The Caesar Cipher

One of the oldest encryption methods is the Caesar cipher. This method is purported to have been used by the ancient roman caesars—thus, the name. It is actually quite simple to do. You choose some number by which to shift each letter of a text. For example, if the text is:

A cat

and you choose to shift by two letters, then the message becomes:

C ecv

Or, if you choose to shift by three letters, it becomes:

D fdw

Julius Caesar was reputed to have used a shift of three to the right. However, you can choose any shifting pattern you wish. You can shift either to the right or to the left by any number of spaces you like. Because this is a very simple method to understand, it makes a good place to start our study of encryption. It is, however, extremely easy to crack. You see, any language has a certain letter and word frequency, meaning that some letters are used more frequently than others). In the English language, the most common single-letter word is A. The most common three-letter word is the. Those two rules alone could help you decrypt a Caesar cipher. For example, if you saw a string of seemingly nonsense letters and noticed that a three-letter word was frequently repeated in the message, you might easily surmise that this word was the—and the odds are highly in favor of this being correct. Furthermore, if you frequently noticed a single-letter word in the text, it is most likely the letter A. You now have found the substitution scheme for A, T, H, and E. You can now either translate all of those letters in the message and attempt to surmise the rest or simply analyze the substitute letters used for A, T, H, and E and derive the substitution cipher that was used for this message. Decrypting a message of this type does not even require a computer. It can be done in less than 10 minutes using pen and paper by someone with no background in cryptography. There are other rules that will help make cracking this code even easier. For example, in the English language the two most common two letter combinations are ee and oo. That gives you even more to work on.

Another reason this algorithm can be easily cracked is an issue known as key space—the number of possible keys that can be used. In this case, when applied to the English alphabet, there are only 26 possible keys since there are only 26 letters in the English alphabet. This means you could simply try each possible key (+1, +2, +3,…+26) until one works. Trying all possible keys is referred to as a brute-force attack.

The substitution scheme you choose (for example, +2, +1) is referred to as a substitution alphabet (for example, B substitutes for A, U substitutes for T). Thus, the Caesar cipher is also referred to as a mono-alphabet substitution method, meaning that it uses a single substitution for the encryption. There are other mono-alphabet algorithms, but the Caesar cipher is the most widely known.

Atbash

In ancient times, Hebrew scribes used the Atbash substitution cipher to encrypt religious works such as the book of Jeremiah. Applying this cipher is fairly simple: Just reverse the order of the letters of the alphabet. This is, by modern standards, a very primitive and easy-to-break cipher.

The Atbash cipher is a Hebrew code that substitutes the first letter of the alphabet for the last and the second letter for the second to the last, and so on. It simply reverses the alphabet. For example, in English, A becomes Z, B becomes Y, C becomes X, and so on. Of course, the Hebrews used a different alphabet, with aleph being the first letter and tav being the last letter. However, I will use English examples to demonstrate this:

Attack at dawn

becomes:

Zggzxp zg wzdm

As you can see, the A is the first letter in the alphabet and is switched with Z, the last letter in the alphabet. Then the T is the 19th letter (and the 7th from the end) and gets swapped with G, the 7th letter from the beginning. This process is continued until the entire message is enciphered.

To decrypt the message, you simply reverse the process, and Z becomes A, B becomes Y, and so on. This is obviously a rather simple cipher and not used in modern times. However, it illustrates the basic concept of cryptography: to perform some permutation on the plain text to render it difficult to read by those who don’t have the key to unscramble the cipher text. The Atbash cipher, like the Caesar cipher, is a single substitution cipher, so each letter in the plain text has a direct, one-to-one relationship with each letter in the cipher text. This means that the same letter and word frequency issues that can be used to crack the Caesar cipher can be used to crack the Atbash cipher.

Multi-Alphabet Substitution

Eventually, a slight improvement on the Caesar cipher, called multi-alphabet substitution, was developed. In this scheme, you select multiple numbers by which to shift letters (that is, multiple substitution alphabets). For example, if you select three substitution alphabets (12, 22, 13), then:

A CAT

becomes:

C ADV

Notice that the fourth letter starts over with another +2, and you can see that the first A was transformed to C and the second A was transformed to D. This makes it more difficult to decipher the underlying text. While this is harder to decrypt than a Caesar cipher, it is not overly difficult. It can be done with simple pen and paper and a bit of effort. It can be cracked very quickly with a computer. In fact, no one would use such a method today to send any truly secure message, for this type of encryption is considered very weak.

At one time multi-alphabet substitution was considered quite secure. In fact, a special version of this, called a Vigenère cipher, was used in the 1800s and early 1900s. The Vigenère cipher was invented in 1553 by Giovan Battista Bellaso. It is a method of encrypting alphabetic text by using a series of different Caesar ciphers based on the letters of a keyword. Figure 8.1 shows the Vigenère cipher.

Match the letter of your keyword on the top with the letter of your plain text on the left to find the cipher text. For example, using the chart shown in Figure 8.1, if you are encrypting the word cat and your key is horse, then the cipher text is jok.

Rail Fence

All the ciphers we have examined so far are substitution ciphers. Another approach to classic cryptography is the transposition cipher. The rail fence cipher may be the most widely known transposition cipher. You simply take the message you wish to encrypt and alter each letter on a different row. So “attack at dawn” is written as:

A t c a d w

t a k t a n

Next, you write down the text reading from left to right as one normally would, thus producing:

atcadwtaktan

In order to decrypt the message, the recipient must write it out on rows:

A t c a d w

t a k t a n

Then the recipient reconstructs the original message. Most texts use two rows as examples; however, this can be done with any number of rows you wish to use.

Enigma

It is really impossible to have a discussion about cryptography and not talk about Enigma. Contrary to popular misconceptions, the Enigma is not a single machine but rather a family of machines. The first version was invented by German engineer Arthur Scherbius near the end of World War I. It was used by several different militaries, not just the Nazi Germans.

Some military texts encrypted using a version of Enigma were broken by Polish cryptanalysts Marian Rejewski, Jerzy Rozycki, and Henryk Zygalski. The three basically reverse engineered a working Enigma machine and used that information to develop tools for breaking Enigma ciphers, including one tool named the cryptologic bomb.

The core of the Enigma machine was the rotors, or disks that were arranged in a circle with 26 letters on them. The rotors were lined up. Essentially, each rotor represented a different single substitution cipher. You can think of the Enigma as a sort of mechanical poly-alphabet cipher. The operator of the Enigma machine would be given a message in plain text and then type that message into Enigma. For each letter that was typed in, Enigma would provide a different cipher text, based on a different substitution alphabet. The recipient would type in the cipher text, and as long as both Enigma machines had the same rotor settings, the recipient’s machine would provide the correct plain text. Figure 8.2 is a picture of an Enigma machine.

There were actually several variations of the Enigma machine. The Naval Enigma machine was eventually cracked by British cryptographers working at the now famous Bletchley Park. Alan Turing and a team of analysts were able to eventually break the Naval Enigma machine. Many historians claim that this shortened World War II by as much as 2 years. This story is the basis for the 2014 movie The Imitation Game.

Binary Operations

Most symmetric ciphers make use of a binary operation called exclusive or (XOR). Before we examine modern symmetric ciphers, we will review basic binary math. Various operations on binary numbers (numbers made of only 0s and 1s) are well known to programmers and programming students. But for those who are not familiar with them, a brief explanation follows. When working with binary numbers, there are three operations not found in normal math: AND, OR, and XOR operations. Each is illustrated next.

Single-Key (Symmetric) Encryption

Basically, single-key encryption means that the same key is used to both encrypt and decrypt a message. This is also referred to as symmetric key encryption. There are two types of symmetric algorithms (or ciphers): stream and block. A block cipher divides the data into blocks (often 64-bit blocks, but newer algorithms sometimes use 128-bit blocks) and encrypts the data one block at a time. Stream ciphers encrypt the data as a stream of bits, one bit at a time.

AES

Advanced Encryption Standard (AES) was the algorithm eventually chosen to replace DES. It is a block cipher that works on 128-bit blocks. It can have one of three key sizes: 128, 192, or 256 bits. The U.S. government selected AES to be the replacement for DES, and it is now the most widely used symmetric key algorithm.

AES, also known as Rijndael block cipher, was officially designated as a replacement for DES in 2001 after a 5-year process involving 15 competing algorithms. AES is designated as FIPS 197. Other algorithms that did not win that competition include such well-known algorithms as Twofish. The importance of AES cannot be overstated. It is widely used around the world and is perhaps the most widely used symmetric cipher. Of all the algorithms in this chapter, AES is the one you should give the most attention to.

As mentioned earlier, AES can have three different key sizes: 128, 192, and 256 bits. The three different implementations of AES are referred to as AES 128, AES 192, and AES 256. The block size can also be 128, 192, or 256 bit. It should be noted that the original Rijndael cipher allowed for variable block and key sizes in 32-bit increments. However, the U.S. government uses these three key sizes with a 128-bit block as the standard for AES.

This algorithm was developed by two Belgian cryptographers, John Daemen and Vincent Rijmen. John Daeman is a Belgian cryptographer who has worked extensively on the cryptanalysis of block ciphers, stream ciphers, and cryptographic hash functions. For those new to security, the brief description given so far is sufficient. However, we will explore the AES algorithm in more detail. Just as with the details of DES, this may be a bit confusing to some readers at first glance and may require a few rereads.

Rijndael uses a substitution-permutation matrix rather than a Feistel network. The Rijndael cipher works by first putting the 128-bit block of plain text into a 4-byte-by-4-byte matrix, termed the state, that changes as the algorithm proceeds through its steps. The first step is to convert the plain text block into binary and then put it into a matrix, as shown in Figure 8.3.

Once you have the original plain text in binary, placed in the 4-byte-by-4-byte matrix, the algorithm consists of a few relatively simple processes that are used during various rounds. The processes are described here:

• AddRoundKey: In this process, each byte of the state is exclusively ORed with the round key. Just as with DES, there is a key schedule algorithm that slightly changes the key each round.

• SubBytes: This involves substitutions of the input bytes (which are the output from the AddRoundKey phase). This is where the contents of the matrix are put through the s-boxes. Each of the s-boxes is 8 bits.

• ShiftRows: This is a transposition step where each row of the state is shifted cyclically a certain number of steps. In this process, the first row is left unchanged. Every byte in the second row is shifted 1 byte to the left (and the far left wraps around). Every byte of the third row is shifted 2 to the left, and every byte of the fourth row is shifted 3 to the left (again with the far left wrapping around). This is shown in Figure 8.4. Notice that in this figure that the bytes are simply labeled by their row and then a letter, such as 1a, 1b, 1c, 1d.

• MixColumns: This is a mixing operation that operates on the columns of the state, combining the 4 bytes in each column. In the MixColumns process, each column of the state is multiplied with a fixed polynomial. Each column in the state (remember the matrix we are working with) is treated as a polynomial within the Galois field (2⁸). The result is multiplied with a fixed polynomial c(x) = 3x³ + x² + x +2 modulo x⁴ + 1. The MixColumns process can also be viewed as a multiplication by the shown particular MDS matrix in the finite field GF(2⁸).

These processes are executed multiple times in the Rijndael cipher. For 128-bit keys, there are 10 rounds. For 192-bit keys, there are 12 rounds. For 256-bit keys, there are 14 rounds.

These last few steps may be leaving you a bit confused if you don’t have a background in number theory. You may be asking questions like “What is a Galois field?” and “What is a fixed polynomial?” A general overview of the math needed to understand this is provided in the next section.

AES Math

A group is an algebraic system consisting of a set, an identity element, one operation, and its inverse operation. Basically, groups are ways to limit math operations, such as addition, to specific sets of numbers. There are several specialized types of groups, briefly described here:

• An abelian group, or commutative group, has an additional axiom a + b = b + a if the operation is addition or ab = ba if the operation is multiplication.

• A cyclic group has elements that are all powers of one of its elements.

• A ring is an algebraic system consisting of a set, an identity element, two operations, and the inverse operation of the first operation.

• A field is an algebraic system consisting of a set, an identity element for each operation, two operations, and their respective inverse operations.

This brings us to the Galois group, or Galois field. GF(p) for any prime, p, this Galois field has p elements that are the residue classes of integers modulo p. That prime number p is the defining element for the field. Now, this is obviously a brief description and does not attempt to get into details. A thorough study of group theory would be needed to get into more detail.

Modification of Symmetric Methods

Just as important as understanding symmetric ciphers is understanding how they are implemented. There are some common modes that can affect how a symmetric cipher functions.

RSA

It isn’t possible to discuss cryptography without at least some discussion of RSA, which is a very widely used encryption algorithm. This public key method was developed in 1977 by three mathematicians: Ron Rivest, Adi Shamir, and Len Adleman. The name RSA is derived from the first letter of each mathematician’s last name Let us take a look at the math involved in RSA. (It should be pointed out that knowing the math behind this, or any other algorithm, is not critical for most security professionals. But some readers will have an interest in going deeper into cryptography, and this will be a good place to start.)

There are a few basic math concepts you need to know about in order to understand RSA. Some (or even all) of this material may be a review:

• Prime numbers: A prime number is divisible by itself and by 1. So 2, 3, 5, 7, 11, 13, 17, and 23 are all prime numbers. (Note that 1 itself is considered a special case and is not prime.)

• Co-prime: This actually does not mean prime; it means two numbers have no common factors. So, for example, the factors of 8 (excluding the special case of 1) are 2 and 4, and the factor of 9 are 3. The numbers 8 and 9 have no common factors, so they are co-prime.

• Euler’s Totient: Pronounced “oilers” totient, or called just the totient, this is the number of integers smaller than n that are co-prime with n. So let us consider the number 10. Since 2 is a factor of 10, it is not co-prime with 10. But 3 is co-prime with 10. The number 4 is not co-prime since both 4 and 10 have 2 as a factor. The number 5 is not since it is a factor of 10. Neither is 6 since both 6 and 10 have 2 as a cofactor. The number 7 is prime, so it is co-prime with 10. The number 8 is not because both 8 and 10 have 2 as a factor. The number 9 is co-prime with 10. So the numbers 3, 7, and 9 are co-prime with 10. We add in 1 as a special case, the Euler’s totient of 10 is 4. Now it just so happens that Leonhard Euler also proved that if the number n is a prime number, then its totient is always n − 1. So the totient of 7 is 6. The totient of 13 is 12.

• Multiplying and co-prime: Now we can easily compute the totient of any number. And we know automatically that the totient of any prime number n is just n − 1. But what if we multiply two primes? For example, we can multiply 5 and 7 to get 35. Well, we can go through all the numbers up to 35 and tally up the numbers that are co-prime with 35. But the larger the numbers get, the more tedious this process becomes. For example, if you have a 20-digit number, manually calculating the totient is almost impossible. Fortunately, Leonhard Euler also proved that if you have a number that is the product of two primes (let’s call them p and q), such as 5 and 7, then the totient of the product of those two numbers (in this case 35) is equal to p − 1 × q − 1 (in this case 4 × 6, or 24).

• Modulus: This is the last concept you need for RSA. There are a few approaches to explaining this concept. We will actually use two of them. First, from a programmer’s perspective, the modulus operation is to divide two numbers but only give the remainder. Programmers often use the symbol % to denote modulo operations. So 10 % 3 is 1. The remainder of 10 divided by 3 is 1. Now, this is not really a mathematical explanation of modulo operations.

Basically, modulo operations take addition and subtraction and limit them by some value. You have actually done this all your life without realizing it. Consider a clock. When you say 2 p.m., what you really mean is 14 mod 12 (or 14 divided by 12; just give me the remainder). Or if it is 2 p.m. now (14 actually) and you tell me you will call me in 36 hours, what I do is 14 + 36 mod 12, or 50 mod 12, which is 2 a.m. ( a bit early for a phone call, but it illustrates our point).

Now if you understand these basic operations, then you are ready to learn RSA. If needed, reread the preceding section (perhaps even more than once) before proceeding.

To create the key, you start by generating two large random primes, p and q, of approximately equal size. You need to pick two numbers so that when multiplied together, the product will be the size you want (2048 bits, 4096 bits, and so on).

Now multiply p and q to get n. Let n = pq.

The next step is to multiply Euler’s totient for each of these primes. Basically, Euler’s totient is the total number of co-prime numbers. Two numbers are considered co-prime if they have no common factors. For example, if the original number is 7, then 5 and 7 would be co-prime. Remember that it just so happens that for prime numbers, this is always the number minus 1. For example, 7 has 6 numbers that are co-prime to it. (If you think about this a bit, you will see that 1, 2, 3, 4, 5, and 6 are all co-prime with 7.)

Let m = (p − 1)(q − 1).

Now we are going to select another number. We will call this number e. We want to pick e so that it is co-prime to m. Choose a small number e, co-prime to m.

We are almost done generating a key. Now we just find a number d that, when multiplied by e and modulo m, would yield 1. (Remember: Modulo means dividing two numbers and returning the remainder. For example, 8 modulo 3 would be 2.)

Find d, such that de % m = 1.

Now you will publish e and n as the public key. Keep d as the secret key. To encrypt, you simply take your message raised to the e power and modulo n:

= me % n

To decrypt, you take the cipher text and raise it to the d power modulo n:

P = Cd % n

The letter e is for encrypt and d is for decrypt. If all this seems a bit complex to you, you must realize that many people work in network security without being familiar with the actual algorithm for RSA (or any other cryptography for that matter). However, if you wish to go deeper into cryptography, then this is a very good start. It involves some fundamental number theory, particularly regarding prime numbers. There are other asymmetric algorithms that work in a different manner. For example, elliptic curve cryptography is one such example.

Let’s look at an example that might help you understand. Of course, RSA would be done with very large integers. To make the math easy to follow, we will use small integers in this example.

Choose two distinct prime numbers, such as p = 61 and q = 53. Compute n = pq:

n = 61 × 53 = 3233

Compute the totient of the product as φ(n) = (p − 1)(q − 1):

φ (3233) = (61 − 1)(53 − 1) = 3120

Choose any number 1 < e < 3120 that is co-prime to 3120. Choosing a prime number for e leaves us only to check that e is not a divisor of 3120. Let e = 17. Compute d, the modular multiplicative inverse of yielding d = 2753.

The public key is (n = 3233, e = 17). For a padded plain text message m, the encryption function is:

m¹⁷ (mod 3233)

The private key is:

(n = 3233, d = 2753)

For an encrypted cipher text, c, the decryption function is:

c²⁷⁵³ (mod 3233)

For those readers new to RSA or new to cryptography in general, it might be helpful to see one more example, with even smaller numbers:

Select primes: p = 17 & q = 11.

Compute n = pq =17 × 11 = 187.

Compute φ(n) = (p − 1)(q − 1) = 16 × 10 = 160.

Select e: gcd(e,160) = 1; choose e = 7.

Determine d: de = 1 mod 160 and d < 160. Value is d = 23 since 23 × 7 = 161 = 10 × 160 + 1.

Publish public key: (7 and 187).

Keep secret private key: 23.

MD5

MD5 is a 128-bit hash that is specified in RFC 1321. It was designed by Ron Rivest in 1991 to replace an earlier hash function, MD4. MD5 produces a 128-bit hash or digest. It has been found to be not as collision resistant as SHA.

SHA

Secure Hash Algorithm (SHA) is perhaps the most widely used hash algorithm today. There are now several versions of SHA. All versions of SHA are considered to be secure and collision free:

• SHA-1: This is a 160-bit hash function that resembles the earlier MD5 algorithm. It was designed by the NSA to be part of the Digital Signature Algorithm (DSA).

• SHA-2: This is actually two similar hash functions, with different block sizes, known as SHA-256 and SHA-512. They differ in the word size; SHA-256 uses 32-byte (256 bits) words, whereas SHA-512 uses 64-byte (512 bits) words. There are also truncated versions of each standardized, known as SHA-224 and SHA-384. These were also designed by the NSA.

• SHA-3: This latest version of SHA was adopted in October 2012.

RIPEMD

RACE Integrity Primitives Evaluation Message Digest (RIPEMD) is a 160-bit hash algorithm developed by Hans Dobbertin, Antoon Bosselaers, and Bart Preneel. There exist 128-, 256-, and 320-bit versions of this algorithm, called RIPEMD-128, RIPEMD-256, and RIPEMD-320, respectively. All these replace the original RIPEMD, which was found to have collision issues.

Rainbow Tables

Since Windows and many other systems store passwords as hashes, many people have had an interest in how to break hashes. As we’ve mentioned, since a hash is not reversible, there is no way to unhash something. In 1980, Martin Hellman described a cryptanalytic technique that reduces the time of cryptanalysis by using precalculated data stored in memory. This technique was improved by Rivest before 1982. Basically, these types of password crackers are working with precalculated hashes of all passwords available within a certain character space, be that a–z or a–zA–z or a–zA–Z0–9, and more. This is called a rainbow table. If you search a rainbow table for a given hash, whatever plain text you find must be the text that was input into the hashing algorithm to produce that specific hash.

Clearly, such a rainbow table would get very large very fast. Assume that the passwords must be limited to keyboard characters. That leaves 52 letters (26 uppercase and 26 lowercase), 10 digits, and roughly 10 symbols, or about 72 characters. As you can imagine, even a 6-character password has a very large number of possible combinations. This means there is a limit to how large a rainbow table can be, and this is why longer passwords are more secure than shorter passwords.

Since the development of rainbow tables, there have been methods designed to thwart such attacks. The most common is salting, in which random bits are added to further secure encryption or hashing. This is most often encountered with hashing to prevent rainbow table attacks.

Essentially, the salt is intermixed with the message that is to be hashed. Consider an example. Say that you have a password that is:

pass001

In binary that is:

01110000 01100001 01110011 01110011 00110000 00110000 00110001

A salt algorithm would insert bits periodically. Let’s assume for our example that we insert bits every fourth bit, giving us:

0111100001 0110100011 0111100111 0111100111 0011100001 0011100001 0011100011

If you convert that to text, you would get:

xZ7#

All this is transparent to the end user, who doesn’t even know that salting is happening or what it is. However, an attacker using a rainbow table to get passwords would get the wrong password.

Historical Steganography

In modern times, steganography involves digital manipulation of files to hide messages. However, the concept of hiding messages is not new. There have been many methods used throughout history.

• The ancient Chinese wrapped notes in wax and swallowed them for transport. This was a crude but effective method of hiding messages.

• In ancient Greece, a messenger’s head would be shaved, a message was written on his head, and then his hair was allowed to grow back. Obviously, this method required some time.

• In 1518 Johannes Trithemius wrote a book on cryptography and described a technique in which a message was hidden by having each letter taken as a word from a specific column.

• During World War II the French Resistance sent messages written on the backs of couriers using invisible ink.

• Microdots are images/undeveloped film the size of a typewriter period that are embedded in innocuous documents. These were said to be used by spies during the Cold War.

• Also during the Cold War, the U.S. Central Intelligence Agency used various devices to hide messages. For example, they developed a tobacco pipe that had a small space to hide microfilm but could still be smoked.

In more recent times, but before the advent of computers, other methods were used to hide messages.

Steganography Methods and Tools

There are a number of tools available for implementing steganography. Many are free or at least have free trial versions. A few of these tools are listed here:

• QuickStego: Easy to use but very limited

• Invisible Secrets: Much more robust, with both a free version and a commercial version available

• MP3Stego: Specifically for hiding payload in MP3 files

• Stealth Files 4: Works with sound files, video files, and image files

• Snow: Hides data in whitespace

• StegVideo: Hides data in a video sequence

• Invisible Secrets: A very versatile steganography tool that has several options

Frequency Analysis

Frequency analysis is a basic tool for breaking most classical ciphers, though it is not useful against modern symmetric or asymmetric cryptography. It is based on the fact that some letters and letter combinations are more common than others. In all languages, certain letters of the alphabet appear more frequently than others. By examining those frequencies, you can derive some information about the key that was used. In English, the words the and and are the two most common three-letter words. The most common single-letter words are I and a. If you see two of the same letters together in a word, it is most likely ee or oo.

Modern Cryptanalysis Methods

Cracking modern cryptographic methods is quite daunting. The level of success depends on a combination of resources, including computational power, time, and data. If you had an infinite amount of any of these, you could crack any modern cipher. But you won’t have an infinite amount.

The following sections describe the basic approaches used to attack block ciphers. There are other methods that are beyond the scope of this book, such as differential cryptanalysis and linear cryptanalysis. For the purposes of understanding basic computer security, it is not necessary that you master these techniques.

MULTIPLE CHOICE QUESTIONS

1. It is important to understand the concepts and application of cryptography. Which of the following most accurately defines encryption?

A. Changing a message so it can only be easily read by the intended recipient

B. Using complex mathematics to conceal a message

C. Changing a message using complex mathematics

D. Applying keys to a message to conceal it

2. Which of the following is the oldest encryption method discussed in this text?

A. PGP

B. Multi-alphabet encryption

C. Caesar cipher

D. Cryptic cipher

3. Many classic ciphers are easy to understand but not secure. What is the main problem with simple substitution?

A. It does not use complex mathematics.

B. It is easily broken with modern computers.

C. It is too simple.

D. It maintains letter and word frequency.

4. Classic ciphers were improved with the addition of multiple shifts (multiple substitution alphabets). Which of the following is an encryption method that uses two or more different shifts?

A. Caesar cipher

B. Multi-alphabet encryption

C. DES

D. PGP

5. Which binary mathematical operation can be used for a simple (but unsecured) encryption method and is in fact a part of modern symmetric ciphers?

A. Bit shift

B. OR

C. XOR

D. Bit swap

6. Why is binary mathematical encryption not secure?

A. It does not change letter or word frequency.

B. It leaves the message intact.

C. It is too simple.

D. The mathematics of it is flawed.

7. Which of the following is most true regarding binary operations and encryption?

A. They are completely useless.

B. They can form a part of viable encryption methods.

C. They are only useful as a teaching method.

D. They can provide secure encryption.

8. What is PGP?

A. Pretty Good Privacy, a public key encryption method

B. Pretty Good Protection, a public key encryption method

C. Pretty Good Privacy, a symmetric key encryption method

D. Pretty Good Protection, a symmetric key encryption method

9. Which of the following methods is available as an add-in for most email clients?

A. DES

B. RSA

C. Caesar cipher

D. PGP

10. Which of the following is a symmetric key system that uses 64-bit blocks?

A. RSA

B. DES

C. PGP

D. Blowfish

11. What is the advantage of a symmetric key system using 64-bit blocks?

A. It is fast.

B. It is unbreakable.

C. It uses asymmetric keys.

D. It is complex.

12. What size key does a DES system use?

A. 64 bit

B. 128 bit

C. 56 bit

D. 256 bit

13. What type of encryption uses different keys to encrypt and decrypt the message?

A. Private key

B. Public key

C. Symmetric

D. Secure

14. Which of the following methods uses a variable-length symmetric key?

A. Blowfish

B. Caesar

C. DES

D. RSA

15. What should you be most careful of when looking for an encryption method to use?

A. Complexity of the algorithm

B. Veracity of the vendor’s claims

C. Speed of the algorithm

D. How long the algorithm has been around

16. Which of the following is most likely to be true of an encryption method that is advertised as unbreakable?

A. It is probably suitable for military use.

B. It may be too expensive for your organization.

C. It is likely to be exaggerated.

D. It is probably one you want to use.

17. Which of the following is most true regarding certified encryption methods?

A. These are the only methods you should use.

B. It depends on the level of certification.

C. It depends on the source of the certification.

D. There is no such thing as certified encryption.

18. Which of the following is most true regarding new encryption methods?

A. Never use them until they have been proven.

B. You can use them, but you must be cautious.

C. Use them only if they are certified.

D. Use them only if they are rated unbreakable.

Exercises

Exercise 8.1: Using the Caesar Cipher

This exercise is well suited for group or classroom exercises.

1. Write a sentence in normal text.

2. Use a Caesar cipher of your own design to encrypt it.

3. Pass it to another person in your group or class.

4. Time how long it takes that person to break the encryption.

5. (Optional) Compute the mean time for the class to break Caesar ciphers.

Exercise 8.2: Using Multi-Alphabet Ciphers

This exercise also works well for group settings and is best used in conjunction with Exercise 8.1.

1. Write a sentence in normal text.

2. Use a multi-alphabet cipher of your own design to encrypt it.

3. Pass it to another person in your group or class.

4. Time how long it takes that person to break the encryption.

5. (Optional) Compute the mean time for the class to break these and compare that to the mean time required to break the Caesar ciphers.

Exercise 8.3: Using PGP

1. Download a PGP attachment for your favorite email client. A web search for PGP and your email client (that is, PGP and Outlook or PGP and Eudora) should locate both modules and instructions.

2. Install and configure the PGP module.

3. Working with a classmate, send encrypted messages back and forth.

Exercise 8.4: Finding Good Encryption Solutions

1. Scan the Web for various commercial encryption algorithms.

2. Find one that you feel may be “snake oil.”

3. Write a brief paper explaining your opinion.

PROJECTS

Project 8.1: RSA Encryption

Using the Web or other resources, write a brief paper about RSA, its history, its methodology, and where it is used. Students with a sufficient math background may choose to delve more deeply into the RSA algorithm’s mathematical basis.

Project 8.2: Programming Caesar Cipher

This project is for students with some programming background.

Write a simple program in any language you prefer (or that your instructor specifies) that can perform a Caesar cipher. In this chapter, you not only saw how this cipher works but were given some ideas on how to use ASCII codes to make this work in any standard programming language.

Project 8.3: Other Encryption Methods

Write a brief essay describing any encryption method not already mentioned in this chapter. In the paper, describe the history and origin of that algorithm. You should also provide some comparisons with other well-known algorithms.