4 polyalphabetic ciphers
Hiteishi Diwanji
Homophonic substitution ciphers:
- Homophonic substitution cipher maps a character to more than one code. For example, “A” could correspond to 3, 15, 23; while for “B” this could be 5, 17, 21.
- This type of substitution makes cryptanalysis or brute force harder but it doesn’t hide all statistical properties.
- With the help of powerful computer this can be broken in a few seconds.
Polyalphabetic Cipher:
- Monoalphabetic substitution rules are used.
- Key decides which monoalphabetic substitution is to be performed.
Vigenere Cipher:
- P=p0p1p2……pn-1
- K=k0k1k2……km-1
- C=C0C1C2……Cn-1
- C=E(K,P) = E[(k0k1k2……km-1, p0p1p2……pn-1)]
=(p0+k0) mod 26, (p1+k1) mod 26, …(pm-1+km-1) mod 26, (pm+k0) mod 26 … K is of length m. For m letters of plaintext , key is used. For the next m letters of plaintext, the key letters are repeated until all plaintext letters are used.
- Pi=(Ci – ki mod m) mod 26
Vigenere Cipher:
- Key : mask
- Plaintext : meetmeatfield
- Ciphertext :
Strength of Vigenere Cipher:
- The letter frequency is masked. But key is repeated so for the same plaintext letter and key letter – same ciphertext letter is achieved.
- For meet and me – me generates same ciphertext ye because key is repeated.
Autokey system:
- Periodic nature of the keyword can be overcomed by nonrepeating keyword that is as long as an autokey system.
- Key word is concatenated with plaintext to provide a running key.
- Key : mask meet meatf
- Plaintext : meetmeatfield
- Ciphertext :
VERNAM cipher:
- Cipher works on binary data(bits)
- ci=pi ki
Where
Pi = ith binary digit of plaintext
Ki = ith binary digit of key
ci = ith binary digit of ciphertext
- = exclusive – or (XOR) operation pi=ci Å ki
One – Time pad:
- The key is used to encrypt and decrypt a single message, then key is discarded.
- Each new message requires a new key of the same length as the new message. Such a scheme, is known as a one-time pad.
- One time pad is unbreakable.
- No statistical relationship to the plaintext.
- Includes space so 27 characters scheme.
One – Time pad Example:
- Plaintext : mr mustard
- key : pxlmvmsydo
- Ciphertext : ANKYODKYUR
- Plaintext : miss scarl
- Key : pftgpmiydg
- Ciphertext : ANKYODKYUR
Cryptanalyst can find keys and can not decide which plaintext is true and which key is correct.
One Time pad – Difficulties in using:
- Practically difficult to generate large quantities of random keys.
- How to distribute key and protect them. Because for every message , equal length of key is to be sent to sender and receiver.
One time pad gives perfect secrecy. But limited use . Used only for highly secure low bandwidth channels.
Transposition Techniques. – permutation of plaintext letters :
Rail fence cipher:
Message “ all gather at lawn”.
Rail fence depth 2
Transposition Techniques. –– Complex scheme:
- permutation of plaintext letters
- Write the message row by row , read column by column. Permutation is done by order of the column.
- Plaintext : discusstheformulainmeeting
- Ciphertext : ssfuntxisemiegduhraenctolmiy
- For key 3214, start with column labeled 1- column 3 in table. Write all the letters then column labeled 2 – column 2 in the table. Then column labeled 3 and 4.
- This type of transposition cipher is not secure because letter frequencies in ciphertext are same as plaintext.
- Cryptanalyst can put ciphertext in a matrix and play with column position.
Stegnography:
- Plaintext message is hidden
- Simple method – An arrangement of words or letters in the message can give real message.
- Text : Driveon Road Open passage.
- Hidden message : First letter of each word – DROP
Stegnography techniques:
- Character Marking – Selected letters are overwritten in pencil and not visible unless hold at an angle to bright light
- Invisible ink – leave no visible trace until heat or some chemical is applied to the paper.
- Pin punctures – pin punctures on selected letters. Not visible unless the paper is held up in light.
Advantage : hide the secret communication
Disadvantage : Lot of overhead to hide few bits of information
you can view video on polyalphabetic ciphers |
Suggested Reading:
- Cryptography and Network Security Principles and Practice by William Stallings, sixth Edition, PEARSON.
- Security in Computing by Charles Pfleeger & Shari Lawrence Pfleeger, fourth Edition, PEARSON.
- Network Security by Charlie Kaufman, Radia Perlman, Mike Speciner, second Edition, PHI.
- The Complete Reference – Network Security by Roberta Bragg, Mark Rhodes-Ousley & Keith Strassberg, Tata McGraw Hill
- Network Security Bible by Eric Cole, Ronald Krutz, James Conley, Wiley
- Hacking 6 Exposed by Stuart McClure, Joel Scambray & George Kurtz , Tata McGraw Hill .
- www.snort.org
- https://nmap.org