Exam SSCP topic 5 question 96 discussion

Actual exam question from ISC's SSCP

Question #: 96
Topic #: 5

[All SSCP Questions]

What is the maximum key size for the RC5 algorithm?

A. 128 bits
B. 256 bits
C. 1024 bits
D. 2040 bits

Show Suggested Answer

Suggested Answer: D 🗳️
RC5 is a fast block cipher created by Ron Rivest and analyzed by RSA Data Security, Inc.
It is a parameterized algorithm with a variable block size, a variable key size, and a variable number of rounds.
Allowable choices for the block size are 32 bits (for experimentation and evaluation purposes only), 64 bits (for use a drop-in replacement for DES), and 128 bits.
The number of rounds can range from 0 to 255, while the key can range from 0 bits to 2040 bits in size.
Please note that some sources such as the latest Shon Harris book mentions that RC5 maximum key size is of 2048, not 2040 bits. I would definitively use RSA as the authoritative source which specifies a key of 2040 bits. It is an error in Shon's book.
The OIG book says:
RC5 was developed by Ron Rivest of RSA and is deployed in many of RSAs products. It is a very adaptable product useful for many applications, ranging from software to hardware implementations. The key for RC5 can vary from 0 to 2040 bits, the number of rounds it executes can be adjusted from 0 to 255, and the length of the input words can also be chosen from 16-, 32-, and 64-bit lengths.
The following answers were incorrect choices:
All of the other answers were wrong.
Reference(s) used for this question:
Schneiter, Andrew (2013-04-15). Official (ISC)2 Guide to the CISSP CBK, Third Edition : Cryptography (Kindle Locations 1098-1101). . Kindle Edition.
Harris, Shon (2012-10-25). CISSP All-in-One Exam Guide, 6th Edition (Kindle Locations 16744-16747). McGraw-Hill. Kindle Edition. http://www.rsa.com/rsalabs/node.asp?id=2251, What are RC5 and RC6, RSA The Security Division of EMC.
From Rivest himself, see http://people.csail.mit.edu/rivest/Rivest-rc5rev.pdf
Also see the draft IETF IPSEC standard which clearly mention that it is in fact 2040 bits as a MAXIMUM key size: http://www.tools.ietf.org/html/draft-ietf-ipsec-esp-rc5-cbc-00 http://en.wikipedia.org/wiki/RC5, Mention a maximum key size of 2040 as well.

by xymavu at April 4, 2025, 11:40 p.m.

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