Crypto

A TCP service hands you the AES-CBC ciphertext of your own input and asks you to return a ciphertext that decrypts to contain admin credentials — solved with a single-byte XOR flip.

A homemade RSA-CRT signer leaks a correct and a faulted signature of the same message; gcd(s − spz, n) factors n (the Bellcore attack), while the trivial shortcut just recovers the message via s^e mod n — and the XOR ‘protection’ undoes itself.

The title ‘All’s Fair’ points at the Playfair cipher; the grid image is the standard 5×5 key square with even columns blanked, and the ciphertext decrypts to ANYTHINGFORTHEFLAG.

An RSA modulus that looks 2054-bit-strong is actually 67 × q — a tiny prime factor makes it trivially factorable (FactorDB / trial division), reconstructing the private key and decrypting the flag.

A 192-bit Huffman stream and a frequency table; the textbook min-heap decode looks perfect (wrapped in dalctf{}, zero leftover bits) yet is wrong — matching the encoder’s tie-break policy recovers the real flag.

A homemade LCG-as-cipher multiplies each flag byte by a deterministic state independent of the plaintext — the known DalCTF{ prefix recovers one state, and replaying the generator divides out the rest. The lost seed is a red herring.

A C program squares each flag byte as a float, then a ‘forgotten’ Quake-style (long)&y type-pun prints the raw IEEE-754 bits as integers; reversing bits→float→sqrt→char recovers the flag.

A commit-reveal rock-paper-scissors whose non-binding sha256(r1 + message + r2) commitment lets you open one hash to any winning move and sweep all 100 rounds.

A P-521 ECDSA oracle derives its nonce from an MD5-based uuid3, so an MD5 collision forces nonce reuse, leaks the private key, and lets us forge a signature for the flag.