CSC 2106: Cryptology And
Coding Theory - BCS
By Muwonge S. Bernard
Makerere University
Faculty of Computing & IT
bmuwonge@cit.mak.ac.ug
bernard.muwonge@yahoo.com
Tel:+256776312246, +256712312246
Cryptography
Cryptographic systems are generally grouped
according to three facts about them:
The mathematical operation that changes the
plaintext into the ciphertext using the
encryption key.
Whether a block or a stream cipher is
produced.
The type of key system used - single or two
key.
Single Key Cryptography
Concealment Cipher
Here, the message is present but concealed in
some way.
The security of such messages totally
dependents upon the concealment trick.
These are easily broken if one is looking for
such devices and they do not lend themselves
to fast ciphering and deciphering, so they are
not used in any serious applications.
Examples
Sir John Trevanion, was locked up in Colcester
Castle. He had every reason to believe that he
would be put to death just as had been his friends
and fellow Royalists. While awaiting his doom,
however, he was one day handed the following
letter by his jailer.
Concealment Cipher
Worthie Sir John:- Hope, that is ye beste comfort of ye
afflicted, cannot much, I fear me, help you now. That I
would saye to you, is this only: if ever I may be able to
requite that I do owe you, stand not upon asking me.
'Tis not much that I can do: but what I can do, bee ye
verie sure I wille. I knowe that, if dethe comes, if
ordinary men fear it, it frights not you, accounting it for
a high honor, to have such a rewarde of your loyalty.
Pray yet that you may be spared this soe bitter, cup. I
fear not that you will grudge any sufferings; only if bie
submission you can turn them away, 'tis the part of a
wise man. Tell me, an if you can, to do for you
anythinge that you wolde have done. The general goes
back on Wednesday. Restinge your servant to
command. - R.T.
Sir John did read the third letter after every
punctuation mark and felt some degree of
relief on knowing