Homepage
Crypto
Index
Glossary
Enigma
Hagelin
Fialka
Nema
Voice
Hand
OTP
EMU
Mixers
Phones
FILL
Codebooks
Algorithms
USA
USSR
UK
Germany
Yugoslavia
Ascom
AT&T
Bosch
Datotek
Gretag
HELL
ITT
Motorola
Mils
OMI
Philips
Racal
Siemens
STK
Tadiran
Telsy
Teltron
Transvertex
TST
Spy radio
Burst encoders
Intercept
Covert
Radio
PC
Telex
Telephones
People
Agencies
Manufacturers
• • • Donate • • •
Kits
Shop
News
Events
Wanted
Contact
About
Links
   Click for homepage
Caesar Cipher
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.

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
X Y Z A B C D E F G H I J K L M N O P Q R S T U V W

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).


ROT13
A special variant of this, 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.

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
N O P Q R S T U V W X Y Z A B C D E F G H I J K L M

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 reciproke, and the Vigenère Cipher, which uses a variable shift.


Security
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].


Modern use
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 Excell [2].


References
  1. Wikipedia, Caesar Cipher
    Retrieved February 2015.

  2. The Register, BA jihadist relied on Jesus-era encryption
    Retrieved February 2015.
Further information
Any links shown in red are currently unavailable. If you like the information on this website, why not make a donation?
Crypto Museum. Created: Sunday 22 February 2015. Last changed: Monday, 13 November 2017 - 09:43 CET.
Click for homepage