In current cryptography, all the algorithms are public, so it becomes critical to protect the private/secret key. The hardware security module (HSM) is a dedicated crypto-processing device that is specifically designed for the protection of the crypto key lifecycle, and increases key handling security, because only public keys are passed to the Bootgen and not the private/secure keys. A standard mode is also available; this mode does not require passing keys.
In some organizations, an infosec staff is responsible for the production release of a secure embedded product. The infosec staff might use a HSM for digital signatures and a separate secure server for encryption. The HSM and secure server typically reside in a secure area. The HSM is a secure key/signature generation device which generates private keys, signs the partitions using the private key, and provides the public part of the RSA key to Bootgen. The private keys reside in the HSM only.
Bootgen in HSM mode uses only RSA public keys and the signatures that were created by the HSM to generate the boot image. The HSM accepts hash values of partitions generated by Bootgen and returns a signature block, based on the hash and the secret RSA key.
In contrast to the HSM mode, Bootgen in its Standard mode uses AES encryption keys and the RSA Secret keys provided through the BIF file, to encrypt and authenticate the partitions in the image, respectively. The output is a single boot image, which is encrypted and authenticated. For authentication, the user has to provide both sets of public and private/secret keys. The private/secret keys are used by the Bootgen to sign the partitions and create signatures. These signatures along with the public keys are embedded into the final boot image.
For more information about the HSM mode for FPGAs, see the HSM Mode.
Using Advanced Key Management Options
The public keys associated with the private keys are ppk.pub and spk.pub. The HSM accepts hash values of partitions generated by Bootgen and returns a signature block, based on the hash and the secret key.