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 2^32 169 170 171Signed Configurations 172--------------------- 173While signing images is useful, it does not provide complete protection 174against several types of attack. For example, it it possible to create a 175FIT with the same signed images, but with the configuration changed such 176that a different one is selected (mix and match attack). It is also possible 177to substitute a signed image from an older FIT version into a newer FIT 178(roll-back attack). 179 180As an example, consider this FIT: 181 182/ { 183 images { 184 kernel-1 { 185 data = <data for kernel1> 186 signature-1 { 187 algo = "sha1,rsa2048"; 188 value = <...kernel signature 1...> 189 }; 190 }; 191 kernel-2 { 192 data = <data for kernel2> 193 signature-1 { 194 algo = "sha1,rsa2048"; 195 value = <...kernel signature 2...> 196 }; 197 }; 198 fdt-1 { 199 data = <data for fdt1>; 200 signature-1 { 201 algo = "sha1,rsa2048"; 202 vaue = <...fdt signature 1...> 203 }; 204 }; 205 fdt-2 { 206 data = <data for fdt2>; 207 signature-1 { 208 algo = "sha1,rsa2048"; 209 vaue = <...fdt signature 2...> 210 }; 211 }; 212 }; 213 configurations { 214 default = "conf-1"; 215 conf-1 { 216 kernel = "kernel-1"; 217 fdt = "fdt-1"; 218 }; 219 conf-1 { 220 kernel = "kernel-2"; 221 fdt = "fdt-2"; 222 }; 223 }; 224}; 225 226Since both kernels are signed it is easy for an attacker to add a new 227configuration 3 with kernel 1 and fdt 2: 228 229 configurations { 230 default = "conf-1"; 231 conf-1 { 232 kernel = "kernel-1"; 233 fdt = "fdt-1"; 234 }; 235 conf-1 { 236 kernel = "kernel-2"; 237 fdt = "fdt-2"; 238 }; 239 conf-3 { 240 kernel = "kernel-1"; 241 fdt = "fdt-2"; 242 }; 243 }; 244 245With signed images, nothing protects against this. Whether it gains an 246advantage for the attacker is debatable, but it is not secure. 247 248To solve this problem, we support signed configurations. In this case it 249is the configurations that are signed, not the image. Each image has its 250own hash, and we include the hash in the configuration signature. 251 252So the above example is adjusted to look like this: 253 254/ { 255 images { 256 kernel-1 { 257 data = <data for kernel1> 258 hash-1 { 259 algo = "sha1"; 260 value = <...kernel hash 1...> 261 }; 262 }; 263 kernel-2 { 264 data = <data for kernel2> 265 hash-1 { 266 algo = "sha1"; 267 value = <...kernel hash 2...> 268 }; 269 }; 270 fdt-1 { 271 data = <data for fdt1>; 272 hash-1 { 273 algo = "sha1"; 274 value = <...fdt hash 1...> 275 }; 276 }; 277 fdt-2 { 278 data = <data for fdt2>; 279 hash-1 { 280 algo = "sha1"; 281 value = <...fdt hash 2...> 282 }; 283 }; 284 }; 285 configurations { 286 default = "conf-1"; 287 conf-1 { 288 kernel = "kernel-1"; 289 fdt = "fdt-1"; 290 signature-1 { 291 algo = "sha1,rsa2048"; 292 value = <...conf 1 signature...>; 293 }; 294 }; 295 conf-2 { 296 kernel = "kernel-2"; 297 fdt = "fdt-2"; 298 signature-1 { 299 algo = "sha1,rsa2048"; 300 value = <...conf 1 signature...>; 301 }; 302 }; 303 }; 304}; 305 306 307You can see that we have added hashes for all images (since they are no 308longer signed), and a signature to each configuration. In the above example, 309mkimage will sign configurations/conf-1, the kernel and fdt that are 310pointed to by the configuration (/images/kernel-1, /images/kernel-1/hash-1, 311/images/fdt-1, /images/fdt-1/hash-1) and the root structure of the image 312(so that it isn't possible to add or remove root nodes). The signature is 313written into /configurations/conf-1/signature-1/value. It can easily be 314verified later even if the FIT has been signed with other keys in the 315meantime. 316 317 318Verification 319------------ 320FITs are verified when loaded. After the configuration is selected a list 321of required images is produced. If there are 'required' public keys, then 322each image must be verified against those keys. This means that every image 323that might be used by the target needs to be signed with 'required' keys. 324 325This happens automatically as part of a bootm command when FITs are used. 326 327 328Enabling FIT Verification 329------------------------- 330In addition to the options to enable FIT itself, the following CONFIGs must 331be enabled: 332 333CONFIG_FIT_SIGNATURE - enable signing and verification in FITs 334CONFIG_RSA - enable RSA algorithm for signing 335 336WARNING: When relying on signed FIT images with required signature check 337the legacy image format is default disabled by not defining 338CONFIG_IMAGE_FORMAT_LEGACY 339 340Testing 341------- 342An easy way to test signing and verification is to use the test script 343provided in test/vboot/vboot_test.sh. This uses sandbox (a special version 344of U-Boot which runs under Linux) to show the operation of a 'bootm' 345command loading and verifying images. 346 347A sample run is show below: 348 349$ make O=sandbox sandbox_config 350$ make O=sandbox 351$ O=sandbox ./test/vboot/vboot_test.sh 352Simple Verified Boot Test 353========================= 354 355Please see doc/uImage.FIT/verified-boot.txt for more information 356 357/home/hs/ids/u-boot/sandbox/tools/mkimage -D -I dts -O dtb -p 2000 358Build keys 359do sha1 test 360Build FIT with signed images 361Test Verified Boot Run: unsigned signatures:: OK 362Sign images 363Test Verified Boot Run: signed images: OK 364Build FIT with signed configuration 365Test Verified Boot Run: unsigned config: OK 366Sign images 367Test Verified Boot Run: signed config: OK 368check signed config on the host 369Signature check OK 370OK 371Test Verified Boot Run: signed config: OK 372Test Verified Boot Run: signed config with bad hash: OK 373do sha256 test 374Build FIT with signed images 375Test Verified Boot Run: unsigned signatures:: OK 376Sign images 377Test Verified Boot Run: signed images: OK 378Build FIT with signed configuration 379Test Verified Boot Run: unsigned config: OK 380Sign images 381Test Verified Boot Run: signed config: OK 382check signed config on the host 383Signature check OK 384OK 385Test Verified Boot Run: signed config: OK 386Test Verified Boot Run: signed config with bad hash: OK 387 388Test passed 389 390 391Hardware Signing with PKCS#11 392----------------------------- 393 394Securely managing private signing keys can challenging, especially when the 395keys are stored on the file system of a computer that is connected to the 396Internet. If an attacker is able to steal the key, they can sign malicious FIT 397images which will appear genuine to your devices. 398 399An alternative solution is to keep your signing key securely stored on hardware 400device like a smartcard, USB token or Hardware Security Module (HSM) and have 401them perform the signing. PKCS#11 is standard for interfacing with these crypto 402device. 403 404Requirements: 405Smartcard/USB token/HSM which can work with the pkcs11 engine 406openssl 407libp11 (provides pkcs11 engine) 408p11-kit (recommended to simplify setup) 409opensc (for smartcards and smartcard like USB devices) 410gnutls (recommended for key generation, p11tool) 411 412The following examples use the Nitrokey Pro. Instructions for other devices may vary. 413 414Notes on pkcs11 engine setup: 415 416Make sure p11-kit, opensc are installed and that p11-kit is setup to use opensc. 417/usr/share/p11-kit/modules/opensc.module should be present on your system. 418 419 420Generating Keys On the Nitrokey: 421 422$ gpg --card-edit 423 424Reader ...........: Nitrokey Nitrokey Pro (xxxxxxxx0000000000000000) 00 00 425Application ID ...: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx 426Version ..........: 2.1 427Manufacturer .....: ZeitControl 428Serial number ....: xxxxxxxx 429Name of cardholder: [not set] 430Language prefs ...: de 431Sex ..............: unspecified 432URL of public key : [not set] 433Login data .......: [not set] 434Signature PIN ....: forced 435Key attributes ...: rsa2048 rsa2048 rsa2048 436Max. PIN lengths .: 32 32 32 437PIN retry counter : 3 0 3 438Signature counter : 0 439Signature key ....: [none] 440Encryption key....: [none] 441Authentication key: [none] 442General key info..: [none] 443 444gpg/card> generate 445Make off-card backup of encryption key? (Y/n) n 446 447Please note that the factory settings of the PINs are 448 PIN = '123456' Admin PIN = '12345678' 449You should change them using the command --change-pin 450 451What keysize do you want for the Signature key? (2048) 4096 452The card will now be re-configured to generate a key of 4096 bits 453Note: There is no guarantee that the card supports the requested size. 454 If the key generation does not succeed, please check the 455 documentation of your card to see what sizes are allowed. 456What keysize do you want for the Encryption key? (2048) 4096 457The card will now be re-configured to generate a key of 4096 bits 458What keysize do you want for the Authentication key? (2048) 4096 459The card will now be re-configured to generate a key of 4096 bits 460Please specify how long the key should be valid. 461 0 = key does not expire 462 <n> = key expires in n days 463 <n>w = key expires in n weeks 464 <n>m = key expires in n months 465 <n>y = key expires in n years 466Key is valid for? (0) 467Key does not expire at all 468Is this correct? (y/N) y 469 470GnuPG needs to construct a user ID to identify your key. 471 472Real name: John Doe 473Email address: john.doe@email.com 474Comment: 475You selected this USER-ID: 476 "John Doe <john.doe@email.com>" 477 478Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? o 479 480 481Using p11tool to get the token URL: 482 483Depending on system configuration, gpg-agent may need to be killed first. 484 485$ p11tool --provider /usr/lib/opensc-pkcs11.so --list-tokens 486Token 0: 487URL: pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29 488Label: OpenPGP card (User PIN (sig)) 489Type: Hardware token 490Manufacturer: ZeitControl 491Model: PKCS#15 emulated 492Serial: 000xxxxxxxxx 493Module: (null) 494 495 496Token 1: 497URL: pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%29 498Label: OpenPGP card (User PIN) 499Type: Hardware token 500Manufacturer: ZeitControl 501Model: PKCS#15 emulated 502Serial: 000xxxxxxxxx 503Module: (null) 504 505Use the portion of the signature token URL after "pkcs11:" as the keydir argument (-k) to mkimage below. 506 507 508Use the URL of the token to list the private keys: 509 510$ p11tool --login --provider /usr/lib/opensc-pkcs11.so --list-privkeys \ 511"pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29" 512Token '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 513Enter PIN: 514Object 0: 515URL: 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 516Type: Private key 517Label: Signature key 518Flags: CKA_PRIVATE; CKA_NEVER_EXTRACTABLE; CKA_SENSITIVE; 519ID: 01 520 521Use the label, in this case "Signature key" as the key-name-hint in your FIT. 522 523Create the fitImage: 524$ ./tools/mkimage -f fit-image.its fitImage 525 526 527Sign the fitImage with the hardware key: 528 529$ ./tools/mkimage -F -k \ 530"model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29" \ 531-K u-boot.dtb -N pkcs11 -r fitImage 532 533 534Future Work 535----------- 536- Roll-back protection using a TPM is done using the tpm command. This can 537be scripted, but we might consider a default way of doing this, built into 538bootm. 539 540 541Possible Future Work 542-------------------- 543- Add support for other RSA/SHA variants, such as rsa4096,sha512. 544- Other algorithms besides RSA 545- More sandbox tests for failure modes 546- Passwords for keys/certificates 547- Perhaps implement OAEP 548- Enhance bootm to permit scripted signature verification (so that a script 549can verify an image but not actually boot it) 550 551 552Simon Glass 553sjg@chromium.org 5541-1-13 555