Introduction to Network Security – Part 7

NOTIFICATION: These examples are provided for educational purposes. The use of this code and/or information is under your own responsibility and risk. The information and/or code is given ‘as is’. I do not take responsibilities of how they are used.

Transposition Ciphers

The main idea of transposition ciphers is to rearrange the order of the letters used in the plaintext. This prevent the attacker to be able to recognise the message by using the frequency of distributions.

Rail Fence Cipher

Encryption

The basic concept of encryption on Rail Fence cipher is the follow:

  1. Select a number of rows greater or equal to two. For this example, we will pick three:
  2. Place each letter of the message in each row, one letter at a time, on one row at a time, from the top to the bottom
    1. Lets assume the plaintext is “SUPERSECRETMESSAGE”
    2. Rearrange the letters on the rows:
  3. After finished, we append one row after another in order, forming the ciphertext.

Decryption

The decryption of a rail fence cipher is almost the reverse process of the encryption.

  1. You will need the ciphertext and the number of rows:
    1. The ciphertext is “SEEEEAURCTSGPSRMSE”
    2. The number of rows is:
      |rows| = 3
  2. Computer the length of the ciphertext. In this case, the ciphertext “SEEEEAURCTSGPSRMSE” is:
    |ciphertext| = 18
  3. Lets calculate the columns that we will have:
    Number of Columns = ( |ciphertext| ÷ |rows| ) + ( |ciphertext| mod |rows| )
    = ( 18 ÷ 3 ) + (18 mod 3 )
    = 6 + 0
    = 6 columns
  4. Now, we have a table of 3 rows by 6 columns:
  5. Let fill up this table with the ciphertext, one letter at a time, from top to down and left to right:
  6. Now recreate the plaintext from this table:
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Introduction to Network Security – Part 4

NOTIFICATION: These examples are provided for educational purposes. The use of this code and/or information is under your own responsibility and risk. The information and/or code is given ‘as is’. I do not take responsibilities of how they are used.

Before we begin talking about encryption, decryption, and ciphers related topic, let go over some terminologies to have in account:

  • Cipher: An algorithm used for encryption.
    Link reference: <http://www.merriam-webster.com/dictionary/cipher>
  • Ciphertext: The encrypted(coded) message.
    Link reference: <http://cryptnet.net/fdp/crypto/crypto-dict/en/crypto-dict.html>
  • Cryptanalysis: Study of the principles and methods of deciphering a ciphertext without having the required key.
    Link reference: <http://en.wikipedia.org/wiki/Cryptanalysis>
  • Cryptography: Study of the principles and methods of encryption.
    Link reference: <http://en.wikipedia.org/wiki/Cryptography>
  • Cryptology: The study of cryptanalysis and cryptography.
    Link reference: <http://www.britannica.com/EBchecked/topic/145058/cryptology>
  • Deciphering: Also known as decryption. The act of transforming a ciphertext to the original plaintext.
    Link reference: <http://www.merriam-webster.com/dictionary/deciphering>
  • Decryption: Also known as deciphering. The act of transforming a ciphertext to the original plaintext.
  • Enciphering: Also known as encryption. The act of transforming a plaintext to a ciphertext.
    Link reference: <http://www.merriam-webster.com/dictionary/enciphering>
  • Encryption: Also know as enciphering. The act of transforming a plaintext to a ciphertext.
  • Plaintext: the original message to be encrypted.
    Link reference: <http://en.wikipedia.org/wiki/Plaintext>
  • Product: stages of transposition and substitutions performed.
    Link reference: <http://www.britannica.com/EBchecked/topic/477942/product-cipher>
  • Secret key: An input required for the encryption and/or decryption algorithms.
  • Substitution: Map each element in a plain text to another element.
    Link reference: <http://substitution.webmasters.sk/>
  • Transposition: Rearrange the elements in the plaintext
    Link reference: <http://mw1.meriam-webster.com/dictionary/transposition%20cipher>

Cryptography

A cryptographic system is characterized by the use of encryption operations, number of keys used for encryption and decryption, and the way in which the plain text is processed.

Encryption Operations: In order to encrypt a plaintext to a chipertext is required to perform multiple stages of transposition and substitution, also known as product.

  • Substitution: We take each element from the plaintext and mapped them to another element
  • Transposition: We  take each element in the plaintext and rearrange its order in such a way that it differ from the original plaintext.

To perform encryption and decryption, we use a key reference. We can categorize the encryption techniques as  symmetric, single, asymmetric, double, and/or public.

The plaintext can be processed by using a method of streams or blocks:

  • Stream: The plaintext is processed as a continuous set of elements in which each element is encrypted one at a time.
  • Blocks: The plaintext is divided in a set of blocks in which each block is encrypted one at a time.

Cryptanalysis

As explained in the terminology list, Cryptanalysis is purpose of decrypt an encrypted ciphertext without the knowledge of the key used for the encryption. One way is to attack the encryption system and recover the key used for the encryption instead of recovering the plaintext from a single ciphertext.
Cryptanalysis attacks are divided in two categories:

  1. Brute-force Attack: Every combination of a possible key is tested on the chipertext until the plaintext is obtained.
  2. Cryptanalytic Attack: The use of knowing some characteristic of the original plaintext such as some used keywords, language, format, plaintext to ciphertext pairs examples, and  knowledge of the possible algorithm used to decrypt the ciphertext.

Unconditional Security

We call unconditional security when a cipher cannot be broken by using a ciphertext and the plaintext that produced the ciphertext regardless of the computational power and time available. Up to day, there are no encryption algorithm that can be unconditional secure with the exception of the one-time pad encryption algorithm <http://www.ibm.com/developerworks/library/s-pads.html> which will be explained in the following postings.

Computational Security

Base on the cost-benefit of braking a cipher, a cipher may not be broker due:

  1. The cost of braking the cipher is greater than the value of the plaintext encrypted
  2. The time required to breaking the cipher exceed the usefulness lifetime of the plaintext encrypted
  3. Depending of the complexity of the cipher, there would be a limitation of computing resources and time.

Brute Force Search

As explained before, we call brute force to try every key combination possible to decrypt the ciphertext into plaintext. Before obtaining success, the attacker must try at least 50 percent of the possible keys; therefore, the probability of success may be proportional to the size of the key.

Lets assume we wish to have to option of using:

  1. DES encoding (56-bit) <http://groups.csail.mit.edu/cag/raw/benchmark/suites/des/README.html>.
  2. Triple DES (168-bit) <http://en.wikipedia.org/wiki/Triple_DES>
  3. AES (Greater than 128 bits) <http://www.aescrypt.com/>

Depending of which encryption we use, the time required to find the right key by brute force could be:

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