caesar

Tasks.md

@@ -282,8 +282,51 @@ Use [regex101.com](https://regex101.com) if you are not already a REGEX expert.
 
 There should be $374$ matches.
 
-A regex that fullfills the requirements is `\b[a-z][AEIOUaeiou]([a-w]|[A-W])`.
+A regex that matches the requirements is `\b[a-z][AEIOUaeiou]([a-w]|[A-W])`.
 
 [Code Example](src/tasks/regex.py)
 
 </details>
+
+## Ancient Cryptography
+
+Difficulty: 2/5
+
+<details>
+<summary>Text</summary>
+
+```text
+Neovim is a refactor, and sometimes redactor, in the tradition of Vim (which itself derives from 
+Stevie). It is not a rewrite but a continuation and extension of Vim. Many clones and derivatives 
+exist, some very clever—but none are Vim. Neovim is built for users who want the good parts of 
+Vim, and more. 
+````
+
+</details>
+
+1. The text above has been cyphered with the Caesar cipher, a timeless classic
+   algorithm to abstract the meaning away from a text and arguably an early
+   form of encryption. Your task is to decipher it back into readable text.
+
+### The caesar cipher
+
+For the caesar cipher, all letters are shifted by the value of the key.
+
+**Examples**
+
+"foo Bar" becomes "gpp Cbs" when shifted by $1$.
+
+
+
+<details>
+<summary>Hints</summary>
+
+
+</details>
+<details>
+<summary>Solution</summary>
+
+
+[Code Example](src/caesar.py)
+
+</details>

src/caesar.py

@@ -0,0 +1,46 @@
+#!/usr/bin/env python3
+import sys
+
+def internal(text: str, key: int) -> str:
+    cyphertext: str = ""
+    for c in text:
+        if ord(c) >= 65 and ord(c) <= 90: # uppercase letters
+            ci = ord(c) - ord('A')
+            add = ord('A')
+            #print(f"{c} is uppercase")
+        elif ord(c) >= 97 and ord(c) <= 122: # uppercase letters
+            ci = ord(c) - ord('a')
+            add = ord('a')
+            #print(f"{c} is lowercase")
+        else:
+            #print(f"not a letter: {c} ({ord(c)})")
+            # character is not a letter, just skip it
+            cyphertext += c
+            continue
+        ci += key
+        ci %= 26    # only 23 letters in the alphabet
+        #print(f"ci for {c}: {ci}")
+        cyphertext += chr(ci + add)
+
+    return cyphertext
+
+
+if len(sys.argv) <= 2 or len(sys.argv) >= 5:
+    print("Takes two arguments: <SOURCE> <KEY> [-e]")
+    exit(1)
+
+source: str = sys.argv[1]
+target: str = ""
+key = int(sys.argv[2])
+
+if len(sys.argv) >= 4 and sys.argv[3] == "-e":
+    print("encrypting...")
+    target = internal(source, -key)
+
+
+else:
+    print("decrypting...")
+    target = internal(source, key)
+
+print("=" * 80)
+print("%s" % target)