The International Data Encryption Algorithm (IDEA) is a symmetric-key block cipher designed by Xuejia Lai and James L. Massey. It operates on 64-bit blocks of data. The algorithm's design specifies a fixed key size of 128 bits. This 128-bit key is used to generate a series of 52 16-bit subkeys that are applied during the eight rounds of encryption. The combination of a 128-bit key and a complex round structure provides its security against common cryptanalytic attacks like differential and linear cryptanalysis.

A. 64 bits: This is the block size of the IDEA cipher, which is the amount of data encrypted at one time, not the length of the secret key.
C. 160 bits: This size is not associated with IDEA. It is commonly the output size for hash functions like SHA-1 or a key size for certain elliptic curve algorithms.
D. 192 bits: This is one of the valid key sizes for the Advanced Encryption Standard (AES), specifically AES-192, but it is not used by IDEA.

1. Lai

X.

& Massey

J. L. (1991). A Proposal for a New Block Encryption Standard. In D. W. Davies (Ed.)

Advances in Cryptology — EUROCRYPT ’91 (Lecture Notes in Computer Science

Vol. 547

pp. 389–404). Springer-Verlag. In Section 2

"Description of the Proposed Cipher

" the authors state

"The proposed cipher is an iterated cipher with a block size of 64 bits and a key of 128 bits." (p. 390). DOI: https://doi.org/10.1007/3-540-46416-636

2. Menezes

A. J.

van Oorschot

P. C.

& Vanstone

S. A. (1996). Handbook of Applied Cryptography. CRC Press. In Chapter 7

"Block Ciphers

" Section 7.6

"IDEA

" it is stated

"IDEA is a 64-bit iterated block cipher with a 128-bit key." (p. 266).

3. Daemen

J.

& Rijmen

V. (2002). The Design of Rijndael: AES - The Advanced Encryption Standard. Springer. In Chapter 2

"Block Cipher Principles

" Section 2.4.3

"IDEA

" the authors note

"IDEA has a block length of 64 bits and a key length of 128 bits." (p. 22).