Using Derived KEK on HS devices¶
This document is only applicable to HS devices.
K3 HS devices have a randomly generated 256 bit key written into the efuses in TI Factory. This key is called a Key Encryption Key (KEK) and is unique to each device. The characteristics of KEK are listed below.
- KEK is a randomly generated symmetric key
- KEK is made using NIST certified tester routine
- KEK is different for each device and is not correlated in any ways with keys issued on others devices.
- KEK is not stored in any database or retained in any manufacturing tester
- KEK is burnt in TI factory.
KEK is fed in hardware into the AES engine inside DMSC. The efuses carrying the KEK are marked as read and write protected. As a result, KEK is only accessible via the AES engine in DMSC. The DMSC AES engine has only a register interface and no DMA interface. The DMSC AES engine has lower throughput compared to the AES engine in SA2UL. Instead of exposing the DMSC AES engine to all cores, System Firmware provides API to obtain a key derived from KEK(DKEK) for encryption/decryption. DKEK can be used with SA2UL or with CPU acceleration to perform encryption/decryption operations.
This document describes the usage of the Derived KEK(DKEK) API. This document must be read along side
Key Derivation Method¶
System Firmware uses CMAC as the Pseudo Random Function(PRF) in counter mode as described in Section 5.1 of NIST SP 800 108 “Recommendation for Key Derivation Using Pseudorandom Functions”. The user can provide the following inputs as defined in the above document.
- Label - A string that identifies the purpose for the derived keying material, which is encoded as a binary string.
- Context - A binary string containing the information related to the derived keying material.
The total length of the Label and Context fields is limited by the size of the TISCI messages(see Secure Messaging Header) to 41 bytes.
System Firmware adds additional context containing the ID of the host requesting the DKEK into the input. As a result, each host requesting the DKEK receives a unique value.
DKEK computation is deterministic on each device. For a given host, label and context, the derived KEK is the same even across reboots on the same device.
On different devices, derived KEK is different for the same input. This is due to a random KEK being programmed into each device in TI factory.
System Firmware always computes a 256 bit key. It is up to the host to use the desired length out of the 256 bits of the key.
Derived KEK does not describe a single key. The key derivation procedure produces different keys per host. The host can derive different keys based on the label and context provided as input.
Using Derived KEK¶
There are two ways to use the derived KEK for encryption/decryption.
- System Firmware programs SA2UL with DKEK. Host performs encryption/decryption with SA2UL.
- System Firmware computes DKEK and returns it to the host. Host can perform encryption/decryption with SA2UL or via CPU.
This section assumes that the user is familiar with using SA2UL for performing encryption/decryption.
Approach 1 - DKEK programmed into SA2UL¶
In this approach, host requests System Firmware to derive a DKEK and program it into the SA2UL DKEK registers using the TISCI_MSG_SA2UL_SET_DKEK API.
DKEK is not directly made available to the host. The host can use DKEK for
encryption/decryption by setting the
USE_DKEK flag in the SA2UL security
When the host is done with the use of DKEK, it can release the DKEK registers by using the TISCI_MSG_SA2UL_RELEASE_DKEK API.
Caveat on Approach 1¶
USE_DKEK flag is used in SA2UL security context, SA2UL checks if the
privid of the initiator is present in the SA2UL DKEK privid register.
SA2UL only checks the privid of the initiator against the SA2UL DKEK privid register
and ignores any attributes such as user/priviliged and secure/non-secure in the transaction.
This creates a tricky situation when secure and non-secure software are running on the same core. Software running on the same core share the same privid but differ in other attributes. It is possible for non-secure software to encrypt or decrypt data with DKEK configured by the secure software.
This can be mitigated by setting the DKEK only when required and releasing the DKEK as soon as the encryption/decryption operation is complete. Alternatively approach 2 described below can be used.
Approach 2 - DKEK supplied via TISCI¶
In this approach, host requests System Firmware to derive a DKEK and supply it in the response using the TISCI_MSG_SA2UL_GET_DKEK API. The host is responsible for maintaining the secrecy of the DKEK supplied by System Firmware.
Once DKEK is available, the host can perform encryption/decryption with it in the same manner as any other symmetric key.
As the SA2UL DKEK registers are not being used, there is no release DKEK API required in this approach.
Comparing the two approaches¶
|Option 1||Option 2|
|DKEK programmed directly into SA2UL DKEK registers.||DKEK is provided to host as a response to a TISCI message.|
|No separate firewalling is required for DKEK.||Host must firewall memory where it is storing DKEK.|
|DKEK can only be used through SA2UL. Host must set the
DKEK can be used in two ways
|Only one host can use this approach at a time. There is only one set of SA2UL DKEK registers. Access to these registers for encryption/decryption is controlled by DKEK privid register. System Firmware programs the DKEK privid register with the privid of the host who has invoked the TISCI_MSG_SA2UL_SET_DKEK API. Only this host can use DKEK via the SA2UL DKEK registers until DKEK is released via TISCI_MSG_SA2UL_RELEASE_DKEK||Multiple hosts can use DKEK at the same time using this approach. Each host manages its own DKEK.|
|TISCI_MSG_SA2UL_SET_DKEK and TISCI_MSG_SA2UL_RELEASE_DKEK API are used in this approach.||TISCI_MSG_SA2UL_GET_DKEK is used in this approach.|
We recommend using Approach 1 wherever possible to maintain the secrecy of DKEK.