1 U-Boot FIT Signature Verification
2 =================================
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.
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.
15 See verified-boot.txt for more general information on verified boot.
20 Some familiarity with public key cryptography is assumed in this section.
22 The procedure for signing is as follows:
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
28 The procedure for verification is:
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
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
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.
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.
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.
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
65 Creating an RSA key pair and certificate
66 ----------------------------------------
67 To create a new public/private key pair, size 2048 bits:
69 $ openssl genpkey -algorithm RSA -out keys/dev.key \
70 -pkeyopt rsa_keygen_bits:2048 -pkeyopt rsa_keygen_pubexp:65537
72 To create a certificate for this containing the public key:
74 $ openssl req -batch -new -x509 -key keys/dev.key -out keys/dev.crt
76 If you like you can look at the public key also:
78 $ openssl rsa -in keys/dev.key -pubout
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.
88 - algo: Algorithm name (e.g. "sha1,rsa2048")
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
95 When the image is signed, the following properties are added (mandatory):
97 - value: The signature data (e.g. 256 bytes for 2048-bit RSA)
99 When the image is signed, the following properties are optional:
101 - timestamp: Time when image was signed (standard Unix time_t format)
103 - signer-name: Name of the signer (e.g. "mkimage")
105 - signer-version: Version string of the signer (e.g. "2013.01")
107 - comment: Additional information about the signer or image
109 - padding: The padding algorithm, it may be pkcs-1.5 or pss,
110 if no value is provided we assume pkcs-1.5
112 For config bindings (see Signed Configurations below), the following
113 additional properties are optional:
115 - sign-images: A list of images to sign, each being a property of the conf
116 node that contains then. The default is "kernel,fdt" which means that these
117 two images will be looked up in the config and signed if present.
119 For config bindings, these properties are added by the signer:
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:
124 hashed-nodes = "/", "/configurations/conf-1", "/images/kernel",
125 "/images/kernel/hash-1", "/images/fdt-1",
126 "/images/fdt-1/hash-1";
128 - hashed-strings: The start and size of the string region of the FIT that
131 Example: See sign-images.its for an example image tree source file and
132 sign-configs.its for config signing.
137 In order to verify an image that has been signed with a public key we need to
138 have a trusted public key. This cannot be stored in the signed image, since
139 it would be easy to alter. For this implementation we choose to store the
140 public key in U-Boot's control FDT (using CONFIG_OF_CONTROL).
142 Public keys should be stored as sub-nodes in a /signature node. Required
145 - algo: Algorithm name (e.g. "sha1,rsa2048" or "sha256,ecdsa256")
147 Optional properties are:
149 - key-name-hint: Name of key used for signing. This is only a hint since it
150 is possible for the name to be changed. Verification can proceed by checking
151 all available signing keys until one matches.
153 - required: If present this indicates that the key must be verified for the
154 image / configuration to be considered valid. Only required keys are
155 normally verified by the FIT image booting algorithm. Valid values are
156 "image" to force verification of all images, and "conf" to force verification
157 of the selected configuration (which then relies on hashes in the images to
160 Each signing algorithm has its own additional properties.
162 For RSA the following are mandatory:
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
170 For 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
175 These parameters can be added to a binary device tree using parameter -K of the
178 tools/mkimage -f fit.its -K control.dtb -k keys -r image.fit
180 Here is an example of a generated device tree node::
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
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
230 rsa,exponent = <0x00 0x10001>;
231 rsa,n0-inverse = <0xb3928b85>;
232 rsa,num-bits = <0x800>;
233 key-name-hint = "dev";
238 Signed Configurations
239 ---------------------
240 While signing images is useful, it does not provide complete protection
241 against several types of attack. For example, it it possible to create a
242 FIT with the same signed images, but with the configuration changed such
243 that a different one is selected (mix and match attack). It is also possible
244 to substitute a signed image from an older FIT version into a newer FIT
247 As an example, consider this FIT:
252 data = <data for kernel1>
254 algo = "sha1,rsa2048";
255 value = <...kernel signature 1...>
259 data = <data for kernel2>
261 algo = "sha1,rsa2048";
262 value = <...kernel signature 2...>
266 data = <data for fdt1>;
268 algo = "sha1,rsa2048";
269 value = <...fdt signature 1...>
273 data = <data for fdt2>;
275 algo = "sha1,rsa2048";
276 value = <...fdt signature 2...>
293 Since both kernels are signed it is easy for an attacker to add a new
294 configuration 3 with kernel 1 and fdt 2:
312 With signed images, nothing protects against this. Whether it gains an
313 advantage for the attacker is debatable, but it is not secure.
315 To solve this problem, we support signed configurations. In this case it
316 is the configurations that are signed, not the image. Each image has its
317 own hash, and we include the hash in the configuration signature.
319 So the above example is adjusted to look like this:
324 data = <data for kernel1>
327 value = <...kernel hash 1...>
331 data = <data for kernel2>
334 value = <...kernel hash 2...>
338 data = <data for fdt1>;
341 value = <...fdt hash 1...>
345 data = <data for fdt2>;
348 value = <...fdt hash 2...>
358 algo = "sha1,rsa2048";
359 value = <...conf 1 signature...>;
366 algo = "sha1,rsa2048";
367 value = <...conf 1 signature...>;
374 You can see that we have added hashes for all images (since they are no
375 longer signed), and a signature to each configuration. In the above example,
376 mkimage will sign configurations/conf-1, the kernel and fdt that are
377 pointed 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
380 written into /configurations/conf-1/signature-1/value. It can easily be
381 verified later even if the FIT has been signed with other keys in the
387 The signature node contains a property ('hashed-nodes') which lists all the
388 nodes that the signature was made over. The image is walked in order and each
389 tag processed as follows:
390 - DTB_BEGIN_NODE: The tag and the following name are included in the signature
391 if the node or its parent are present in 'hashed-nodes'
392 - DTB_END_NODE: The tag is included in the signature if the node or its parent
393 are present in 'hashed-nodes'
394 - DTB_PROPERTY: The tag, the length word, the offset in the string table, and
395 the data are all included if the current node is present in 'hashed-nodes'
396 and the property name is not 'data'.
397 - DTB_END: The tag is always included in the signature.
398 - DTB_NOP: The tag is included in the signature if the current node is present
401 In addition, the signature contains a property 'hashed-strings' which contains
402 the offset and length in the string table of the strings that are to be
403 included in the signature (this is done last).
405 IMPORTANT: To verify the signature outside u-boot, it is vital to not only
406 calculate the hash of the image and verify the signature with that, but also to
407 calculate the hashes of the kernel, fdt, and ramdisk images and check those
408 match the hash values in the corresponding 'hash*' subnodes.
413 FITs are verified when loaded. After the configuration is selected a list
414 of required images is produced. If there are 'required' public keys, then
415 each image must be verified against those keys. This means that every image
416 that might be used by the target needs to be signed with 'required' keys.
418 This happens automatically as part of a bootm command when FITs are used.
420 For Signed Configurations, the default verification behavior can be changed by
421 the following optional property in /signature node in U-Boot's control FDT.
423 - required-mode: Valid values are "any" to allow verified boot to succeed if
424 the selected configuration is signed by any of the 'required' keys, and "all"
425 to allow verified boot to succeed if the selected configuration is signed by
426 all of the 'required' keys.
428 This property can be added to a binary device tree using fdtput as shown in
431 fdtput -t s control.dtb /signature required-mode any
432 fdtput -t s control.dtb /signature required-mode all
435 Enabling FIT Verification
436 -------------------------
437 In addition to the options to enable FIT itself, the following CONFIGs must
440 CONFIG_FIT_SIGNATURE - enable signing and verification in FITs
441 CONFIG_RSA - enable RSA algorithm for signing
443 WARNING: When relying on signed FIT images with required signature check
444 the legacy image format is default disabled by not defining
445 CONFIG_LEGACY_IMAGE_FORMAT
450 An easy way to test signing and verification is to use the test script
451 provided in test/vboot/vboot_test.sh. This uses sandbox (a special version
452 of U-Boot which runs under Linux) to show the operation of a 'bootm'
453 command loading and verifying images.
455 A sample run is show below:
457 $ make O=sandbox sandbox_config
459 $ O=sandbox ./test/vboot/vboot_test.sh
462 Simple Verified Boot Test
463 =========================
465 Please see doc/uImage.FIT/verified-boot.txt for more information
467 /home/hs/ids/u-boot/sandbox/tools/mkimage -D -I dts -O dtb -p 2000
470 Build FIT with signed images
471 Test Verified Boot Run: unsigned signatures:: OK
473 Test Verified Boot Run: signed images: OK
474 Build FIT with signed configuration
475 Test Verified Boot Run: unsigned config: OK
477 Test Verified Boot Run: signed config: OK
478 check signed config on the host
481 Test Verified Boot Run: signed config: OK
482 Test Verified Boot Run: signed config with bad hash: OK
484 Build FIT with signed images
485 Test Verified Boot Run: unsigned signatures:: OK
487 Test Verified Boot Run: signed images: OK
488 Build FIT with signed configuration
489 Test Verified Boot Run: unsigned config: OK
491 Test Verified Boot Run: signed config: OK
492 check signed config on the host
495 Test Verified Boot Run: signed config: OK
496 Test Verified Boot Run: signed config with bad hash: OK
501 Software signing: keydir vs keyfile
502 -----------------------------------
504 In the simplest case, signing is done by giving mkimage the 'keyfile'. This is
505 the path to a file containing the signing key.
507 The alternative is to pass the 'keydir' argument. In this case the filename of
508 the key is derived from the 'keydir' and the "key-name-hint" property in the
509 FIT. In this case the "key-name-hint" property is mandatory, and the key must
510 exist in "<keydir>/<key-name-hint>.<ext>" Here the extension "ext" is
511 specific to the signing algorithm.
514 Hardware Signing with PKCS#11 or with HSM
515 -----------------------------------------
517 Securely managing private signing keys can challenging, especially when the
518 keys are stored on the file system of a computer that is connected to the
519 Internet. If an attacker is able to steal the key, they can sign malicious FIT
520 images which will appear genuine to your devices.
522 An alternative solution is to keep your signing key securely stored on hardware
523 device like a smartcard, USB token or Hardware Security Module (HSM) and have
524 them perform the signing. PKCS#11 is standard for interfacing with these crypto
528 Smartcard/USB token/HSM which can work with some openssl engine
531 For pkcs11 engine usage:
532 libp11 (provides pkcs11 engine)
533 p11-kit (recommended to simplify setup)
534 opensc (for smartcards and smartcard like USB devices)
535 gnutls (recommended for key generation, p11tool)
537 For generic HSMs respective openssl engine must be installed and locateable by
538 openssl. This may require setting up LD_LIBRARY_PATH if engine is not installed
539 to openssl's default search paths.
541 PKCS11 engine support forms "key id" based on "keydir" and with
542 "key-name-hint". "key-name-hint" is used as "object" name (if not defined in
543 keydir). "keydir" (if defined) is used to define (prefix for) which PKCS11 source
544 is being used for lookup up for the key.
546 PKCS11 engine key ids:
547 "pkcs11:<keydir>;object=<key-name-hint>;type=<public|private>"
548 or, if keydir contains "object="
549 "pkcs11:<keydir>;type=<public|private>"
551 "pkcs11:object=<key-name-hint>;type=<public|private>",
553 Generic HSM engine support forms "key id" based on "keydir" and with
554 "key-name-hint". If "keydir" is specified for mkimage it is used as a prefix in
555 "key id" and is appended with "key-name-hint".
557 Generic engine key ids:
558 "<keydir><key-name-hint>"
562 In order to set the pin in the HSM, an environment variable "MKIMAGE_SIGN_PIN"
565 The following examples use the Nitrokey Pro using pkcs11 engine. Instructions
566 for other devices may vary.
568 Notes on pkcs11 engine setup:
570 Make sure p11-kit, opensc are installed and that p11-kit is setup to use opensc.
571 /usr/share/p11-kit/modules/opensc.module should be present on your system.
574 Generating Keys On the Nitrokey:
578 Reader ...........: Nitrokey Nitrokey Pro (xxxxxxxx0000000000000000) 00 00
579 Application ID ...: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
580 Version ..........: 2.1
581 Manufacturer .....: ZeitControl
582 Serial number ....: xxxxxxxx
583 Name of cardholder: [not set]
584 Language prefs ...: de
585 Sex ..............: unspecified
586 URL of public key : [not set]
587 Login data .......: [not set]
588 Signature PIN ....: forced
589 Key attributes ...: rsa2048 rsa2048 rsa2048
590 Max. PIN lengths .: 32 32 32
591 PIN retry counter : 3 0 3
592 Signature counter : 0
593 Signature key ....: [none]
594 Encryption key....: [none]
595 Authentication key: [none]
596 General key info..: [none]
599 Make off-card backup of encryption key? (Y/n) n
601 Please note that the factory settings of the PINs are
602 PIN = '123456' Admin PIN = '12345678'
603 You should change them using the command --change-pin
605 What keysize do you want for the Signature key? (2048) 4096
606 The card will now be re-configured to generate a key of 4096 bits
607 Note: There is no guarantee that the card supports the requested size.
608 If the key generation does not succeed, please check the
609 documentation of your card to see what sizes are allowed.
610 What keysize do you want for the Encryption key? (2048) 4096
611 The card will now be re-configured to generate a key of 4096 bits
612 What keysize do you want for the Authentication key? (2048) 4096
613 The card will now be re-configured to generate a key of 4096 bits
614 Please specify how long the key should be valid.
615 0 = key does not expire
616 <n> = key expires in n days
617 <n>w = key expires in n weeks
618 <n>m = key expires in n months
619 <n>y = key expires in n years
620 Key is valid for? (0)
621 Key does not expire at all
622 Is this correct? (y/N) y
624 GnuPG needs to construct a user ID to identify your key.
627 Email address: john.doe@email.com
629 You selected this USER-ID:
630 "John Doe <john.doe@email.com>"
632 Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? o
635 Using p11tool to get the token URL:
637 Depending on system configuration, gpg-agent may need to be killed first.
639 $ p11tool --provider /usr/lib/opensc-pkcs11.so --list-tokens
641 URL: pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29
642 Label: OpenPGP card (User PIN (sig))
644 Manufacturer: ZeitControl
645 Model: PKCS#15 emulated
651 URL: pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%29
652 Label: OpenPGP card (User PIN)
654 Manufacturer: ZeitControl
655 Model: PKCS#15 emulated
659 Use the portion of the signature token URL after "pkcs11:" as the keydir argument (-k) to mkimage below.
662 Use the URL of the token to list the private keys:
664 $ p11tool --login --provider /usr/lib/opensc-pkcs11.so --list-privkeys \
665 "pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29"
666 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
669 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
672 Flags: CKA_PRIVATE; CKA_NEVER_EXTRACTABLE; CKA_SENSITIVE;
675 Use the label, in this case "Signature key" as the key-name-hint in your FIT.
678 $ ./tools/mkimage -f fit-image.its fitImage
681 Sign the fitImage with the hardware key:
683 $ ./tools/mkimage -F -k \
684 "model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29" \
685 -K u-boot.dtb -N pkcs11 -r fitImage
690 - Roll-back protection using a TPM is done using the tpm command. This can
691 be scripted, but we might consider a default way of doing this, built into
697 - Add support for other RSA/SHA variants, such as rsa4096,sha512.
698 - Other algorithms besides RSA
699 - More sandbox tests for failure modes
700 - Passwords for keys/certificates
701 - Perhaps implement OAEP
702 - Enhance bootm to permit scripted signature verification (so that a script
703 can verify an image but not actually boot it)