ascii

cryptography/ascii/DESCRIPTION.md

@@ -0,0 +1,63 @@
+Computers might think in binary, but on your terminal, you see letters!
+How does this happen?
+Through _encodings_.
+
+An encoding is a mapping from some sequence of binary digits (bits) to a letter.
+The mapping itself is just something made up by some people somewhere, and there have been many such mappings throughout history.
+For example, the mapping that powers the modern internet, including the all-important emojis that you send to your friends and earn by completing pwn.college dojos, is [UTF-8](https://en.wikipedia.org/wiki/UTF-8).
+UTF-8 describes how one or more bytes (each byte is 8 bits) corresponds to a character.
+Luckily for our purposes as an English-language educational resource, the encoding of the Latin alphabet in UTF-8 is quite simple, and harkens back to another encoding standard called [_ASCII_](https://en.wikipedia.org/wiki/ASCII).
+
+ASCII is, by computing standards, an ancient encoding, dating back to 1963.
+Modern ASCII is pretty simple: every character is one byte (8 bits), uppercase letters are `0x40+letter_index` (e.g., A is `0x41`, F is `0x46`, and Z is `0x5a`), lowercase letters are `0x60+letter_index` (A is `0x61`, F is `0x66`, and Z is `0x5a`), and numbers (yes, the numeric characters you're seeing are _not_ bytes of those values, they are ASCII-encoded number characters) are `0x30+number`, so 0 is `0x30` and 7 is `0x37`.
+Special characters are sprinkled around the mapping as well: forward slash (`/` is `0x2f`), space is 0x20, and newline is 0x0a.
+You can see the whole ASCII table with `man ascii`!
+
+One cool thing is that, since ASCII (and UTF-8) puts byte values to characters, we can do operations like XOR!
+This has obvious implications for cryptography.
+
+In this level, we'll explore these implications byte by byte.
+The challenge will give you one letter a time, along with a key to "decrypt" (XOR) the letter with.
+You give us the result of the XOR.
+For example:
+
+```console
+hacker@dojo:~$ /challenge/run
+Challenge number 0...
+- Encrypted Character: A
+- XOR Key: 0x01
+- Decrypted Character?
+```
+
+How would you approach this?
+You can `man ascii` and find the entry for A:
+
+```none
+Oct   Dec   Hex   Char
+──────────────────────
+101   65    41    A
+```
+
+So A is `0x41` in hex.
+You would XOR that with `0x01`
+The result here would be: `0x41 ^ 0x01 == 0x40`, and, according to `man ascii`:
+
+```none
+Oct   Dec   Hex   Char
+──────────────────────
+100   64    40    @
+```
+
+It's the @ character!
+
+```console
+hacker@dojo:~$ /challenge/run
+Challenge number 0...
+- Encrypted Character: A
+- XOR Key: 0x01
+- Decrypted Character? @
+Correct! Moving on.
+```
+
+Now it's your turn!
+Can you XOR things up and get the flag?

cryptography/ascii/run

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+#!/opt/pwn.college/python
+
+import random
+import string
+import sys
+
+if not sys.stdin.isatty():
+	print("You must interact with me directly. No scripting this!")
+	sys.exit(1)
+
+for n in range(1, 10):
+	print(f"Challenge number {n}...")
+	pt_chr, ct_chr = random.sample(
+		string.digits + string.ascii_letters + string.punctuation,
+		2
+	)
+	key = ord(pt_chr) ^ ord(ct_chr)
+
+	print(f"- Encrypted Character: {ct_chr}")
+	print(f"- XOR Key: 0x{'%02x'%key}")
+	answer = input("- Decrypted Character? ").strip()
+	if answer != pt_chr:
+		print("Incorrect!")
+		sys.exit(1)
+
+	print("Correct! Moving on.")
+
+print("You have mastered XORing ASCII! Your flag:")
+print(open("/flag").read())

cryptography/module.yml

@@ -3,6 +3,8 @@ description: Solve various cryptography challenges ranging from decoding base64
 challenges:
 - id: xor
   name: XOR
+- id: ascii
+  name: ASCII
 - id: level-1
   name: Base64
 - id: level-2