code: func PBKDF2(password, salt []byte, iter, keyLen int, h func() hash.Hash) []byte {
  	prf := hmac.New(h, password)
  	hashLen := prf.Size()
  	numBlocks := (keyLen + hashLen - 1) / hashLen
  
  	var buf [4]byte
  	dk := make([]byte, 0, numBlocks*hashLen)
  	U := make([]byte, hashLen)
  	for block := 1; block <= numBlocks; block++ {
  		// N.B.: || means concatenation, ^ means XOR
  		// for each block T_i = U_1 ^ U_2 ^ ... ^ U_iter
  		// U_1 = PRF(password, salt || uint(i))
  		prf.Reset()
  		prf.Write(salt)
  		buf[0] = byte(block >> 24)
  		buf[1] = byte(block >> 16)
  		buf[2] = byte(block >> 8)
  		buf[3] = byte(block)
  		prf.Write(buf[:4])
  		dk = prf.Sum(dk)
  		T := dk[len(dk)-hashLen:]
  		copy(U, T)
  
  		// U_n = PRF(password, U_(n-1))
  		for n := 2; n <= iter; n++ {
  			prf.Reset()
  			prf.Write(U)
  			U = U[:0]
  			U = prf.Sum(U)
  			for x := range U {
  				T[x] ^= U[x]
  			}
  		}
  	}
  	return dk[:keyLen]
  }
docs: // PBKDF2 implements Password-Based Key Derivation Function 2), aimed to reduce
  // the vulnerability of encrypted keys to brute force attacks.
  // http://code.google.com/p/go/source/browse/pbkdf2/pbkdf2.go?repo=crypto