CTF TOPIC · 6 FLAGS · 25 MIN

Ciphers, hashes and keys

Vigenere, known-plaintext attacks, hashes, Morse, tiny RSA, encoding vs encryption.

A step up from classical ciphers. Vigenere repeats a key of several Caesar shifts; if you know part of the plaintext you can recover the key (a crib). Hashes (MD5, SHA-256) are one-way, so you crack them by hashing guesses and comparing. Morse encodes letters as dots and dashes. Textbook RSA with tiny numbers is breakable by factoring n. And remember: encoding (base64, hex) is not encryption, because it has no key.

A first example

Vigenere key 'duck' ; md5('abc') compared to a leak ; RSA c = m^e mod n.

First moves

1. Is it actually encrypted, or just encoded?
2. Hashes: hash guesses and compare, never 'decrypt'
3. Vigenere: use the crib flag to recover key letters
4. RSA: check whether n is small enough to factor

The challenges

• Vigenère with a known key · Medium · 30 XP
• Find the key yourself · Hard · 40 XP
• Hashes are one-way · Medium · 30 XP
• Dots and dashes · Easy · 20 XP
• RSA with tiny numbers · Hard · 40 XP
• Encoding is not encryption · Easy · 20 XP