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Caesar cipher

Caesar cipher, is one of the simplest and most widely known encryption techniques. The transformation can be represented by aligning two alphabets, the cipher alphabet is the plain alphabet rotated left or right by some number of positions.

When encrypting, a person looks up each letter of the message in the 'plain' line and writes down the corresponding letter in the 'cipher' line. Deciphering is done in reverse.
The encryption can also be represented using modular arithmetic by first transforming the letters into numbers, according to the scheme, A = 0, B = 1,..., Z = 25. Encryption of a letter x by a shift n can be described mathematically as

Plaintext: welsh
cipher variations:
xfmti ygnuj zhovk aipwl bjqxm
ckryn dlszo emtap fnubq govcr
hpwds iqxet jryfu kszgv ltahw
mubix nvcjy owdkz pxela qyfmb
rzgnc sahod tbipe ucjqf vdkrg

Decryption is performed similarly,

(There are different definitions for the modulo operation. In the above, the result is in the range 0...25. I.e., if x+n or x-n are not in the range 0...25, we have to subtract or add 26.)
Read more ...
Atbash Cipher

Atbash is an ancient encryption system created in the Middle East. It was originally used in the Hebrew language.
The Atbash cipher is a simple substitution cipher that relies on transposing all the letters in the alphabet such that the resulting alphabet is backwards.
The first letter is replaced with the last letter, the second with the second-last, and so on.
An example plaintext to ciphertext using Atbash:
Plain: welsh
Cipher: dvohs

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Baconian Cipher

To encode a message, each letter of the plaintext is replaced by a group of five of the letters 'A' or 'B'. This replacement is done according to the alphabet of the Baconian cipher, shown below.
a   AAAAA   g    AABBA     m    ABABB   s    BAAAB     y    BABBA
b   AAAAB   h    AABBB     n    ABBAA   t    BAABA     z    BABBB
c   AAABA   i    ABAAA     o    ABBAB   u    BAABB 
d   AAABB   j    BBBAA     p    ABBBA   v    BBBAB
e   AABAA   k    ABAAB     q    ABBBB   w    BABAA
f   AABAB   l    ABABA     r    BAAAA   x    BABAB

Plain: welsh
Cipher: BABAA AABAA ABABA BAAAB AABBB

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Affine Cipher
In the affine cipher the letters of an alphabet of size m are first mapped to the integers in the range 0..m - 1. It then uses modular arithmetic to transform the integer that each plaintext letter corresponds to into another integer that correspond to a ciphertext letter. The encryption function for a single letter is

where modulus m is the size of the alphabet and a and b are the key of the cipher. The value a must be chosen such that a and m are coprime.
Considering the specific case of encrypting messages in English (i.e. m = 26), there are a total of 286 non-trivial affine ciphers, not counting the 26 trivial Caesar ciphers. This number comes from the fact there are 12 numbers that are coprime with 26 that are less than 26 (these are the possible values of a). Each value of a can have 26 different addition shifts (the b value) ; therefore, there are 12*26 or 312 possible keys.
Plaintext: welsh
cipher variations:
xfmtipnidwhvenkzdaxyrlwhmjtsratjklclrgvq
dzcfevhypsnpuzgfxqjuygnujqojexiwfolaebyz
smxinkutsbuklmdmshwreadgfwizqtoqvahgyrkv
zhovkrpkfyjxgpmbfczatnyjolvutcvlmnentixs
fbehgxjaruprwbihzslwaipwlsqlgzkyhqncgdab
uozkpmwvudwmnofoujytgcfihykbsvqsxcjiatmx
bjqxmtrmhalzirodhebcvpalqnxwvexnopgpvkzu
hdgjizlctwrtydkjbunyckrynusnibmajspeifcd
wqbmroyxwfyopqhqwlaviehkjamduxsuzelkcvoz
dlszovtojcnbktqfjgdexrcnspzyxgzpqrirxmbw
jfilkbnevytvafmldwpaemtapwupkdoclurgkhef
ysdotqazyhaqrsjsyncxkgjmlcofwzuwbgnmexqb
fnubqxvqlepdmvshlifgztepurbazibrstktzody
lhknmdpgxavxchonfyrcgovcrywrmfqenwtimjgh
aufqvscbajcstuluapezmiloneqhybwydipogzsd
hpwdszxsngrfoxujnkhibvgrwtdcbkdtuvmvbqfa
njmpofrizcxzejqphateiqxetaytohsgpyvkolij
cwhsxuedcleuvwnwcrgboknqpgsjadyafkrqibuf
jryfubzupithqzwlpmjkdxityvfedmfvwxoxdshc
plorqhtkbezbglsrjcvgkszgvcavqjuiraxmqnkl
eyjuzwgfengwxypyetidqmpsriulcfachmtskdwh
ltahwdbwrkvjsbynrolmfzkvaxhgfohxyzqzfuje
rnqtsjvmdgbdinutleximubixecxslwktczospmn
galwbyihgpiyzaragvkfsorutkwnehcejovumfyj
nvcjyfdytmxludaptqnohbmxczjihqjzabsbhwlg
tpsvulxofidfkpwvngzkowdkzgezunymvebqurop
icnydakjirkabctcixmhuqtwvmypgjeglqxwohal
pxelahfavoznwfcrvspqjdozeblkjslbcdudjyni
vruxwnzqhkfhmryxpibmqyfmbigbwpaoxgdswtqr
kepafcmlktmcdevekzojwsvyxoarilginszyqjcn
rzgncjhcxqbpyhetxurslfqbgdnmlundefwflapk
xtwzypbsjmhjotazrkdosahodkidyrcqzifuyvst
mgrcheonmvoefgxgmbqlyuxazqctknikpubaslep
tbipeljezsdrajgvzwtunhsdifponwpfghyhncrm
zvybardulojlqvcbtmfqucjqfmkfatesbkhwaxuv
oitejgqpoxqghiziodsnawzcbsevmpkmrwdcungr
vdkrgnlgbuftclixbyvwpjufkhrqpyrhijajpeto
bxadctfwnqlnsxedvohswelshomhcvgudmjyczwx
qkvglisrqzsijkbkqfupcybedugxormotyfewpit

The decryption function is

where a - 1 is the modular multiplicative inverse of a modulo m. I.e., it satisfies the equation

The multiplicative inverse of a only exists if a and m are coprime. Hence without the restriction on a decryption might not be possible. It can be shown as follows that decryption function is the inverse of the encryption function,

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ROT13 Cipher
Applying ROT13 to a piece of text merely requires examining its alphabetic characters and replacing each one by the letter 13 places further along in the alphabet, wrapping back to the beginning if necessary. A becomes N, B becomes O, and so on up to M, which becomes Z, then the sequence continues at the beginning of the alphabet: N becomes A, O becomes B, and so on to Z, which becomes M. Only those letters which occur in the English alphabet are affected; numbers, symbols, whitespace, and all other characters are left unchanged. Because there are 26 letters in the English alphabet and 26 = 2 * 13, the ROT13 function is its own inverse:

ROT13(ROT13(x)) = x for any basic Latin-alphabet text x


An example plaintext to ciphertext using ROT13:

Plain: welsh
Cipher: jryfu

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Polybius Square

A Polybius Square is a table that allows someone to translate letters into numbers. To give a small level of encryption, this table can be randomized and shared with the recipient. In order to fit the 26 letters of the alphabet into the 25 spots created by the table, the letters i and j are usually combined.
1 2 3 4 5
1 A B C D E
2 F G H I/J K
3 L M N O P
4 Q R S T U
5 V W X Y Z

Basic Form:
Plain: welsh
Cipher: 2551133432

Extended Methods:
Method #1

Plaintext: welsh
method variations:
bkqxngpvcsmuahxrzfnc

Method #2
Bifid cipher
The message is converted to its coordinates in the usual manner, but they are written vertically beneath:
w e l s h 
2 5 1 3 3 
5 1 3 4 2 
They are then read out in rows:
2513351342
Then divided up into pairs again, and the pairs turned back into letters using the square:
Plain: welsh
Cipher: wlxli

Read more ...
Method #3

Plaintext: welsh
method variations:
zanog anogz nogza
ogzan gzano

Read more ...[RUS] , [EN]

 

Permutation Cipher
In classical cryptography, a permutation cipher is a transposition cipher in which the key is a permutation. To apply a cipher, a random permutation of size E is generated (the larger the value of E the more secure the cipher). The plaintext is then broken into segments of size E and the letters within that segment are permuted according to this key.
In theory, any transposition cipher can be viewed as a permutation cipher where E is equal to the length of the plaintext; this is too cumbersome a generalisation to use in actual practice, however.
The idea behind a permutation cipher is to keep the plaintext characters unchanged, butalter their positions by rearrangement using a permutation
This cipher is defined as:
Let m be a positive integer, and K consist of all permutations of {1,...,m}
For a key (permutation) , define:
The encryption function
The decryption function
A small example, assuming m = 6, and the key is the permutation :

The first row is the value of i, and the second row is the corresponding value of (i)
The inverse permutation, is constructed by interchanging the two rows, andrearranging the columns so that the first row is in increasing order, Therefore, is:

Total variation formula:

e = 2,718281828 , n - plaintext length

Plaintext: welsh

all 120 cipher variations:
welsh welhs weslh weshl wehsl wehls wlesh wlehs wlseh wlshe wlhse
wlhes wsleh wslhe wselh wsehl wshel wshle whlse whles whsle whsel
whesl whels ewlsh ewlhs ewslh ewshl ewhsl ewhls elwsh elwhs elswh
elshw elhsw elhws eslwh eslhw eswlh eswhl eshwl eshlw ehlsw ehlws
ehslw ehswl ehwsl ehwls lewsh lewhs leswh leshw lehsw lehws lwesh
lwehs lwseh lwshe lwhse lwhes lsweh lswhe lsewh lsehw lshew lshwe
lhwse lhwes lhswe lhsew lhesw lhews selwh selhw sewlh sewhl sehwl
sehlw slewh slehw slweh slwhe slhwe slhew swleh swlhe swelh swehl
swhel swhle shlwe shlew shwle shwel shewl shelw helsw helws heslw
heswl hewsl hewls hlesw hlews hlsew hlswe hlwse hlwes hslew hslwe
hselw hsewl hswel hswle hwlse hwles hwsle hwsel hwesl hwels

Read more ...[1] , [2] , [3]

History of cryptography
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