Example - 1

Consider a system that has four bus masters and a decoder. The decoder is supposed to decode every bus transaction and assign a corresponding security identifier. The security identifier is used to determine accesses to the assets. The bus transaction that contains the security information is Bus_transaction [15:14], and the bits 15 through 14 contain the security identifier information. The table below provides bus masters as well as their security identifiers and trust assumptions:Bus MasterSecurity Identifier DecodingTrust AssumptionsMaster_0"00"UntrustedMaster_1"01"TrustedMaster_2"10"UntrustedMaster_3"11"UntrustedThe assets are the AES-Key registers for encryption or decryption. The key is 128 bits implemented as a set of four 32-bit registers. The AES_KEY_ACCESS_POLICY is used to define which agents with a security identifier in the transaction can access the AES-key registers. The size of the security identifier is 4 bits (i.e., bit 3 through 0). Each bit in these 4 bits defines a security identifier. There are only 4 security identifiers that are allowed accesses to the AES-key registers. The number of the bit when set (i.e., "1") allows respective action from an agent whose identity matches the number of the bit. If clear (i.e., "0"), disallows the respective action to that corresponding agent.RegisterField descriptionAES_ENC_DEC_KEY_0AES key [0:31] for encryption or decryptionDefault 0x00000000AES_ENC_DEC_KEY_1AES key [32:63] for encryption or decryptionDefault 0x00000000AES_ENC_DEC_KEY_2AES key [64:95] for encryption or decryptionDefault 0x00000000AES_ENC_DEC_KEY_3AES key [96:127] for encryption or decryptionDefault 0x00000000AES_KEY_ACCESS_POLCY[31:4] Default 0x00000000[3:0]-0x01 agent with Security Identified "1" has access to AES_ENC_DEC_KEY_0 through AES_ENC_DEC_KEY_3 registers

The following Pseudo code outlines the process of checking the value of the Security Identifier within the AES_KEY_ACCESS_POLICY register:

Allow access to AES-Key registers
Deny access to AES-Key registersIf (AES_KEY_ACCESS_POLICY[Security_Identifier] == "1")Else

Below is a decoder's Pseudo code that only checks for bit [14] of the bus transaction to determine what Security Identifier it must assign.

Security_Identifier == "1"
Security_Identifier == "0"If (Bus_transaction[14] == "1") Else

The security identifier is two bits, but the decoder code above only checks the value of one bit. Two Masters have their bit 0 set to "1" - Master_1 and Master_3. Master_1 is trusted, while Master_3 is not. The code above would therefore allow an untrusted agent, Master_3, access to the AES-Key registers in addition to intended trusted Master_1.The decoder should check for the entire size of the security identifier in the bus-transaction signal to assign a corresponding security identifier. The following is good Pseudo code:

Security_Identifier == "0"
Security_Identifier == "1"
Security_Identifier == "2"
Security_Identifier == "3"If (Bus_transaction[15:14] == "00") If (Bus_transaction[15:14] == "01") If (Bus_transaction[15:14] == "10") If (Bus_transaction[15:14] == "11")