Guided tour puzzle (GTP) protocol is a cryptographic protocol for mitigating application layer denial of service attacks. It aims to overcome the shortcoming of computation-based puzzle protocols, in which clients are required to compute hard CPU- or memory-bound puzzles that favor clients with abundant computational resources. Guided tour puzzle protocol can be seen as a form of proof-of-work (POW) protocol.
Overview The protocol steps of the guided tour puzzle protocol is similar to that of Client Puzzle Protocol. All clients are required to complete a guided tour puzzle prior to receiving service from the server, if the server suspects it is currently under denial of service attack or its load exceeds a pre-defined threshold. Simply put, a guided tour puzzle is a tour that needs to be completed by taking multiple round-trips to a set of special nodes, called tour guides, in a sequential order. It is called a guided tour, because the order in which the tour guides are visited is unknown to the client, and each tour guide has to direct the client towards the next tour guide for the client to complete the tour in correct order. A single tour guide may appear multiple times in a tour, so the term stop is used to denote a single appearance of a tour guide in a tour. A client knows which tour guide is at the next stop, only after completing its visit to the current stop. Solving a guided tour puzzle is essentially equal to completing a guided tour in the correct order. Starting from the first stop, the client contacts each stop and receives a reply. Each reply contains a unique token. The token in the reply message from the current stop is used for computing the address of the next stop tour guide. The address of the first stop tour guide is computed using the token contained in the server's first reply message that informs the client of the start of a puzzle process. The client must send the token received from the current stop tour guide to the next stop tour guide, which will use it as an input to its token calculation function. The token received from the last stop tour guide plus the token from the server's puzzle message are sent to the server as the proof of completion of a tour. The server can efficiently validate these two tokens, and grants service to the client only after proving their validity.
Protocol steps
Before the guided tour puzzle can start, N {\displaystyle N} tour guides has to be set up in the system, where N ≥ 2 {\displaystyle N\geq 2} . Meanwhile, the server establishes a shared secret k j s {\displaystyle k_{js}} with each tour guide G j {\displaystyle G_{j}} using a secure channel, where 0 ≤ j < N {\displaystyle 0\leq j<N} . The server keeps a short-lived secret K s {\displaystyle K_{s}} for computing the first hash value that is returned to the client as part of a puzzle message. A puzzle message also contains the length of the tour L {\displaystyle L} , which is used to control the difficulty of a guided tour puzzle. The figure shows an example of a guided tour when N = 2 {\displaystyle N=2} and L = 5 {\displaystyle L=5} . The details of the each protocol step of the guided tour puzzle protocol is explained in the following.
Service request: A client x {\displaystyle x} sends a service request to the server. If the server load is normal, the client's request is serviced as usual; if the server is overloaded, then it proceeds to the initial puzzle generation step. Initial puzzle generation: the server replies to the client x {\displaystyle x} with a puzzle message that informs the client to complete a guided tour. The puzzle message contains the length of the tour L {\displaystyle L} and a hash value h 0 {\displaystyle h_{0}} . The server computes h 0 {\displaystyle h_{0}} using the following formula:
h 0 = h a s h ( A x | | L | | t s | | K s ) {\displaystyle h_{0}=hash(A_{x}\;||\;L\;||\;ts\;||\;K_{s})}
where, | | {\displaystyle ||} means concatenation, A x {\displaystyle A_{x}} is the address (or any unique value) of the client x {\displaystyle x} , t s {\displaystyle ts} is a coarse timestamp, and h a s h {\displaystyle hash} is a cryptographic hash function such as SHA-1. Puzzle solving: A client computes the index of the tour guide at the l {\displaystyle l} -th stop of its tour using the following formula:
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