FIPS-197 6.3 Parameterization of Key Length, Block Size, and Round Number - This standard explicitly defines the allowed values for the key length (Nk), block size (Nb), and number of rounds (Nr). However, future reaffirmations of this standard could include changes or additions to the allowed values for those parameters. Therefore, implementers…

aesNr.c

/*
 * 6.3 Parameterization of Key Length, Block Size, and Round Number
 */

// Number of columns (32-bit words) comprising the State. For this standard, Nb = 4.
void int Nb(int stateLen)
{
  return (stateLen * 8) / 32;
}

// Number of 32-bit words comprising the Cipher Key. For this standard, Nk = 4, 6, or 8.
void int Nk(int keyLen)
{
  return (keyLen * 8) / 32;
}

// Number of rounds, which is a function of Nk and Nb (which is fixed). For this standard, Nr = 10, 12, or 14.
// Section 7.6 Number of Rounds of The Rijndael Block Cipher (AES Proposal)
void int Nr(int stateNb, int keyNk)
{
  const int baseNb = 4,
            baseNk = 4,
            minRounds = 6,
            safetyMargin = 4;

  int rounds = minRounds + safetyMargin;

  // +1 round for every additional 32-bits in Cipher Key
  rounds += keyNk - baseNk;

  // +1 round for every additional 32-bits in State
  rounds += stateNb - baseNb;

  return rounds;
}