AES encryption is a self-authenticating algorithm with a symmetric key, meaning that the key to encrypt is the same as the one to decrypt. This key must be protected as it is secret (hence storage to the internal key space). There is an alternative form of authentication in the form of Rivest-Shamir-Adleman (RSA). RSA is an asymmetric algorithm, meaning that the key to verify is not the same key used to sign. A pair of keys are needed for authentication.
- Signing is done using Secret Key/ Private Key
- Verification is done using a Public Key
This public key does not need to be protected, and does not need special secure storage. This form of authentication can be used with encryption to provide both authenticity and confidentiality. RSA can be used with either encrypted or unencrypted partitions.
RSA not only has the advantage of using a public key, it also has the advantage of authenticating prior to decryption. The hash of the RSA Public key must be stored in the eFUSE. AMD SoC devices support authenticating the partition data before it is sent to the AES decryption engine. This method can be used to help prevent attacks on the decryption engine itself by ensuring that the partition data is authentic before performing any decryption.
In AMD SoCs, two pairs of public and secret keys are used - primary and secondary. The function of the primary public/secret key pair is to authenticate the secondary public/secret key pair. The function of the secondary key is to sign/verify partitions.
The first letter of the acronyms used to describe the keys is either P for primary or S for secondary. The second letter of the acronym used to describe the keys is either P for public or S for secret. There are four possible keys:
- PPK = Primary Public Key
- PSK = Primary Secret Key
- SPK = Secondary Public Key
- SSK = Secondary Secret Key
Bootgen can create a authentication certificate in two ways:
- Supply the PSK and SSK. The SPK signature is calculated on-the-fly using these two inputs.
- Supply the PPK and SSK and the SPK signature as inputs. This is used in cases where the PSK is not known.
The primary key is hashed and stored in the eFUSE. This hash is compared against the hash of the primary key stored in the boot image by the FSBL. This hash can be written to the PS eFUSE memory using standalone driver provided along with Vitis.
The following is an example BIF file:
For device-specific Authentication information, see the following: