code: def hkdf(self, chaining_key, input_key_material, dhlen=64):
          
  
          if len(chaining_key) != self.HASHLEN:
              raise HashError("Incorrect chaining key length")
          if len(input_key_material) not in (0, 32, dhlen):
              raise HashError("Incorrect input key material length")
          temp_key = self.hmac_hash(chaining_key, input_key_material)
          output1 = self.hmac_hash(temp_key, b'\x01')
          output2 = self.hmac_hash(temp_key, output1 + b'\x02')
          return output1, output2
docs: Hash-based key derivation function
  
          Takes a ``chaining_key'' byte sequence of len HASHLEN, and an
          ``input_key_material'' byte sequence with length either zero
          bytes, 32 bytes or dhlen bytes.
  
          Returns two byte sequences of length HASHLEN