babyrsa

Challenge author

Will Song (SIGPwny)

Write-up author

Vanilla (Batman's Kitchen)

Points

200

Category

crypto

Message intercepted. Break it.

babyrsa.zip

We are given the encryption (with no padding) of a short message with a 4096-bit RSA key with e = 5. Recall that RSA encrypts with c = m^e mod n. Because m is short and e is small, m^e < n. Because it never wraps around, we can take the fifth root as in normal arithmetic. This yields the plaintext m with the flag.