mbedtls_shadow_key_derive()

int mbedtls_shadow_key_derive	(	uint32_t	slot_id,
		unsigned int	keybits,
		uint8_t const *	label,
		size_t	label_size,
		uint8_t const *	context,
		size_t	context_size,
		uint8_t *	output,
		size_t	output_size )

#include <crypto/nrf_cc310_mbedcrypto/include/mbedtls/cc3xx_kmu.h>

Function to use CMAC to derive a key stored in KMU/Kdr.

The KDF is using a PRF function described in the Special publication 800-108: Recommendation for Key Derivation Using Pseudorandom Functions https://csrc.nist.gov/publications/detail/sp/800-108/final.

This algorithm is described in chapter 5.1 - KDF in Counter Mode

The format of the PRF (the input) is as follows: PRF (KI, i || Label || 0x00 || Context || L)

KI: The Key derivation key i : The counter value for each iteration of the PRF represented as one byte. label: A string identifying the purpose of the derived key that is up to 64 bytes long. 0x00: a single byte delimiter. Context: Fixed information about the derived keying material that is up to 64 bytes long. L : The length of derived key material in bits represented as two bytes.

Note

On nRF52840 only slot_id == 0 is valid, pointing to the Kdr key (also known as a HUK key) loaded into the CryptoCell.

Parameters

slot_id	Identifier of the key slot.
keybits	Key size in bits.
label	Label to use for KDF.
label_size	Size of the label to use for KDF.
context	Context info to use for KDF.
context_size	Context info size to use for KDF.
output	Output buffer.
output_size	Size of output buffer in bytes.

Returns

0 on success, otherwise a negative number.