text: /*
  chacha-merged.c version 20080118
  D. J. Bernstein
  Public domain.
  */
  
  typedef unsigned char u8;
  typedef unsigned int u32;
  
  typedef struct
  {
    u32 input[16]; /* could be compressed */
  } chacha_ctx;
  
  #define U8C(v) (v##U)
  #define U32C(v) (v##U)
  
  #define U8V(v) ((u8)(v) & U8C(0xFF))
  #define U32V(v) ((u32)(v) & U32C(0xFFFFFFFF))
  
  #define ROTL32(v, n) \
    (U32V((v) << (n)) | ((v) >> (32 - (n))))
  
  #define U8TO32_LITTLE(p) \
    (((u32)((p)[0])      ) | \
     ((u32)((p)[1]) <<  8) | \
     ((u32)((p)[2]) << 16) | \
     ((u32)((p)[3]) << 24))
  
  #define U32TO8_LITTLE(p, v) \
    do { \
      (p)[0] = U8V((v)      ); \
      (p)[1] = U8V((v) >>  8); \
      (p)[2] = U8V((v) >> 16); \
      (p)[3] = U8V((v) >> 24); \
    } while (0)
  
  #define ROTATE(v,c) (ROTL32(v,c))
  #define XOR(v,w) ((v) ^ (w))
  #define PLUS(v,w) (U32V((v) + (w)))
  #define PLUSONE(v) (PLUS((v),1))
  
  #define QUARTERROUND(a,b,c,d) \
    a = PLUS(a,b); d = ROTATE(XOR(d,a),16); \
    c = PLUS(c,d); b = ROTATE(XOR(b,c),12); \
    a = PLUS(a,b); d = ROTATE(XOR(d,a), 8); \
    c = PLUS(c,d); b = ROTATE(XOR(b,c), 7);
  
  static const char sigma[16] = "expand 32-byte k";
  static const char tau[16] = "expand 16-byte k";
  
  static void
  chacha_keysetup(chacha_ctx *x,const u8 *k,u32 kbits,u32 ivbits)
  {
    const char *constants;
  
    x->input[4] = U8TO32_LITTLE(k + 0);
    x->input[5] = U8TO32_LITTLE(k + 4);
    x->input[6] = U8TO32_LITTLE(k + 8);
    x->input[7] = U8TO32_LITTLE(k + 12);
    if (kbits == 256) { /* recommended */
      k += 16;
      constants = sigma;
    } else { /* kbits == 128 */
      constants = tau;
    }
    x->input[8] = U8TO32_LITTLE(k + 0);
    x->input[9] = U8TO32_LITTLE(k + 4);
    x->input[10] = U8TO32_LITTLE(k + 8);
    x->input[11] = U8TO32_LITTLE(k + 12);
    x->input[0] = U8TO32_LITTLE(constants + 0);
    x->input[1] = U8TO32_LITTLE(constants + 4);
    x->input[2] = U8TO32_LITTLE(constants + 8);
    x->input[3] = U8TO32_LITTLE(constants + 12);
  }
  
  static void
  chacha_ivsetup(chacha_ctx *x,const u8 *iv)
  {
    x->input[12] = 0;
    x->input[13] = 0;
    x->input[14] = U8TO32_LITTLE(iv + 0);
    x->input[15] = U8TO32_LITTLE(iv + 4);
  }
  
  static void
  chacha_encrypt_bytes(chacha_ctx *x,const u8 *m,u8 *c,u32 bytes)
  {
    u32 x0, x1, x2, x3, x4, x5, x6, x7, x8, x9, x10, x11, x12, x13, x14, x15;
    u32 j0, j1, j2, j3, j4, j5, j6, j7, j8, j9, j10, j11, j12, j13, j14, j15;
    u8 *ctarget = NULL;
    u8 tmp[64];
    u_int i;
  
    if (!bytes) return;
  
    j0 = x->input[0];
    j1 = x->input[1];
    j2 = x->input[2];
    j3 = x->input[3];
    j4 = x->input[4];
    j5 = x->input[5];
    j6 = x->input[6];
    j7 = x->input[7];
    j8 = x->input[8];
    j9 = x->input[9];
    j10 = x->input[10];
    j11 = x->input[11];
    j12 = x->input[12];
    j13 = x->input[13];
    j14 = x->input[14];
    j15 = x->input[15];
  
    for (;;) {
      if (bytes < 64) {
        for (i = 0;i < bytes;++i) tmp[i] = m[i];
        m = tmp;
        ctarget = c;
        c = tmp;
      }
      x0 = j0;
      x1 = j1;
      x2 = j2;
      x3 = j3;
      x4 = j4;
      x5 = j5;
      x6 = j6;
      x7 = j7;
      x8 = j8;
      x9 = j9;
      x10 = j10;
      x11 = j11;
      x12 = j12;
      x13 = j13;
      x14 = j14;
      x15 = j15;
      for (i = 20;i > 0;i -= 2) {
        QUARTERROUND( x0, x4, x8,x12)
        QUARTERROUND( x1, x5, x9,x13)
        QUARTERROUND( x2, x6,x10,x14)
        QUARTERROUND( x3, x7,x11,x15)
        QUARTERROUND( x0, x5,x10,x15)
        QUARTERROUND( x1, x6,x11,x12)
        QUARTERROUND( x2, x7, x8,x13)
        QUARTERROUND( x3, x4, x9,x14)
      }
      x0 = PLUS(x0,j0);
      x1 = PLUS(x1,j1);
      x2 = PLUS(x2,j2);
      x3 = PLUS(x3,j3);
      x4 = PLUS(x4,j4);
      x5 = PLUS(x5,j5);
      x6 = PLUS(x6,j6);
      x7 = PLUS(x7,j7);
      x8 = PLUS(x8,j8);
      x9 = PLUS(x9,j9);
      x10 = PLUS(x10,j10);
      x11 = PLUS(x11,j11);
      x12 = PLUS(x12,j12);
      x13 = PLUS(x13,j13);
      x14 = PLUS(x14,j14);
      x15 = PLUS(x15,j15);
  
  #ifndef KEYSTREAM_ONLY
      x0 = XOR(x0,U8TO32_LITTLE(m + 0));
      x1 = XOR(x1,U8TO32_LITTLE(m + 4));
      x2 = XOR(x2,U8TO32_LITTLE(m + 8));
      x3 = XOR(x3,U8TO32_LITTLE(m + 12));
      x4 = XOR(x4,U8TO32_LITTLE(m + 16));
      x5 = XOR(x5,U8TO32_LITTLE(m + 20));
      x6 = XOR(x6,U8TO32_LITTLE(m + 24));
      x7 = XOR(x7,U8TO32_LITTLE(m + 28));
      x8 = XOR(x8,U8TO32_LITTLE(m + 32));
      x9 = XOR(x9,U8TO32_LITTLE(m + 36));
      x10 = XOR(x10,U8TO32_LITTLE(m + 40));
      x11 = XOR(x11,U8TO32_LITTLE(m + 44));
      x12 = XOR(x12,U8TO32_LITTLE(m + 48));
      x13 = XOR(x13,U8TO32_LITTLE(m + 52));
      x14 = XOR(x14,U8TO32_LITTLE(m + 56));
      x15 = XOR(x15,U8TO32_LITTLE(m + 60));
  #endif
  
      j12 = PLUSONE(j12);
      if (!j12) {
        j13 = PLUSONE(j13);
        /* stopping at 2^70 bytes per nonce is user's responsibility */
      }
  
      U32TO8_LITTLE(c + 0,x0);
      U32TO8_LITTLE(c + 4,x1);
      U32TO8_LITTLE(c + 8,x2);
      U32TO8_LITTLE(c + 12,x3);
      U32TO8_LITTLE(c + 16,x4);
      U32TO8_LITTLE(c + 20,x5);
      U32TO8_LITTLE(c + 24,x6);
      U32TO8_LITTLE(c + 28,x7);
      U32TO8_LITTLE(c + 32,x8);
      U32TO8_LITTLE(c + 36,x9);
      U32TO8_LITTLE(c + 40,x10);
      U32TO8_LITTLE(c + 44,x11);
      U32TO8_LITTLE(c + 48,x12);
      U32TO8_LITTLE(c + 52,x13);
      U32TO8_LITTLE(c + 56,x14);
      U32TO8_LITTLE(c + 60,x15);
  
      if (bytes <= 64) {
        if (bytes < 64) {
          for (i = 0;i < bytes;++i) ctarget[i] = c[i];
        }
        x->input[12] = j12;
        x->input[13] = j13;
        return;
      }
      bytes -= 64;
      c += 64;
  #ifndef KEYSTREAM_ONLY
      m += 64;
  #endif
    }
  }
  