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Transposition cipher
The Caesar Cipher is one of the most basic methods for
encrypting and decrypting a text.
The method is named after Julius Caesar (55BC) and is a substitution
cipher in which each letter of the plaintext is shifted up or down the alphabet by a fixed number of positions.
The method is also known as Caesar's Cipher, Caesar's Code,
Caesar Shift and Shift Cipher.
Julius Caesar, who used it in his private correspondence, shifted each letter of the plaintext down by 3 positions.
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In this case, The letter D becomes an A, E becomes B, etc. At the receiving end,
all the recipient has to do, is shift each letter by the same fixed number in
reverse direction. This means that A becomes D, B becomes E, etc. For this reason, the cipher is sometimes called a 'shift cipher' or a 'rotation',
in which case the Caesar Cipher is identified as ROT3 (rotation by 3 positions).
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In Caesar's days, the ROT3 algorithm probably offered resonable security
as most of Caesar's opponents were illiterate and even if they were able
to read,
they would probably have thought that it was written in an unknown foreign
language. Today, the Caesar Cipher offers absolutely no security
whatsoever. It is easily broken by hand and even the more complex
Vigenère Cipher
can not withstand frequency analysis and mathematical key length tests [1].
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A well known variant – which is also used in some computer software – is ROT13.
As this rotates the alphabet by 13 positions (exactly half the available 26 letters),
the cipher becomes reversible or reciproke.
In this case the letter A becomes
an N and the N (13 places further) becomes an A.
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There are many variants of the Caesar Cipher. Caesar's nephew Augustus for
example, used a right-shift of one position (A becomes B, B becomes C, etc.).
Other variants are the Reverse Caesar Cipher, which is always
reciprocal, and the Vigenère Cipher,
which uses a variable shift.
Furthermore a combination of the latter two is known as
the Beaufort Cipher.
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The ROT13 method is still widely used on Usenet, often for obscuring offensive
text rather than for serious encryption. Nevertheless, some well known software
vendors have used ROT13 in the past for storing passwords.
Despite the fact the Caesar Cipher offers virtually no protection today,
Rajib Karim was convicted in the UK in 2011 of 'terrorism offences'. He used
a variant of the Caesar Cipher when discussing plots to blow up British Airways
planes, rather than a more sophisticated program such as Mujhaddin Secrets
or PGP. Although he had access to the latter, he didn't trust it and
came up with this own scheme, implemented in Microsoft Excel [2].
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© Crypto Museum. Created: Sunday 22 February 2015. Last changed: Thursday, 18 February 2021 - 18:35 CET.
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