/* hmac.h - TinyCrypt interface to an HMAC implementation */

/*
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*/

/**
    @file
    @brief Interface to an HMAC implementation.

    Overview:   HMAC is a message authentication code based on hash functions.
                TinyCrypt hard codes SHA-256 as the hash function. A message
                authentication code based on hash functions is also called a
                keyed cryptographic hash function since it performs a
                transformation specified by a key in an arbitrary length data
                set into a fixed length data set (also called tag).

    Security:   The security of the HMAC depends on the length of the key and
                on the security of the hash function. Note that HMAC primitives
                are much less affected by collision attacks than their
                corresponding hash functions.

    Requires:   SHA-256

    Usage:      1) call tc_hmac_set_key to set the HMAC key.

                2) call tc_hmac_init to initialize a struct hash_state before
                processing the data.

                3) call tc_hmac_update to process the next input segment;
                tc_hmac_update can be called as many times as needed to process
                all of the segments of the input; the order is important.

                4) call tc_hmac_final to out put the tag.
*/

#ifndef __TC_HMAC_H__
#define __TC_HMAC_H__

#include <tinycrypt/sha256.h>

#ifdef __cplusplus
extern "C" {
#endif

struct tc_hmac_state_struct
{
    /* the internal state required by h */
    struct tc_sha256_state_struct hash_state;
    /* HMAC key schedule */
    uint8_t key[2*TC_SHA256_BLOCK_SIZE];
};
typedef struct tc_hmac_state_struct* TCHmacState_t;

/**
    @brief HMAC set key procedure
    Configures ctx to use key
    @return returns TC_CRYPTO_SUCCESS (1)
            returns TC_CRYPTO_FAIL (0) if
                  ctx == NULL or
                  key == NULL or
                  key_size == 0
    @param ctx IN/OUT -- the struct tc_hmac_state_struct to initial
    @param key IN -- the HMAC key to configure
    @param key_size IN -- the HMAC key size
*/
int tc_hmac_set_key(TCHmacState_t ctx, const uint8_t* key,
                    unsigned int key_size);

/**
    @brief HMAC init procedure
    Initializes ctx to begin the next HMAC operation
    @return returns TC_CRYPTO_SUCCESS (1)
           returns TC_CRYPTO_FAIL (0) if: ctx == NULL or key == NULL
    @param ctx IN/OUT -- struct tc_hmac_state_struct buffer to init
*/
int tc_hmac_init(TCHmacState_t ctx);

/**
    @brief HMAC update procedure
    Mixes data_length bytes addressed by data into state
    @return returns TC_CRYPTO_SUCCCESS (1)
            returns TC_CRYPTO_FAIL (0) if: ctx == NULL or key == NULL
    @note Assumes state has been initialized by tc_hmac_init
    @param ctx IN/OUT -- state of HMAC computation so far
    @param data IN -- data to incorporate into state
    @param data_length IN -- size of data in bytes
*/
int tc_hmac_update(TCHmacState_t ctx, const void* data,
                   unsigned int data_length);

/**
    @brief HMAC final procedure
    Writes the HMAC tag into the tag buffer
    @return returns TC_CRYPTO_SUCCESS (1)
            returns TC_CRYPTO_FAIL (0) if:
                  tag == NULL or
                  ctx == NULL or
                  key == NULL or
                  taglen != TC_SHA256_DIGEST_SIZE
    @note ctx is erased before exiting. This should never be changed/removed.
    @note Assumes the tag bufer is at least sizeof(hmac_tag_size(state)) bytes
    state has been initialized by tc_hmac_init
    @param tag IN/OUT -- buffer to receive computed HMAC tag
    @param taglen IN -- size of tag in bytes
    @param ctx IN/OUT -- the HMAC state for computing tag
*/
int tc_hmac_final(uint8_t* tag, unsigned int taglen, TCHmacState_t ctx);

#ifdef __cplusplus
}
#endif

#endif /*__TC_HMAC_H__*/
