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" or "sha256,ecdsa256") 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 170For 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 175These 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 }; 236 237 238Signed 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 { 251 kernel-1 { 252 data = <data for kernel1> 253 signature-1 { 254 algo = "sha1,rsa2048"; 255 value = <...kernel signature 1...> 256 }; 257 }; 258 kernel-2 { 259 data = <data for kernel2> 260 signature-1 { 261 algo = "sha1,rsa2048"; 262 value = <...kernel signature 2...> 263 }; 264 }; 265 fdt-1 { 266 data = <data for fdt1>; 267 signature-1 { 268 algo = "sha1,rsa2048"; 269 value = <...fdt signature 1...> 270 }; 271 }; 272 fdt-2 { 273 data = <data for fdt2>; 274 signature-1 { 275 algo = "sha1,rsa2048"; 276 value = <...fdt signature 2...> 277 }; 278 }; 279 }; 280 configurations { 281 default = "conf-1"; 282 conf-1 { 283 kernel = "kernel-1"; 284 fdt = "fdt-1"; 285 }; 286 conf-2 { 287 kernel = "kernel-2"; 288 fdt = "fdt-2"; 289 }; 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 { 297 default = "conf-1"; 298 conf-1 { 299 kernel = "kernel-1"; 300 fdt = "fdt-1"; 301 }; 302 conf-2 { 303 kernel = "kernel-2"; 304 fdt = "fdt-2"; 305 }; 306 conf-3 { 307 kernel = "kernel-1"; 308 fdt = "fdt-2"; 309 }; 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 315To solve this problem, we support signed configurations. In this case it 316is 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 { 323 kernel-1 { 324 data = <data for kernel1> 325 hash-1 { 326 algo = "sha1"; 327 value = <...kernel hash 1...> 328 }; 329 }; 330 kernel-2 { 331 data = <data for kernel2> 332 hash-1 { 333 algo = "sha1"; 334 value = <...kernel hash 2...> 335 }; 336 }; 337 fdt-1 { 338 data = <data for fdt1>; 339 hash-1 { 340 algo = "sha1"; 341 value = <...fdt hash 1...> 342 }; 343 }; 344 fdt-2 { 345 data = <data for fdt2>; 346 hash-1 { 347 algo = "sha1"; 348 value = <...fdt hash 2...> 349 }; 350 }; 351 }; 352 configurations { 353 default = "conf-1"; 354 conf-1 { 355 kernel = "kernel-1"; 356 fdt = "fdt-1"; 357 signature-1 { 358 algo = "sha1,rsa2048"; 359 value = <...conf 1 signature...>; 360 }; 361 }; 362 conf-2 { 363 kernel = "kernel-2"; 364 fdt = "fdt-2"; 365 signature-1 { 366 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, 376mkimage 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 379(so that it isn't possible to add or remove root nodes). The signature is 380written into /configurations/conf-1/signature-1/value. It can easily be 381verified later even if the FIT has been signed with other keys in the 382meantime. 383 384 385Verification 386------------ 387FITs are verified when loaded. After the configuration is selected a list 388of required images is produced. If there are 'required' public keys, then 389each image must be verified against those keys. This means that every image 390that might be used by the target needs to be signed with 'required' keys. 391 392This happens automatically as part of a bootm command when FITs are used. 393 394For Signed Configurations, the default verification behavior can be changed by 395the following optional property in /signature node in U-Boot's control FDT. 396 397- required-mode: Valid values are "any" to allow verified boot to succeed if 398the selected configuration is signed by any of the 'required' keys, and "all" 399to allow verified boot to succeed if the selected configuration is signed by 400all of the 'required' keys. 401 402This property can be added to a binary device tree using fdtput as shown in 403below examples:: 404 405 fdtput -t s control.dtb /signature required-mode any 406 fdtput -t s control.dtb /signature required-mode all 407 408 409Enabling FIT Verification 410------------------------- 411In addition to the options to enable FIT itself, the following CONFIGs must 412be enabled: 413 414CONFIG_FIT_SIGNATURE - enable signing and verification in FITs 415CONFIG_RSA - enable RSA algorithm for signing 416 417WARNING: When relying on signed FIT images with required signature check 418the legacy image format is default disabled by not defining 419CONFIG_LEGACY_IMAGE_FORMAT 420 421 422Testing 423------- 424An easy way to test signing and verification is to use the test script 425provided in test/vboot/vboot_test.sh. This uses sandbox (a special version 426of U-Boot which runs under Linux) to show the operation of a 'bootm' 427command loading and verifying images. 428 429A sample run is show below: 430 431$ make O=sandbox sandbox_config 432$ make O=sandbox 433$ O=sandbox ./test/vboot/vboot_test.sh 434 435 436Simple Verified Boot Test 437========================= 438 439Please see doc/uImage.FIT/verified-boot.txt for more information 440 441/home/hs/ids/u-boot/sandbox/tools/mkimage -D -I dts -O dtb -p 2000 442Build keys 443do sha1 test 444Build FIT with signed images 445Test Verified Boot Run: unsigned signatures:: OK 446Sign images 447Test Verified Boot Run: signed images: OK 448Build FIT with signed configuration 449Test Verified Boot Run: unsigned config: OK 450Sign images 451Test Verified Boot Run: signed config: OK 452check signed config on the host 453Signature check OK 454OK 455Test Verified Boot Run: signed config: OK 456Test Verified Boot Run: signed config with bad hash: OK 457do sha256 test 458Build FIT with signed images 459Test Verified Boot Run: unsigned signatures:: OK 460Sign images 461Test Verified Boot Run: signed images: OK 462Build FIT with signed configuration 463Test Verified Boot Run: unsigned config: OK 464Sign images 465Test Verified Boot Run: signed config: OK 466check signed config on the host 467Signature check OK 468OK 469Test Verified Boot Run: signed config: OK 470Test Verified Boot Run: signed config with bad hash: OK 471 472Test passed 473 474 475Software signing: keydir vs keyfile 476----------------------------------- 477 478In the simplest case, signing is done by giving mkimage the 'keyfile'. This is 479the path to a file containing the signing key. 480 481The alternative is to pass the 'keydir' argument. In this case the filename of 482the key is derived from the 'keydir' and the "key-name-hint" property in the 483FIT. In this case the "key-name-hint" property is mandatory, and the key must 484exist in "<keydir>/<key-name-hint>.<ext>" Here the extension "ext" is 485specific to the signing algorithm. 486 487 488Hardware Signing with PKCS#11 or with HSM 489----------------------------------------- 490 491Securely managing private signing keys can challenging, especially when the 492keys are stored on the file system of a computer that is connected to the 493Internet. If an attacker is able to steal the key, they can sign malicious FIT 494images which will appear genuine to your devices. 495 496An alternative solution is to keep your signing key securely stored on hardware 497device like a smartcard, USB token or Hardware Security Module (HSM) and have 498them perform the signing. PKCS#11 is standard for interfacing with these crypto 499device. 500 501Requirements: 502Smartcard/USB token/HSM which can work with some openssl engine 503openssl 504 505For pkcs11 engine usage: 506libp11 (provides pkcs11 engine) 507p11-kit (recommended to simplify setup) 508opensc (for smartcards and smartcard like USB devices) 509gnutls (recommended for key generation, p11tool) 510 511For generic HSMs respective openssl engine must be installed and locateable by 512openssl. This may require setting up LD_LIBRARY_PATH if engine is not installed 513to openssl's default search paths. 514 515PKCS11 engine support forms "key id" based on "keydir" and with 516"key-name-hint". "key-name-hint" is used as "object" name (if not defined in 517keydir). "keydir" (if defined) is used to define (prefix for) which PKCS11 source 518is being used for lookup up for the key. 519 520PKCS11 engine key ids: 521 "pkcs11:<keydir>;object=<key-name-hint>;type=<public|private>" 522or, if keydir contains "object=" 523 "pkcs11:<keydir>;type=<public|private>" 524or 525 "pkcs11:object=<key-name-hint>;type=<public|private>", 526 527Generic HSM engine support forms "key id" based on "keydir" and with 528"key-name-hint". If "keydir" is specified for mkimage it is used as a prefix in 529"key id" and is appended with "key-name-hint". 530 531Generic engine key ids: 532 "<keydir><key-name-hint>" 533or 534 "<key-name-hint>" 535 536In order to set the pin in the HSM, an environment variable "MKIMAGE_SIGN_PIN" 537can be specified. 538 539The following examples use the Nitrokey Pro using pkcs11 engine. Instructions 540for other devices may vary. 541 542Notes on pkcs11 engine setup: 543 544Make sure p11-kit, opensc are installed and that p11-kit is setup to use opensc. 545/usr/share/p11-kit/modules/opensc.module should be present on your system. 546 547 548Generating Keys On the Nitrokey: 549 550$ gpg --card-edit 551 552Reader ...........: Nitrokey Nitrokey Pro (xxxxxxxx0000000000000000) 00 00 553Application ID ...: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx 554Version ..........: 2.1 555Manufacturer .....: ZeitControl 556Serial number ....: xxxxxxxx 557Name of cardholder: [not set] 558Language prefs ...: de 559Sex ..............: unspecified 560URL of public key : [not set] 561Login data .......: [not set] 562Signature PIN ....: forced 563Key attributes ...: rsa2048 rsa2048 rsa2048 564Max. PIN lengths .: 32 32 32 565PIN retry counter : 3 0 3 566Signature counter : 0 567Signature key ....: [none] 568Encryption key....: [none] 569Authentication key: [none] 570General key info..: [none] 571 572gpg/card> generate 573Make off-card backup of encryption key? (Y/n) n 574 575Please note that the factory settings of the PINs are 576 PIN = '123456' Admin PIN = '12345678' 577You should change them using the command --change-pin 578 579What keysize do you want for the Signature key? (2048) 4096 580The card will now be re-configured to generate a key of 4096 bits 581Note: There is no guarantee that the card supports the requested size. 582 If the key generation does not succeed, please check the 583 documentation of your card to see what sizes are allowed. 584What keysize do you want for the Encryption key? (2048) 4096 585The card will now be re-configured to generate a key of 4096 bits 586What keysize do you want for the Authentication key? (2048) 4096 587The card will now be re-configured to generate a key of 4096 bits 588Please specify how long the key should be valid. 589 0 = key does not expire 590 <n> = key expires in n days 591 <n>w = key expires in n weeks 592 <n>m = key expires in n months 593 <n>y = key expires in n years 594Key is valid for? (0) 595Key does not expire at all 596Is this correct? (y/N) y 597 598GnuPG needs to construct a user ID to identify your key. 599 600Real name: John Doe 601Email address: john.doe@email.com 602Comment: 603You selected this USER-ID: 604 "John Doe <john.doe@email.com>" 605 606Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? o 607 608 609Using p11tool to get the token URL: 610 611Depending on system configuration, gpg-agent may need to be killed first. 612 613$ p11tool --provider /usr/lib/opensc-pkcs11.so --list-tokens 614Token 0: 615URL: pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29 616Label: OpenPGP card (User PIN (sig)) 617Type: Hardware token 618Manufacturer: ZeitControl 619Model: PKCS#15 emulated 620Serial: 000xxxxxxxxx 621Module: (null) 622 623 624Token 1: 625URL: pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%29 626Label: OpenPGP card (User PIN) 627Type: Hardware token 628Manufacturer: ZeitControl 629Model: PKCS#15 emulated 630Serial: 000xxxxxxxxx 631Module: (null) 632 633Use the portion of the signature token URL after "pkcs11:" as the keydir argument (-k) to mkimage below. 634 635 636Use the URL of the token to list the private keys: 637 638$ p11tool --login --provider /usr/lib/opensc-pkcs11.so --list-privkeys \ 639"pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29" 640Token '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 641Enter PIN: 642Object 0: 643URL: 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 644Type: Private key 645Label: Signature key 646Flags: CKA_PRIVATE; CKA_NEVER_EXTRACTABLE; CKA_SENSITIVE; 647ID: 01 648 649Use the label, in this case "Signature key" as the key-name-hint in your FIT. 650 651Create the fitImage: 652$ ./tools/mkimage -f fit-image.its fitImage 653 654 655Sign the fitImage with the hardware key: 656 657$ ./tools/mkimage -F -k \ 658"model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29" \ 659-K u-boot.dtb -N pkcs11 -r fitImage 660 661 662Future Work 663----------- 664- Roll-back protection using a TPM is done using the tpm command. This can 665be scripted, but we might consider a default way of doing this, built into 666bootm. 667 668 669Possible Future Work 670-------------------- 671- Add support for other RSA/SHA variants, such as rsa4096,sha512. 672- Other algorithms besides RSA 673- More sandbox tests for failure modes 674- Passwords for keys/certificates 675- Perhaps implement OAEP 676- Enhance bootm to permit scripted signature verification (so that a script 677can verify an image but not actually boot it) 678 679 680Simon Glass 681sjg@chromium.org 6821-1-13 683