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Crypto Cipher

Challenge 04 Jul 2025 Pro Exclusive Solution Available

Start the machine, hack the system, and find the hidden flags to complete this challenge and earn points!

1
Flags
5
Points
47%
Success Rate
Start Your Challenge
~1-2 min setup
Dedicated server
Private instance
Industry standard
This solution is for Flags Mode

This walkthrough explains how to hack the lab and capture the flags. For help with Learning Mode questions, use the Request Hint button next to each question.

Challenge

Crypto Cipher - Complete Solution Walkthrough

Step 1: Initial Analysis and Reconnaissance

  1. Begin by carefully examining the ciphertext provided on the challenge page: Zzc2OTlvMnMtMzVvZi00czhxLXNjcXctOTB5Njd0N3MyMDJz
  2. Observe the character set: A-Z, a-z, 0-9, +, /, and = padding at the end
  3. This pattern is characteristic of Base64 encoding, which uses 64 characters to represent binary data
  4. The challenge mentions a classical cipher, indicating we need to decode this first to reveal the actual encrypted message
  5. Note that Base64 is not encryption - it's encoding that can be easily reversed

Step 2: Base64 Decoding Process

  1. Use an online tool like CyberChef (gchq.github.io/CyberChef) or command line utilities
  2. For command line decoding:
echo "Zzc2OTlvMnMtMzVvZi00czhxLXNjcXctOTB5Njd0N3MyMDJz" | base64 -d
  1. Alternative method using Python:
import base64
encoded = "Zzc2OTlvMnMtMzVvZi00czhxLXNjcXctOTB5Njd0N3MyMDJz"
decoded = base64.b64decode(encoded).decode('utf-8')
print(decoded)
  1. The decoded result reveals the actual ciphertext: g7699o2s-35of-4s8q-scqw-90y67t7s202s
  2. This is the message that has been encrypted using the classical cipher

Step 3: Cipher Analysis and Pattern Recognition

  1. Analyze the structure of the decoded ciphertext: g7699o2s-35of-4s8q-scqw-90y67t7s202s
  2. Notice the UUID-like structure with hyphens in the expected positions (8-4-4-4-12 characters)
  3. This structure suggests a polyalphabetic substitution cipher with a short key
  4. The presence of numbers and hyphens mixed with letters indicates the cipher only affects alphabetic characters
  5. Classical polyalphabetic ciphers include Vigenère, Beaufort, and others
  6. The most common and likely candidate is the Vigenère cipher due to its historical significance

Step 4: Understanding the Vigenère Cipher

  1. The Vigenère cipher is a polyalphabetic substitution cipher invented in the 16th century
  2. It uses a repeating keyword to shift letters in the plaintext
  3. Each letter in the key determines how much to shift the corresponding letter in the message
  4. For example, if the key is 'ABC' and the message is 'HELLO':
    • H + A = H (no shift)
    • E + B = F (shift by 1)
    • L + C = O (shift by 2)
    • L + A = L (no shift, key repeats)
    • O + B = P (shift by 1)
  5. To decrypt, we subtract the key from the ciphertext
  6. Important Note on Non-Alphabetic Characters: The Vigenère cipher only affects alphabetic characters (A-Z, a-z). Numbers, hyphens, spaces, and other special characters remain unchanged during both encryption and decryption. However, the key position advances for ALL characters, including non-alphabetic ones. This means that even though numbers and hyphens are not encrypted, they still consume key positions, which affects the decryption of subsequent letters. This is a critical implementation detail that can make the difference between a successful and failed decryption attempt.

Step 5: Cryptanalysis and Key Discovery

  1. Use frequency analysis to identify the cipher type and potential key length
  2. Try common keywords that might be used as keys: 'crypto', 'flag', 'secret', 'key', 'password', etc.
  3. Use online Vigenère cipher breakers like dcode.fr, cryptii.com, or rumkin.com
  4. Input the ciphertext and try different key lengths and common words
  5. Systematically test keys of different lengths, starting with short keys (3-6 characters)
  6. Look for meaningful patterns in the decrypted output

Step 6: Key Testing and Verification

  1. After trying various keys, test the key 'crypto':
  2. This key makes sense given the context (and the name!) of the challenge
  3. Verify this key by attempting to decrypt a small portion of the ciphertext manually
  4. Use the Vigenère decryption formula: Plaintext = (Ciphertext - Key) mod 26
  5. For letters, convert to numbers (A=0, B=1, ..., Z=25), perform subtraction, then convert back
  6. Test the first few characters to confirm the key works

Step 7: Complete Decryption Process

  1. Use the confirmed key 'crypto' to decrypt the entire ciphertext
  2. Apply the Vigenère decryption algorithm systematically:
Ciphertext: g7699o2s-35of-4s8q-scqw-90y67t7s202s
Key:       cryptocryptocryptocryptocryptocryptocrypto

For each position, subtract the key letter from the ciphertext letter (modulo 26)
  1. Testing Different Vigenère Implementations: Different Vigenère cipher implementations handle non-alphabetic characters differently. Some advance the key for all characters, while others only advance for alphabetic characters. We need to test both approaches to determine which implementation was used:
❌ INCORRECT APPROACH (Key only advances for letters):
Position 1: g (ciphertext) - c (key) = g - c = 6 - 2 = 4 → e
Position 2: 7 (number) → 7 (unchanged, key stays at 'c')
Position 3: 6 (number) → 6 (unchanged, key stays at 'c')
Position 4: 9 (number) → 9 (unchanged, key stays at 'c')
Position 5: 9 (number) → 9 (unchanged, key stays at 'c')
Position 6: o (ciphertext) - r (key) = o - r = 14 - 17 = -3 → 23 → x
Position 7: 2 (number) → 2 (unchanged, key stays at 'r')
Position 8: s (ciphertext) - y (key) = s - y = 18 - 24 = -6 → 20 → u
Position 9: - (hyphen) → - (unchanged, key stays at 'y')
Position 10: 3 (number) → 3 (unchanged, key stays at 'y')

Result: e7699x2u-35of-4s8q-scqw-90y67t7s202s
❌ This produces 'x' at position 6, which is not valid in a UUID!
  1. Correct Approach - Key Advances for ALL Characters: This implementation advances the key position for every character, which is what was used in this challenge:
✅ CORRECT APPROACH (Key advances for all characters):
Position 1: g (ciphertext) - c (key) = g - c = 6 - 2 = 4 → e
Position 2: 7 (number) → 7 (unchanged, but key advances to 'r')
Position 3: 6 (number) → 6 (unchanged, but key advances to 'y')
Position 4: 9 (number) → 9 (unchanged, but key advances to 'p')
Position 5: 9 (number) → 9 (unchanged, but key advances to 't')
Position 6: o (ciphertext) - o (key) = o - o = 14 - 14 = 0 → a
Position 7: 2 (number) → 2 (unchanged, but key advances to 'c')
Position 8: s (ciphertext) - r (key) = s - r = 18 - 17 = 1 → b
Position 9: - (hyphen) → - (unchanged, but key advances to 'y')
Position 10: 3 (number) → 3 (unchanged, but key advances to 'p')

Result: e7699a2b-35ad-4d8c-bebd-90a67f7b202e
✅ This produces 'a' at position 6, which is valid in a UUID!
  1. The decrypted result reveals the flag: e7699a2b-35ad-4d8c-bebd-90a67f7b202e
  2. This appears to be a valid UUID format

Step 8: Flag Verification and Validation

  1. Verify that the decrypted result is a valid UUID format (8-4-4-4-12 characters)
  2. Confirm that the flag follows the expected pattern: xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
  3. The flag e7699a2b-35ad-4d8c-bebd-90a67f7b202e matches this format exactly
  4. Validate that all characters are hexadecimal (0-9, a-f)
  5. Check that the hyphens are in the correct positions (after 8, 12, 16, and 20 characters)
  6. The total length should be 36 characters (32 hex chars + 4 hyphens)

Technical Details and Cryptanalysis

  • Vigenère Cipher: A polyalphabetic substitution cipher that uses a repeating keyword to shift letters. Each letter in the key determines the shift amount for the corresponding letter in the plaintext.
  • Base64 Encoding: Used to represent binary data as ASCII text using 64 characters (A-Z, a-z, 0-9, +, /) with = padding. This is encoding, not encryption.
  • Pattern Analysis: The repeating pattern in the ciphertext indicates a short key length. The length of the repeating segment can help determine the key length.
  • Frequency Analysis: Helps identify the type of cipher and potential key characteristics by analyzing letter frequency patterns.
  • Key Length Determination: The Kasiski examination can be used to find the key length by looking for repeated patterns in the ciphertext.

Learning Points and Security Implications

  • Classical Ciphers: Are vulnerable to modern cryptanalysis techniques due to their mathematical weaknesses and predictable patterns.
  • Pattern Recognition: Is crucial for identifying cipher types and key characteristics. Repeating patterns often indicate polyalphabetic ciphers with short keys.
  • Encoding vs Encryption: Base64 is encoding, not encryption - it can be easily reversed and provides no security. Never rely on encoding for confidentiality.
  • Key Length: Shorter keys in polyalphabetic ciphers create repeating patterns that make cryptanalysis easier. Longer keys provide better security but are harder to manage.
  • Historical Context: Understanding classical ciphers helps appreciate the evolution of cryptography and why modern algorithms are necessary.

Tools and Resources Used

  • Base64 Decoder: Online tools (CyberChef, base64decode.org) or command line utilities (base64 -d)
  • Vigenère Cipher Breaker: Online cryptanalysis tools (dcode.fr, cryptii.com, rumkin.com)
  • Pattern Analysis: Manual examination of ciphertext patterns and frequency analysis
  • Python Scripts: Custom scripts for automated decryption and analysis
  • Cryptanalysis Tools: Frequency analyzers and statistical analysis tools

Challenge Summary and Methodology

  1. Reconnaissance: Analyze the provided ciphertext and identify encoding methods
  2. Decoding: Remove the Base64 encoding layer to reveal the actual ciphertext
  3. Pattern Analysis: Identify repeating patterns and determine cipher characteristics
  4. Cipher Identification: Recognize the Vigenère cipher based on pattern analysis
  5. Cryptanalysis: Use frequency analysis and key testing to discover the key
  6. Decryption: Apply the Vigenère decryption algorithm with the discovered key
  7. Verification: Validate the decrypted result as a proper UUID format
  8. Submission: Submit the verified flag to complete the challenge