bearssl_kdf.h

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/*
 * Copyright (c) 2018 Thomas Pornin <pornin@bolet.org>
 *
 * Permission is hereby granted, free of charge, to any person obtaining 
 * a copy of this software and associated documentation files (the
 * "Software"), to deal in the Software without restriction, including
 * without limitation the rights to use, copy, modify, merge, publish,
 * distribute, sublicense, and/or sell copies of the Software, and to
 * permit persons to whom the Software is furnished to do so, subject to
 * the following conditions:
 *
 * The above copyright notice and this permission notice shall be 
 * included in all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, 
 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND 
 * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
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 * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */

#ifndef BR_BEARSSL_KDF_H__
#define BR_BEARSSL_KDF_H__

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

#include "bearssl_hash.h"
#include "bearssl_hmac.h"

#ifdef __cplusplus
extern "C" {
#endif

/** \file bearssl_kdf.h
 *
 * # Key Derivation Functions
 *
 * KDF are functions that takes a variable length input, and provide a
 * variable length output, meant to be used to derive subkeys from a
 * master key.
 *
 * ## HKDF
 *
 * HKDF is a KDF defined by [RFC 5869](https://tools.ietf.org/html/rfc5869).
 * It is based on HMAC, itself using an underlying hash function. Any
 * hash function can be used, as long as it is compatible with the rules
 * for the HMAC implementation (i.e. output size is 64 bytes or less, hash
 * internal state size is 64 bytes or less, and the internal block length is
 * a power of 2 between 16 and 256 bytes). HKDF has two phases:
 *
 *  - HKDF-Extract: the input data in ingested, along with a "salt" value.
 *
 *  - HKDF-Expand: the output is produced, from the result of processing
 *    the input and salt, and using an extra non-secret parameter called
 *    "info".
 *
 * The "salt" and "info" strings are non-secret and can be empty. Their role
 * is normally to bind the input and output, respectively, to conventional
 * identifiers that qualifu them within the used protocol or application.
 *
 * The implementation defined in this file uses the following functions:
 *
 *  - `br_hkdf_init()`: initialize an HKDF context, with a hash function,
 *    and the salt. This starts the HKDF-Extract process.
 *
 *  - `br_hkdf_inject()`: inject more input bytes. This function may be
 *    called repeatedly if the input data is provided by chunks.
 *
 *  - `br_hkdf_flip()`: end the HKDF-Extract process, and start the
 *    HKDF-Expand process.
 *
 *  - `br_hkdf_produce()`: get the next bytes of output. This function
 *    may be called several times to obtain the full output by chunks.
 *    For correct HKDF processing, the same "info" string must be
 *    provided for each call.
 *
 * Note that the HKDF total output size (the number of bytes that
 * HKDF-Expand is willing to produce) is limited: if the hash output size
 * is _n_ bytes, then the maximum output size is _255*n_.
 */

/**
 * \brief HKDF context.
 *
 * The HKDF context is initialized with a hash function implementation
 * and a salt value. Contents are opaque (callers should not access them
 * directly). The caller is responsible for allocating the context where
 * appropriate. Context initialisation and usage incurs no dynamic
 * allocation, so there is no release function.
 */
typedef struct {
#ifndef BR_DOXYGEN_IGNORE
        union {
                br_hmac_context hmac_ctx;
                br_hmac_key_context prk_ctx;
        } u;
        unsigned char buf[64];
        size_t ptr;
        size_t dig_len;
        unsigned chunk_num;
#endif
} br_hkdf_context;

/**
 * \brief HKDF context initialization.
 *
 * The underlying hash function and salt value are provided. Arbitrary
 * salt lengths can be used.
 *
 * HKDF makes a difference between a salt of length zero, and an
 * absent salt (the latter being equivalent to a salt consisting of
 * bytes of value zero, of the same length as the hash function output).
 * If `salt_len` is zero, then this function assumes that the salt is
 * present but of length zero. To specify an _absent_ salt, use
 * `BR_HKDF_NO_SALT` as `salt` parameter (`salt_len` is then ignored).
 *
 * \param hc              HKDF context to initialise.
 * \param digest_vtable   pointer to the hash function implementation vtable.
 * \param salt            HKDF-Extract salt.
 * \param salt_len        HKDF-Extract salt length (in bytes).
 */
void br_hkdf_init(br_hkdf_context *hc, const br_hash_class *digest_vtable,
        const void *salt, size_t salt_len);

/**
 * \brief The special "absent salt" value for HKDF.
 */
#define BR_HKDF_NO_SALT   (&br_hkdf_no_salt)

#ifndef BR_DOXYGEN_IGNORE
extern const unsigned char br_hkdf_no_salt;
#endif

/**
 * \brief HKDF input injection (HKDF-Extract).
 *
 * This function injects some more input bytes ("key material") into
 * HKDF. This function may be called several times, after `br_hkdf_init()`
 * but before `br_hkdf_flip()`.
 *
 * \param hc        HKDF context.
 * \param ikm       extra input bytes.
 * \param ikm_len   number of extra input bytes.
 */
void br_hkdf_inject(br_hkdf_context *hc, const void *ikm, size_t ikm_len);

/**
 * \brief HKDF switch to the HKDF-Expand phase.
 *
 * This call terminates the HKDF-Extract process (input injection), and
 * starts the HKDF-Expand process (output production).
 *
 * \param hc   HKDF context.
 */
void br_hkdf_flip(br_hkdf_context *hc);

/**
 * \brief HKDF output production (HKDF-Expand).
 *
 * Produce more output bytes from the current state. This function may be
 * called several times, but only after `br_hkdf_flip()`.
 *
 * Returned value is the number of actually produced bytes. The total
 * output length is limited to 255 times the output length of the
 * underlying hash function.
 *
 * \param hc         HKDF context.
 * \param info       application specific information string.
 * \param info_len   application specific information string length (in bytes).
 * \param out        destination buffer for the HKDF output.
 * \param out_len    the length of the requested output (in bytes).
 * \return  the produced output length (in bytes).
 */
size_t br_hkdf_produce(br_hkdf_context *hc,
        const void *info, size_t info_len, void *out, size_t out_len);

#ifdef __cplusplus
}
#endif

#endif