1 U-Boot FIT Signature Verification 2 ================================= 3 4 Introduction 5 ------------ 6 FIT supports hashing of images so that these hashes can be checked on 7 loading. This protects against corruption of the image. However it does not 8 prevent the substitution of one image for another. 9 10 The signature feature allows the hash to be signed with a private key such 11 that it can be verified using a public key later. Provided that the private 12 key is kept secret and the public key is stored in a non-volatile place, 13 any image can be verified in this way. 14 15 See verified-boot.txt for more general information on verified boot. 16 17 18 Concepts 19 -------- 20 Some familiarity with public key cryptography is assumed in this section. 21 22 The 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 28 The 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 37 The signing is generally performed by mkimage, as part of making a firmware 38 image for the device. The verification is normally done in U-Boot on the 39 device. 40 41 42 Algorithms 43 ---------- 44 In principle any suitable algorithm can be used to sign and verify a hash. 45 At present only one class of algorithms is supported: SHA1 hashing with RSA. 46 This works by hashing the image to produce a 20-byte hash. 47 48 While it is acceptable to bring in large cryptographic libraries such as 49 openssl on the host side (e.g. mkimage), it is not desirable for U-Boot. 50 For the run-time verification side, it is important to keep code and data 51 size as small as possible. 52 53 For this reason the RSA image verification uses pre-processed public keys 54 which can be used with a very small amount of code - just some extraction 55 of data from the FDT and exponentiation mod n. Code size impact is a little 56 under 5KB on Tegra Seaboard, for example. 57 58 It is relatively straightforward to add new algorithms if required. If 59 another RSA variant is needed, then it can be added to the table in 60 image-sig.c. If another algorithm is needed (such as DSA) then it can be 61 placed alongside rsa.c, and its functions added to the table in image-sig.c 62 also. 63 64 65 Creating an RSA key pair and certificate 66 ---------------------------------------- 67 To create a new public/private key pair, size 2048 bits: 68 69 $ openssl genpkey -algorithm RSA -out keys/dev.key \ 70 -pkeyopt rsa_keygen_bits:2048 -pkeyopt rsa_keygen_pubexp:65537 71 72 To create a certificate for this containing the public key: 73 74 $ openssl req -batch -new -x509 -key keys/dev.key -out keys/dev.crt 75 76 If you like you can look at the public key also: 77 78 $ openssl rsa -in keys/dev.key -pubout 79 80 81 Device Tree Bindings 82 -------------------- 83 The following properties are required in the FIT's signature node(s) to 84 allow the signer to operate. These should be added to the .its file. 85 Signature nodes sit at the same level as hash nodes and are called 86 signature-1, signature-2, etc. 87 88 - algo: Algorithm name (e.g. "sha1,rsa2048") 89 90 - key-name-hint: Name of key to use for signing. The keys will normally be in 91 a single directory (parameter -k to mkimage). For a given key <name>, its 92 private key is stored in <name>.key and the certificate is stored in 93 <name>.crt. 94 95 When 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 99 When 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 108 109 For config bindings (see Signed Configurations below), the following 110 additional properties are optional: 111 112 - sign-images: A list of images to sign, each being a property of the conf 113 node that contains then. The default is "kernel,fdt" which means that these 114 two images will be looked up in the config and signed if present. 115 116 For config bindings, these properties are added by the signer: 117 118 - hashed-nodes: A list of nodes which were hashed by the signer. Each is 119 a string - the full path to node. A typical value might be: 120 121 hashed-nodes = "/", "/configurations/conf-1", "/images/kernel", 122 "/images/kernel/hash-1", "/images/fdt-1", 123 "/images/fdt-1/hash-1"; 124 125 - hashed-strings: The start and size of the string region of the FIT that 126 was hashed 127 128 Example: See sign-images.its for an example image tree source file and 129 sign-configs.its for config signing. 130 131 132 Public Key Storage 133 ------------------ 134 In order to verify an image that has been signed with a public key we need to 135 have a trusted public key. This cannot be stored in the signed image, since 136 it would be easy to alter. For this implementation we choose to store the 137 public key in U-Boot's control FDT (using CONFIG_OF_CONTROL). 138 139 Public keys should be stored as sub-nodes in a /signature node. Required 140 properties are: 141 142 - algo: Algorithm name (e.g. "sha1,rsa2048") 143 144 Optional properties are: 145 146 - key-name-hint: Name of key used for signing. This is only a hint since it 147 is possible for the name to be changed. Verification can proceed by checking 148 all available signing keys until one matches. 149 150 - required: If present this indicates that the key must be verified for the 151 image / configuration to be considered valid. Only required keys are 152 normally verified by the FIT image booting algorithm. Valid values are 153 "image" to force verification of all images, and "conf" to force verification 154 of the selected configuration (which then relies on hashes in the images to 155 verify those). 156 157 Each signing algorithm has its own additional properties. 158 159 For RSA the following are mandatory: 160 161 - rsa,num-bits: Number of key bits (e.g. 2048) 162 - rsa,modulus: Modulus (N) as a big-endian multi-word integer 163 - rsa,exponent: Public exponent (E) as a 64 bit unsigned integer 164 - rsa,r-squared: (2^num-bits)^2 as a big-endian multi-word integer 165 - rsa,n0-inverse: -1 / modulus[0] mod 2^32 166 167 168 Signed Configurations 169 --------------------- 170 While signing images is useful, it does not provide complete protection 171 against several types of attack. For example, it it possible to create a 172 FIT with the same signed images, but with the configuration changed such 173 that a different one is selected (mix and match attack). It is also possible 174 to substitute a signed image from an older FIT version into a newer FIT 175 (roll-back attack). 176 177 As an example, consider this FIT: 178 179 / { 180 images { 181 kernel-1 { 182 data = <data for kernel1> 183 signature-1 { 184 algo = "sha1,rsa2048"; 185 value = <...kernel signature 1...> 186 }; 187 }; 188 kernel-2 { 189 data = <data for kernel2> 190 signature-1 { 191 algo = "sha1,rsa2048"; 192 value = <...kernel signature 2...> 193 }; 194 }; 195 fdt-1 { 196 data = <data for fdt1>; 197 signature-1 { 198 algo = "sha1,rsa2048"; 199 vaue = <...fdt signature 1...> 200 }; 201 }; 202 fdt-2 { 203 data = <data for fdt2>; 204 signature-1 { 205 algo = "sha1,rsa2048"; 206 vaue = <...fdt signature 2...> 207 }; 208 }; 209 }; 210 configurations { 211 default = "conf-1"; 212 conf-1 { 213 kernel = "kernel-1"; 214 fdt = "fdt-1"; 215 }; 216 conf-1 { 217 kernel = "kernel-2"; 218 fdt = "fdt-2"; 219 }; 220 }; 221 }; 222 223 Since both kernels are signed it is easy for an attacker to add a new 224 configuration 3 with kernel 1 and fdt 2: 225 226 configurations { 227 default = "conf-1"; 228 conf-1 { 229 kernel = "kernel-1"; 230 fdt = "fdt-1"; 231 }; 232 conf-1 { 233 kernel = "kernel-2"; 234 fdt = "fdt-2"; 235 }; 236 conf-3 { 237 kernel = "kernel-1"; 238 fdt = "fdt-2"; 239 }; 240 }; 241 242 With signed images, nothing protects against this. Whether it gains an 243 advantage for the attacker is debatable, but it is not secure. 244 245 To solve this problem, we support signed configurations. In this case it 246 is the configurations that are signed, not the image. Each image has its 247 own hash, and we include the hash in the configuration signature. 248 249 So the above example is adjusted to look like this: 250 251 / { 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 }; 302 303 304 You can see that we have added hashes for all images (since they are no 305 longer signed), and a signature to each configuration. In the above example, 306 mkimage will sign configurations/conf-1, the kernel and fdt that are 307 pointed to by the configuration (/images/kernel-1, /images/kernel-1/hash-1, 308 /images/fdt-1, /images/fdt-1/hash-1) and the root structure of the image 309 (so that it isn't possible to add or remove root nodes). The signature is 310 written into /configurations/conf-1/signature-1/value. It can easily be 311 verified later even if the FIT has been signed with other keys in the 312 meantime. 313 314 315 Verification 316 ------------ 317 FITs are verified when loaded. After the configuration is selected a list 318 of required images is produced. If there are 'required' public keys, then 319 each image must be verified against those keys. This means that every image 320 that might be used by the target needs to be signed with 'required' keys. 321 322 This happens automatically as part of a bootm command when FITs are used. 323 324 325 Enabling FIT Verification 326 ------------------------- 327 In addition to the options to enable FIT itself, the following CONFIGs must 328 be enabled: 329 330 CONFIG_FIT_SIGNATURE - enable signing and verification in FITs 331 CONFIG_RSA - enable RSA algorithm for signing 332 333 WARNING: When relying on signed FIT images with required signature check 334 the legacy image format is default disabled by not defining 335 CONFIG_IMAGE_FORMAT_LEGACY 336 337 Testing 338 ------- 339 An easy way to test signing and verification is to use the test script 340 provided in test/vboot/vboot_test.sh. This uses sandbox (a special version 341 of U-Boot which runs under Linux) to show the operation of a 'bootm' 342 command loading and verifying images. 343 344 A sample run is show below: 345 346 $ make O=sandbox sandbox_config 347 $ make O=sandbox 348 $ O=sandbox ./test/vboot/vboot_test.sh 349 Simple Verified Boot Test 350 ========================= 351 352 Please see doc/uImage.FIT/verified-boot.txt for more information 353 354 /home/hs/ids/u-boot/sandbox/tools/mkimage -D -I dts -O dtb -p 2000 355 Build keys 356 do sha1 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 do sha256 test 371 Build FIT with signed images 372 Test Verified Boot Run: unsigned signatures:: OK 373 Sign images 374 Test Verified Boot Run: signed images: OK 375 Build FIT with signed configuration 376 Test Verified Boot Run: unsigned config: OK 377 Sign images 378 Test Verified Boot Run: signed config: OK 379 check signed config on the host 380 Signature check OK 381 OK 382 Test Verified Boot Run: signed config: OK 383 Test Verified Boot Run: signed config with bad hash: OK 384 385 Test passed 386 387 388 Hardware Signing with PKCS#11 389 ----------------------------- 390 391 Securely managing private signing keys can challenging, especially when the 392 keys are stored on the file system of a computer that is connected to the 393 Internet. If an attacker is able to steal the key, they can sign malicious FIT 394 images which will appear genuine to your devices. 395 396 An alternative solution is to keep your signing key securely stored on hardware 397 device like a smartcard, USB token or Hardware Security Module (HSM) and have 398 them perform the signing. PKCS#11 is standard for interfacing with these crypto 399 device. 400 401 Requirements: 402 Smartcard/USB token/HSM which can work with the pkcs11 engine 403 openssl 404 libp11 (provides pkcs11 engine) 405 p11-kit (recommended to simplify setup) 406 opensc (for smartcards and smartcard like USB devices) 407 gnutls (recommended for key generation, p11tool) 408 409 The following examples use the Nitrokey Pro. Instructions for other devices may vary. 410 411 Notes on pkcs11 engine setup: 412 413 Make sure p11-kit, opensc are installed and that p11-kit is setup to use opensc. 414 /usr/share/p11-kit/modules/opensc.module should be present on your system. 415 416 417 Generating Keys On the Nitrokey: 418 419 $ gpg --card-edit 420 421 Reader ...........: Nitrokey Nitrokey Pro (xxxxxxxx0000000000000000) 00 00 422 Application ID ...: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx 423 Version ..........: 2.1 424 Manufacturer .....: ZeitControl 425 Serial number ....: xxxxxxxx 426 Name of cardholder: [not set] 427 Language prefs ...: de 428 Sex ..............: unspecified 429 URL of public key : [not set] 430 Login data .......: [not set] 431 Signature PIN ....: forced 432 Key attributes ...: rsa2048 rsa2048 rsa2048 433 Max. PIN lengths .: 32 32 32 434 PIN retry counter : 3 0 3 435 Signature counter : 0 436 Signature key ....: [none] 437 Encryption key....: [none] 438 Authentication key: [none] 439 General key info..: [none] 440 441 gpg/card> generate 442 Make off-card backup of encryption key? (Y/n) n 443 444 Please note that the factory settings of the PINs are 445 PIN = '123456' Admin PIN = '12345678' 446 You should change them using the command --change-pin 447 448 What keysize do you want for the Signature key? (2048) 4096 449 The card will now be re-configured to generate a key of 4096 bits 450 Note: There is no guarantee that the card supports the requested size. 451 If the key generation does not succeed, please check the 452 documentation of your card to see what sizes are allowed. 453 What keysize do you want for the Encryption key? (2048) 4096 454 The card will now be re-configured to generate a key of 4096 bits 455 What keysize do you want for the Authentication key? (2048) 4096 456 The card will now be re-configured to generate a key of 4096 bits 457 Please specify how long the key should be valid. 458 0 = key does not expire 459 <n> = key expires in n days 460 <n>w = key expires in n weeks 461 <n>m = key expires in n months 462 <n>y = key expires in n years 463 Key is valid for? (0) 464 Key does not expire at all 465 Is this correct? (y/N) y 466 467 GnuPG needs to construct a user ID to identify your key. 468 469 Real name: John Doe 470 Email address: john.doe (a] email.com 471 Comment: 472 You selected this USER-ID: 473 "John Doe <john.doe (a] email.com>" 474 475 Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? o 476 477 478 Using p11tool to get the token URL: 479 480 Depending on system configuration, gpg-agent may need to be killed first. 481 482 $ p11tool --provider /usr/lib/opensc-pkcs11.so --list-tokens 483 Token 0: 484 URL: pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29 485 Label: OpenPGP card (User PIN (sig)) 486 Type: Hardware token 487 Manufacturer: ZeitControl 488 Model: PKCS#15 emulated 489 Serial: 000xxxxxxxxx 490 Module: (null) 491 492 493 Token 1: 494 URL: pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%29 495 Label: OpenPGP card (User PIN) 496 Type: Hardware token 497 Manufacturer: ZeitControl 498 Model: PKCS#15 emulated 499 Serial: 000xxxxxxxxx 500 Module: (null) 501 502 Use the portion of the signature token URL after "pkcs11:" as the keydir argument (-k) to mkimage below. 503 504 505 Use the URL of the token to list the private keys: 506 507 $ p11tool --login --provider /usr/lib/opensc-pkcs11.so --list-privkeys \ 508 "pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29" 509 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 510 Enter PIN: 511 Object 0: 512 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 513 Type: Private key 514 Label: Signature key 515 Flags: CKA_PRIVATE; CKA_NEVER_EXTRACTABLE; CKA_SENSITIVE; 516 ID: 01 517 518 Use the label, in this case "Signature key" as the key-name-hint in your FIT. 519 520 Create the fitImage: 521 $ ./tools/mkimage -f fit-image.its fitImage 522 523 524 Sign the fitImage with the hardware key: 525 526 $ ./tools/mkimage -F -k \ 527 "model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29" \ 528 -K u-boot.dtb -N pkcs11 -r fitImage 529 530 531 Future Work 532 ----------- 533 - Roll-back protection using a TPM is done using the tpm command. This can 534 be scripted, but we might consider a default way of doing this, built into 535 bootm. 536 537 538 Possible Future Work 539 -------------------- 540 - Add support for other RSA/SHA variants, such as rsa4096,sha512. 541 - Other algorithms besides RSA 542 - More sandbox tests for failure modes 543 - Passwords for keys/certificates 544 - Perhaps implement OAEP 545 - Enhance bootm to permit scripted signature verification (so that a script 546 can verify an image but not actually boot it) 547 548 549 Simon Glass 550 sjg (a] chromium.org 551 1-1-13 552
