Cryptography Algorithm

Simple Substitution Cipher.py

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+import sys, random
+
+LETTERS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
+
+def main():
+    message = input('Enter message: ')
+    key = 'LFWOAYUISVKMNXPBDCRJTQEGHZ'
+    resp = input('Encrypt/Decrypt [e/d]: ')
+
+    checkValidKey(key)
+
+    if resp.lower().startswith('e'):
+        mode = 'encrypt'
+        translated = encryptMessage(key, message)
+    elif resp.lower().startswith('d'):
+        mode = 'decrypt'
+        translated = decryptMessage(key, message)
+
+    print('\n%sion: \n%s' % (mode.title(), translated))
+    
+def checkValidKey(key):
+    keyList = list(key)
+    lettersList = list(LETTERS)
+    keyList.sort()
+    lettersList.sort()
+
+    if keyList != lettersList:
+        sys.exit('Error in the key or symbol set.')
+
+def encryptMessage(key, message):
+    return translateMessage(key, message, 'encrypt')
+
+def decryptMessage(key, message):
+    return translateMessage(key, message, 'decrypt')
+
+def translateMessage(key, message, mode):
+    translated = ''
+    charsA = LETTERS
+    charsB = key
+
+    if mode == 'decrypt':
+        charsA, charsB = charsB, charsA
+        
+    for symbol in message:
+        if symbol.upper() in charsA:
+            symIndex = charsA.find(symbol.upper())
+            if symbol.isupper():
+                translated += charsB[symIndex].upper()
+            else:
+                translated += charsB[symIndex].lower()
+        else:
+            translated += symbol
+
+    return translated
+
+def getRandomKey():
+    key = list(LETTERS)
+    random.shuffle(key)
+    return ''.join(key)
+
+if __name__ == '__main__':
+    main()