PicoCTF_2017: Broadcast

Category: Cryptography Points: 120 Description:

You stumbled upon a group Message. Can you figure out what they were sending? The string sent is ascii encoded as a hex number (submit the ascii string as the flag)

Hint:

The same message, with a small exponent, is being encrypted with several different n values

Write-up

This challenge involves the RSA Hastad Broadcast attack. In a true TLDR fashion, this attack refers to how if a small exponent is used and when a significant number of ciphertexts and public keys have been intercepted, the ciphertext can then be easily cracked.

Sample pythonic script.

$ ./solve.py 
Flag: broadcast_with_small_e_is_killer_20472673112

Therefore, the flag is broadcast_with_small_e_is_killer_20472673112.