tomcrypt/src/misc/hkdf/hkdf.c

#include <assert.h>
#include <stdio.h>
#include <stdlib.h>

#include <tomcrypt.h>

#ifndef MIN
#define MIN(a,b) ((a)<(b))?(a):(b)
#endif

/* This is mostly just a wrapper around hmac_memory */
int hkdf_extract(int hash_idx, const unsigned char *salt, unsigned long  saltlen,
                               const unsigned char *in,   unsigned long  inlen,
                                     unsigned char *out,  unsigned long *outlen)
{
   /* libtomcrypt chokes on a zero length HMAC key, so we need to check for
      that.  HMAC specifies that keys shorter than the hash's blocksize are
      0 padded to the block size.  HKDF specifies that a NULL salt is to be
      substituted with a salt comprised of hashLen 0 bytes.  HMAC's padding
      means that in either case the HMAC is actually using a blocksize long
      zero filled key.  Unless blocksize < hashLen (which wouldn't make any
      sense), we can use a single 0 byte as the HMAC key and still generate
      valid results for HKDF. */
   if (salt == NULL || saltlen == 0) {
      return hmac_memory(hash_idx, "",   1,       in, inlen, out, outlen); 
   } else {
      return hmac_memory(hash_idx, salt, saltlen, in, inlen, out, outlen);
   }
}

int hkdf_expand(int hash_idx, const unsigned char *info, unsigned long infolen,
                              const unsigned char *in,   unsigned long inlen,
                                    unsigned char *out,  unsigned long outlen)
{
   const unsigned long hashsize = hash_descriptor[hash_idx].hashsize;
   int err;
   unsigned char N;
   unsigned long Noutlen, outoff;

   unsigned char *T,  *dat;
   unsigned long Tlen, datlen;

   /* RFC5869 parameter restrictions */
   if (inlen < hashsize || outlen > hashsize * 255)
      return CRYPT_INVALID_ARG;
   if (info == NULL && infolen != 0)
      return CRYPT_INVALID_ARG;
   assert(out != NULL);

   Tlen = hashsize + infolen + 1;
   T = XMALLOC(Tlen); /* Replace with static buffer? */
   if (T == NULL) {
      return CRYPT_MEM;
   }
   XMEMCPY(T + hashsize, info, infolen);

   /* HMAC data T(1) doesn't include a previous hash value */
   dat    = T    + hashsize;
   datlen = Tlen - hashsize;

   N = 0;
   outoff = 0; /* offset in out to write to */
   while (1) { /* an exit condition breaks mid-loop */
      Noutlen = MIN(hashsize, outlen - outoff);
      T[Tlen - 1] = ++N;
      if ((err = hmac_memory(hash_idx, in, inlen, dat, datlen,
                             out + outoff, &Noutlen)) != CRYPT_OK) {
         zeromem(T, Tlen);
         XFREE(T);
         return err;
      }
      outoff += Noutlen;

      if (outoff >= outlen) /* loop exit condition */
         break;

      /* All subsequent HMAC data T(N) DOES include the previous hash value */
      XMEMCPY(T, out + hashsize * (N-1), hashsize);
      if (N == 1) {
         dat = T;
         datlen = Tlen;
      }
   }
   zeromem(T, Tlen);
   XFREE(T);
   return CRYPT_OK;
}

/* all in one step */
int hkdf(int hash_idx, const unsigned char *salt, unsigned long saltlen,
                       const unsigned char *info, unsigned long infolen,
                       const unsigned char *in,   unsigned long inlen,
                             unsigned char *out,  unsigned long outlen)
{
   unsigned long hashsize = hash_descriptor[hash_idx].hashsize;
   int err;
   unsigned char *extracted = XMALLOC(hashsize); /* replace with static buffer? */
   if (extracted == NULL) {
      return CRYPT_MEM;
   }
   if ((err = hkdf_extract(hash_idx, salt, saltlen, in, inlen, extracted, &hashsize)) != 0) {
      zeromem(extracted, hashsize);
      XFREE(extracted);
      return err;
   }
   err = hkdf_expand(hash_idx, extracted, hashsize, info, infolen, out, outlen);
   zeromem(extracted, hashsize);
   XFREE(extracted);
   return err;
}


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