hcrypto.engines
Class PolySubstitutionEngine

java.lang.Object
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  +--hcrypto.cipher.CipherEngine
        |
        +--hcrypto.cipher.BlockCipher
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              +--hcrypto.engines.PolySubstitutionEngine

public class PolySubstitutionEngine

extends BlockCipher

This class implements a PolySubstitution cipher algorithm (which many authors call a Homophonic Substitution cipher). The key is represented by a keyword, which is inserted at the beginning of the decoding alphabet.

More precisely, the cipher alphabet is likely to be considerably larger than the plaintext alphabet with certain plaintext characters mapped into several different ciphertext characters, that is, the encryption mapping is one-to-many. On the other hand, each ciphertext character represents a unique plaintext character. The keyword will be expanded to a string with length equal to the size of the cipher alphabet with all characters in the plaintext alphabet. The kth character of the cipher alphabet will be decoded into the kth character of the keyword. This map also uniquely determines the set of characters that each plaintext character can be mapped into.

The encoding algorithm maintains a variable that, for each character in the plaintext message, stores the position of that character. The value of that variable modulo the number of ciphertext characters that plaintext character can be mapped into determines which ciphertext character it is mapped into.

For example, suppose that the plaintext alphabet is a..z of size 26 and the cipher alphabet is a..zA..Z of size 52. Suppose the user chooses the keyword: "nowisthetimetolearncryptography" of length 31. The 10 characters "bdfjkquvxz" are not contained in this keyword so add them to the end of it. Now add the first 11 characters of the resulting keyword to the end of itself to get a new keyword of length 52. Thus we have the following table for decoding:

 abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ  -- The cipher alphabet
 nowisthetimetolearncryptographybdfjkquvxznowisthetim  -- The decoding alphabet
 

Thus for encryption, for example, "a" can be mapped into "q" or "B" and "t" can be mapped into any of the letters of "fimxUX". If the plaintext message starts with the word "that", the first "t" of "that" would be encoded as "f", the second would be encoded as "i", and so on. Since there are six different encodings for "t", after six encodings, the seventh occurrence of 't' would be encoded as "f". The PolySubstution cipher is one that requires that two different alphabets be used for the plaintext and ciphertext alphabets respectively. As just explained, the ciphertext alphabet must be larger than the plaintext alphabet. The following code segment provides an example of how to create and use a Polysubstitution cipher:

      Cipher cipher = Cipher.getInstance("PolySubstitution");
      HistoricalKey key = HistoricalKey.getInstance("PolySubstitution", cipher.getProvider());
      key.init("nowisthetimetolearncryptography/az/azAZ");
      cipher.init(key);
      String c1encrypt = cipher.encrypt("this is a secret message");
 

In this case, the plaintext alphabet consists of the letters a..z, and the ciphertext alphabet consists of the letters a..zA..Z. The decoding alphabet would be constructed from the keyphrase as described above.

See also:

PolySubstitutionKey
Alphabet

Fields inherited from class hcrypto.cipher.BlockCipher

alphabet, blocksize, cipherAlphabet

Fields inherited from class hcrypto.cipher.CipherEngine

alphabetRangeOptions

Constructor Summary

PolySubstitutionEngine()
Creates a PolySubstitutionEngine and sets the alphabetRangeOptions instance variable to "111111", which translates to all six alphabet options.

Method Summary

java.lang.String	engineDecode(java.lang.String s) Returns an decoded String for the specified encode String.
java.lang.String	engineEncode(java.lang.String s) Returns an encoded String for the specified String.
protected void	engineInit(HistoricalKey hKey) Initializes the PolySubstitutionEngine with the specified hKey.

Methods inherited from class hcrypto.cipher.BlockCipher

decodeShift, encodeShift, engineDecrypt, engineDecryptFile, engineDecryptFile, engineEncrypt, engineEncryptFile, engineEncryptFile

Methods inherited from class hcrypto.cipher.CipherEngine

getAlphabetRangeOptions

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

PolySubstitutionEngine

public PolySubstitutionEngine()

Creates a PolySubstitutionEngine and sets the alphabetRangeOptions instance variable to "111111", which translates to all six alphabet options.

Method Detail

engineInit

protected void engineInit(HistoricalKey hKey)
                   throws java.lang.Exception

Initializes the PolySubstitutionEngine with the specified hKey.

Overrides:

engineInit in class BlockCipher

Parameters:

hKey - a PolySubstitutionKey.

engineEncode

public java.lang.String engineEncode(java.lang.String s)
                              throws java.lang.Exception

Returns an encoded String for the specified String. Characters which are not part of the chosen alphabet set are ignored.

Overrides:

engineEncode in class BlockCipher

Parameters:

s - the String to be encoded

engineDecode

public java.lang.String engineDecode(java.lang.String s)
                              throws java.lang.Exception

Returns an decoded String for the specified encode String. Characters which are not part of the chosen alphabet set are ignored.

Overrides:

engineDecode in class BlockCipher

Parameters:

s - the String to be decoded.