added simple commandline tool to perform caeser cipher en/decoding. I usually use it in CTFs

caeser_cipher/caeser.py

@@ -0,0 +1,75 @@
+from __future__ import print_function
+import os
+import string
+import argparse
+
+try:
+    maketrans = string.maketrans  # python2
+except AttributeError:
+    maketrans = str.maketrans  # python3
+
+
+def caeser_cipher(string_: str, offset: int, decode: bool, file_: string) -> None:
+    """ Caeser Cipher implementation, reads file or string.  Also decodes.
+
+    Default implementation is ROT13 encoding.
+
+    To decode, specify the same offset you used to encode and your ciphertext / file.
+
+    :param string_: string to encode / decode
+    :param offset:  # of chars to rotate by
+    :param decode:  decode instead of encode
+    :param file_:   file to read in then encode/decode
+    """
+    if file_ and os.path.exists(file_):
+        with open(file_, 'r') as f:
+            string_ = f.read()
+
+    if decode:
+        offset *= -1
+
+    lower_offset_alphabet = string.ascii_lowercase[offset:] + string.ascii_lowercase[:offset]
+    lower_translation_table = maketrans(string.ascii_lowercase, lower_offset_alphabet)
+
+    upper_offset_alphabet = string.ascii_uppercase[offset:] + string.ascii_uppercase[:offset]
+    upper_translation_table = maketrans(string.ascii_uppercase, upper_offset_alphabet)
+
+    lower_converted = string_.translate(lower_translation_table)
+    final_converted = lower_converted.translate(upper_translation_table)
+
+    if file_:
+        extension = 'dec' if decode else 'enc'
+        with open("{}.{}".format(file_, extension), 'w') as f:
+            print(final_converted, file=f)
+    else:
+        print(final_converted)
+
+
+def check_offset_range(value: int) -> int:
+    """ Validates that value is in the allowable range.
+
+    :param value:  integer to validate
+    :return:  valid integer
+    :raises: argparse.ArgumentTypeError
+    """
+    value = int(value)
+    if value < -25 or value > 25:
+        raise argparse.ArgumentTypeError("{} is an invalid offset".format(value))
+    return value
+
+
+if __name__ == '__main__':
+    parser = argparse.ArgumentParser(description="Simple Caeser Cipher [En,De]coder")
+
+    parser.add_argument('-d', '--decode', action='store_true', dest='decode',
+                        help='decode ciphertext (offset should equal what was used to encode)', default=False)
+    parser.add_argument('-o', '--offset', dest='offset', default=13, type=check_offset_range,
+                        help='number of characters to shift')
+
+    group = parser.add_mutually_exclusive_group(required=True)
+    group.add_argument('-f', '--file', dest='file', help='file to encode', default=None)
+    group.add_argument('-s', '--string', dest='string', help='string to encode', default=None)
+
+    args = parser.parse_args()
+
+    caeser_cipher(args.string, args.offset, args.decode, args.file)