E x e r c i s e • 1 : Apply frequency test to verify the randomness of the output of the following text: 1. The following plaintext: In cryptography, a transposition cipher (also known as a permutation cipher) is a method of encryption which scrambles the positions of characters (transposition) without changing the characters themselves. Transposition ciphers reorder units of plaintext (typically characters or groups of characters) according to a regular system to produce a ciphertext which is a permutation of the plaintext. They differ from substitution ciphers, which do not change the position of units of plaintext but instead change the units themselves. Despite the difference between transposition and substitution operations, they are often combined, as in historical ciphers like the ADFGVX cipher or complex high-quality encryption methods like the modern Advanced Encryption Standard (AES). laintexts can be rearranged into a ciphertext using a key, scrambling the order of characters like the shuffled pieces of a jigsaw puzzle. The resulting message is hard to decipher without the key because there are many ways the characters can be arranged. For example, the plaintext "THIS IS WIKIPEDIA" could be encrypted to "TWDIP SIHII IKASE". To decipher the encrypted message without the key, an attacker could try to guess possible words and phrases like DIATHESIS, DISSIPATE, WIDTH, etc., but it would take them some time to reconstruct the plaintext because there are many combinations of letters and words. By contrast, someone with the key could reconstruct the message easily: In practice, a message this short and with a predictable keyword would be broken almost immediately with cryptanalysis techniques. Transposition ciphers have several vulnerabilities (see the section on "Detection and cryptanalysis" below), and small mistakes in the encipherment process can render the entire ciphertext meaningless. However, given the right conditions - long messages (e.g., over 100–200 letters), unpredictable contents, unique keys per message, strong transposition methods, and so on - guessing the right words could be computationally impossible without further information. In their book on codebreaking historical ciphers, Elonka Dunin and Klaus Schmeh describe double columnar transposition (see below) as "one of the best manual ciphers known". 2. The ciphertext after applying Vigenere encryption (choose any key). 3. The ciphertext after applying playfair encryption (choose any key). 4. The ciphertext after applying RC4 encryption (choose any key). 5. The ciphertext after applying DES (ECB) encryption (choose any key). Note: Remember this key because you will use it in the second part. E x e r c i s e • 2 : Now, we want to determine the key from the encrypted document using a brute-force attack. 1. Select “Analysis\Symmetric Encryption (modern)\DES (ECB)” from menu. 2. Enter the key used in step 5 in the previous exercise except the last two character. 3. Click “Start”. 4. The first one returns readable results. Click “Accept selection”. The original plaintext shows up. 5. Expose less from the above key, and repeat the steps from 5 to 8, What is your observation? E x e r c i s e • 3 : 1. Encrypt the word “hello” using DES (ECB) and RC4. 2. Then, encrypt the word “Hello” using DES (ECB) and RC4. Use the same password used in the previous step. 3. What is your observation? Before starting the explanation, put the steps of how to solve a question on cryptool on each exercise
E x e r c i s e • 1 :
Apply frequency test to verify the randomness of the output of the
following text:
1. The following plaintext:
In cryptography, a transposition cipher (also known as a permutation
cipher) is a method of encryption which scrambles the positions of
characters (transposition) without changing the characters themselves.
Transposition ciphers reorder units of plaintext (typically characters or
groups of characters) according to a regular system to produce a
ciphertext which is a permutation of the plaintext. They differ from
substitution ciphers, which do not change the position of units of
plaintext but instead change the units themselves. Despite the difference
between transposition and substitution operations, they are often
combined, as in historical ciphers like the ADFGVX cipher or complex
high-quality encryption methods like the modern Advanced Encryption
Standard (AES).
laintexts can be rearranged into a ciphertext using a key, scrambling the
order of characters like the shuffled pieces of a jigsaw puzzle. The
resulting message is hard to decipher without the key because there are
many ways the characters can be arranged.
For example, the plaintext "THIS IS WIKIPEDIA" could be encrypted to
"TWDIP SIHII IKASE". To decipher the encrypted message without the
key, an attacker could try to guess possible words and phrases like
DIATHESIS, DISSIPATE, WIDTH, etc., but it would take them some time to
reconstruct the plaintext because there are many combinations of letters
and words. By contrast, someone with the key could reconstruct the
message easily:
In practice, a message this short and with a predictable keyword would
be broken almost immediately with cryptanalysis techniques.
Transposition ciphers have several vulnerabilities (see the section on
"Detection and cryptanalysis" below), and small mistakes in the encipherment process can render the entire ciphertext meaningless.
However, given the right conditions - long messages (e.g., over 100–200
letters), unpredictable contents, unique keys per message, strong
transposition methods, and so on - guessing the right words could be
computationally impossible without further information. In their book on
codebreaking historical ciphers, Elonka Dunin and Klaus Schmeh
describe double columnar transposition (see below) as "one of the best
manual ciphers known".
2. The ciphertext after applying Vigenere encryption (choose
any key).
3. The ciphertext after applying playfair encryption (choose
any key).
4. The ciphertext after applying RC4 encryption (choose any
key).
5. The ciphertext after applying DES (ECB) encryption
(choose any key). Note: Remember this key because you
will use it in the second part.
E x e r c i s e • 2 :
Now, we want to determine the key from the encrypted document
using a brute-force attack.
1. Select “Analysis\Symmetric Encryption (modern)\DES
(ECB)” from menu.
2. Enter the key used in step 5 in the previous exercise except
the last two character.
3. Click “Start”.
4. The first one returns readable results. Click “Accept
selection”. The original plaintext shows up.
5. Expose less from the above key, and repeat the steps from 5
to 8, What is your observation?
E x e r c i s e • 3 :
1. Encrypt the word “hello” using DES (ECB) and RC4.
2. Then, encrypt the word “Hello” using DES (ECB) and RC4.
Use the same password used in the previous step.
3. What is your observation?
Before starting the explanation, put the steps of how to solve a question on cryptool on each exercise
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