POSTIT

Service for BambiCTF #6

Idea

RSA signatures, checked with strcmp in C

• terminal-based "Post-Its"
• register with name and public key
• get all user names
• to add a note, sign the challenge with your private key
• request posts for a user:
  • get the public key and a token to sign
  • if signature correct, get all messages from that user

Checker only uses keys with e = 3, then can forge signatures for short messages (but not 512 bits).

signature = ceil((m + \x00 * foo) ^ (1/e)) When checking: signature^e = m + \x00 + ..., and strcmp accepts

Exploits

Check out the exploit implementation in checker/src/checker.py.