Rsactftool

RsaCtfTool

Multi-attack RSA solver for weak RSA scenarios. Best used as a triage engine: feed known key material, run constrained attacks first, escalate only when signal supports it.

Core workflow

1. Normalize input (.pem, OpenSSH pubkey, or raw n/e).
2. Inspect key parameters and weakness indicators.
3. Select attack families by signal (not brute-force all blindly).
4. Recover private key and/or decrypt ciphertext.
5. Verify output format and cryptographic consistency.

Quick commands

# Recover private key (auto attack selection)
RsaCtfTool --publickey key.pub --private

# Force a specific attack
RsaCtfTool --publickey key.pub --attack wiener --private

# Decrypt a ciphertext file once key is recoverable
RsaCtfTool --publickey key.pub --decryptfile ciphertext.bin

# Build pubkey from n/e (for problem specifications that give integers only)
RsaCtfTool --createpub -n <n> -e <e>

# Dump key parameters for triage
RsaCtfTool --dumpkey --ext --key key.pub

Attack selection heuristics

• Start with non-factorization attacks when evidence indicates weak d, broadcast, shared n, or partial leakage.
• Prioritize factorization when n structure suggests close primes, smoothness, reused factors, or vulnerable keygen.
• Use scenario-specific methods when custom prime construction or non-standard modulus generation is suspected.

Practical attack loop

# 1) Parse and inspect
RsaCtfTool --dumpkey --ext --key key.pub

# 2) Run fast specific checks first
RsaCtfTool --publickey key.pub --attack wiener --private
RsaCtfTool --publickey key.pub --attack hastads --private
RsaCtfTool --publickey key.pub --attack common_factors --private

# 3) Escalate to broader runs
RsaCtfTool --publickey key.pub --private

# 4) If key recovered, decrypt material
RsaCtfTool --publickey key.pub --decryptfile ct.bin

Output validation checklist

• Decrypted bytes decode cleanly (UTF-8/ASCII) or have expected binary structure.
• PKCS#1 padding assumptions are consistent with implementation.
• If multiple candidate plaintexts appear, test all against oracle verification or remote service.
• Never trust a single lucky decode without re-encryption consistency checks.

Boundaries

• Tool scope is RSA semiprime attacks on weak public keys (standard asymmetric key scenarios).
• Hash cracking (MD5/SHA/bcrypt) is intentionally out of scope here (covered by hashcat skills).

Resources

• references/factorization-attacks.md — RSA attacks based on factoring n (Fermat, Pollard variants, ECM, QS, etc.).
• references/non-factorization-attacks.md — RSA attacks without directly factoring n (Wiener, Hastad, Boneh-Durfee, partial leaks, lattice).
• references/challenge-specific-attacks.md — scenario-specific attacks for non-standard RSA key patterns and triage patterns.