Playfair encrypt error encoding "BAAAH

[mjump-northeastern]

We discovered that the encipher method of Playfair does not encrypt BAAAH correctly when using the key "KINGSABCDEFHLMOPQRTUVWXYZ". It returns "CBCVLW" when it should be "CBVCLW". However if I write the grid out:

KINGS
ABCDE
FHLMO
PQRTU
VWXYZ

BAAAH -> BAXAH -> BAXAHX -> CBVCLW

[omar-ashinawy]

When using two online cipher calculators, the result was: CBCVBF. The two websites are: https://www.dcode.fr/playfair-cipher and https://planetcalc.com/7751/

[eragon-saphira]

This error is coming here because it is being padded with X

[omar-ashinawy]

I implemented it myself using X padding and it gave me: CBCVBF.

[eragon-saphira]

The plaintext BAAAH is padded with X. So we split as BA AA HX. Now BA are in same row so we rightshift as CB,next AA same characters so change to AX next A=(1,0) X= (4,2) so we take (1,2) and (4,0) which is CV again for HX H=(2,1) X=(4,2) so we take (2,2) and (4,1) which is LV.Correct me if I have made a mistake here and if so please tell me the correct steps

[omar-ashinawy]

I think the mistake is when you padded the end of the string first. When padding a playfair plaintext, you follow the following steps in order:

1. Divide the string into pairs: in our case 'BAAAH' becomes: 'BA', 'AA', and 'H'. Don't worry for the single letter 'H' at the end for now.
2. Check every pair to determine if the pair contains one repeated letter: 'BA' is OK, 'AA' has repeated A, 'H' is OK.
3. Put 'X' between the two repeated letters in every pair: 'BAAAH' becomes: 'BA', 'AX', 'AH'
4. Here comes the step that you did first: Check if every pair contains exactly two letters. After filling 'X', the string is already good to go so we don't need to pad the end of the string.
   Then the final plaintext: 'BA', 'AX', 'AH'.
   You can learn more from STALLINGS: CRYPTOGRAPHY AND NETWORK SECURITY PRINCIPLES AND PRACTICE
   

[eragon-saphira]

Got it.I had misunderstood that the repeated one's second character is replaced by X.Thanks for the help and sorry for the trouble

[omar-ashinawy]

You are welcome.