Trusted Protection Module

A Trusted Platform Module (TPM) is a small piece of hardware designed to provide various security functionalities. It offers numerous features, such as storing secrets, ‘measuring’ boot, and may act as an external cryptographic engine.

The Trusted Computing Group (TCG) delivers a document called TPM Interface Specifications (TIS) which describes the architecture of such devices and how they are supposed to behave as well as various details around the concepts. Additionally they provide a Library Specification.

TPM chips are either compliant with the initial specification or the v2.0+ specification: See TPM v1.2 vs TPM 2.0 for details.

Please also read: Gateworks TPM Wiki when used with Secure Boot

Microchip ATTPM20P

Gateworks has an optional TPM on the Venice and Malibu family SBCs:

  • Malibu GW8901
  • Venice:
    • GW74xx (revision B+)
    • GW73xx (revision F+)
    • GW72xx (revision F+)
    • GW71xx (revision E+)

The TPM used is a Microchip ATTPM20P-H6MA1-10 TPM connected to the SPI bus and is compliant to the Trusted Computing Group (TCG) Trusted Platform Module (TPM) Version 2.0

This provides cryptographic support for:

  • HMAC
  • AES-128
  • SHA-1
  • SHA-256
  • ECC BN_P256, ECCNIST_P256
  • RSA 1024-2048 bit keys

Linux Driver

The TIS compliant TPM devices are supported by the TCG SPI Linux driver:


This driver provides access via:

  • /dev/tpm0
  • /dev/tpmrm0

A solid TPM 2.0 software stack is available for Linux:


  • Install packages
    apt install tpm2-tools tpm2-abrmd
  • Show tpm capabilities/properties:
    root@jammy-malibu:~# tpm2_getcap properties-fixed
      raw: 0x322E3000
      value: "2.0"
      raw: 0
      raw: 0x77
      value: 1.19
      raw: 0x42
      raw: 0x7DE
      raw: 0x4D434850
      value: "MCHP"
      raw: 0x0
      value: ""
  • Read the PCR Values:
    root@jammy-venice:~# tpm2_pcrread
        0 : 0x0000000000000000000000000000000000000000
        1 : 0x0000000000000000000000000000000000000000
        2 : 0x0000000000000000000000000000000000000000
        3 : 0x0000000000000000000000000000000000000000
        4 : 0x0000000000000000000000000000000000000000
        5 : 0x0000000000000000000000000000000000000000
        6 : 0x0000000000000000000000000000000000000000
        7 : 0x0000000000000000000000000000000000000000
        8 : 0x0000000000000000000000000000000000000000
        9 : 0x0000000000000000000000000000000000000000
        10: 0x0000000000000000000000000000000000000000
        11: 0x0000000000000000000000000000000000000000
        12: 0x0000000000000000000000000000000000000000
        13: 0x0000000000000000000000000000000000000000
        14: 0x0000000000000000000000000000000000000000
        15: 0x0000000000000000000000000000000000000000
        16: 0x0000000000000000000000000000000000000000
        23: 0x0000000000000000000000000000000000000000
  • Clearing the TPM
  • Generate Random Number
    tpm2_getrandom --hex 8

Please also read: Gateworks TPM Wiki when used with Secure Boot

PCR Values

A cryptographic hash (sometimes called a 'digest') is a kind of 'signature' for a set of data. For example the SHA-256 algorithm can be used to generate an almost-unique 256-bit (32-byte) signature (aka 'hash' or 'digest') for a file. Note that this signature/hash/digest is not 'encryption' - it is a one way cryptographic function and is a fixed size for any source of data.

Starting from a root of trust (typically the SoC BOOT ROM) each software stage during the boot process is supposed to to some measurements and store them in a safe place. A 'measure' is just a signature/hash/digest of a memory region. This value can be sent to the TPM as a measure which will merge with measurement with the previous ones.

The hardware feature used to store and merge these measurements is called Platform Configuration Registers (PCR). At power-up a PCR is set to a known value (typically either 0x00's or 0xff's) and sending a new value to the TPM is called 'extending a PCR' because the chosen register will extend its value with the one received. This way a PCR can only evolve in one direction and never go back unless the platform is reset. Each software stage will be in charge of extending a set of PCRs with digests of the next software stage. Once in Linux for example user software may ask the TPM to deliver its secrects but the only way to get them is having all PCRs matching a known pattern which can only be obtained by extending the PCRs in the right order with the right digets.

If the stored PCR values in the TPM do not match the currently booting system PCRs, access will not be granted. For example, someone trying to boot a Ubuntu Live CD would not be able to access the TPM key as the PCRs generated from the original disk and stored in the TPM will not match the newly generated PCRs from the boot CD. PCRs use hashing and thus any new value is concatenated with the old and then hashed. This new hash will replace the old hash. The definition of each specific PCR register can be found online.

Please also read: Gateworks TPM Wiki when used with Secure Boot

TPM Key Flow

A general flow for loading a key into the TPM: (arguments needed for below commands specific to each application )

tpm2_createpolicy  # Create PCR Policy
tpm2_createprimary  # Create primary TPM object
tpm2_create  # Create TPM Object with Secret
tpm2_load  # Load object into the TPM
tpm2_evictcontrol  # Make TPM Object Persistant
rm files #remove your working files

Please also read: Gateworks TPM Wiki when used with Secure Boot


U-Boot has TPM support as well:

  • drivers/tpm/tpm2_tis_spi.c (CONFIG_TPM,CONFIG_TPM2_TIS_SPI)

Usage Example:

u-boot=> tpm device
device 0: tpm@0 v2.0: VendorID 0x1114, DeviceID 0x3205, RevisionID 0x01 [open]
u-boot=> tpm info
tpm@0 v2.0: VendorID 0x1114, DeviceID 0x3205, RevisionID 0x01 [open]
u-boot=> tpm init
u-boot=> tpm startup TPM2_SU_CLEAR
u-boot=> tpm self_test full                  
u-boot=> tpm self_test continue

At this point you can pursue measured boot byt extending the PCR as needed:

u-boot=> tpm extend 0 $loadaddr # extend PCR 0 using digest loaded to $loadaddr

Please also read: Gateworks TPM Wiki when used with Secure Boot

Seee also:

Additional Resources

Last modified 12 days ago Last modified on 04/01/2024 11:26:11 PM
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