Classical Encryption Techniques: An Overview of Substitution and Transposition Methods, Slides of Computer Security

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Computers Security

Classical Encryption Techniques

Key Terms

• Encryption

 The art of making information obscure so that it is not readable without special knowledge

• Plain Text

 Original message

• Cipher Text

 Encoded message

• Encryption / Enciphering

 Process of converting plain text into ciphertext

• Decryption / Deciphering

 Process of converting ciphertext into plain text

• Cipher

 A scheme used to convert plaintext into ciphertext

Categorizing Cryptographic system

• Three aspects for categorization

 Operations

 All cryptographic systems / encryption algorithm are based on two general principle  Substitution

• Example PIEAS  QJFBT  Transposition
• Example PIEAS  EPSIA  Product systems

 Number of Keys

 Same key for sender and receiver

• Symmetric , Single key, Secret key, Conventional Key Encryption Different Keys for sender and receiver
• Asymmetric ,Two Key ,Public Key Encryption

cryptography..cont..

• Plaintext Processing

 Block Cipher

 Algorithms that operates on block of data

 Stream Cipher

 Algorithms that operates on stream of data

Symmetric Cipher Model

• Also termed as conventional encryption

• Same key is used by sender and receiver

• Elements of symmetric model

 Plaintext

 Encryption algorithm

 Secret key

 Input to the encryption algorithm For each key a different output is produced for same plaintext

 Ciphertext

 Decryption algorithm

 Encryption algorithm in reverse order

symmetric cipher model. .cont..

symmetric cipher cont..

• Strength of Easycipher

 Simply by looking the plain text and cipher text one can deduce both the key and the algorithm

• Requirements for secure conventional encryption

 The opponent should be unable to discover the key even if he has plaintext ciphertext pairs  The key should be communicated securely

• Making ciphers public

 Common applications can implement them  Example email, online transactions, authentication application  Can be implemented at hardware level  Improves the efficiency of the system

A generic symmetric model

Plain Text X=[X 1 ,X 2 ,X 3 ,…..XM]

Secret Key K=[ K 1 ,K 2 ,K 3 ….KJ]

Cipher Text Y=[Y 1 ,Y 2 ,Y 3 …..YN]

Encryption

Y=E K (X)

Decryption

X=D K (Y)

Cryptanalyst Knows

• Encryption Algorithm
• Ciphertext

Can generate

• Approximate plaintext
• Approximate key

Cryptanalysis Attack

Securing Encryption

• Simple rule

 Don’t provide enough information for determining the

corresponding plaintext

• Simple rule not possible

 Most of the encrypted message contain patterns

 There are only few exceptional algorithm that don’t

generate a pattern in cipher text

• Alternate Solutions

 Increase the cost of breaking cipher than value of

information

 Increase the time to break the cipher such that the

information becomes useless

Classical Encryption Techniques

• An overview of class encryption techniques with

 Potential strength

 Potential weaknesses

 Attacks

• Transposition and substitution encryption

techniques

• Combination of substitution and transposition

techniques

Encryption using Substitution

• Letter replaced by another latter

• Ceaser Cipher

 Rule

 Replace each letter with letter standing three places down

 Example:

Plain Text Cipher

 Alphabets are wrapped  Transformation scheme a b c d e f g h I j k l m n o p q r s t u v w x y z D E F G H I J K L M N O P Q R S T U V W X Y Z A B C

meet me after the toga party

PHHW PH DIWHU WKH WRJD SDUWB

ceaser cipher .. cont..

• Ceaser cipher can be

made strong by applying

 Permutation

 Making the plain text

unknown