3. Substitution Ciphers
• Introduction
Substitution Ciphers are the ciphers where a alphabet is
replaced by another different ciphers. This ciphers mainly
uses the alphabets to encrypt messages for confidentiality in
communication.
4. Mono-alphabetic Ciphers
• The Mono-alphabetic Substitution cipher
is where each character in the plaintext is
replaced by a corresponding one from a
cipher alphabet. There are 3 types of
Mono-alphabetic Ciphers.
– 1) Caesar Ciphers
– 2) Affine Ciphers
– 3) Atbash Ciphers
5. Caesar Ciphers
• This is a video
which briefly
describes about
the working of a
Caesar ciphers.
(Youtube,2012)
6. Homophonic Ciphers
• A homophonic substitution cipher maps each
plaintext letter of a message to one or more
cipher text symbols. Homophonic ciphers hide
letter frequencies of plaintext messages which
are mapped to one or more cipher text units
called homophones.
• This method is based on the mono alphabetic
substitution. The original text is not only
encrypted by one but is also replaced with
multiple secret symbols.
7. Homophonic Ciphers(Contd.)
•
•
A Homophonic substitution cipher is like a simple substitution cryptosystem, in which a single character of plaintext can map to one of several
characters of cipher text. A plaintext message M=m1m2... is enciphered
as C=c1c2..., where each ci is picked at random from the set of
homophones f(mi).
Letter
Homophones
A
17 19 34 41 56 60 67 83
I
08 22 53 65 88 90
L
03 44 76
N
02 09 15 27 32 40 59
O
01 11 23 28 42 54 70 80
P
33 91
T
05 10 20 29 45 58 64 78 99
One possible encipherment of the message is:
M=P L A I N P I L O T
C = 91 44 56 65 59 33 08 76 28 78
8. Poly-graphic Ciphers
• “In a polygraphic substitution cipher plaintext letters
are substituted in large groups instead of
substituting letters individually”.(wordIQ, n.d)
• The first advantage is that the frequency distribution
is much flatter than that of individual letter.
• Second , the large numbers of symbols requires
correspondingly more cipher text to productively
analyze letter frequencies.
9. Playfair Ciphers
• The playfair cipher encrypts pair of
letters(diagraph), instead of single letters.
• It was invented in 1854 by Charles Wheatstone
for telegraph secrecy.
• It is the earliest practical Polygraphic
Substitution ciphers.
• The cipher used a 5 by five table containing a
key word.
10. Poly-alphabetic Ciphers
• In poly-alphabetic cipher, each plaintext
character may be replaced by more than one
character
• Since there are only 26 alphabets this process
will require using a different representation than
the alphabets
• Alphabets „A‟ through „Z‟ are replaced by 00,
01, 02, …, 25
• We need two digits in this representation since
we need to know how to reverse the process at
the decryption side (Babylon,1997).
11. Vignere Ciphers
• The most common method used as a
type of Poly-Alphabetic Cipher is
Vigenère cipher
• Vigenère cipher starts with a 26 x 26
matrix of alphabets in sequence. First
row starts with „A‟, second row starts
with „B‟, etc.
12. Vigenere(Contd.)
• E.g., Message = SEE ME
IN MALL
• Take keyword as
INFOSEC
• Vigenère cipher works as
follows:
SEEME I NMALL
I NFOSEC I NFO
------------------------------------A RJ AWMPUNQZ
13. Conclusion
• We have considered mainly the key
aspects of Substitution Ciphers and their
workings as well as their algorithms.
14. References
•
Brown, 1996, Introduction to Cryptography, [online], Available at:
http://williamstallings.com/Extras/Security-Notes/lectures/classical.html [Accessed at 1st
Jan 2012]
•
All Science Projects, 2011, [online], Available at: http://www.all-science-fairprojects.com/science_fair_projects_encyclopedia/Caesar_cipher [Accessed at 1 Jan 2012]
•
Chargrill, 2006,PlayFair Cipher ,[online] http://www.perlmonks.org/?node_id=554404
accessed on Jan 11
•
WordIQ, n.d, Substitution cipher- Definition, [online]
http://www.wordiq.com/definition/Substitution_cipher#Polygraphic_substitution accessed on
Jan 11
•
University of Adelaide,2012,[online],Available at :
https://www.eleceng.adelaide.edu.au/personal/dabbott/wiki/index.php/Cipher_Crossoff_List#Homophonic_Substitution_Ciphers,[Accessed on 10 Jan 2012]
•
Mrgeorged, 2009. Top Gear The Stig revealed Full. [video online],Available
at:<http://www.youtube.com/watch#!v=eTapK5dRaw4>[Accessed 23 June 2009].
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