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 and certificate
66 -----------------------------------
67 To create a new public key, size 2048 bits:
69 $ openssl genrsa -F4 -out keys/dev.key 2048
71 To create a certificate for this:
73 $ openssl req -batch -new -x509 -key keys/dev.key -out keys/dev.crt
75 If you like you can look at the public key also:
77 $ openssl rsa -in keys/dev.key -pubout
82 The following properties are required in the FIT's signature node(s) to
83 allow thes signer to operate. These should be added to the .its file.
84 Signature nodes sit at the same level as hash nodes and are called
85 signature@1, signature@2, etc.
87 - algo: Algorithm name (e.g. "sha1,rs2048")
89 - key-name-hint: Name of key to use for signing. The keys will normally be in
90 a single directory (parameter -k to mkimage). For a given key <name>, its
91 private key is stored in <name>.key and the certificate is stored in
94 When the image is signed, the following properties are added (mandatory):
96 - value: The signature data (e.g. 256 bytes for 2048-bit RSA)
98 When the image is signed, the following properties are optional:
100 - timestamp: Time when image was signed (standard Unix time_t format)
102 - signer-name: Name of the signer (e.g. "mkimage")
104 - signer-version: Version string of the signer (e.g. "2013.01")
106 - comment: Additional information about the signer or image
109 Example: See sign-images.its for an example image tree source file.
114 In order to verify an image that has been signed with a public key we need to
115 have a trusted public key. This cannot be stored in the signed image, since
116 it would be easy to alter. For this implementation we choose to store the
117 public key in U-Boot's control FDT (using CONFIG_OF_CONTROL).
119 Public keys should be stored as sub-nodes in a /signature node. Required
122 - algo: Algorithm name (e.g. "sha1,rs2048")
124 Optional properties are:
126 - key-name-hint: Name of key used for signing. This is only a hint since it
127 is possible for the name to be changed. Verification can proceed by checking
128 all available signing keys until one matches.
130 - required: If present this indicates that the key must be verified for the
131 image / configuration to be considered valid. Only required keys are
132 normally verified by the FIT image booting algorithm. Valid values are
133 "image" to force verification of all images, and "conf" to force verfication
134 of the selected configuration (which then relies on hashes in the images to
137 Each signing algorithm has its own additional properties.
139 For RSA the following are mandatory:
141 - rsa,num-bits: Number of key bits (e.g. 2048)
142 - rsa,modulus: Modulus (N) as a big-endian multi-word integer
143 - rsa,r-squared: (2^num-bits)^2 as a big-endian multi-word integer
144 - rsa,n0-inverse: -1 / modulus[0] mod 2^32
149 FITs are verified when loaded. After the configuration is selected a list
150 of required images is produced. If there are 'required' public keys, then
151 each image must be verified against those keys. This means that every image
152 that might be used by the target needs to be signed with 'required' keys.
154 This happens automatically as part of a bootm command when FITs are used.
157 Enabling FIT Verification
158 -------------------------
159 In addition to the options to enable FIT itself, the following CONFIGs must
162 CONFIG_FIT_SIGNATURE - enable signing and verfication in FITs
163 CONFIG_RSA - enable RSA algorithm for signing
168 An easy way to test signing and verfication is to use the test script
169 provided in test/vboot/vboot_test.sh. This uses sandbox (a special version
170 of U-Boot which runs under Linux) to show the operation of a 'bootm'
171 command loading and verifying images.
173 A sample run is show below:
175 $ make O=sandbox sandbox_config
177 $ O=sandbox ./test/vboot/vboot_test.sh
178 Simple Verified Boot Test
179 =========================
181 Please see doc/uImage.FIT/verified-boot.txt for more information
184 Build FIT with signed images
185 Test Verified Boot Run: unsigned signatures:: OK
187 Test Verified Boot Run: signed images: OK
188 Build FIT with signed configuration
189 Test Verified Boot Run: unsigned config: OK
191 Test Verified Boot Run: signed config: OK
198 - Roll-back protection using a TPM is done using the tpm command. This can
199 be scripted, but we might consider a default way of doing this, built into
205 - Add support for other RSA/SHA variants, such as rsa4096,sha512.
206 - Other algorithms besides RSA
207 - More sandbox tests for failure modes
208 - Passwords for keys/certificates
209 - Perhaps implement OAEP
210 - Enhance bootm to permit scripted signature verification (so that a script
211 can verify an image but not actually boot it)