Asymmetric Hardware Root of Trust Secure Boot

Versal ACAP Technical Reference Manual (AM011)

Document ID
AM011
Release Date
2022-04-26
Revision
1.4 English

The Versal device A-HWRoT boot mode is built upon the use of RSA-4096 or ECDSA P-384 asymmetric authentication algorithms using SHA-3/384, and allows the use of both primary and secondary public keys for signature verification (PPK and SPK, respectively). The following table lists the characteristics of each public key type.

Table 1. Public Key Types
Public Key Number Location Revocable
Primary (PPK) 3 External memory with hash in eFUSEs Yes
Secondary (SPK) 256 Boot image Yes
The Versal device allows for the use of three PPKs, each of which is revocable. To reduce the number of fuses required, the full public key is stored in external memory (e.g., flash) while 256 bits of a SHA-3/384 hash of each key is securely stored inside the device using eFUSEs. During the secure boot process, the RCU first validates the integrity of the full public key stored externally by hashing it (SHA-3/384) and taking 256 bits of that hash and comparing against the value stored in eFUSEs.

There are also 256 SPKs available, each of which are also revocable. The SPK is delivered inside the authenticated boot image, and is consequently protected by the PPK, which is the primary purpose of the PPK. The SPK is intended to authenticate everything else.