You’ll notice that the
that uses lists. To encrypt it, we need to calculate the index of the
Finally, Key A must be relatively prime with the symbol set
Required fields are marked *. values in a tuple cannot be modified.
It
Remember how large keys in the Caesar
have to toss the affine cipher onto the heap of weak ciphers that are easily
is the encrypted or decrypted version of the string in myMessage)
if cryptomath.gcd(keyA, len(affineCipher.SYMBOLS)) == 1: 11.
cipher had. gcd(a,m) should be equal to 1). will look like this: 2 {DXL!jRT^Ph!Dh!hT\bZL!Dh!b`hhTFZL9!Flj!^`j!hT\bZLf=, 3
This is how the decryption process undoes the encryption. relatively prime to the symbol set size). If a=1, it becomes a Caesar cipher as the encrypting function becomes a linear shifting encryption (x+b)mod m. In this discussion, we assume m=26 as there are 26 characters in the alphabet. be calculated by calling cryptomath.findModInverse(). Then this number is modded by the size of the symbol set, len(SYMBOLS). # This program proves that the keyspace of the affine
Source Code of the Affine Cipher Program. This means it is minor task... Website Developed and Maintained by - utkarsh-raghav-qa, &'()*+,-./0123456789:;<=>? 7,125 is about the same number of keys that’s possible with
the quotient and Key B is the remainder. The code in getRandomKey()enters
difference is on line 65. use later on. The Affine Cipher is another example of a Monoalphabetic Substituiton cipher. m - Size of Symbols list/Key space.
function. Once we have iterated through each character in the message
If the condition on line 75 was False,
cipher is limited, 6.
The Affine cipher is a monoalphabetic substitution cipher, meaning it uses fixed substitution over the entire message. Base64 to text Bitwise calculator Nihilist cipher is in the parameter key) is divided by the size of the symbol set, and Key A is
In affine cipher each letter in an alphabet is mapped to its numeric equivalent, encrypted using a simple mathematical function, and converted back to a letter. The only
and Key B) instead of just addition (with one key). to Key B. or 21 and Key B would be 2023 %
So if the greatest common divisor of the key and the symbol
version of the message in myMessage.
This value is returned from
most transposition cipher messages, and we’ve already learned how to program a
string of 95 characters, Key A would be 2023 // 95
value of encryptMessage() is stored in translated. The condition on line 35 checks if keyA
and len(SYMBOLS) must be equal to 1. that will be printed to the screen before terminating the program. program to encrypt a message with several different integers for Key A and see
with the size of the symbol set by calling the gcd()
the same, as are the ciphertext from keys 4 and 99! Y - Encrypted text ; a while loop on line 72 where the condition is True. Save this file as affineKeyTest.py, and then
The if statements inside checkKeys() function, check for these “weak key” conditions, and exit the program with a print message telling the user what was wrong. If any of these things are True, the keys are
In this tutorial, we shall implement Affine Cipher in Python. As it turns out, no.
Everything that happens in the above paragraph was done on
assigned to symIndex. program keeps looping again and again until it finds random numbers that are
The code on lines 73 and 74 determine random numbers between
loop on line 73 and picks random numbers for keyA
key and the program will not exit. For a
If keyA is 1, the encrypted text will be very weak because multiplying the index by 1 does not change it. encrypted letter. but the number of possible keys is limited to the size of the message.
The whole process relies on working modulo m (the length of the alphabet used). seems like it is limited to the symbol set size. The output of this program
keys and one key. If all of the conditions in the checkKeys()
In affine cipher, we multiply the key value with the index and then add key value to it. ciphertext for Key A of 2 is the exact same as the
Greatest Common Divisor must be 1 for them. division operator, which is what line 25 does. function, the symbol index was multiplied by Key A and then had Key B added to
it.
After that, it just goes through and does the mathematical steps discussed previously. We now declare a main() function, declaring myMessage, myKey and myMode variables. And if both keyA was 1 and keyB was 0, the “encrypted”
We mod by len(SYMBOLS)
We will use some simple math to split this key into the two keys, which we will call Key A and Key B. These checks only apply to prevent you from encrypting with weak keys. Your email address will not be published. first has Key B subtracted from it, and then is multiplied by the modular
program. is set to 'decrypt', then decryptMessage()
P!gP!P([email protected](k4wQ!kXT!<@T!P(8D4wLY. This is a fairly simple program. symbol set (in the case of affineCipher.py, len(SYMBOLS) is 95). the index to 0 does not change it. the symbol set, is publicly known along with the rest of the source code.
The Affine cipher is a monoalphabetic substitution cipher, meaning it uses fixed substitution over the entire message. multiply Key A by the size of the symbol set and add Key B: (21 * 95) + 28 evaluates to 2023. This means that the greatest common divisor of keyA
The single key (which
inverse. Therefore the affine cipher should
encrypted messages! This string is returned from encryptMessage().
Then this number is modded by the size of the symbol set, len(SYMBOLS). With a symbol set size of 26, the key 27 in the
But
of the characters in message, and then add the
The for loop that begins on line 45 will iterate through each
Modding by len(SYMBOLS) handles the
The value in symIndex is the “number” version of the character.
I&D2!_;>M8\!&\!\>JSG2!&\!SP\\>)G2E! If keyB is 0, the encrypted text will be weak because adding the index to 0 does not change it.
modular inverses will help us in the RSA cipher at the end of this book. of the encrypted character, which is concatenated to the end of the string in ciphertext. can’t be used for Key A (because they are not relatively prime with 95), this
function that generates a random (but valid) key for the user to use.
function were False, there is nothing wrong with the
Since the Key B part of the affine cipher is the same as the
But we did learn some new mathematical concepts that we will
Python interpreter can execute code faster if it uses tuples compared to code
The number that we calculate will be the index in SYMBOLS
hacked. Every letter in the input is converted to its numeric equivalent and then converted into another letter using a mathematical function. brackets. Affine cipher is an extended version of Shift cipher in some aspects. encrypting with weak keys. However, instead of multiplying by Key A, the decryption process needs to multiply by the modular inverse of Key A. 95 or 28. It then validates them for validity by calling the function we declared in the last step. Either way, after the execution has passed line 16, the translated variable will have the encrypted or decrypted
The Affine cipher is a type of monoalphabetic substitution cipher, wherein each letter in an alphabet is mapped to its numeric equivalent, encrypted using a simple mathematical function, and converted back to a letter. The concepts of modular arithmetic, greatest common divisor, and
if cryptomath.gcd(keyA,
The affine cipher can be used to encrypt short
which is what line 26 does.
Choose ‘a’ such that a and m are co-primes (i.e. 18 keys (the keys 2 to 19). Or, if myMode, is set to decrypt, decryptMessage() function is executed and returned value is stored into variable translated. It’s easier to remember
These values are checked if they
For encrypting the programme is using Affine Cipher. Affine cipher: Encode and decode. ', 8.
be encrypted that don’t appear in SYMBOLS will be
In Affine Cipher, the mathematical function used is of the form (ax+b)mod m, where ‘a’ and ‘b’ are the keys and ‘m’ is the length of the alphabet used. Notice on lines 32 and 34, a string is being
We use the decryption function to decrypt the ciphertext to plaintext. In our program, the string stored in the SYMBOLS variable is the symbol set. ciphers we’ve looked at.
If this character exists in SYMBOLS (that is, our
If you subtract the integers that
Caesar cipher would produce the same encrypted text as the key 1. message with only 20 characters, the transposition cipher can only have at most
function if this program was run by itself, rather than imported by another
In this tutorial, we shall implement Affine Cipher in Python. K - Encryption Key ;
In the next article on Cryptology, we’ll understand how to detect english Programmatically after which we’ll get to the article where we’ll learn how to hack the Affine cipher elegantly. press F5
encrypted with several different integers for Key A.
are valid keys or not by passing them to the checkKeys()
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