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rsa

Challenge​

Imported from local notes.md.

Solution​

Original Notes​

rsa

Challenge Summary​

  • Given: rsa.txt with a base64 ciphertext and a public modulus.
  • Goal: Recover the flag for the crypto challenge rsa.
  • Constraints: Flag format is squ1rrel{...}.

Initial Recon / Triage​

  • The challenge description was rivest shamir adleman type shi, which strongly suggested RSA.
  • The provided public key value was only the modulus n, so the first job was to recover the prime factors.
  • The ciphertext length matched a 2048-bit RSA block.

Hypotheses & Approach​

  • Because the modulus factors were extremely close, Fermat factorization was the right first attack.
  • After factoring, use the standard public exponent e = 65537, derive d, and decrypt the ciphertext.
  • The decrypted payload looked odd enough to invite extra decoding, but it was already the literal flag body.

Execution Steps (Reproducible)​

Stage 1​

Commands:

cd /root/squ1rrel2026CTF/rsa
python3 artifacts/solve.py

Results:

  • Fermat factorization succeeded immediately.
  • Recovered primes p and q with q - p = 3320.
  • Derived the private exponent and decrypted the RSA block.

Stage 2​

Commands:

cd /root/squ1rrel2026CTF/rsa
python3 artifacts/solve.py | tail -n 8

Results:

  • Solver output included:
    • q - p = 3320
    • payload = aoei;jrgnapeicgiw,fgjpisugfr
  • Several post-processing attempts were tested, but the payload itself was the correct flag content.

Artifacts Produced​

  • artifacts/solve.py - RSA factor/decrypt helper.
  • artifacts/solve_output.txt - saved solver output.
  • artifacts/transform_sweep.txt - attempted follow-up transforms.

Flag​

squ1rrel{aoei;jrgnapeicgiw,fgjpisugfr}