What is the maximum allowable key size of the Rijndael encryption algorithm?

The Rijndael algorithm, selected by the U.S. National Institute of Standards and Technology (NIST) to become the Advanced Encryption Standard (AES), was designed with variable block and key lengths. The specification for Rijndael allows for key sizes of 128, 160, 192, 224, and 256 bits. Therefore, the maximum allowable key size for the original Rijndael algorithm is 256 bits. While the AES standard (FIPS 197) only specifies the use of 128, 192, and 256-bit keys, the underlying Rijndael algorithm's upper limit is 256 bits.

A. 128 bits: This is a valid key size for Rijndael and the minimum size specified for AES, but it is not the maximum.

B. 192 bits: This is an intermediate, valid key size for both Rijndael and AES, but it is not the maximum.

D. 512 bits: This key size is not supported by the Rijndael specification. The algorithm was not designed to operate with keys of this length.

1. Daemen

J.

& Rijmen

V. (2002). The Design of Rijndael: AES - The Advanced Encryption Standard. Springer-Verlag. In Chapter 3

Section 3.4

"Key-Block-Round Combinations

" the authors state

"In Rijndael

the block length and the key length can be independently specified to be 128

192 or 256 bits." Note: The initial submission also included 160 and 224-bit variants

but the core design's maximum is consistently cited as 256 bits.

2. National Institute of Standards and Technology (NIST). (2001). FIPS PUB 197: Advanced Encryption Standard (AES). Section 2

"DEFINITIONS

" and Section 5

"AES ALGORITHM SPECIFICATION

" specify that the AES algorithm uses the Rijndael algorithm with a fixed block size of 128 bits and key sizes of 128

192

or 256 bits. This document confirms that 256 bits is the maximum key size adopted for the standard from the Rijndael family.

3. Stallings

W. (2017). Cryptography and Network Security: Principles and Practice (7th ed.). Pearson. In Chapter 6

"Advanced Encryption Standard

" Section 6.1

"Finite Field Arithmetic

" the text discusses the AES selection process and notes that the original Rijndael proposal allowed for key and block sizes in 32-bit steps from 128 to 256 bits.