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/**@file
 * @defgroup nrf_oberon_chacha ChaCha20 APIs
 * @ingroup nrf_oberon_chacha_poly
 * @{
 * @brief Type declaration and APIs for the Chacha20 stream cipher algorithm.
 *
 * ChaCha20 is a stream cipher developed by Daniel J. Bernstein based on the 20-round cipher
 * Salsa20/20.
 *
 * A 256-bit key is expanded into 2^64 randomly accessible streams, each
 * containing 2^64 randomly accessible 64-byte (512-bit) blocks.
 *
 * The changes from Salsa20/20 to ChaCha20 are designed to improve diffusion per
 * round, conjecturally increasing resistance to cryptanalysis, while
 * preserving - and often improving - time per round.
 *
 * @see [RFC 7539 - ChaCha20 and Poly1305 for IETF Protocols](http://tools.ietf.org/html/rfc7539)
 * @see [The ChaCha family of stream ciphers](http://cr.yp.to/chacha.html)
 */

#ifndef OCRYPTO_CHACHA20_H
#define OCRYPTO_CHACHA20_H

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

#ifdef __cplusplus
extern "C" {
#endif

/**
 * Length of the encryption key.
 */
#define ocrypto_chacha20_KEY_BYTES (32)

/**
 * Maximum length of the nonce.
 */
#define ocrypto_chacha20_NONCE_BYTES_MAX (12)


/**
 * ChaCha20 cipher stream generator.
 *
 * The encryption key @p k, the nonce @p n, and the initial block counter
 * @p count are used to generate a pseudo random cipher stream.
 *
 * Possible applications include key generation and random number generation.
 *
 * @param[out] c     Generated cipher stream.
 * @param      c_len Length of @p c.
 * @param      n     Nonce.
 * @param      n_len Nonce length. 0 <= @p n_len <= @c ocrypto_chacha20_NONCE_BYTES_MAX.
 * @param      k     Encryption key.
 * @param      count Initial block counter.
 *
 * @remark When reusing an encryption key @p k, a different nonce @p n or
 *         initial block counter @p count must be used.
 *
 * @remark This function is equivalent to @c chacha20_stream_xor with a
 *         message @p m consisting of @p c_len zeroes.
 */
void ocrypto_chacha20_stream(
    uint8_t *c, size_t c_len,
    const uint8_t *n, size_t n_len,
    const uint8_t k[ocrypto_chacha20_KEY_BYTES],
    uint32_t count);

/**
 * ChaCha20 cipher stream encoder.
 *
 * The message @p m is encrypted by applying the XOR operation with a pseudo
 * random cipher stream derived from the encryption key @p k, the nonce @p n, and
 * the initial block counter @p count.
 *
 * Calling the function a second time with the generated ciphertext as input
 * message @p m decrypts it back to the original message.
 *
 * @param[out] c     Generated ciphertext. Same length as input message.
 * @param      m     Input message.
 * @param      m_len Length of @p c and @p m.
 * @param      n     Nonce.
 * @param      n_len Nonce length. 0 <= @p n_len <= @c ocrypto_chacha20_NONCE_BYTES_MAX.
 * @param      k     Encryption key.
 * @param      count Initial block counter.
 *
 * @remark @p c and @p m can point to the same address.
 *
 * @remark When reusing an encryption key @p k for a different message @p m, a
 *         different nonce @p n or initial block counter @p count must be used.
 */
void ocrypto_chacha20_stream_xor(
    uint8_t *c,
    const uint8_t *m, size_t m_len,
    const uint8_t *n, size_t n_len,
    const uint8_t k[ocrypto_chacha20_KEY_BYTES],
    uint32_t count);

#ifdef __cplusplus
}
#endif

#endif /* #ifndef OCRYPTO_CHACHA20_H */

/** @} */
