OmniCTF 2026 Quals Crypto Writeup: 3 Challenges Solved
OmniCTF 2026 Quals shipped a crypto track built around one recurring lesson: the algebra is scarier than the actual attack surface. Three challenges, all rated medium, and every one dresses a mundane linear-algebra collapse in a costume the reader is expected to spend hours investigating. dual_linera uses two-modulus LWE where the error e is shared across both moduli. CRT is a linear map, so e survives the reconstruction and 18 samples plus LLL on a 20x20 lattice recover the 96-bit secret. Whiskerfield-Meowtin offers a “CuteSecure-DH” whose modulus is 65537^16 (Fermat prime to the 16th power) and whose public value is hand-crafted as one byte-flip away from a multiple of 65537. Patch the byte, drive the shared secret to zero, decrypt under the LCG stream cipher. Orbital-Strike-Cannon wraps a 9-unknown affine state in non-associative octonion multiplication, a published-stream “broken RNG”, and 7 satellites (5 real, 2 fake), but once you fix the association order it becomes 8x8 matrix multiplication, and once you write the samples as linear equations a per-satellite RREF sorts real from fake without needing the private real_ids. ...