Summary  255

Cryptographic Lifecycle

With the exception of the one-time pad, all cryptographic systems have a limited life span. Moore’s law, a commonly cited trend in the advancement of computing power, states that the processing capabilities of a state-of-the-art microprocessor will double approximately every two years. This means that, eventually, processors will reach the amount of strength required to simply guess the encryption keys used for a communication.

Security professionals must keep this cryptographic lifecycle in mind when selecting an encryption algorithm and have appropriate governance controls in place to ensure that the algorithms, protocols, and key lengths selected are sufficient to preserve the integrity of a cryptosystem for however long it is necessary to keep the information it is protecting secret. Security professionals can use the following algorithm and protocol governance controls:

■■Specifying the cryptographic algorithms (such as AES, 3DES, and RSA) acceptable for use in an organization

■■Identifying the acceptable key lengths for use with each algorithm based on the sensitivity of information transmitted

■■Enumerating the secure transaction protocols (such as TLS) that may be used

For example, if you’re designing a cryptographic system to protect the security of business plans that you expect to execute next week, you don’t need to worry about the theoretical risk that a processor capable of decrypting them might be developed a decade from now. On the other hand, if you’re protecting the confidentiality of information that could be used to construct a nuclear bomb, it’s virtually certain that you’ll still want that information to remain secret 10 years in the future!

Summary

Cryptographers and cryptanalysts are in a never-ending race to develop more secure cryptosystems and advanced cryptanalytic techniques designed to circumvent those systems.

Cryptography dates back as early as Caesar and has been an ongoing topic of study for many years. In this chapter, you learned some of the fundamental concepts underlying the field of cryptography and gained a basic understanding of the terminology used by cryptographers.

This chapter also examined the similarities and differences between symmetric key cryptography (where communicating parties use the same key) and asymmetric key cryptography (where each communicator has a pair of public and private keys). You learned how hashing may be used to guarantee integrity and how hashes play a role in the digital signature process that guarantees nonrepudiation.

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We then analyzed some of the symmetric algorithms currently available and their strengths and weaknesses. We wrapped up the chapter by taking a look at the cryptographic lifecycle and the role of algorithm/protocol governance in enterprise security.

The next chapter expands this discussion to cover contemporary public key cryptographic algorithms. Additionally, some of the common cryptanalytic techniques used to defeat both types of cryptosystems will be explored.

Exam Essentials

Understand the role that confidentiality, integrity, and nonrepudiation play in cryptosystems.   Confidentiality is one of the major goals of cryptography. It protects the secrecy of data while it is both at rest and in transit. Integrity provides the recipient of a message with the assurance that data was not altered (intentionally or unintentionally) between the

time it was created and the time it was accessed. Nonrepudiation provides undeniable proof that the sender of a message actually authored it. It prevents the sender from subsequently denying that they sent the original message.

Know how cryptosystems can be used to achieve authentication goals.   Authentication provides assurances as to the identity of a user. One possible scheme that uses authentication

is the challenge-response protocol, in which the remote user is asked to encrypt a message using a key known only to the communicating parties. Authentication can be achieved with both symmetric and asymmetric cryptosystems.

Be familiar with the basic terminology of cryptography.   When a sender wants to transmit a private message to a recipient, the sender takes the plaintext (unencrypted) message and encrypts it using an algorithm and a key. This produces a ciphertext message that is transmitted to the recipient. The recipient then uses a similar algorithm and key to decrypt the ciphertext and re-create the original plaintext message for viewing.

Understand the difference between a code and a cipher and explain the basic types

of ciphers.   Codes are cryptographic systems of symbols that operate on words or phrases and are sometimes secret but don’t always provide confidentiality. Ciphers, however, are always meant to hide the true meaning of a message. Know how the following types of ciphers work: transposition ciphers, substitution ciphers (including one-time pads), stream ciphers, and block ciphers.

Know the requirements for successful use of a one-time pad.   For a one-time pad to be successful, the key must be generated randomly without any known pattern. The key must be at least as long as the message to be encrypted. The pads must be protected against physical disclosure, and each pad must be used only one time and then discarded.

Understand split knowledge.   Split knowledge means that the information or privilege required to perform an operation is divided among multiple users. This ensures that no single person has sufficient privileges to compromise the security of the environment. M of N Control is an example of split knowledge used in key recovery and other sensitive tasks.

Understand work function (work factor).   Work function, or work factor, is a way to measure the strength of a cryptography system by measuring the effort in terms of cost and/or time to decrypt messages. Usually the time and effort required to perform a complete bruteforce attack against an encryption system is what a work function rating represents. The security and protection offered by a cryptosystem is directly proportional to the value of its work function/factor.

Understand the importance of key security.   Cryptographic keys provide the necessary element of secrecy to a cryptosystem. Modern cryptosystems utilize keys that are at least 128 bits long to provide adequate security.

Know the differences between symmetric and asymmetric cryptosystems.   Symmetric key cryptosystems (or secret key cryptosystems) rely on the use of a shared secret key. They are much faster than asymmetric algorithms, but they lack support for scalability, easy key distribution, and nonrepudiation. Asymmetric cryptosystems use public-private key pairs for communication between parties but operate much more slowly than symmetric algorithms.

Be able to explain the basic operational modes of symmetric cryptosystems.   Symmetric cryptosystems operate in several discrete modes: Electronic Code Book (ECB) mode, Cipher Block Chaining (CBC) mode, Cipher Feedback (CFB) mode, Output Feedback (OFB) mode, Counter (CTR) mode, Galois/Counter mode (GCM), and Counter with Cipher Block Chaining Message Authentication Code mode (CCM). ECB mode is considered the least secure and is used only for short messages. 3DES uses three iterations of DES with two or three different keys to increase the effective key strength to 112 or 168 bits, respectively.

Know the Advanced Encryption Standard (AES).   The Advanced Encryption Standard (AES) uses the Rijndael algorithm and is the U.S. government standard for the secure exchange of sensitive but unclassified data. AES uses key lengths of 128, 192, and 256 bits and a fixed block size of 128 bits to achieve a much higher level of security than that provided by the older DES algorithm.

Written Lab

1.What is the major hurdle preventing the widespread adoption of one-time pad cryptosystems to ensure data confidentiality?

2.Encrypt the message “I will pass the CISSP exam and become certified next month” using columnar transposition with the keyword SECURE.

3.Decrypt the message “F R Q J U D W X O D W L R Q V B R X J R W L W” using the Caesar ROT3 substitution cipher.

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Review Questions

1.Ryan is responsible for managing the cryptographic keys used by his organization. Which of the following statements are correct about how he should select and manage those keys? (Choose all that apply.)

A.Keys should be sufficiently long to protect against future attacks if the data is expected to remain sensitive.

B.Keys should be chosen using an approach that generates them from a predictable pattern.

C.Keys should be maintained indefinitely.

D.Longer keys provide greater levels of security.

2.John recently received an email message from Bill. What cryptographic goal would need to be met to convince John that Bill was actually the sender of the message?

A.Nonrepudiation

B.Confidentiality

C.Availability

D.Integrity

3.You are implementing AES encryption for files that your organization plans to store in a cloud storage service and wish to have the strongest encryption possible. What key length should you choose?

A.192 bits

B.256 bits

C.512 bits

D.1,024 bits

4.You are creating a security product that must facilitate the exchange of symmetric encryption keys between two parties that have no way to securely exchange keys in person. What algorithm might you use to facilitate the exchange?

A.Rijndael

B.Blowfish

C.Vernam

D.Diffie–Hellman

5.What occurs when the relationship between the plaintext and the key is complicated enough that an attacker can’t merely continue altering the plaintext and analyzing the resulting ciphertext to determine the key? (Choose all that apply.)

A.Confusion

B.Transposition

C.Polymorphism

D.Diffusion

6.Randy is implementing an AES-based cryptosystem for use within his organization. He would like to better understand how he might use this cryptosystem to achieve his goals. Which of the following goals are achievable with AES? (Choose all that apply.)

A.Nonrepudiation

B.Confidentiality

C.Authentication

D.Integrity

7.Brian encountered encrypted data left on one of his systems by attackers who were communicating with one another. He has tried many cryptanalytic techniques and was unable to decrypt the data. He believes that the data may be protected with an unbreakable system. When correctly implemented, what is the only cryptosystem known to be unbreakable?

A.Transposition cipher

B.Substitution cipher

C.Advanced Encryption Standard

D.One-time pad

8.Helen is planning to use a one-time pad to meet a unique cryptographic requirement in her organization. She is trying to identify the requirements for using this cryptosystem. Which of the following are requirements for the use of a one-time pad? (Choose all that apply.)

A.The encryption key must be at least one-half the length of the message to be encrypted.

B.The encryption key must be randomly generated.

C.Each one-time pad must be used only once.

D.The one-time pad must be physically protected against disclosure.

9.Brian administers a symmetric cryptosystem used by 20 users, each of whom has the ability to communicate privately with any other user. One of those users lost control of their account and Brian believes that user’s keys were compromised. How many keys must he change?

A.1

B.2

C.19

D.190

10.Which one of the following cipher types operates on large pieces of a message rather than individual characters or bits of a message?

A.Stream cipher

B.Caesar cipher

C.Block cipher

D.ROT3 cipher

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11.James is the administrator for his organization’s symmetric key cryptographic system. He issues keys to users when the need arises. Mary and Beth recently approached him and presented a need to be able to exchange encrypted files securely. How many keys must James generate?

A.One

B.Two

C.Three

D.Four

12.Dave is developing a key escrow system that requires multiple people to retrieve a key but does not depend on every participant being present. What type of technique is he using?

A.Split knowledge

B.M of N Control

C.Work function

D.Zero-knowledge proof

13.What is used to increase the strength of cryptography by creating a unique ciphertext every time the same message is encrypted with the same key?

A.Initialization vector

B.Vigenère cipher

C.Steganography

D.Stream cipher

14.Tammy is choosing a mode of operation for a symmetric cryptosystem that she will be using in her organization. She wants to choose a mode that is capable of providing both confidentiality and data authenticity. What mode would best meet her needs?

A.ECB

B.GCM

C.OFB

D.CTR

15.Julie is designing a highly secure system and is concerned about the storage of unencrypted data in RAM. What use case is she considering?

A.Data in motion

B.Data at rest

C.Data in destruction

D.Data in use

16.Renee conducted an inventory of encryption algorithms used in her organization and found that they are using all of the algorithms below. Which of these algorithms should be discontinued? (Choose all that apply.)

A.AES

B.DES

C.3DES

D.RC5

17.Which one of the following encryption algorithm modes suffers from the undesirable characteristic of errors propagating between blocks?

A.Electronic Code Book

B.Cipher Block Chaining

C.Output Feedback

D.Counter

18.Which one of the following key distribution methods is most cumbersome when users are located in different geographic locations?

A.Diffie–Hellman

B.Public key encryption

C.Offline

D.Escrow

19.Victoria is choosing an encryption algorithm for use within her organization and would like to choose the most secure symmetric algorithm from a list of those supported by the software package she intends to use. If the package supports the following algorithms, which would be the best option?

A.AES-256

B.3DES

C.RC4

D.Skipjack

20.The Jones Institute has six employees and uses a symmetric key encryption system to ensure confidentiality of communications. If each employee needs to communicate privately with every other employee, how many keys are necessary?

A.1

B.6

C.15

D.30