1U-Boot Verified Boot
2====================
3
4Introduction
5------------
6Verified boot here means the verification of all software loaded into a
7machine during the boot process to ensure that it is authorised and correct
8for that machine.
9
10Verified boot extends from the moment of system reset to as far as you wish
11into the boot process. An example might be loading U-Boot from read-only
12memory, then loading a signed kernel, then using the kernel's dm-verity
13driver to mount a signed root filesystem.
14
15A key point is that it is possible to field-upgrade the software on machines
16which use verified boot. Since the machine will only run software that has
17been correctly signed, it is safe to read software from an updatable medium.
18It is also possible to add a secondary signed firmware image, in read-write
19memory, so that firmware can easily be upgraded in a secure manner.
20
21
22Signing
23-------
24Verified boot uses cryptographic algorithms to 'sign' software images.
25Images are signed using a private key known only to the signer, but can
26be verified using a public key. As its name suggests the public key can be
27made available without risk to the verification process. The private and
28public keys are mathematically related. For more information on how this
29works look up "public key cryptography" and "RSA" (a particular algorithm).
30
31The signing and verification process looks something like this:
32
33
34      Signing                                      Verification
35      =======                                      ============
36
37 +--------------+                   *
38 | RSA key pair |                   *             +---------------+
39 | .key  .crt   |                   *             | Public key in |
40 +--------------+       +------> public key ----->| trusted place |
41       |                |           *             +---------------+
42       |                |           *                    |
43       v                |           *                    v
44   +---------+          |           *              +--------------+
45   |         |----------+           *              |              |
46   | signer  |                      *              |    U-Boot    |
47   |         |----------+           *              |  signature   |--> yes/no
48   +---------+          |           *              | verification |
49      ^                 |           *              |              |
50      |                 |           *              +--------------+
51      |                 |           *                    ^
52 +----------+           |           *                    |
53 | Software |           +----> signed image -------------+
54 |  image   |                       *
55 +----------+                       *
56
57
58The signature algorithm relies only on the public key to do its work. Using
59this key it checks the signature that it finds in the image. If it verifies
60then we know that the image is OK.
61
62The public key from the signer allows us to verify and therefore trust
63software from updatable memory.
64
65It is critical that the public key be secure and cannot be tampered with.
66It can be stored in read-only memory, or perhaps protected by other on-chip
67crypto provided by some modern SOCs. If the public key can ben changed, then
68the verification is worthless.
69
70
71Chaining Images
72---------------
73The above method works for a signer providing images to a run-time U-Boot.
74It is also possible to extend this scheme to a second level, like this:
75
761. Master private key is used by the signer to sign a first-stage image.
772. Master public key is placed in read-only memory.
782. Secondary private key is created and used to sign second-stage images.
793. Secondary public key is placed in first stage images
804. We use the master public key to verify the first-stage image. We then
81use the secondary public key in the first-stage image to verify the second-
82state image.
835. This chaining process can go on indefinitely. It is recommended to use a
84different key at each stage, so that a compromise in one place will not
85affect the whole change.
86
87
88Flattened Image Tree (FIT)
89--------------------------
90The FIT format is alreay widely used in U-Boot. It is a flattened device
91tree (FDT) in a particular format, with images contained within. FITs
92include hashes to verify images, so it is relatively straightforward to
93add signatures as well.
94
95The public key can be stored in U-Boot's CONFIG_OF_CONTROL device tree in
96a standard place. Then when a FIT it loaded it can be verified using that
97public key. Multiple keys and multiple signatures are supported.
98
99See signature.txt for more information.
100
101
102Simon Glass
103sjg@chromium.org
1041-1-13
105