CNT 5412 Network Security And
Cryptography Exam Study Guide
Caesar Cipher - ANSWER Encryption: cipher = (plaintext + key) mod 26
There are only 26 possible variations.
Monoalphabetic CIpher - ANSWER Consists of an arbitrary mapping of one letter to
another.
There are 26! possible pairings. Or 4 * 10^26
Vigenere Cipher - ANSWER It is a method of encrypting alphabetic text by using a series
of different Caesar Ciphers based on the letters.
When the key is shorter than the text, the key must be repeated as many times as
necessary to provide padding for the encryption.
Using the Probability Distribution of characters: - ANSWER In the English language, E is
the most common, with the digrams most is "th' 'he', trigrams being 'the', 'and'
Breaking the vigenere cipher - ANSWER E is the most common letter, and it usually
appears following H or preceding R. H, usually follows T. I follows N, using digrams and
trigrams.
Index of Coincidence - ANSWER Count the total number of characters in the ciphertext =
c, and the number of occurrences of each character c(sub)I. If I = 1, ci is the number of
occurrences of the character 'B' in the ciphertext. The character frequency is the ratio
of the number of occurrences of a given character by the total number of characters.
Transposition Ciphers - ANSWER Plaintext: attackatdawn
3142 is the key
CIpher: output the letters vertically using key order
atta
ckat
dawn
CIPHER: TKAATNACDTAW
Symmetric Schemes - ANSWER If the encryption and decryption keys are equal
Asymmetric Schemes - ANSWER If the encryption and decryption keys differ, and the
, decryption key cannot be computed (computationally infeasible) from knowledge of the
algorithm and encryption key
Vernam One-Time Pad - ANSWER If the Vigenere Cipher has a key at least as long as the
plaintext, and is chosen at random and is used only once.
The plaintext is xor'ed with a string of random bits, where the result is a random bit
string.
Examples of a One-Time Key - ANSWER Plaintext: T E D
ASCII: 0101 0100 0100 0101 0100 0100
Key xor: 0110 0110 0111 0011 0001 0010
Cipher: 0011 0010 0011 0110 0101 0110
ASCII of Cipher: 2 6 V
Modern Ciphers: - ANSWER Operate on binary plaintext, use binary keys of fixed length
Different Types of modern ciphers: - ANSWER Public Key/Asymmetric Ciphers
Symmetric Ciphers such as: Stream Ciphers (RC4, A5/x, HELIX, SEAL) or Block Ciphers
(Triple-DES, Blowfish, AES)
Modern Cipher Operations (2): - ANSWER Substitution: Substitute a code symbol for
another. FOr each of the 2^k possible values of input, the k-bit output. To specify a
completely random it would be k * 2^k bits
Permutation: transpose or reorder the symbols present in the code. For each of the k
input bits, the output position to where it goes. Each bit of the output gets its value from
exaclt one of the bits of input.
BOTH ARE NEEDED
Substitution basics: - ANSWER Shifts a Vigenere Cipher, xor
10110111 -> 11001010
Permutation Basics: - ANSWER 10110111 -> 11111100
Hash Algorithms: - ANSWER Also known as message digests or one way transformation.
It is a mathematical transformation that takes a message of arbitrary length and
computes it from a fixed-length(short) number.
Use the short hand: hash of a message m = h(m)
More Hash algorithms: - ANSWER For any message m, it is easy to compute h(m). This
means that in order to be practical it cant take a lot of processing to compute the hash.
Cryptography Exam Study Guide
Caesar Cipher - ANSWER Encryption: cipher = (plaintext + key) mod 26
There are only 26 possible variations.
Monoalphabetic CIpher - ANSWER Consists of an arbitrary mapping of one letter to
another.
There are 26! possible pairings. Or 4 * 10^26
Vigenere Cipher - ANSWER It is a method of encrypting alphabetic text by using a series
of different Caesar Ciphers based on the letters.
When the key is shorter than the text, the key must be repeated as many times as
necessary to provide padding for the encryption.
Using the Probability Distribution of characters: - ANSWER In the English language, E is
the most common, with the digrams most is "th' 'he', trigrams being 'the', 'and'
Breaking the vigenere cipher - ANSWER E is the most common letter, and it usually
appears following H or preceding R. H, usually follows T. I follows N, using digrams and
trigrams.
Index of Coincidence - ANSWER Count the total number of characters in the ciphertext =
c, and the number of occurrences of each character c(sub)I. If I = 1, ci is the number of
occurrences of the character 'B' in the ciphertext. The character frequency is the ratio
of the number of occurrences of a given character by the total number of characters.
Transposition Ciphers - ANSWER Plaintext: attackatdawn
3142 is the key
CIpher: output the letters vertically using key order
atta
ckat
dawn
CIPHER: TKAATNACDTAW
Symmetric Schemes - ANSWER If the encryption and decryption keys are equal
Asymmetric Schemes - ANSWER If the encryption and decryption keys differ, and the
, decryption key cannot be computed (computationally infeasible) from knowledge of the
algorithm and encryption key
Vernam One-Time Pad - ANSWER If the Vigenere Cipher has a key at least as long as the
plaintext, and is chosen at random and is used only once.
The plaintext is xor'ed with a string of random bits, where the result is a random bit
string.
Examples of a One-Time Key - ANSWER Plaintext: T E D
ASCII: 0101 0100 0100 0101 0100 0100
Key xor: 0110 0110 0111 0011 0001 0010
Cipher: 0011 0010 0011 0110 0101 0110
ASCII of Cipher: 2 6 V
Modern Ciphers: - ANSWER Operate on binary plaintext, use binary keys of fixed length
Different Types of modern ciphers: - ANSWER Public Key/Asymmetric Ciphers
Symmetric Ciphers such as: Stream Ciphers (RC4, A5/x, HELIX, SEAL) or Block Ciphers
(Triple-DES, Blowfish, AES)
Modern Cipher Operations (2): - ANSWER Substitution: Substitute a code symbol for
another. FOr each of the 2^k possible values of input, the k-bit output. To specify a
completely random it would be k * 2^k bits
Permutation: transpose or reorder the symbols present in the code. For each of the k
input bits, the output position to where it goes. Each bit of the output gets its value from
exaclt one of the bits of input.
BOTH ARE NEEDED
Substitution basics: - ANSWER Shifts a Vigenere Cipher, xor
10110111 -> 11001010
Permutation Basics: - ANSWER 10110111 -> 11111100
Hash Algorithms: - ANSWER Also known as message digests or one way transformation.
It is a mathematical transformation that takes a message of arbitrary length and
computes it from a fixed-length(short) number.
Use the short hand: hash of a message m = h(m)
More Hash algorithms: - ANSWER For any message m, it is easy to compute h(m). This
means that in order to be practical it cant take a lot of processing to compute the hash.
