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+U-Boot Verified Boot
+====================
+
+Introduction
+------------
+Verified boot here means the verification of all software loaded into a
+machine during the boot process to ensure that it is authorised and correct
+for that machine.
+
+Verified boot extends from the moment of system reset to as far as you wish
+into the boot process. An example might be loading U-Boot from read-only
+memory, then loading a signed kernel, then using the kernel's dm-verity
+driver to mount a signed root filesystem.
+
+A key point is that it is possible to field-upgrade the software on machines
+which use verified boot. Since the machine will only run software that has
+been correctly signed, it is safe to read software from an updatable medium.
+It is also possible to add a secondary signed firmware image, in read-write
+memory, so that firmware can easily be upgraded in a secure manner.
+
+
+Signing
+-------
+Verified boot uses cryptographic algorithms to 'sign' software images.
+Images are signed using a private key known only to the signer, but can
+be verified using a public key. As its name suggests the public key can be
+made available without risk to the verification process. The private and
+public keys are mathematically related. For more information on how this
+works look up "public key cryptography" and "RSA" (a particular algorithm).
+
+The signing and verification process looks something like this:
+
+
+      Signing                                      Verification
+      =======                                      ============
+
+ +--------------+                   *
+ | RSA key pair |                   *             +---------------+
+ | .key  .crt   |                   *             | Public key in |
+ +--------------+       +------> public key ----->| trusted place |
+       |                |           *             +---------------+
+       |                |           *                    |
+       v                |           *                    v
+   +---------+          |           *              +--------------+
+   |         |----------+           *              |              |
+   | signer  |                      *              |    U-Boot    |
+   |         |----------+           *              |  signature   |--> yes/no
+   +---------+          |           *              | verification |
+      ^                 |           *              |              |
+      |                 |           *              +--------------+
+      |                 |           *                    ^
+ +----------+           |           *                    |
+ | Software |           +----> signed image -------------+
+ |  image   |                       *
+ +----------+                       *
+
+
+The signature algorithm relies only on the public key to do its work. Using
+this key it checks the signature that it finds in the image. If it verifies
+then we know that the image is OK.
+
+The public key from the signer allows us to verify and therefore trust
+software from updatable memory.
+
+It is critical that the public key be secure and cannot be tampered with.
+It can be stored in read-only memory, or perhaps protected by other on-chip
+crypto provided by some modern SOCs. If the public key can ben changed, then
+the verification is worthless.
+
+
+Chaining Images
+---------------
+The above method works for a signer providing images to a run-time U-Boot.
+It is also possible to extend this scheme to a second level, like this:
+
+1. Master private key is used by the signer to sign a first-stage image.
+2. Master public key is placed in read-only memory.
+2. Secondary private key is created and used to sign second-stage images.
+3. Secondary public key is placed in first stage images
+4. We use the master public key to verify the first-stage image. We then
+use the secondary public key in the first-stage image to verify the second-
+state image.
+5. This chaining process can go on indefinitely. It is recommended to use a
+different key at each stage, so that a compromise in one place will not
+affect the whole change.
+
+
+Flattened Image Tree (FIT)
+--------------------------
+The FIT format is alreay widely used in U-Boot. It is a flattened device
+tree (FDT) in a particular format, with images contained within. FITs
+include hashes to verify images, so it is relatively straightforward to
+add signatures as well.
+
+The public key can be stored in U-Boot's CONFIG_OF_CONTROL device tree in
+a standard place. Then when a FIT it loaded it can be verified using that
+public key. Multiple keys and multiple signatures are supported.
+
+See signature.txt for more information.
+
+
+Simon Glass
+sjg@chromium.org
+1-1-13