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/**
 * @file
 * @brief Interface to a SHA-256 implementation.
 *
 *  Overview:   SHA-256 is a NIST approved cryptographic hashing algorithm
 *              specified in FIPS 180. A hash algorithm maps data of arbitrary
 *              size to data of fixed length.
 *
 *  Security:   SHA-256 provides 128 bits of security against collision attacks
 *              and 256 bits of security against pre-image attacks. SHA-256 does
 *              NOT behave like a random oracle, but it can be used as one if
 *              the string being hashed is prefix-free encoded before hashing.
 *
 *  Usage:      1) call tc_sha256_init to initialize a struct
 *              tc_sha256_state_struct before hashing a new string.
 *
 *              2) call tc_sha256_update to hash the next string segment;
 *              tc_sha256_update can be called as many times as needed to hash
 *              all of the segments of a string; the order is important.
 *
 *              3) call tc_sha256_final to out put the digest from a hashing
 *              operation.
 */

#ifndef EPID_MEMBER_TINY_MATH_SHA256_H_
#define EPID_MEMBER_TINY_MATH_SHA256_H_

#include <stddef.h>
#include <stdint.h>

/// Block size
#define SHA256_BLOCK_SIZE (64)
/// Digest size
#define SHA256_DIGEST_SIZE (32)
/// Number of blocks in state
#define SHA256_STATE_BLOCKS (SHA256_DIGEST_SIZE / 4)

/// The SHA state
/// \cond
typedef struct sha256_state {
  unsigned int iv[SHA256_STATE_BLOCKS];
  uint64_t bits_hashed;
  uint8_t leftover[SHA256_BLOCK_SIZE];
  size_t leftover_offset;
} sha256_state;
/// \endcond

/**
 *  @brief SHA256 initialization procedure
 *  Initializes s
 *  @param s Sha256 state struct
 */
void tc_sha256_init(sha256_state* s);

/**
 *  @brief SHA256 update procedure
 *  Hashes data_length bytes addressed by data into state s

 *  @note Assumes s has been initialized by tc_sha256_init
 *  @warning The state buffer 'leftover' is left in memory after processing
 *           If your application intends to have sensitive data in this
 *           buffer, remind to erase it after the data has been processed
 *  @param s Sha256 state struct
 *  @param data message to hash
 *  @param datalen length of message to hash
 */
void tc_sha256_update(sha256_state* s, const uint8_t* data, size_t datalen);

/**
 *  @brief SHA256 final procedure
 *  Inserts the completed hash computation into digest
 *  @return returns 1
 *          returns 0 if:
 *                s == NULL,
 *                s->iv == NULL,
 *                digest == NULL
 *  @note Assumes: s has been initialized by tc_sha256_init
 *        digest points to at least SHA256_DIGEST_SIZE bytes
 *  @warning The state buffer 'leftover' is left in memory after processing
 *           If your application intends to have sensitive data in this
 *           buffer, remind to erase it after the data has been processed
 *  @param digest unsigned eight bit integer
 *  @param s Sha256 state struct
 */
void tc_sha256_final(uint8_t* digest, sha256_state* s);

#endif  // EPID_MEMBER_TINY_MATH_SHA256_H_