When we use LPC55Sxx PRINCE feature, we need enable PRINCE sub-region “crypto” by setting SR_ENABLE register. If we “crypto” enable discontinuous sub-regions and erase part of them, we may find we can’t erase/read/write other “crypto” sub-regions any more. This article will discuss how to resolve this phenomenon. Figure 1 Testing Steps According to LPC55Sxx UM, each PRINCE region has its SR_ENABLEx register. This register enables PRINCE encryption and decryption of data for each sub-region of crypto region 0. Each bit in this field enables a sub-region of crypto region 0 at offset 8kB*n, where n is the bit number. For example, when we set SR_ENABLE0=0X00000005, PRINCE region 0 sub-region 1 and sub-region 3 are set as encryption region. When read data out from these sub-regions, PRINCE will decrypt the data automatically. Now we will test discontinuous sub-region erase/read/write. Board: LPC55S16-EVK IDE: Keil MDK v5.29 Step 1: PRINCE initialization: Enable PRINCE region 0 and two discontinuous sub-regions; generate key, IV code; enable crypto. //set SR_ENABLE , SR_ENABLE=0X28000000,enable sub-regions(0x30000-0x32000,0x34000-0x36000) crypto 。 status=PRINCE_SetRegionSREnable(PRINCE( prince_region_t )region0,0X28000000); //select PRINCE crypto for region0 PRINCE_SetRegionBaseAddress ( PRINCE_Type *base, prince_region_t region0, uint32_t 0X0) //generate PRINCE region0 crypto key Status=FFR_KeystoreGetKC(&flashInstance,&keyCode[0],kFFR_KeyTypePrinceRegion0); status=PUF_GetHwKey(PUF,keyCode,sizeof(keyCode),kPUF_KeySlot2, rand()); //generate PRINCE region0 crypto IV_code status=PRINCE_GenNewIV(kPRINCE_Region0,&prince_iv_code[0],true,&flashInstance) //load IV code to PRINCE status=PRINCE_LoadIV(kPRINCE_Region0,&prince_iv_code[0]) //enable PRINCE encryption PRINCE_EncryptEnable(PRINCE) Step 2: Select two discontinuous sub-regions ( 0x30000-0x32000,0x34000-0x36000). Erase one of them (0x30000-0x32000), then write data to this sub-region. Output: Erasing and Writing are all successful. See Figure 2. //Erase 0x30000-0x32000 sub-region status=PRINCE_FlashEraseWithChecker(&flashInstance,0x30000,0x2000,kFLASH_ApiEraseKey); //Write 0x30000-0x32000 sub-region status=PRINCE_FlashProgramWithChecker(&flashInstance,0x30000,(uint8_t *)prince_iv_code,0x2000); Step 3: Erase and Write the other sub-region ( 0x34000-0x36000 ) Output: Erasing and Writing are failed. See Figure 2. //Erasing 0x34000-0x36000 sub-region status=PRINCE_FlashEraseWithChecker(&flashInstance,0x34000, 0x2000,kFLASH_ApiEraseKey); //Write 0x34000-0x36000 sub-region status=PRINCE_FlashProgramWithChecker(&flashInstance,0x34000, (uint8_t *)prince_iv_code,0x2000); Error Analysis According to UM11126( 49.16.1 Functional details ) , each crypto region has its own SKEY and IV code. SKEY and IV are used together by the PRINCE when encrypting or decrypting the data in the sub-regions of crypto region. For Instance, For PRINCE region1, each time after we execute erasing operation, new Skey1 and IV1 are generated, thus when executing erase/read/write operation to another sub-region, the old IV1 and new IV1 don’t match, which causes PRINCE can’t decrypt correctly. Suggestion We suggest user using SR_ENABLE to set continuous crypto sub-regions. When erasing operation is needed, erasing all the crypto sub-regions together, avoid erasing part of the sub-regions. One sub-region size is 8K, make sure the erasing/writing address 8K aligned. Thanks for the suggestion from johnwu
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