Consider a system of seven registers for storing and configuring an AES key for encryption or decryption.
Four 32-bit registers are used to store a 128-bit AES key. The names of those registers are AES_ENC_DEC_KEY_0, AES_ENC_DEC_KEY_1, AES_ENC_DEC_KEY_2, and AES_ENC_DEC_KEY_3. Collectively these are referred to as the AES Key registers.
Three 32-bit registers are used to define access control for the AES-key registers. The names of those registers are AES_KEY_CONTROL_POLICY, AES_KEY_READ_POLICY, and AES_KEY_WRITE_POLICY. Collectively these registers are referred to as the Policy registers, and their functions are explained next.
The preceding three policy registers encode access control at the bit level. Therefore a maximum of 32 agents can be defined (1 bit per agent). The value of the bit when set (i.e., "1") allows the respective action from an agent whose identity corresponds to the number of the bit. If clear (i.e., "0"), it disallows the respective action to that corresponding agent. For example, if bit 0 is set to "1" in the AES_KEY_READ_POLICY register, then agent 0 has permission to read the AES-key registers.
Consider that there are 4 agents named Agent 1, Agent 2, Agent 3, and Agent 4. For access control purposes Agent 1 is assigned to bit 1, Agent 2 to bit 2, Agent 3 to bit 3, and Agent 4 to bit 4. All agents are trusted except for Agent 3 who is untrusted. Also consider the register values in the below table.
The AES_KEY_CONTROL_POLICY register defines which agents can write to the AES_KEY_READ_POLICY or AES_KEY_WRITE_POLICY registers.
The AES_KEY_READ_POLICY register defines which agents can read the AES-key registers.
The AES_KEY_WRITE_POLICY register defines which agents can write the AES key registers.
The AES_KEY_CONTROL_POLICY register value is 0x00000018. In binary, the lower 8 bits will be 0001 1000, meaning that:
The AES_KEY_READ_POLICY register value is 0x00000002. In binary, the lower 8 bits will be 0000 0010, meaning that:
The AES_KEY_WRITE_POLICY register value is 0x00000004. In binary, the lower 8 bits will be 0000 0100, meaning that:
The configured access control policy for Agents 1,2,3,4 is summarized in the table below.
At this point, Agents 3 and 4 can only configure which agents can read AES keys and which agents can write AES keys. Agents 3 and 4 cannot read or write AES keys - just configure access control.
Now, recall Agent 3 is untrusted. As explained above, the value of the AES_KEY_CONTROL_POLICY register gives agent 3 access to write to the AES_KEY_WRITE_POLICY register. Agent 3 can use this write access to add itself to the AES_KEY_WRITE_POLICY register. This is accomplished by Agent 3 writing the value 0x0000000C. In binary, the lower 8 bits are 0000 1100, meaning that bits 2 and 3 will be set. Thus, Agent 3 will be given the ability to write to the AES Key registers. If Agent 3 does not set bit 2, this would prevent Agent 2 from having write access, which could cause operational failures that could expose the attack (or be used for a denial of service).
If the AES_KEY_CONTROL_POLICY register value is 0x00000010, the lower 8 bits will be 0001 0000. This will give Agent 4, a trusted agent, write access to AES_KEY_WRITE_POLICY, but Agent 3, who is untrusted, will not have write access. The Policy register values should therefore be as follows:
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