text: #ifndef PBKDF2_SHA256_INCLUDE
  #define PBKDF2_SHA256_INCLUDE
  
  #define SHA256_BLOCKLEN  64ul //size of message block buffer
  #define SHA256_DIGESTLEN 32ul //size of digest in uint8_t
  #define SHA256_DIGESTINT 8ul  //size of digest in uint32_t
  
  #ifndef PBKDF2_SHA256_STATIC
  #define PBKDF2_SHA256_DEF extern
  #else
  #define PBKDF2_SHA256_DEF static
  #endif
  
  #include <stdint.h>
  
  typedef struct sha256_ctx_t
  {
  	uint64_t len;                 // processed message length
  	uint32_t h[SHA256_DIGESTINT]; // hash state
  	uint8_t buf[SHA256_BLOCKLEN]; // message block buffer
  } SHA256_CTX;
  
  PBKDF2_SHA256_DEF void sha256_init(SHA256_CTX *ctx);
  PBKDF2_SHA256_DEF void sha256_update(SHA256_CTX *ctx, const uint8_t *m, uint32_t mlen);
  // resets state: calls sha256_init
  PBKDF2_SHA256_DEF void sha256_final(SHA256_CTX *ctx, uint8_t *md);
  
  typedef struct hmac_sha256_ctx_t
  {
  	uint8_t buf[SHA256_BLOCKLEN]; // key block buffer, not needed after init
  	uint32_t h_inner[SHA256_DIGESTINT];
  	uint32_t h_outer[SHA256_DIGESTINT];
  	SHA256_CTX sha;
  } HMAC_SHA256_CTX;
  
  PBKDF2_SHA256_DEF void hmac_sha256_init(HMAC_SHA256_CTX *hmac, const uint8_t *key, uint32_t keylen);
  PBKDF2_SHA256_DEF void hmac_sha256_update(HMAC_SHA256_CTX *hmac, const uint8_t *m, uint32_t mlen);
  // resets state to hmac_sha256_init
  PBKDF2_SHA256_DEF void hmac_sha256_final(HMAC_SHA256_CTX *hmac, uint8_t *md);
  
  PBKDF2_SHA256_DEF void pbkdf2_sha256(HMAC_SHA256_CTX *ctx,
      const uint8_t *key, uint32_t keylen, const uint8_t *salt, uint32_t saltlen, uint32_t rounds,
      uint8_t *dk, uint32_t dklen);
  
  #endif // PBKDF2_SHA256_INCLUDE
  
  //------------------------------------------------------------------------------
  
  #ifdef PBKDF2_SHA256_IMPLEMENTATION
  
  #include <string.h>
  
  //#define ROR(n,k) ((n >> k) | (n << (32 - k)))
  
  static uint32_t ror(uint32_t n, uint32_t k)
  {
  	return (n >> k) | (n << (32 - k));
  }
  
  #define ROR(n,k) ror(n,k)
  
  #define CH(x,y,z)  (z ^ (x & (y ^ z)))
  #define MAJ(x,y,z) ((x & y) | (z & (x | y)))
  #define S0(x)      (ROR(x, 2) ^ ROR(x,13) ^ ROR(x,22))
  #define S1(x)      (ROR(x, 6) ^ ROR(x,11) ^ ROR(x,25))
  #define R0(x)      (ROR(x, 7) ^ ROR(x,18) ^ (x>>3))
  #define R1(x)      (ROR(x,17) ^ ROR(x,19) ^ (x>>10))
  
  static const uint32_t K[64] =
  {
  	0x428a2f98, 0x71374491, 0xb5c0fbcf, 0xe9b5dba5, 0x3956c25b, 0x59f111f1, 0x923f82a4, 0xab1c5ed5,
  	0xd807aa98, 0x12835b01, 0x243185be, 0x550c7dc3, 0x72be5d74, 0x80deb1fe, 0x9bdc06a7, 0xc19bf174,
  	0xe49b69c1, 0xefbe4786, 0x0fc19dc6, 0x240ca1cc, 0x2de92c6f, 0x4a7484aa, 0x5cb0a9dc, 0x76f988da,
  	0x983e5152, 0xa831c66d, 0xb00327c8, 0xbf597fc7, 0xc6e00bf3, 0xd5a79147, 0x06ca6351, 0x14292967,
  	0x27b70a85, 0x2e1b2138, 0x4d2c6dfc, 0x53380d13, 0x650a7354, 0x766a0abb, 0x81c2c92e, 0x92722c85,
  	0xa2bfe8a1, 0xa81a664b, 0xc24b8b70, 0xc76c51a3, 0xd192e819, 0xd6990624, 0xf40e3585, 0x106aa070,
  	0x19a4c116, 0x1e376c08, 0x2748774c, 0x34b0bcb5, 0x391c0cb3, 0x4ed8aa4a, 0x5b9cca4f, 0x682e6ff3,
  	0x748f82ee, 0x78a5636f, 0x84c87814, 0x8cc70208, 0x90befffa, 0xa4506ceb, 0xbef9a3f7, 0xc67178f2
  };
  
  static void sha256_transform(SHA256_CTX *s, const uint8_t *buf)
  {
  	uint32_t t1, t2, a, b, c, d, e, f, g, h, m[64];
  	uint32_t i, j;
  	
  	for (i = 0, j = 0; i < 16; i++, j += 4)
  	{
  		m[i] = (uint32_t) buf[j] << 24 | (uint32_t) buf[j + 1] << 16 |
  		       (uint32_t) buf[j + 2] << 8 | (uint32_t) buf[j + 3];
  	}
  	for (; i < 64; i++)
  	{
  		m[i] = R1(m[i - 2]) + m[i - 7] + R0(m[i - 15]) + m[i - 16];
  	}
  	a = s->h[0];
  	b = s->h[1];
  	c = s->h[2];
  	d = s->h[3];
  	e = s->h[4];
  	f = s->h[5];
  	g = s->h[6];
  	h = s->h[7];
  	for (i = 0; i < 64; i++)
  	{
  		t1 = h + S1(e) + CH(e, f, g) + K[i] + m[i];
  		t2 = S0(a) + MAJ(a, b, c);
  		h = g;
  		g = f;
  		f = e;
  		e = d + t1;
  		d = c;
  		c = b;
  		b = a;
  		a = t1 + t2;
  	}
  	s->h[0] += a;
  	s->h[1] += b;
  	s->h[2] += c;
  	s->h[3] += d;
  	s->h[4] += e;
  	s->h[5] += f;
  	s->h[6] += g;
  	s->h[7] += h;
  }
  
  PBKDF2_SHA256_DEF void sha256_init(SHA256_CTX *s)
  {
  	s->len = 0;
  	
  	s->h[0] = 0x6a09e667;
  	s->h[1] = 0xbb67ae85;
  	s->h[2] = 0x3c6ef372;
  	s->h[3] = 0xa54ff53a;
  	s->h[4] = 0x510e527f;
  	s->h[5] = 0x9b05688c;
  	s->h[6] = 0x1f83d9ab;
  	s->h[7] = 0x5be0cd19;
  }
  
  PBKDF2_SHA256_DEF void sha256_final(SHA256_CTX *s, uint8_t *md)
  {
  	uint32_t r = s->len % SHA256_BLOCKLEN;
  	
  	//pad
  	s->buf[r++] = 0x80;
  	if (r > 56)
  	{
  		memset(s->buf + r, 0, SHA256_BLOCKLEN - r);
  		r = 0;
  		sha256_transform(s, s->buf);
  	}
  	memset(s->buf + r, 0, 56 - r);
  	s->len *= 8;
  	s->buf[56] = s->len >> 56;
  	s->buf[57] = s->len >> 48;
  	s->buf[58] = s->len >> 40;
  	s->buf[59] = s->len >> 32;
  	s->buf[60] = s->len >> 24;
  	s->buf[61] = s->len >> 16;
  	s->buf[62] = s->len >> 8;
  	s->buf[63] = s->len;
  	sha256_transform(s, s->buf);
  	
  	for (uint32_t i = 0; i < SHA256_DIGESTINT; i++)
  	{
  		md[4 * i    ] = s->h[i] >> 24;
  		md[4 * i + 1] = s->h[i] >> 16;
  		md[4 * i + 2] = s->h[i] >> 8;
  		md[4 * i + 3] = s->h[i];
  	}
  	sha256_init(s);
  }
  
  PBKDF2_SHA256_DEF void sha256_update(SHA256_CTX *s, const uint8_t *m, uint32_t len)
  {
  	const uint8_t *p = m;
  	uint32_t r = s->len % SHA256_BLOCKLEN;
  	
  	s->len += len;
  	if (r)
  	{
  		if (len + r < SHA256_BLOCKLEN)
  		{
  			memcpy(s->buf + r, p, len);
  			return;
  		}
  		memcpy(s->buf + r, p, SHA256_BLOCKLEN - r);
  		len -= SHA256_BLOCKLEN - r;
  		p += SHA256_BLOCKLEN - r;
  		sha256_transform(s, s->buf);
  	}
  	for (; len >= SHA256_BLOCKLEN; len -= SHA256_BLOCKLEN, p += SHA256_BLOCKLEN)
  	{
  		sha256_transform(s, p);
  	}
  	memcpy(s->buf, p, len);
  }
  
  #define INNER_PAD '\x36'
  #define OUTER_PAD '\x5c'
  
  PBKDF2_SHA256_DEF void hmac_sha256_init(HMAC_SHA256_CTX *hmac, const uint8_t *key, uint32_t keylen)
  {
  	SHA256_CTX *sha = &hmac->sha;
  	
  	if (keylen <= SHA256_BLOCKLEN)
  	{
  		memcpy(hmac->buf, key, keylen);
  		memset(hmac->buf + keylen, '\0', SHA256_BLOCKLEN - keylen);
  	}
  	else
  	{
  		sha256_init(sha);
  		sha256_update(sha, key, keylen);
  		sha256_final(sha, hmac->buf);
  		memset(hmac->buf + SHA256_DIGESTLEN, '\0', SHA256_BLOCKLEN - SHA256_DIGESTLEN);
  	}
  	
  	uint32_t i;
  	for (i = 0; i < SHA256_BLOCKLEN; i++)
  	{
  		hmac->buf[ i ] = hmac->buf[ i ] ^ OUTER_PAD;
  	}
  	
  	sha256_init(sha);
  	sha256_update(sha, hmac->buf, SHA256_BLOCKLEN);
  	// copy outer state
  	memcpy(hmac->h_outer, sha->h, SHA256_DIGESTLEN);
  	
  	for (i = 0; i < SHA256_BLOCKLEN; i++)
  	{
  		hmac->buf[ i ] = (hmac->buf[ i ] ^ OUTER_PAD) ^ INNER_PAD;
  	}
  	
  	sha256_init(sha);
  	sha256_update(sha, hmac->buf, SHA256_BLOCKLEN);
  	// copy inner state
  	memcpy(hmac->h_inner, sha->h, SHA256_DIGESTLEN);
  }
  
  PBKDF2_SHA256_DEF void hmac_sha256_update(HMAC_SHA256_CTX *hmac, const uint8_t *m, uint32_t mlen)
  {
  	sha256_update(&hmac->sha, m, mlen);
  }
  
  PBKDF2_SHA256_DEF void hmac_sha256_final(HMAC_SHA256_CTX *hmac, uint8_t *md)
  {
  	SHA256_CTX *sha = &hmac->sha;
  	sha256_final(sha, md);
  	
  	// reset sha to outer state
  	memcpy(sha->h, hmac->h_outer, SHA256_DIGESTLEN);
  	sha->len = SHA256_BLOCKLEN;
  	
  	sha256_update(sha, md, SHA256_DIGESTLEN);
  	sha256_final(sha, md); // md = D(outer || D(inner || msg))
  	
  	// reset sha to inner state -> reset hmac
  	memcpy(sha->h, hmac->h_inner, SHA256_DIGESTLEN);
  	sha->len = SHA256_BLOCKLEN;
  }
  
  PBKDF2_SHA256_DEF void pbkdf2_sha256(HMAC_SHA256_CTX *hmac,
      const uint8_t *key, uint32_t keylen, const uint8_t *salt, uint32_t saltlen, uint32_t rounds,
      uint8_t *dk, uint32_t dklen)
  {
  	uint32_t hlen = SHA256_DIGESTLEN;
  	uint32_t l = dklen / hlen + ((dklen % hlen) ? 1 : 0);
  	uint32_t r = dklen - (l - 1) * hlen;
  	
  	hmac_sha256_init(hmac, key, keylen);
  	
  	uint8_t *U = hmac->buf;
  	uint8_t *T = dk;
  	uint8_t count[4];
  	
  	uint32_t i, j, k;
  	uint32_t len = hlen;
  	for (i = 1; i <= l; i++)
  	{
  		if (i == l) { len = r; }
  		count[0] = (i >> 24) & 0xFF;
  		count[1] = (i >> 16) & 0xFF;
  		count[2] = (i >>  8) & 0xFF;
  		count[3] = (i) & 0xFF;
  		hmac_sha256_update(hmac, salt, saltlen);
  		hmac_sha256_update(hmac, count, 4);
  		hmac_sha256_final(hmac, U);
  		memcpy(T, U, len);
  		for (j = 1; j < rounds; j++)
  		{
  			hmac_sha256_update(hmac, U, hlen);
  			hmac_sha256_final(hmac, U);
  			for (k = 0; k < len; k++)
  			{
  				T[k] ^= U[k];
  			}
  		}
  		T += len;
  	}
  	
  }
  
  #endif // PBKDF2_SHA256_IMPLEMENTATION
  
  /*
  ------------------------------------------------------------------------------
  This software is available under 2 licenses -- choose whichever you prefer.
  ------------------------------------------------------------------------------
  ALTERNATIVE A - Public Domain (www.unlicense.org)
  This is free and unencumbered software released into the public domain.
  Anyone is free to copy, modify, publish, use, compile, sell, or distribute this
  software, either in source code form or as a compiled binary, for any purpose,
  commercial or non-commercial, and by any means.
  In jurisdictions that recognize copyright laws, the author or authors of this
  software dedicate any and all copyright interest in the software to the public
  domain. We make this dedication for the benefit of the public at large and to
  the detriment of our heirs and successors. We intend this dedication to be an
  overt act of relinquishment in perpetuity of all present and future rights to
  this software under copyright law.
  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 BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
  ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
  WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
  ------------------------------------------------------------------------------
  ALTERNATIVE B - MIT License
  Copyright (c) 2019 monolifed
  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 BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
  SOFTWARE.
  ------------------------------------------------------------------------------
  */
  