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Tabula recta
Tabula recta is a cryptographic term invented by Johannes Trithemius[?] in 1518. The tabula recta was a square table of alphabets, each one made by shifting the previous one to the left, like this:
|ABCDEFGHIJKLMNOPQRSTUVWXYZ
-+--------------------------
A|ABCDEFGHIJKLMNOPQRSTUVWXYZ
B|BCDEFGHIJKLMNOPQRSTUVWXYZA
C|CDEFGHIJKLMNOPQRSTUVWXYZAB
D|DEFGHIJKLMNOPQRSTUVWXYZABC
E|EFGHIJKLMNOPQRSTUVWXYZABCD
F|FGHIJKLMNOPQRSTUVWXYZABCDE
G|GHIJKLMNOPQRSTUVWXYZABCDEF
H|HIJKLMNOPQRSTUVWXYZABCDEFG
I|IJKLMNOPQRSTUVWXYZABCDEFGH
J|JKLMNOPQRSTUVWXYZABCDEFGHI
K|KLMNOPQRSTUVWXYZABCDEFGHIJ
L|LMNOPQRSTUVWXYZABCDEFGHIJK
M|MNOPQRSTUVWXYZABCDEFGHIJKL
N|NOPQRSTUVWXYZABCDEFGHIJKLM
O|OPQRSTUVWXYZABCDEFGHIJKLMN
P|PQRSTUVWXYZABCDEFGHIJKLMNO
Q|QRSTUVWXYZABCDEFGHIJKLMNOP
R|RSTUVWXYZABCDEFGHIJKLMNOPQ
S|STUVWXYZABCDEFGHIJKLMNOPQR
T|TUVWXYZABCDEFGHIJKLMNOPQRS
U|UVWXYZABCDEFGHIJKLMNOPQRST
V|VWXYZABCDEFGHIJKLMNOPQRSTU
W|WXYZABCDEFGHIJKLMNOPQRSTUV
X|XYZABCDEFGHIJKLMNOPQRSTUVW
Y|YZABCDEFGHIJKLMNOPQRSTUVWX
Z|ZABCDEFGHIJKLMNOPQRSTUVWXY
Trithemius used the tabula recta to define a polyalphabetic cipher which was equivalent to Leon Battista Alberti's cipher disk. The tabula recta is often referred to in discussing pre-computer ciphers, including the Vigenère Cipher and Vigenère's less well-known (but much stronger) autokey cipher. All polyalphabetic ciphers based on Caesar ciphers can be described in terms of the tabula recta.
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