0x99c2454c 0x6ff117b3 0xd830683b
  137. 0x3f81c9cf 0x1ca38a91 0x0c3392e4
  138. 0xd817c625 0x7b8e9a24 0x175b89ea
  139. 0xad79f3dc 0x4d50d7b4 0x9d4e90f8
  140. 0xad9e2939 0xc165d6a4 0x0ada7e1b
  141. 0xfb1bf495 0xfc3131c2 0xb8c6e604
  142. 0xc2761124 0xf63de4a6 0x0e9565f9
  143. 0xc8e53761 0x7e7a37a5 0xe99dcdae
  144. 0x9aff7e1e 0xbd44b13d 0x6b0e6aa4
  145. 0x038907e4 0x8e0d6850 0xef51bc20
  146. 0xf73c94af 0x88bea7b1 0xcbbb1b30
  147. 0xd024b7f3>;
  148. rsa,modulus = <0xc0711d6cb 0x9e86db7f 0x45986dbe
  149. 0x023f1e8c9 0xe1a4c4d0 0x8a0dfdc9
  150. 0x023ba0c48 0x06815f6a 0x5caa0654
  151. 0x07078c4b7 0x3d154853 0x40729023
  152. 0x0b007c8fe 0x5a3647e5 0x23b41e20
  153. 0x024720591 0x66915305 0x0e0b29b0
  154. 0x0de2ad30d 0x8589430f 0xb1590325
  155. 0x0fb9f5d5e 0x9eba752a 0xd88e6de9
  156. 0x056b3dcc6 0x9a6b8e61 0x6784f61f
  157. 0x000f39c21 0x5eec6b33 0xd78e4f78
  158. 0x0921a305f 0xaa2cc27e 0x1ca917af
  159. 0x06e1134f4 0xd48cac77 0x4e914d07
  160. 0x0f707aa5a 0x0d141f41 0x84677f1d
  161. 0x0ad47a049 0x028aedb6 0xd5536fcf
  162. 0x03fef1e4f 0x133a03d2 0xfd7a750a
  163. 0x0f9159732 0xd207812e 0x6a807375
  164. 0x06434230d 0xc8e22dad 0x9f29b3d6
  165. 0x07c44ac2b 0xfa2aad88 0xe2429504
  166. 0x041febd41 0x85d0d142 0x7b194d65
  167. 0x06e5d55ea 0x41116961 0xf3181dde
  168. 0x068bf5fbc 0x3dd82047 0x00ee647e
  169. 0x0d7a44ab3>;
  170. rsa,exponent = <0x00 0x10001>;
  171. rsa,n0-inverse = <0xb3928b85>;
  172. rsa,num-bits = <0x800>;
  173. key-name-hint = "dev";
  174. };
  175. };
  176. Signed Configurations
  177. ---------------------
  178. While signing images is useful, it does not provide complete protection
  179. against several types of attack. For example, it it possible to create a
  180. FIT with the same signed images, but with the configuration changed such
  181. that a different one is selected (mix and match attack). It is also possible
  182. to substitute a signed image from an older FIT version into a newer FIT
  183. (roll-back attack).
  184. As an example, consider this FIT:
  185. / {
  186. images {
  187. kernel-1 {
  188. data = <data for kernel1>
  189. signature-1 {
  190. algo = "sha1,rsa2048";
  191. value = <...kernel signature 1...>
  192. };
  193. };
  194. kernel-2 {
  195. data = <data for kernel2>
  196. signature-1 {
  197. algo = "sha1,rsa2048";
  198. value = <...kernel signature 2...>
  199. };
  200. };
  201. fdt-1 {
  202. data = <data for fdt1>;
  203. signature-1 {
  204. algo = "sha1,rsa2048";
  205. vaue = <...fdt signature 1...>
  206. };
  207. };
  208. fdt-2 {
  209. data = <data for fdt2>;
  210. signature-1 {
  211. algo = "sha1,rsa2048";
  212. vaue = <...fdt signature 2...>
  213. };
  214. };
  215. };
  216. configurations {
  217. default = "conf-1";
  218. conf-1 {
  219. kernel = "kernel-1";
  220. fdt = "fdt-1";
  221. };
  222. conf-2 {
  223. kernel = "kernel-2";
  224. fdt = "fdt-2";
  225. };
  226. };
  227. };
  228. Since both kernels are signed it is easy for an attacker to add a new
  229. configuration 3 with kernel 1 and fdt 2:
  230. configurations {
  231. default = "conf-1";
  232. conf-1 {
  233. kernel = "kernel-1";
  234. fdt = "fdt-1";
  235. };
  236. conf-2 {
  237. kernel = "kernel-2";
  238. fdt = "fdt-2";
  239. };
  240. conf-3 {
  241. kernel = "kernel-1";
  242. fdt = "fdt-2";
  243. };
  244. };
  245. With signed images, nothing protects against this. Whether it gains an
  246. advantage for the attacker is debatable, but it is not secure.
  247. To solve this problem, we support signed configurations. In this case it
  248. is the configurations that are signed, not the image. Each image has its
  249. own hash, and we include the hash in the configuration signature.
  250. So the above example is adjusted to look like this:
  251. / {
  252. images {
  253. kernel-1 {
  254. data = <data for kernel1>
  255. hash-1 {
  256. algo = "sha1";
  257. value = <...kernel hash 1...>
  258. };
  259. };
  260. kernel-2 {
  261. data = <data for kernel2>
  262. hash-1 {
  263. algo = "sha1";
  264. value = <...kernel hash 2...>
  265. };
  266. };
  267. fdt-1 {
  268. data = <data for fdt1>;
  269. hash-1 {
  270. algo = "sha1";
  271. value = <...fdt hash 1...>
  272. };
  273. };
  274. fdt-2 {
  275. data = <data for fdt2>;
  276. hash-1 {
  277. algo = "sha1";
  278. value = <...fdt hash 2...>
  279. };
  280. };
  281. };
  282. configurations {
  283. default = "conf-1";
  284. conf-1 {
  285. kernel = "kernel-1";
  286. fdt = "fdt-1";
  287. signature-1 {
  288. algo = "sha1,rsa2048";
  289. value = <...conf 1 signature...>;
  290. };
  291. };
  292. conf-2 {
  293. kernel = "kernel-2";
  294. fdt = "fdt-2";
  295. signature-1 {
  296. algo = "sha1,rsa2048";
  297. value = <...conf 1 signature...>;
  298. };
  299. };
  300. };
  301. };
  302. You can see that we have added hashes for all images (since they are no
  303. longer signed), and a signature to each configuration. In the above example,
  304. mkimage will sign configurations/conf-1, the kernel and fdt that are
  305. pointed to by the configuration (/images/kernel-1, /images/kernel-1/hash-1,
  306. /images/fdt-1, /images/fdt-1/hash-1) and the root structure of the image
  307. (so that it isn't possible to add or remove root nodes). The signature is
  308. written into /configurations/conf-1/signature-1/value. It can easily be
  309. verified later even if the FIT has been signed with other keys in the
  310. meantime.
  311. Verification
  312. ------------
  313. FITs are verified when loaded. After the configuration is selected a list
  314. of required images is produced. If there are 'required' public keys, then
  315. each image must be verified against those keys. This means that every image
  316. that might be used by the target needs to be signed with 'required' keys.
  317. This happens automatically as part of a bootm command when FITs are used.
  318. Enabling FIT Verification
  319. -------------------------
  320. In addition to the options to enable FIT itself, the following CONFIGs must
  321. be enabled:
  322. CONFIG_FIT_SIGNATURE - enable signing and verification in FITs
  323. CONFIG_RSA - enable RSA algorithm for signing
  324. WARNING: When relying on signed FIT images with required signature check
  325. the legacy image format is default disabled by not defining
  326. CONFIG_LEGACY_IMAGE_FORMAT
  327. Testing
  328. -------
  329. An easy way to test signing and verification is to use the test script
  330. provided in test/vboot/vboot_test.sh. This uses sandbox (a special version
  331. of U-Boot which runs under Linux) to show the operation of a 'bootm'
  332. command loading and verifying images.
  333. A sample run is show below:
  334. $ make O=sandbox sandbox_config
  335. $ make O=sandbox
  336. $ O=sandbox ./test/vboot/vboot_test.sh
  337. Simple Verified Boot Test
  338. =========================
  339. Please see doc/uImage.FIT/verified-boot.txt for more information
  340. /home/hs/ids/u-boot/sandbox/tools/mkimage -D -I dts -O dtb -p 2000
  341. Build keys
  342. do sha1 test
  343. Build FIT with signed images
  344. Test Verified Boot Run: unsigned signatures:: OK
  345. Sign images
  346. Test Verified Boot Run: signed images: OK
  347. Build FIT with signed configuration
  348. Test Verified Boot Run: unsigned config: OK
  349. Sign images
  350. Test Verified Boot Run: signed config: OK
  351. check signed config on the host
  352. Signature check OK
  353. OK
  354. Test Verified Boot Run: signed config: OK
  355. Test Verified Boot Run: signed config with bad hash: OK
  356. do sha256 test
  357. Build FIT with signed images
  358. Test Verified Boot Run: unsigned signatures:: OK
  359. Sign images
  360. Test Verified Boot Run: signed images: OK
  361. Build FIT with signed configuration
  362. Test Verified Boot Run: unsigned config: OK
  363. Sign images
  364. Test Verified Boot Run: signed config: OK
  365. check signed config on the host
  366. Signature check OK
  367. OK
  368. Test Verified Boot Run: signed config: OK
  369. Test Verified Boot Run: signed config with bad hash: OK
  370. Test passed
  371. Hardware Signing with PKCS#11 or with HSM
  372. -----------------------------------------
  373. Securely managing private signing keys can challenging, especially when the
  374. keys are stored on the file system of a computer that is connected to the
  375. Internet. If an attacker is able to steal the key, they can sign malicious FIT
  376. images which will appear genuine to your devices.
  377. An alternative solution is to keep your signing key securely stored on hardware
  378. device like a smartcard, USB token or Hardware Security Module (HSM) and have
  379. them perform the signing. PKCS#11 is standard for interfacing with these crypto
  380. device.
  381. Requirements:
  382. Smartcard/USB token/HSM which can work with some openssl engine
  383. openssl
  384. For pkcs11 engine usage:
  385. libp11 (provides pkcs11 engine)
  386. p11-kit (recommended to simplify setup)
  387. opensc (for smartcards and smartcard like USB devices)
  388. gnutls (recommended for key generation, p11tool)
  389. For generic HSMs respective openssl engine must be installed and locateable by
  390. openssl. This may require setting up LD_LIBRARY_PATH if engine is not installed
  391. to openssl's default search paths.
  392. PKCS11 engine support forms "key id" based on "keydir" and with
  393. "key-name-hint". "key-name-hint" is used as "object" name (if not defined in
  394. keydir). "keydir" (if defined) is used to define (prefix for) which PKCS11 source
  395. is being used for lookup up for the key.
  396. PKCS11 engine key ids:
  397. "pkcs11:<keydir>;object=<key-name-hint>;type=<public|private>"
  398. or, if keydir contains "object="
  399. "pkcs11:<keydir>;type=<public|private>"
  400. or
  401. "pkcs11:object=<key-name-hint>;type=<public|private>",
  402. Generic HSM engine support forms "key id" based on "keydir" and with
  403. "key-name-hint". If "keydir" is specified for mkimage it is used as a prefix in
  404. "key id" and is appended with "key-name-hint".
  405. Generic engine key ids:
  406. "<keydir><key-name-hint>"
  407. or
  408. "<key-name-hint>"
  409. As mkimage does not at this time support prompting for passwords HSM may need
  410. key preloading wrapper to be used when invoking mkimage.
  411. The following examples use the Nitrokey Pro using pkcs11 engine. Instructions
  412. for other devices may vary.
  413. Notes on pkcs11 engine setup:
  414. Make sure p11-kit, opensc are installed and that p11-kit is setup to use opensc.
  415. /usr/share/p11-kit/modules/opensc.module should be present on your system.
  416. Generating Keys On the Nitrokey:
  417. $ gpg --card-edit
  418. Reader ...........: Nitrokey Nitrokey Pro (xxxxxxxx0000000000000000) 00 00
  419. Application ID ...: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
  420. Version ..........: 2.1
  421. Manufacturer .....: ZeitControl
  422. Serial number ....: xxxxxxxx
  423. Name of cardholder: [not set]
  424. Language prefs ...: de
  425. Sex ..............: unspecified
  426. URL of public key : [not set]
  427. Login data .......: [not set]
  428. Signature PIN ....: forced
  429. Key attributes ...: rsa2048 rsa2048 rsa2048
  430. Max. PIN lengths .: 32 32 32
  431. PIN retry counter : 3 0 3
  432. Signature counter : 0
  433. Signature key ....: [none]
  434. Encryption key....: [none]
  435. Authentication key: [none]
  436. General key info..: [none]
  437. gpg/card> generate
  438. Make off-card backup of encryption key? (Y/n) n
  439. Please note that the factory settings of the PINs are
  440. PIN = '123456' Admin PIN = '12345678'
  441. You should change them using the command --change-pin
  442. What keysize do you want for the Signature key? (2048) 4096
  443. The card will now be re-configured to generate a key of 4096 bits
  444. Note: There is no guarantee that the card supports the requested size.
  445. If the key generation does not succeed, please check the
  446. documentation of your card to see what sizes are allowed.
  447. What keysize do you want for the Encryption key? (2048) 4096
  448. The card will now be re-configured to generate a key of 4096 bits
  449. What keysize do you want for the Authentication key? (2048) 4096
  450. The card will now be re-configured to generate a key of 4096 bits
  451. Please specify how long the key should be valid.
  452. 0 = key does not expire
  453. <n> = key expires in n days
  454. <n>w = key expires in n weeks
  455. <n>m = key expires in n months
  456. <n>y = key expires in n years
  457. Key is valid for? (0)
  458. Key does not expire at all
  459. Is this correct? (y/N) y
  460. GnuPG needs to construct a user ID to identify your key.
  461. Real name: John Doe
  462. Email address: john.doe@email.com
  463. Comment:
  464. You selected this USER-ID:
  465. "John Doe <john.doe@email.com>"
  466. Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? o
  467. Using p11tool to get the token URL:
  468. Depending on system configuration, gpg-agent may need to be killed first.
  469. $ p11tool --provider /usr/lib/opensc-pkcs11.so --list-tokens
  470. Token 0:
  471. URL: pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29
  472. Label: OpenPGP card (User PIN (sig))
  473. Type: Hardware token
  474. Manufacturer: ZeitControl
  475. Model: PKCS#15 emulated
  476. Serial: 000xxxxxxxxx
  477. Module: (null)
  478. Token 1:
  479. URL: pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%29
  480. Label: OpenPGP card (User PIN)
  481. Type: Hardware token
  482. Manufacturer: ZeitControl
  483. Model: PKCS#15 emulated
  484. Serial: 000xxxxxxxxx
  485. Module: (null)
  486. Use the portion of the signature token URL after "pkcs11:" as the keydir argument (-k) to mkimage below.
  487. Use the URL of the token to list the private keys:
  488. $ p11tool --login --provider /usr/lib/opensc-pkcs11.so --list-privkeys \
  489. "pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29"
  490. Token '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
  491. Enter PIN:
  492. Object 0:
  493. URL: 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
  494. Type: Private key
  495. Label: Signature key
  496. Flags: CKA_PRIVATE; CKA_NEVER_EXTRACTABLE; CKA_SENSITIVE;
  497. ID: 01
  498. Use the label, in this case "Signature key" as the key-name-hint in your FIT.
  499. Create the fitImage:
  500. $ ./tools/mkimage -f fit-image.its fitImage
  501. Sign the fitImage with the hardware key:
  502. $ ./tools/mkimage -F -k \
  503. "model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29" \
  504. -K u-boot.dtb -N pkcs11 -r fitImage
  505. Future Work
  506. -----------
  507. - Roll-back protection using a TPM is done using the tpm command. This can
  508. be scripted, but we might consider a default way of doing this, built into
  509. bootm.
  510. Possible Future Work
  511. --------------------
  512. - Add support for other RSA/SHA variants, such as rsa4096,sha512.
  513. - Other algorithms besides RSA
  514. - More sandbox tests for failure modes
  515. - Passwords for keys/certificates
  516. - Perhaps implement OAEP
  517. - Enhance bootm to permit scripted signature verification (so that a script
  518. can verify an image but not actually boot it)
  519. Simon Glass
  520. sjg@chromium.org
  521. 1-1-13