ECB — AES electronic codebook mode encryption

The AES electronic codebook mode encryption (ECB) can be used for a range of cryptographic functions like hash generation, digital signatures, and keystream generation for data encryption/decryption. The ECB encryption block supports 128 bit AES encryption (encryption only, not decryption).

The main features of ECB are:

• 128-bit AES encryption
• Supports standard AES ECB block encryption
• Memory-to-memory operations using Scatter/Gather DMA

The inputs and outputs of the ECB are illustrated below.

AES ECB uses EasyDMA with scatter-gather to access to memory for in-place operations on cleartext and ciphertext during encryption. ECB uses the same AES core as the CCM and AAR blocks and is an asynchronous operation which may not complete if the AES core is busy.

AES ECB performs a 128 bit AES block encrypt. At the START task, cleartext is loaded into the ECB from memory described by the scatter/gather job list pointed to by INPTR and the ciphertext is written into memory described by the job list pointed to by OUTPTR. When the last cleartext byte has been encrypted and written to OUTPTR, the END event is triggered.

The following figure illustrates how the input and output job lists can be configured. For more details of the joblists, see EasyDMA.

The AES key is set by writing the KEY.VALUE key registers. The same key can be used to encrypt multiple blocks by triggering the START task multiple times.

AES ECB can be stopped by triggering the STOP task.

          {0x02, 0x13, 0x24, 0x35, 0x46, 0x57, 0x68, 0x79, 0xAC, 0xBD, 0xCE, 0xDF, 0xE0, 0xF1, 0x02, 0x13}
      

          {0x99, 0xAD, 0x1B, 0x52, 0x26, 0xA3, 0x7E, 0x3E, 0x05, 0x8E, 0x3B, 0x8E, 0x27, 0xC2, 0xC6, 0x66}
      

Shared resources

EasyDMA

This peripheral implements EasyDMA with scatter-gather functionality for reading from and writing to memory without CPU involvement.

The scatter-gather functionality allows EasyDMA to collect data from multiple memory regions, instead of one contigous block. The memory regions are described by a job list. The job list consists of one or more job entries that consist of a 32-bit address field, 8-bit attribute field, and 24-bit length field. A job list ends with a zero filled job entry. The attribute field must be set to 11.

If INPTR or OUTPTR pointers or the entries in the job lists are not pointing to memory connected to the DMA bus, an EasyDMA transfer may result in a HardFault or memory corruption. See Memory for more information about the different memory regions and DMA connectivity.

The EasyDMA will have finished accessing the RAM when the END or ERROR events are generated.

For instances supporting DMA error detection, the ERRORSTATUS register will report if a bus error has occurred during DMA access. To see if DMA error detection is supported, see the the instance's configuration in Instantiation.

Example

The figure below shows an example of a job list with three job entries. Each of the entries point to a memory address, and the length field describes how many bytes of data is stored at that address. There are three blocks of memory in use

• FIRSTDATA, an array of length 3
• SECONDDATA, an array of length 2
• THIRDDATA, an array of length 11

The data pointed to from the job list is what is fed into the module and processed according to the peripheral's operation. The entries of the job list comprises pointers to the individual arrays, as well as their sizes. Job entries with length greater than one are processed in little endian order.

Registers

TASKS_START

TASKS_STOP

SUBSCRIBE_START

SUBSCRIBE_STOP

EVENTS_END

EVENTS_ERROR

PUBLISH_END

PUBLISH_ERROR

INTENSET

INTENCLR

ERRORSTATUS

KEY.VALUE[n]

IN

IN.PTR

OUT

OUT.PTR

CSAA

CSAA.REFLECTOR

CSAA.INITIATOR

CSAA.MODE