added double whitespace behaviour notion

README.md

@@ -163,10 +163,14 @@ After all plaintext characters are processed, add PL random characters in front
 of the ciphertext, where PL = KL mod 10 + 1, where KL is the initial key phrase
 length. None of the prepended characters must be a whitespace.
 
+As the final step, replace all occurrences of double whitespace with `__` in
+order to make sure the whitespace is transmitted correctly.
+
 Decryption
 ----------
 Remove first PL characters from the ciphertext, where PL = KL mod 10 + 1, where
-KL is the initial key phrase length.
+KL is the initial key phrase length. Also, replace all `__` occurrences with
+double whitespaces.
 
 Then, for each ciphertext digraph C1C2, do the following:
 

ip36.py

@@ -91,7 +91,7 @@ def ipfencrypt(msg:str, key:str):
     prefix = ''
     for i in range(preflen):
         prefix += secrets.choice(pfgrid[1:])
-    return prefix + ''.join(enc)
+    return prefix + ''.join(enc).replace('  ', '__')
 
 # Interleaved Playfair decryption method
 def ipfdecrypt(enc:str, key:str):
@@ -99,7 +99,7 @@ def ipfdecrypt(enc:str, key:str):
     # remove the prefix, then add a space if the cryptogram
     # had been stripped and ended with a space
     preflen = (len(key) % 10) + 1
-    enc = enc[preflen:].rstrip()
+    enc = enc[preflen:].rstrip().replace('__', '  ')
     if len(enc) & 1 == 1:
         enc += ' '
     # split into digraphs