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