/*
 * public domain sha256 crypt implementation
 *
 * original sha crypt design: http://people.redhat.com/drepper/SHA-crypt.txt
 * in this implementation at least 32bit int is assumed,
 * key length is limited, the $5$ prefix is mandatory, '\n' and ':' is rejected
 * in the salt and rounds= setting must contain a valid iteration count,
 * on error "*" is returned.
 */
#include <ctype.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <stdint.h>

/* public domain sha256 implementation based on fips180-3 */

struct sha256 {
  uint64_t len;    /* processed message length */
  uint32_t h[8];   /* hash state */
  uint8_t buf[64]; /* message block buffer */
};

static uint32_t ror(uint32_t n, int k) {
  return (n >> k) | (n << (32 - 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 processblock(struct sha256* s, const uint8_t* buf) {
  uint32_t W[64], t1, t2, a, b, c, d, e, f, g, h;
  int i;

  for (i = 0; i < 16; i++) {
    W[i] = (uint32_t)buf[4 * i] << 24;
    W[i] |= (uint32_t)buf[4 * i + 1] << 16;
    W[i] |= (uint32_t)buf[4 * i + 2] << 8;
    W[i] |= buf[4 * i + 3];
  }
  for (; i < 64; i++)
    W[i] = R1(W[i - 2]) + W[i - 7] + R0(W[i - 15]) + W[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] + W[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;
}

static void pad(struct sha256* s) {
  unsigned r = s->len % 64;

  s->buf[r++] = 0x80;
  if (r > 56) {
    memset(s->buf + r, 0, 64 - r);
    r = 0;
    processblock(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;
  processblock(s, s->buf);
}

static void sha256_init(struct sha256* 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;
}

static void sha256_sum(struct sha256* s, uint8_t* md) {
  int i;

  pad(s);
  for (i = 0; i < 8; 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];
  }
}

static void sha256_update(struct sha256* s, const void* m, unsigned long len) {
  const uint8_t* p = m;
  unsigned r = s->len % 64;

  s->len += len;
  if (r) {
    if (len < 64 - r) {
      memcpy(s->buf + r, p, len);
      return;
    }
    memcpy(s->buf + r, p, 64 - r);
    len -= 64 - r;
    p += 64 - r;
    processblock(s, s->buf);
  }
  for (; len >= 64; len -= 64, p += 64)
    processblock(s, p);
  memcpy(s->buf, p, len);
}

static const unsigned char b64[] =
    "./0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";

static char* to64(char* s, unsigned int u, int n) {
  while (--n >= 0) {
    *s++ = b64[u % 64];
    u /= 64;
  }
  return s;
}

/* key limit is not part of the original design, added for DoS protection.
 * rounds limit has been lowered (versus the reference/spec), also for DoS
 * protection. runtime is O(klen^2 + klen*rounds) */
#define KEY_MAX 256
#define SALT_MAX 16
#define ROUNDS_DEFAULT 5000
#define ROUNDS_MIN 1000
#define ROUNDS_MAX 9999999

/* hash n bytes of the repeated md message digest */
static void hashmd(struct sha256* s, unsigned int n, const void* md) {
  unsigned int i;

  for (i = n; i > 32; i -= 32)
    sha256_update(s, md, 32);
  sha256_update(s, md, i);
}

static char* sha256crypt(const char* key, const char* setting, char* output) {
  struct sha256 ctx;
  unsigned char md[32], kmd[32], smd[32];
  unsigned int i, r, klen, slen;
  char rounds[20] = "";
  const char* salt;
  char* p;

  /* reject large keys */
  klen = strnlen(key, KEY_MAX + 1);
  if (klen > KEY_MAX)
    return 0;

  /* setting: $5$rounds=n$salt$ (rounds=n$ and closing $ are optional) */
  if (strncmp(setting, "$5$", 3) != 0)
    return 0;
  salt = setting + 3;

  r = ROUNDS_DEFAULT;
  if (strncmp(salt, "rounds=", sizeof "rounds=" - 1) == 0) {
    unsigned long u;
    char* end;

    /*
     * this is a deviation from the reference:
     * bad rounds setting is rejected if it is
     * - empty
     * - unterminated (missing '$')
     * - begins with anything but a decimal digit
     * the reference implementation treats these bad
     * rounds as part of the salt or parse them with
     * strtoul semantics which may cause problems
     * including non-portable hashes that depend on
     * the host's value of ULONG_MAX.
     */
    salt += sizeof "rounds=" - 1;
    if (!isdigit(*salt))
      return 0;
    u = strtoul(salt, &end, 10);
    if (*end != '$')
      return 0;
    salt = end + 1;
    if (u < ROUNDS_MIN)
      r = ROUNDS_MIN;
    else if (u > ROUNDS_MAX)
      r = ROUNDS_MAX;
    else
      r = u;
    /* needed when rounds is zero prefixed or out of bounds */
    sprintf(rounds, "rounds=%u$", r);
  }

  for (i = 0; i < SALT_MAX && salt[i] && salt[i] != '$'; i++)
    /* reject characters that interfere with /etc/shadow parsing */
    if (salt[i] == '\n' || salt[i] == ':')
      return 0;
  slen = i;

  /* B = sha(key salt key) */
  sha256_init(&ctx);
  sha256_update(&ctx, key, klen);
  sha256_update(&ctx, salt, slen);
  sha256_update(&ctx, key, klen);
  sha256_sum(&ctx, md);

  /* A = sha(key salt repeat-B alternate-B-key) */
  sha256_init(&ctx);
  sha256_update(&ctx, key, klen);
  sha256_update(&ctx, salt, slen);
  hashmd(&ctx, klen, md);
  for (i = klen; i > 0; i >>= 1)
    if (i & 1)
      sha256_update(&ctx, md, sizeof md);
    else
      sha256_update(&ctx, key, klen);
  sha256_sum(&ctx, md);

  /* DP = sha(repeat-key), this step takes O(klen^2) time */
  sha256_init(&ctx);
  for (i = 0; i < klen; i++)
    sha256_update(&ctx, key, klen);
  sha256_sum(&ctx, kmd);

  /* DS = sha(repeat-salt) */
  sha256_init(&ctx);
  for (i = 0; i < 16 + md[0]; i++)
    sha256_update(&ctx, salt, slen);
  sha256_sum(&ctx, smd);

  /* iterate A = f(A,DP,DS), this step takes O(rounds*klen) time */
  for (i = 0; i < r; i++) {
    sha256_init(&ctx);
    if (i % 2)
      hashmd(&ctx, klen, kmd);
    else
      sha256_update(&ctx, md, sizeof md);
    if (i % 3)
      sha256_update(&ctx, smd, slen);
    if (i % 7)
      hashmd(&ctx, klen, kmd);
    if (i % 2)
      sha256_update(&ctx, md, sizeof md);
    else
      hashmd(&ctx, klen, kmd);
    sha256_sum(&ctx, md);
  }

  /* output is $5$rounds=n$salt$hash */
  p = output;
  p += sprintf(p, "$5$%s%.*s$", rounds, slen, salt);
  static const unsigned char perm[][3] = {
      0,  10, 20, 21, 1,  11, 12, 22, 2,  3,  13, 23, 24, 4,  14,
      15, 25, 5,  6,  16, 26, 27, 7,  17, 18, 28, 8,  9,  19, 29};
  for (i = 0; i < 10; i++)
    p = to64(p, (md[perm[i][0]] << 16) | (md[perm[i][1]] << 8) | md[perm[i][2]],
             4);
  p = to64(p, (md[31] << 8) | md[30], 3);
  *p = 0;
  return output;
}

char* __crypt_sha256(const char* key, const char* setting, char* output) {
  static const char testkey[] = "Xy01@#\x01\x02\x80\x7f\xff\r\n\x81\t !";
  static const char testsetting[] = "$5$rounds=1234$abc0123456789$";
  static const char testhash[] =
      "$5$rounds=1234$abc0123456789$3VfDjPt05VHFn47C/"
      "ojFZ6KRPYrOjj1lLbH.dkF3bZ6";
  char testbuf[128];
  char *p, *q;

  p = sha256crypt(key, setting, output);
  /* self test and stack cleanup */
  q = sha256crypt(testkey, testsetting, testbuf);
  if (!p || q != testbuf || memcmp(testbuf, testhash, sizeof testhash))
    return "*";
  return p;
}