Weak RSA Key
DOWNLOADABLE RSA-encrypted message (n, e, c). The key generation is flawed: p and q are chosen very close. Factor n with Fermat factorization, compute d, decrypt the message. The backup decoy flag in the file is not the RSA solution.
Scenario
An intercepted RSA-encrypted message. The public key (n, e) and ciphertext (c) are given. In key generation the primes p and q are chosen very close; this is a classic RSA weakness. Fermat factorization, searching for an a near the square root of n, finds p and q in seconds. Then phi=(p-1)(q-1), d=e^-1 mod phi, m=c^d mod n yields the plaintext. Skill: real cryptanalysis (close-prime detection + factorization). The file has a 'backup' decoy flag but it is not the RSA solution; reporting it breaks soundness.
Anti-forensics techniques
- Weak RSA: close primes
- Breaking via Fermat factorization
- Decoy backup flag (red herring)
Provided artifacts
- RSA parameters and ciphertext (n, e, c)
Sample questions
- q1: The decrypted plaintext (flag)?
- q2: Which weakness/method was used? (e.g. fermat / close primes)
- trap1: Do NOT report the backup decoy flag (DFB{rsa_tuzak}) as the solution.
Soundness trap
Scoring
Cryptanalysis accuracy (factoring+decrypt) + decoy-flag resistance (soundness).
Download and solve
The answer key is hidden (scored set); the flag is encrypted, you must decrypt it.