mirror of
https://github.com/AsahiLinux/u-boot
synced 2024-11-07 13:44:29 +00:00
263 lines
12 KiB
Text
263 lines
12 KiB
Text
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+=======================================================+
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+ i.MX Secure and Encrypted Boot using HABv4 +
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+=======================================================+
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1. Introduction
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----------------
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The i.MX family of applications processors provides the High Assurance Boot
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(HAB) feature in the on-chip ROM. The ROM is responsible for loading the
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initial program image (U-Boot) from the boot media and HAB enables the ROM
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to authenticate and/or decrypt the program image by using cryptography
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operations.
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This feature is supported in i.MX 50, i.MX 53, i.MX 6, i.MX 7 series and
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i.MX 8M, i.MX 8MM devices.
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Step-by-step guides are available under doc/imx/habv4/guides/ directory,
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users familiar with HAB and CST PKI tree generation should refer to these
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documents instead.
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1.1 The HABv4 Secure Boot Architecture
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---------------------------------------
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The HABv4 secure boot feature uses digital signatures to prevent unauthorized
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software execution during the device boot sequence. In case a malware takes
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control of the boot sequence, sensitive data, services and network can be
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impacted.
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The HAB authentication is based on public key cryptography using the RSA
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algorithm in which image data is signed offline using a series of private
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keys. The resulting signed image data is then verified on the i.MX processor
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using the corresponding public keys. The public keys are included in the CSF
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binary and the SRK Hash is programmed in the SoC fuses for establishing the
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root of trust.
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The diagram below illustrate the secure boot process overview:
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Host PC + CST i.MX + HAB
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+----------+ +----------+
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---> | U-Boot | | Compare |
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| +----------+ +----------+
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| | ^ ^
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| v Reference / \ Generated
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| +----------+ Hash / \ Hash
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| | Hash | Private / \
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| +----------+ Key / \
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| | | +----------+ +----------+
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| v | | Verify | | Hash |
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| +----------+ | +----------+ +----------+
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| | Sign | <--- SRK ^ ^
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| +----------+ HASH \ /
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| | | CSF \ / U-Boot
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| v v \ /
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| +----------+ +----------+ +----------+
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| | U-Boot | | | | U-Boot |
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---> | + | -----> | i.MX | -----> | + |
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| CSF | | | | CSF |
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+----------+ +----------+ +----------+
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The U-Boot image to be programmed into the boot media needs to be properly
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constructed i.e. it must contain a proper Command Sequence File (CSF).
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The CSF is a binary data structure interpreted by the HAB to guide
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authentication process, this is generated by the Code Signing Tool[1].
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The CSF structure contains the commands, SRK table, signatures and
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certificates.
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Details about the Secure Boot and Code Signing Tool (CST) can be found in
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the application note AN4581[2] and in the secure boot guides.
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1.2 The HABv4 Encrypted Boot Architecture
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------------------------------------------
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The HAB Encrypted Boot feature available in CAAM supported devices adds an
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extra security operation to the bootloading sequence. It uses cryptographic
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techniques (AES-CCM) to obscure the U-Boot data, so it cannot be seen or used
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by unauthorized users. This mechanism protects the U-Boot code residing on
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flash or external memory and also ensures that the final image is unique
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per device.
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The process can be divided into two protection mechanisms. The first mechanism
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is the bootloader code encryption which provides data confidentiality and the
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second mechanism is the digital signature, which authenticates the encrypted
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image.
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Keep in mind that the encrypted boot makes use of both mechanisms whatever the
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order is (sign and then encrypt, or encrypt and then sign), both operations
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can be applied on the same region with exception of the U-Boot Header (IVT,
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boot data and DCD) which can only be signed, not encrypted.
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The diagram below illustrate the encrypted boot process overview:
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Host PC + CST i.MX + HAB
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+------------+ +--------------+
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| U-Boot | | U-Boot |
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+------------+ +--------------+
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| ^
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v DEK +--------------+
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+------------+ | ----> | Decrypt |
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| Encrypt | <--- | +--------------+
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+------------+ DEK | ^
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| Private | |
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v Key +------+ +--------------+
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+------------+ | | CAAM | | Authenticate |
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| Sign | <--- +------+ +--------------+
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+------------+ DEK ^ ^
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| + OTPMK DEK \ / U-Boot
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| | Blob \ / + CSF
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v v \ /
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+------------+ +----------+ +------------+
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| Enc U-Boot | | | | Enc U-Boot |
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| + CSF | ----> | i.MX | -------> | + CSF |
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| + DEK Blob | | | | + DEK Blob |
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+------------+ +----------+ +------------+
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^ |
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---------------------
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DEK Blob
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(CAAM)
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The Code Signing Tool automatically generates a random AES Data Encryption Key
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(DEK) when encrypting an image. This key is used in both encrypt and decrypt
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operations and should be present in the final image structure encapsulated
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by a CAAM blob.
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The OTP Master Key (OTPMK) is used to encrypt and wrap the DEK in a blob
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structure. The OTPMK is unique per device and can be accessed by CAAM only.
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To further add to the security of the DEK, the blob is decapsulated and
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decrypted inside a secure memory partition that can only be accessed by CAAM.
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During the design of encrypted boot using DEK blob, it is necessary to inhibit
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any modification or replacement of DEK blob with a counterfeit one allowing
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execution of malicious code. The PRIBLOB setting in CAAM allows secure boot
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software to have its own private blobs that cannot be decapsulated or
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encapsulated by any other user code, including any software running in trusted
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mode.
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Details about DEK Blob generation and PRIBLOB setting can be found in the
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encrypted boot guide and application note AN12056[3] .
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2. Generating a PKI tree
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-------------------------
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The first step is to generate the private keys and public keys certificates.
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The HAB architecture is based in a Public Key Infrastructure (PKI) tree.
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The Code Signing Tools package contains an OpenSSL based key generation script
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under keys/ directory. The hab4_pki_tree.sh script is able to generate a PKI
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tree containing up to 4 Super Root Keys (SRK) as well as their subordinated
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IMG and CSF keys.
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A new PKI tree can be generated by following the example below:
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- Generating 2048-bit PKI tree on CST v3.1.0:
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$ ./hab4_pki_tree.sh
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...
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Do you want to use an existing CA key (y/n)?: n
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Do you want to use Elliptic Curve Cryptography (y/n)?: n
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Enter key length in bits for PKI tree: 2048
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Enter PKI tree duration (years): 5
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How many Super Root Keys should be generated? 4
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Do you want the SRK certificates to have the CA flag set? (y/n)?: y
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The diagram below illustrate the PKI tree:
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+---------+
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| CA |
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+---------+
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---------------------------------------------------
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v v v v
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+--------+ +--------+ +--------+ +--------+
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| SRK1 | | SRK2 | | SRK3 | | SRK4 |
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+--------+ +--------+ +--------+ +--------+
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/ \ / \ / \ / \
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v v v v v v v v
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+----+ +----+ +----+ +----+ +----+ +----+ +----+ +----+
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|CSF1| |IMG1| |CSF2| |IMG2| |CSF3| |IMG3| |CSF4| |IMG4|
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+----+ +----+ +----+ +----+ +----+ +----+ +----+ +----+
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After running the script users can check the private keys under keys/ directory
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and their respective X.509v3 public key certificates under crts/ directory.
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Those files will be used during the signing and authentication process.
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2.1 Generating a fast authentication PKI tree
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----------------------------------------------
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Starting in HAB v4.1.2 users can use a single SRK key to authenticate the both
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CSF and IMG contents. This reduces the number of key pair authentications that
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must occur during the ROM/HAB boot stage, thus providing a faster boot process.
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The script hab4_pki_tree.sh is also able to generate a Public Key Infrastructure
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(PKI) tree which only contains SRK Keys, users should not set the CA flag when
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generating the SRK certificates.
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- Generating 2048-bit fast authentication PKI tree on CST v3.1.0:
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$ ./hab4_pki_tree.sh
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...
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Do you want to use an existing CA key (y/n)?: n
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Do you want to use Elliptic Curve Cryptography (y/n)?: n
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Enter key length in bits for PKI tree: 2048
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Enter PKI tree duration (years): 5
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How many Super Root Keys should be generated? 4
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Do you want the SRK certificates to have the CA flag set? (y/n)?: n
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The diagram below illustrate the PKI tree generated:
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+---------+
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| CA |
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+---------+
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---------------------------------------------------
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v v v v
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+--------+ +--------+ +--------+ +--------+
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| SRK1 | | SRK2 | | SRK3 | | SRK4 |
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+--------+ +--------+ +--------+ +--------+
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2.2 Generating a SRK Table and SRK Hash
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----------------------------------------
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The next step is to generated the SRK Table and its respective SRK Table Hash
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from the SRK public key certificates created in one of the steps above.
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In the HAB architecture, the SRK Table is included in the CSF binary and the
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SRK Hash is programmed in the SoC SRK_HASH[255:0] fuses.
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On the target device during the authentication process the HAB code verify the
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SRK Table against the SoC SRK_HASH fuses, in case the verification success the
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root of trust is established and the HAB code can progress with the image
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authentication.
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The srktool can be used for generating the SRK Table and its respective SRK
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Table Hash.
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- Generating SRK Table and SRK Hash in Linux 64-bit machines:
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$ ../linux64/bin/srktool -h 4 -t SRK_1_2_3_4_table.bin -e \
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SRK_1_2_3_4_fuse.bin -d sha256 -c \
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SRK1_sha256_2048_65537_v3_ca_crt.pem,\
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SRK2_sha256_2048_65537_v3_ca_crt.pem,\
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SRK3_sha256_2048_65537_v3_ca_crt.pem,\
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SRK4_sha256_2048_65537_v3_ca_crt.pem
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The SRK_1_2_3_4_table.bin and SRK_1_2_3_4_fuse.bin files can be used in further
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steps as explained in HAB guides available under doc/imx/habv4/guides/
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directory.
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References:
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[1] CST: i.MX High Assurance Boot Reference Code Signing Tool.
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[2] AN4581: "Secure Boot on i.MX 50, i.MX 53, i.MX 6 and i.MX 7 Series using
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HABv4" - Rev 2.
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[3] AN12056: "Encrypted Boot on HABv4 and CAAM Enabled Devices" - Rev. 1
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