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Zeroization Programmer's Guide

As of version 2.1, the Caliptra subsystem complies with the OCP L.O.C.K. specification for secure cryptographic key management. Zeroization of sensitive keys is a core component of the protocol, which for the fuse_ctrl means that it needs to support overwriting already programmed and locked partitions. This document describes how provisioners can interact with the direct-access interface (DAI) to achieve the zeroization of a partition.

Enabling Zeroization

Zeroization is a build-time parameter that governs for each partition whether it can be zeroized during its life cycle. It can be found in the MMAP alongside the other conventional parameters that instrument a partition. Every partition type except LIFE_CYCLE can be zeroizable regardless of whether it is a hardware or software partition. When a partition is marked as zeroizable, the partition generation script inserts a 64-bit fuse at the end of the partition that serves as an indicator for the fuse_ctrl to detect when a partition is zeroized or in the process of being zeroized. The detection criteria is explained later in the document.

Zeroization Flow

Zeroizing a partition is completely handled by firmware and resembles an ordinary in-field provisioning flow.

  1. The DAI has a dedicated ZEROIZE command through which a word (either 32-bit for software fuses or 64-bit for hardware fuses including the digest and zeroization fields) in a partition can be zeroized. The entire address space of a partition is zeroizable which contrasts with the other DAI commands. For example, the digest field is never writable through the WRITE command in a hardware partition.

    Side effect: The fuse_ctrl detects the first successful word zeroization and disables periodic consistency checks for the corresponding partition as these can potentially fail when interrupting an ongoing zeroization procedure. Integrity checks for hardware partitions can proceed normally until the next reset as they only act on buffered data.

    Although the fuses of a partition can be zeroized in any order, it is recommended to first zeroize the zeroization field at the end of a partition to mark it as zeroized or being zeroized in the case the flow is interrupted and needs to be resumed at a later point.

    Side effect: A fuse macro usually signals an error if a write attempts to clear an already set data or ECC bit. Such an unintended bit flip can occur in the ECC part of a word during a zeroization (when only the data part is zeroized), hence the ZEROIZE command disables ECC when zeroizing a word, setting all bits in both the data and ECC part of a fuse word.

  2. A successful zeroization of a fuse word results an all-1s word being returned to software in the DIRECT_ACCESS_RDATA registers, independent of any potential stuck-at-0 fuses in the word. The ZEROIZE command is idempotent, i.e., it can be retried multiple times such that a zeroization of an already zeroized word has no effect.

  3. When firmware determines that a partition has been sufficiently zeroized, it should reset the fuse_ctrl such that the zeroized data is also reflected in the buffer registers.

    Side effect: Upon initialization, the fuse_ctrl will first read the zeroization field of a partition to determine whether it is in a zeroized state. If so, no periodic consistency and integrity checks will be executed for the partition.

Zeroization Detection

As mentioned above, the periodic consistency and integrity checks need to be disabled for a zeroized partition. Furthermore when reading out the content of both buffered and unbuffered partitions, the ECC checks need to be disabled when dealing with zeroized partitions. This means that the zeroization indicator field has to be checked first before performing the remaining initialization steps. A partition is said to be in a zeroized state if and only if the number of set bits in the indicator field is greater or equal the macro-specific value ZeroizationValidBound. This bound should include a margin that accounts for defects where certain fuse bits are perpetually stuck at 0.