• BK: Blob Key, a random 256-bit AES-CCM key that encrypts the data on a CAAM blob.

  • BKEK: Blob Key Encryption Key, a 256-bit AES-ECB derived key that encrypts the BK in CAAM blobs.

  • CA: Certificate Authority, the entity that issues digital certificates.

  • CAAM: Cryptographic Accelerator and Assurance Module, a hardware module on the System-On-Chip which provides hardware-accelerated crypto capabilities.

  • CSF: Command Sequence File, a binary blob attached to signed U-Boot images that contains the signatures, certificates, and commands to configure the CAAM for the decryption and authentication processes.

  • DEK: Data Encryption Key, secret key used in the encryption of boot artifacts (such as U-Boot, Linux images, device tree blobs and bootscripts).

  • HAB: High Assurance Boot, firmware in the processor’s boot ROM in charge of authenticating boot images.

  • NVTK: Non-Volatile Test Key, a 256-bit key hardwired into the CAAM that is used on open (not secure enabled) devices as a replacement for the OTPMK. The NVTK value is public knowledge and is common to all parts, so its usage is not secure: it should only be used for testing.

  • OTP: One-Time Programmable bits, also referred to as electronic fuses or eFuses.

  • OTPMK: One-Time Programmable Master Key, a unique 256-bit key stored by the CPU manufacturer on the CPU’s OTP bits and used by the CAAM only on closed (secure boot enabled) devices.

  • PKI: Public Key Infrastructure, a set of certificates and private keys that Digi Embedded Yocto uses to sign the firmware images.

  • RSA: The Rivest-Shamir-Adleman cryptosystem for public-key encryption.

  • SRK: Super Root Keys, stored as hashes in the CPU’s OTP bits and used by the HAB for image authentication.