1U-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. 45At present only one class of algorithms is supported: SHA1 hashing with RSA. 46This works by hashing the image to produce a 20-byte hash. 47 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 59another RSA variant is needed, then it can be added to the table in 60image-sig.c. If another algorithm is needed (such as DSA) then it can be 61placed alongside rsa.c, and its functions added to the table in image-sig.c 62also. 63 64 65Creating an RSA key pair and certificate 66---------------------------------------- 67To 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 72To 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 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 84allow the signer to operate. These should be added to the .its file. 85Signature nodes sit at the same level as hash nodes and are called 86signature-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 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 108 109- padding: The padding algorithm, it may be pkcs-1.5 or pss, 110 if no value is provided we assume pkcs-1.5 111 112For config bindings (see Signed Configurations below), the following 113additional properties are optional: 114 115- 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. 118 119For 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: 123 124 hashed-nodes = "/", "/configurations/conf-1", "/images/kernel", 125 "/images/kernel/hash-1", "/images/fdt-1", 126 "/images/fdt-1/hash-1"; 127 128- 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. 133 134 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: 144 145- algo: Algorithm name (e.g. "sha1,rsa2048") 146 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 156"image" to force verification of all images, and "conf" to force verification 157of 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 166- rsa,exponent: Public exponent (E) as a 64 bit unsigned integer 167- 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 170These parameters can be added to a binary device tree using parameter -K of the 171mkimage command:: 172 173 tools/mkimage -f fit.its -K control.dtb -k keys -r image.fit 174 175Here is an example of a generated device tree node:: 176 177 signature { 178 key-dev { 179 required = "conf"; 180 algo = "sha256,rsa2048"; 181 rsa,r-squared = <0xb76d1acf 0xa1763ca5 0xeb2f126 182 0x742edc80 0xd3f42177 0x9741d9d9 183 0x35bb476e 0xff41c718 0xd3801430 184 0xf22537cb 0xa7e79960 0xae32a043 185 0x7da1427a 0x341d6492 0x3c2762f5 186 0xaac04726 0x5b262d96 0xf984e86d 187 0xb99443c7 0x17080c33 0x940f6892 188 0xd57a95d1 0x6ea7b691 0xc5038fa8 189 0x6bb48a6e 0x73f1b1ea 0x37160841 190 0xe05715ce 0xa7c45bbd 0x690d82d5 191 0x99c2454c 0x6ff117b3 0xd830683b 192 0x3f81c9cf 0x1ca38a91 0x0c3392e4 193 0xd817c625 0x7b8e9a24 0x175b89ea 194 0xad79f3dc 0x4d50d7b4 0x9d4e90f8 195 0xad9e2939 0xc165d6a4 0x0ada7e1b 196 0xfb1bf495 0xfc3131c2 0xb8c6e604 197 0xc2761124 0xf63de4a6 0x0e9565f9 198 0xc8e53761 0x7e7a37a5 0xe99dcdae 199 0x9aff7e1e 0xbd44b13d 0x6b0e6aa4 200 0x038907e4 0x8e0d6850 0xef51bc20 201 0xf73c94af 0x88bea7b1 0xcbbb1b30 202 0xd024b7f3>; 203 rsa,modulus = <0xc0711d6cb 0x9e86db7f 0x45986dbe 204 0x023f1e8c9 0xe1a4c4d0 0x8a0dfdc9 205 0x023ba0c48 0x06815f6a 0x5caa0654 206 0x07078c4b7 0x3d154853 0x40729023 207 0x0b007c8fe 0x5a3647e5 0x23b41e20 208 0x024720591 0x66915305 0x0e0b29b0 209 0x0de2ad30d 0x8589430f 0xb1590325 210 0x0fb9f5d5e 0x9eba752a 0xd88e6de9 211 0x056b3dcc6 0x9a6b8e61 0x6784f61f 212 0x000f39c21 0x5eec6b33 0xd78e4f78 213 0x0921a305f 0xaa2cc27e 0x1ca917af 214 0x06e1134f4 0xd48cac77 0x4e914d07 215 0x0f707aa5a 0x0d141f41 0x84677f1d 216 0x0ad47a049 0x028aedb6 0xd5536fcf 217 0x03fef1e4f 0x133a03d2 0xfd7a750a 218 0x0f9159732 0xd207812e 0x6a807375 219 0x06434230d 0xc8e22dad 0x9f29b3d6 220 0x07c44ac2b 0xfa2aad88 0xe2429504 221 0x041febd41 0x85d0d142 0x7b194d65 222 0x06e5d55ea 0x41116961 0xf3181dde 223 0x068bf5fbc 0x3dd82047 0x00ee647e 224 0x0d7a44ab3>; 225 rsa,exponent = <0x00 0x10001>; 226 rsa,n0-inverse = <0xb3928b85>; 227 rsa,num-bits = <0x800>; 228 key-name-hint = "dev"; 229 }; 230 }; 231 232 233Signed Configurations 234--------------------- 235While signing images is useful, it does not provide complete protection 236against several types of attack. For example, it it possible to create a 237FIT with the same signed images, but with the configuration changed such 238that a different one is selected (mix and match attack). It is also possible 239to substitute a signed image from an older FIT version into a newer FIT 240(roll-back attack). 241 242As an example, consider this FIT: 243 244/ { 245 images { 246 kernel-1 { 247 data = <data for kernel1> 248 signature-1 { 249 algo = "sha1,rsa2048"; 250 value = <...kernel signature 1...> 251 }; 252 }; 253 kernel-2 { 254 data = <data for kernel2> 255 signature-1 { 256 algo = "sha1,rsa2048"; 257 value = <...kernel signature 2...> 258 }; 259 }; 260 fdt-1 { 261 data = <data for fdt1>; 262 signature-1 { 263 algo = "sha1,rsa2048"; 264 vaue = <...fdt signature 1...> 265 }; 266 }; 267 fdt-2 { 268 data = <data for fdt2>; 269 signature-1 { 270 algo = "sha1,rsa2048"; 271 vaue = <...fdt signature 2...> 272 }; 273 }; 274 }; 275 configurations { 276 default = "conf-1"; 277 conf-1 { 278 kernel = "kernel-1"; 279 fdt = "fdt-1"; 280 }; 281 conf-2 { 282 kernel = "kernel-2"; 283 fdt = "fdt-2"; 284 }; 285 }; 286}; 287 288Since both kernels are signed it is easy for an attacker to add a new 289configuration 3 with kernel 1 and fdt 2: 290 291 configurations { 292 default = "conf-1"; 293 conf-1 { 294 kernel = "kernel-1"; 295 fdt = "fdt-1"; 296 }; 297 conf-2 { 298 kernel = "kernel-2"; 299 fdt = "fdt-2"; 300 }; 301 conf-3 { 302 kernel = "kernel-1"; 303 fdt = "fdt-2"; 304 }; 305 }; 306 307With signed images, nothing protects against this. Whether it gains an 308advantage for the attacker is debatable, but it is not secure. 309 310To solve this problem, we support signed configurations. In this case it 311is the configurations that are signed, not the image. Each image has its 312own hash, and we include the hash in the configuration signature. 313 314So the above example is adjusted to look like this: 315 316/ { 317 images { 318 kernel-1 { 319 data = <data for kernel1> 320 hash-1 { 321 algo = "sha1"; 322 value = <...kernel hash 1...> 323 }; 324 }; 325 kernel-2 { 326 data = <data for kernel2> 327 hash-1 { 328 algo = "sha1"; 329 value = <...kernel hash 2...> 330 }; 331 }; 332 fdt-1 { 333 data = <data for fdt1>; 334 hash-1 { 335 algo = "sha1"; 336 value = <...fdt hash 1...> 337 }; 338 }; 339 fdt-2 { 340 data = <data for fdt2>; 341 hash-1 { 342 algo = "sha1"; 343 value = <...fdt hash 2...> 344 }; 345 }; 346 }; 347 configurations { 348 default = "conf-1"; 349 conf-1 { 350 kernel = "kernel-1"; 351 fdt = "fdt-1"; 352 signature-1 { 353 algo = "sha1,rsa2048"; 354 value = <...conf 1 signature...>; 355 }; 356 }; 357 conf-2 { 358 kernel = "kernel-2"; 359 fdt = "fdt-2"; 360 signature-1 { 361 algo = "sha1,rsa2048"; 362 value = <...conf 1 signature...>; 363 }; 364 }; 365 }; 366}; 367 368 369You can see that we have added hashes for all images (since they are no 370longer signed), and a signature to each configuration. In the above example, 371mkimage will sign configurations/conf-1, the kernel and fdt that are 372pointed to by the configuration (/images/kernel-1, /images/kernel-1/hash-1, 373/images/fdt-1, /images/fdt-1/hash-1) and the root structure of the image 374(so that it isn't possible to add or remove root nodes). The signature is 375written into /configurations/conf-1/signature-1/value. It can easily be 376verified later even if the FIT has been signed with other keys in the 377meantime. 378 379 380Verification 381------------ 382FITs are verified when loaded. After the configuration is selected a list 383of required images is produced. If there are 'required' public keys, then 384each image must be verified against those keys. This means that every image 385that might be used by the target needs to be signed with 'required' keys. 386 387This happens automatically as part of a bootm command when FITs are used. 388 389For Signed Configurations, the default verification behavior can be changed by 390the following optional property in /signature node in U-Boot's control FDT. 391 392- required-mode: Valid values are "any" to allow verified boot to succeed if 393the selected configuration is signed by any of the 'required' keys, and "all" 394to allow verified boot to succeed if the selected configuration is signed by 395all of the 'required' keys. 396 397This property can be added to a binary device tree using fdtput as shown in 398below examples:: 399 400 fdtput -t s control.dtb /signature required-mode any 401 fdtput -t s control.dtb /signature required-mode all 402 403 404Enabling FIT Verification 405------------------------- 406In addition to the options to enable FIT itself, the following CONFIGs must 407be enabled: 408 409CONFIG_FIT_SIGNATURE - enable signing and verification in FITs 410CONFIG_RSA - enable RSA algorithm for signing 411 412WARNING: When relying on signed FIT images with required signature check 413the legacy image format is default disabled by not defining 414CONFIG_LEGACY_IMAGE_FORMAT 415 416 417Testing 418------- 419An easy way to test signing and verification is to use the test script 420provided in test/vboot/vboot_test.sh. This uses sandbox (a special version 421of U-Boot which runs under Linux) to show the operation of a 'bootm' 422command loading and verifying images. 423 424A sample run is show below: 425 426$ make O=sandbox sandbox_config 427$ make O=sandbox 428$ O=sandbox ./test/vboot/vboot_test.sh 429 430 431Simple Verified Boot Test 432========================= 433 434Please see doc/uImage.FIT/verified-boot.txt for more information 435 436/home/hs/ids/u-boot/sandbox/tools/mkimage -D -I dts -O dtb -p 2000 437Build keys 438do sha1 test 439Build FIT with signed images 440Test Verified Boot Run: unsigned signatures:: OK 441Sign images 442Test Verified Boot Run: signed images: OK 443Build FIT with signed configuration 444Test Verified Boot Run: unsigned config: OK 445Sign images 446Test Verified Boot Run: signed config: OK 447check signed config on the host 448Signature check OK 449OK 450Test Verified Boot Run: signed config: OK 451Test Verified Boot Run: signed config with bad hash: OK 452do sha256 test 453Build FIT with signed images 454Test Verified Boot Run: unsigned signatures:: OK 455Sign images 456Test Verified Boot Run: signed images: OK 457Build FIT with signed configuration 458Test Verified Boot Run: unsigned config: OK 459Sign images 460Test Verified Boot Run: signed config: OK 461check signed config on the host 462Signature check OK 463OK 464Test Verified Boot Run: signed config: OK 465Test Verified Boot Run: signed config with bad hash: OK 466 467Test passed 468 469 470Hardware Signing with PKCS#11 or with HSM 471----------------------------------------- 472 473Securely managing private signing keys can challenging, especially when the 474keys are stored on the file system of a computer that is connected to the 475Internet. If an attacker is able to steal the key, they can sign malicious FIT 476images which will appear genuine to your devices. 477 478An alternative solution is to keep your signing key securely stored on hardware 479device like a smartcard, USB token or Hardware Security Module (HSM) and have 480them perform the signing. PKCS#11 is standard for interfacing with these crypto 481device. 482 483Requirements: 484Smartcard/USB token/HSM which can work with some openssl engine 485openssl 486 487For pkcs11 engine usage: 488libp11 (provides pkcs11 engine) 489p11-kit (recommended to simplify setup) 490opensc (for smartcards and smartcard like USB devices) 491gnutls (recommended for key generation, p11tool) 492 493For generic HSMs respective openssl engine must be installed and locateable by 494openssl. This may require setting up LD_LIBRARY_PATH if engine is not installed 495to openssl's default search paths. 496 497PKCS11 engine support forms "key id" based on "keydir" and with 498"key-name-hint". "key-name-hint" is used as "object" name (if not defined in 499keydir). "keydir" (if defined) is used to define (prefix for) which PKCS11 source 500is being used for lookup up for the key. 501 502PKCS11 engine key ids: 503 "pkcs11:<keydir>;object=<key-name-hint>;type=<public|private>" 504or, if keydir contains "object=" 505 "pkcs11:<keydir>;type=<public|private>" 506or 507 "pkcs11:object=<key-name-hint>;type=<public|private>", 508 509Generic HSM engine support forms "key id" based on "keydir" and with 510"key-name-hint". If "keydir" is specified for mkimage it is used as a prefix in 511"key id" and is appended with "key-name-hint". 512 513Generic engine key ids: 514 "<keydir><key-name-hint>" 515or 516 "<key-name-hint>" 517 518As mkimage does not at this time support prompting for passwords HSM may need 519key preloading wrapper to be used when invoking mkimage. 520 521The following examples use the Nitrokey Pro using pkcs11 engine. Instructions 522for other devices may vary. 523 524Notes on pkcs11 engine setup: 525 526Make sure p11-kit, opensc are installed and that p11-kit is setup to use opensc. 527/usr/share/p11-kit/modules/opensc.module should be present on your system. 528 529 530Generating Keys On the Nitrokey: 531 532$ gpg --card-edit 533 534Reader ...........: Nitrokey Nitrokey Pro (xxxxxxxx0000000000000000) 00 00 535Application ID ...: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx 536Version ..........: 2.1 537Manufacturer .....: ZeitControl 538Serial number ....: xxxxxxxx 539Name of cardholder: [not set] 540Language prefs ...: de 541Sex ..............: unspecified 542URL of public key : [not set] 543Login data .......: [not set] 544Signature PIN ....: forced 545Key attributes ...: rsa2048 rsa2048 rsa2048 546Max. PIN lengths .: 32 32 32 547PIN retry counter : 3 0 3 548Signature counter : 0 549Signature key ....: [none] 550Encryption key....: [none] 551Authentication key: [none] 552General key info..: [none] 553 554gpg/card> generate 555Make off-card backup of encryption key? (Y/n) n 556 557Please note that the factory settings of the PINs are 558 PIN = '123456' Admin PIN = '12345678' 559You should change them using the command --change-pin 560 561What keysize do you want for the Signature key? (2048) 4096 562The card will now be re-configured to generate a key of 4096 bits 563Note: There is no guarantee that the card supports the requested size. 564 If the key generation does not succeed, please check the 565 documentation of your card to see what sizes are allowed. 566What keysize do you want for the Encryption key? (2048) 4096 567The card will now be re-configured to generate a key of 4096 bits 568What keysize do you want for the Authentication key? (2048) 4096 569The card will now be re-configured to generate a key of 4096 bits 570Please specify how long the key should be valid. 571 0 = key does not expire 572 <n> = key expires in n days 573 <n>w = key expires in n weeks 574 <n>m = key expires in n months 575 <n>y = key expires in n years 576Key is valid for? (0) 577Key does not expire at all 578Is this correct? (y/N) y 579 580GnuPG needs to construct a user ID to identify your key. 581 582Real name: John Doe 583Email address: john.doe@email.com 584Comment: 585You selected this USER-ID: 586 "John Doe <john.doe@email.com>" 587 588Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? o 589 590 591Using p11tool to get the token URL: 592 593Depending on system configuration, gpg-agent may need to be killed first. 594 595$ p11tool --provider /usr/lib/opensc-pkcs11.so --list-tokens 596Token 0: 597URL: pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29 598Label: OpenPGP card (User PIN (sig)) 599Type: Hardware token 600Manufacturer: ZeitControl 601Model: PKCS#15 emulated 602Serial: 000xxxxxxxxx 603Module: (null) 604 605 606Token 1: 607URL: pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%29 608Label: OpenPGP card (User PIN) 609Type: Hardware token 610Manufacturer: ZeitControl 611Model: PKCS#15 emulated 612Serial: 000xxxxxxxxx 613Module: (null) 614 615Use the portion of the signature token URL after "pkcs11:" as the keydir argument (-k) to mkimage below. 616 617 618Use the URL of the token to list the private keys: 619 620$ p11tool --login --provider /usr/lib/opensc-pkcs11.so --list-privkeys \ 621"pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29" 622Token '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 623Enter PIN: 624Object 0: 625URL: 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 626Type: Private key 627Label: Signature key 628Flags: CKA_PRIVATE; CKA_NEVER_EXTRACTABLE; CKA_SENSITIVE; 629ID: 01 630 631Use the label, in this case "Signature key" as the key-name-hint in your FIT. 632 633Create the fitImage: 634$ ./tools/mkimage -f fit-image.its fitImage 635 636 637Sign the fitImage with the hardware key: 638 639$ ./tools/mkimage -F -k \ 640"model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29" \ 641-K u-boot.dtb -N pkcs11 -r fitImage 642 643 644Future Work 645----------- 646- Roll-back protection using a TPM is done using the tpm command. This can 647be scripted, but we might consider a default way of doing this, built into 648bootm. 649 650 651Possible Future Work 652-------------------- 653- Add support for other RSA/SHA variants, such as rsa4096,sha512. 654- Other algorithms besides RSA 655- More sandbox tests for failure modes 656- Passwords for keys/certificates 657- Perhaps implement OAEP 658- Enhance bootm to permit scripted signature verification (so that a script 659can verify an image but not actually boot it) 660 661 662Simon Glass 663sjg@chromium.org 6641-1-13 665