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 thes 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,rs2048") 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 109For config bindings (see Signed Configurations below), the following 110additional properties are optional: 111 112- sign-images: A list of images to sign, each being a property of the conf 113node that contains then. The default is "kernel,fdt" which means that these 114two images will be looked up in the config and signed if present. 115 116For 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@1", 122 "/images/kernel@1/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 128Example: See sign-images.its for an example image tree source file and 129sign-configs.its for config signing. 130 131 132Public Key Storage 133------------------ 134In order to verify an image that has been signed with a public key we need to 135have a trusted public key. This cannot be stored in the signed image, since 136it would be easy to alter. For this implementation we choose to store the 137public key in U-Boot's control FDT (using CONFIG_OF_CONTROL). 138 139Public keys should be stored as sub-nodes in a /signature node. Required 140properties are: 141 142- algo: Algorithm name (e.g. "sha1,rs2048") 143 144Optional properties are: 145 146- key-name-hint: Name of key used for signing. This is only a hint since it 147is possible for the name to be changed. Verification can proceed by checking 148all available signing keys until one matches. 149 150- required: If present this indicates that the key must be verified for the 151image / configuration to be considered valid. Only required keys are 152normally verified by the FIT image booting algorithm. Valid values are 153"image" to force verification of all images, and "conf" to force verfication 154of the selected configuration (which then relies on hashes in the images to 155verify those). 156 157Each signing algorithm has its own additional properties. 158 159For 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 168Signed Configurations 169--------------------- 170While signing images is useful, it does not provide complete protection 171against several types of attack. For example, it it possible to create a 172FIT with the same signed images, but with the configuration changed such 173that a different one is selected (mix and match attack). It is also possible 174to substitute a signed image from an older FIT version into a newer FIT 175(roll-back attack). 176 177As 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 223Since both kernels are signed it is easy for an attacker to add a new 224configuration 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 242With signed images, nothing protects against this. Whether it gains an 243advantage for the attacker is debatable, but it is not secure. 244 245To solved this problem, we support signed configurations. In this case it 246is the configurations that are signed, not the image. Each image has its 247own hash, and we include the hash in the configuration signature. 248 249So 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 304You can see that we have added hashes for all images (since they are no 305longer signed), and a signature to each configuration. In the above example, 306mkimage will sign configurations/conf@1, the kernel and fdt that are 307pointed 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 310written into /configurations/conf@1/signature@1/value. It can easily be 311verified later even if the FIT has been signed with other keys in the 312meantime. 313 314 315Verification 316------------ 317FITs are verified when loaded. After the configuration is selected a list 318of required images is produced. If there are 'required' public keys, then 319each image must be verified against those keys. This means that every image 320that might be used by the target needs to be signed with 'required' keys. 321 322This happens automatically as part of a bootm command when FITs are used. 323 324 325Enabling FIT Verification 326------------------------- 327In addition to the options to enable FIT itself, the following CONFIGs must 328be enabled: 329 330CONFIG_FIT_SIGNATURE - enable signing and verfication in FITs 331CONFIG_RSA - enable RSA algorithm for signing 332 333WARNING: When relying on signed FIT images with required signature check 334the legacy image format is default disabled by not defining 335CONFIG_IMAGE_FORMAT_LEGACY 336 337Testing 338------- 339An easy way to test signing and verfication is to use the test script 340provided in test/vboot/vboot_test.sh. This uses sandbox (a special version 341of U-Boot which runs under Linux) to show the operation of a 'bootm' 342command loading and verifying images. 343 344A sample run is show below: 345 346$ make O=sandbox sandbox_config 347$ make O=sandbox 348$ O=sandbox ./test/vboot/vboot_test.sh 349Simple Verified Boot Test 350========================= 351 352Please 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 355Build keys 356do sha1 test 357Build FIT with signed images 358Test Verified Boot Run: unsigned signatures:: OK 359Sign images 360Test Verified Boot Run: signed images: OK 361Build FIT with signed configuration 362Test Verified Boot Run: unsigned config: OK 363Sign images 364Test Verified Boot Run: signed config: OK 365check signed config on the host 366Signature check OK 367OK 368Test Verified Boot Run: signed config: OK 369Test Verified Boot Run: signed config with bad hash: OK 370do sha256 test 371Build FIT with signed images 372Test Verified Boot Run: unsigned signatures:: OK 373Sign images 374Test Verified Boot Run: signed images: OK 375Build FIT with signed configuration 376Test Verified Boot Run: unsigned config: OK 377Sign images 378Test Verified Boot Run: signed config: OK 379check signed config on the host 380Signature check OK 381OK 382Test Verified Boot Run: signed config: OK 383Test Verified Boot Run: signed config with bad hash: OK 384 385Test passed 386 387 388Future Work 389----------- 390- Roll-back protection using a TPM is done using the tpm command. This can 391be scripted, but we might consider a default way of doing this, built into 392bootm. 393 394 395Possible Future Work 396-------------------- 397- Add support for other RSA/SHA variants, such as rsa4096,sha512. 398- Other algorithms besides RSA 399- More sandbox tests for failure modes 400- Passwords for keys/certificates 401- Perhaps implement OAEP 402- Enhance bootm to permit scripted signature verification (so that a script 403can verify an image but not actually boot it) 404 405 406Simon Glass 407sjg@chromium.org 4081-1-13 409