Null or Never

cryptohack

Task: RSA with e=3, the flag is right-zero-padded to 100 bytes so m = FLAG_int * 256^57; the full message is ~2^342, just above n^(1/3) ~ 2^341, so a naive cube root fails. Solution: exploit the known flag prefix 'crypto{' and suffix '}' to write m = A + B*y with only a 35-byte unknown y (~2^280 < n^(1/3)); the monic cubic (A+B*y)^3 - c ≡ 0 mod n has a small root y, recovered by Howgrave-Graham/Coppersmith univariate small-roots via fpylll LLL.