Programmer’s Guide
Initialization
The software can update the KMAC/SHA3 configurations only when the IP is in the idle state.
The software should check STATUS.sha3_idle before updating the configurations.
The software must first program CFG_SHADOWED.msg_endianness and CFG_SHADOWED.state_endianness at the initialization stage.
These determine the byte order of incoming messages (msg_endianness) and the Keccak state output (state_endianness).
Software Initiated KMAC/SHA3 process
This section describes the expected software process to run the KMAC/SHA3 HWIP.
At first, the software configures CFG_SHADOWED.kmac_en for KMAC operation.
If KMAC is enabled, the software should configure CFG_SHADOWED.mode to cSHAKE and CFG_SHADOWED.kstrength to 128 or 256 bit security strength.
The software also updates PREFIX registers if cSHAKE mode is used.
Current design does not convert cSHAKE mode to SHAKE even if PREFIX is empty string.
It is the software’s responsibility to change the CFG_SHADOWED.mode to SHAKE in case of empty PREFIX.
The KMAC/SHA3 HWIP uses PREFIX registers as it is.
It means that the software should update PREFIX with encoded values.
If CFG_SHADOWED.kmac_en is set, the software should update the secret key.
The software prepares two shares of the secret key and selects its length in KEY_LEN then writes the shares of the secret key to KEY_SHARE0 and KEY_SHARE1 .
The two shares of the secret key are the values that represent the secret key value when they are XORed together.
The software can XOR the unmasked secret key with entropy.
The XORed value is a share and the entropy used is the other share.
After configuring, the software notifies the KMAC/SHA3 engine to accept incoming messages by issuing Start command into CMD .
If Start command is not issued, the incoming message is discarded.
If KMAC is enabled, the software pushes the right_encode(output_length) value at the end of the message.
For example, if the desired output length is 256 bit, the software writes 0x00020100 to MSG_FIFO.
After the software pushes all messages, it issues Process command to CMD for SHA3 engine to complete the sponge absorbing process.
SHA3 hashing engine pads the incoming message as defined in the SHA3 specification.
After the SHA3 engine completes the sponge absorbing step, it generates kmac_done interrupt.
Or the software can poll the STATUS.squeeze bit until it becomes 1.
In this stage, the software may run the Keccak round manually.
If the desired digest length is greater than the Keccak rate, the software issues Run command for the Keccak round logic to run one full round after the software reads the current available Keccak state.
At this stage, KMAC/SHA3 does not raise an interrupt when the Keccak round completes the software initiated manual run.
The software should check STATUS.squeeze register field for the readiness of STATE value.
After the software reads all the digest values, it issues Done command to CMD register to clear the internal states.
Done command clears the Keccak state, FSM in SHA3 and KMAC, and a few internal variables.
Secret key and other software programmed values won’t be reset.
Endianness
This KMAC HWIP operates in little-endian. Internal SHA3 hashing engine receives in 64-bit granularity. The data written to SHA3 is assumed to be little endian.
The software may write/read the data in big-endian order if CFG_SHADOWED.msg_endianness or CFG_SHADOWED.state_endianness is set.
If the endianness bit is 1, the data is assumed to be big-endian.
So, the internal logic byte-swap the data.
For example, when the software writes 0xDEADBEEF with endianness as 1, the logic converts it to 0xEFBEADDE then writes into MSG_FIFO.
The software managed secret key, and the prefix are always little-endian values.
For example, if the software configures the function name N in KMAC operation, it writes encode_string("KMAC").
The encode_string("KMAC") represents 0x01 0x20 0x4b 0x4d 0x41 0x43 in byte order.
The software writes 0x4d4b2001 into PREFIX0 and 0x????4341 into PREFIX1 .
Upper 2 bytes can vary depending on the customization input string S.
Error Handling
When the KMAC HW IP encounters an error, it raises the kmac_err IRQ.
SW can then read the ERR_CODE CSR to obtain more information about the error.
Having handled that IRQ, SW is expected to clear the kmac_err bit in the INTR_STATE CSR before exiting the ISR.
When SW has handled the error condition, it is expected to set the err_processed bit in the CMD CSR.
The internal SHA3 engine then flushes its FIFOs and state, which may take a few cycles.
The KMAC HW IP is ready for operation again as soon as the sha3_idle bit in the STATUS CSR is set; SW must not change the configuration of or send commands to the KMAC HW IP before that.
If the error occurred while the KMAC HW IP was being used from SW (i.e., not via an HW application interface), the kmac_done IRQ is raised when the KMAC HW IP is ready again.
KMAC/SHA3 context switching
This version of KMAC/SHA3 HWIP does not support the software context switching. A context switching scheme would allow software to save the current hashing engine state and initiate a new high priority hashing operation. It could restore the previous hashing state later and continue the operation.