cryptofreemedium

Protocol Analysis 6: Sneedham-Chucker

metactf

Task: a DawgCTF protocol analysis challenge implementing the classic Needham-Schroeder Public Key Protocol between Sneed (initiator) and Chuck (responder), with the flag sent as {FLAG}h(nA+nB) at the end. Solution: Lowe's man-in-the-middle attack — use our own attacker identity 'mallory' as Alice's X, relay nA to Bob as-if from sneed, bounce {nA,nB}pubA back to Alice so she encrypts nB to us under pubM, then re-encrypt {nB}pubB to trigger the flag send. The flag is symmetric-encrypted with ChaCha20-Poly1305 under key=SHA256(nA_hex+nB_hex) and nonce=key[:12].

$ ls tags/ techniques/
rsamitmcertificateprotocol_analysisdawgctfneedham_schroederlowe_attackpublic_key_protocolnonce_exchangechacha20_poly1305
lowe_mitmneedham_schroeder_public_key_attackre_encryption_relaynonce_reflectioncross_session_key_reuseaead_with_deterministic_nonce

Protocol Analysis 6: Sneedham-Chucker — metactf

Description

Chuck needs to send Sneed an urgent message regarding the name of their store, but he doesn't want any city slickers listening in. There may be a flag abound.

Source: https://github.com/UMBCCyberDawgs/dawgctf-sp26/blob/main/Protocol%20Analysis%20(1-9)/Protocol_Analysis_chals.pdf

Sixth entry in the DawgCTF Protocol Analysis series. The task description is a direct hint to "Sneed's Feed and Seed (Formerly Chuck's)" from The Simpsons episode Lemon of Troy — the "name of their store" is the key plot point of the running internet joke.

Protocol

From the PDF manual:

Protocol key:
    A = sneed
    B = chuck
    X = any name
   nX = nonce of entity X
 h(x + y) = hash of data x and y (+ means concatenation without pipes or colons)

Alice (sneed)                     Bob (chuck)
                                  send: pubB, B, certB
recv: pubX, X, certX
send: {nA, pubA, A, certA}pubX
                                  recv: {nA, pubA, A, certA}pubB
                                  send: {nA, nB}pubA
recv: {nA, nX}pubA
send: {nX}pubX
                                  recv: {nB}pubB
                                  send: {[FLAG]}h(nA+nB)
recv: {[FLAG]}h(nA+nX)

This is the textbook Needham-Schroeder Public Key Protocol with certificates tacked onto the identity announcements, plus a symmetrically encrypted payload at the end. Chuck is the initiator (Bob column — sends his cert first), Sneed is the responder (Alice column).

Service primer

All operations are HTTP POST against https://protocols.live.

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