RSA Angriffe

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Schnelle Triage

Sammeln:

  • n, e, c (und alle zusätzlichen Ciphertexte)
  • Jegliche Beziehungen zwischen Nachrichten (gleicher Plaintext? gemeinsamer Modulus? strukturierter Plaintext?)
  • Any leaks (partial p/q, bits of d, dp/dq, known padding)

Dann versuchen:

  • Faktorisierungsprüfung (Factordb / sage: factor(n) für relativ kleine n)
  • Muster bei kleinem Exponenten (e=3, broadcast)
  • Gemeinsamer Modulus / wiederholte Primfaktoren
  • Gittermethoden (Coppersmith/LLL), wenn etwas fast bekannt ist

Common RSA attacks

Common modulus

If two ciphertexts c1, c2 encrypt the same message under the same modulus n but with different exponents e1, e2 (and gcd(e1,e2)=1), you can recover m using the extended Euclidean algorithm:

m = c1^a * c2^b mod n where a*e1 + b*e2 = 1.

Example outline:

  1. Compute (a, b) = xgcd(e1, e2) so a*e1 + b*e2 = 1
  2. If a < 0, interpret c1^a as inv(c1)^{-a} mod n (same for b)
  3. Multiply and reduce modulo n

Shared primes across moduli

If you have multiple RSA moduli from the same challenge, check whether they share a prime:

  • gcd(n1, n2) != 1 implies a catastrophic key-generation failure.

This shows up frequently in CTFs as “we generated many keys quickly” or “bad randomness”.

Håstad broadcast / low exponent

If the same plaintext is sent to multiple recipients with small e (often e=3) and no proper padding, you can recover m via CRT and integer root.

Technical condition:

If you have e ciphertexts of the same message under pairwise-coprime moduli n_i:

  • Use CRT to recover M = m^e over the product N = Π n_i
  • If m^e < N, then M is the true integer power, and m = integer_root(M, e)

Wiener attack: small private exponent

If d is too small, continued fractions can recover it from e/n.

Textbook RSA pitfalls

If you see:

  • No OAEP/PSS, raw modular exponentiation
  • Deterministic encryption

then algebraic attacks and oracle abuse become much more likely.

Tools

  • RsaCtfTool: https://github.com/Ganapati/RsaCtfTool
  • SageMath (CRT, roots, CF): https://www.sagemath.org/

If you see two ciphertexts under the same modulus with messages that are algebraically related (e.g., m2 = a*m1 + b), look for “related-message” attacks such as Franklin–Reiter. These typically require:

  • same modulus n
  • same exponent e
  • known relationship between plaintexts

In practice this is often solved with Sage by setting up polynomials modulo n and computing a GCD.

Lattices / Coppersmith

Reach for this when you have partial bits, structured plaintext, or close relations that make the unknown small.

Lattice methods (LLL/Coppersmith) show up whenever you have partial information:

  • Partially known plaintext (structured message with unknown tail)
  • Partially known p/q (high bits leaked)
  • Small unknown differences between related values

What to recognize

Typical hints in challenges:

  • “We leaked the top/bottom bits of p”
  • “The flag is embedded like: m = bytes_to_long(b\"HTB{\" + unknown + b\"}\")”
  • “We used RSA but with a small random padding”

Tooling

In practice you’ll use Sage for LLL and a known template for the specific instance.

Good starting points:

  • Sage CTF crypto templates: https://github.com/defund/coppersmith
  • A survey-style reference: https://martinralbrecht.wordpress.com/2013/05/06/coppersmiths-method/

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