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+This draft is intended to specify the meaning of ‘secure’ for a Tor
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+circuit protocol, hopefully in enough detail that
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+mathematically-inclined cryptographers can use this definition to
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+prove that a Tor circuit protocol (or component thereof) is secure
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+under reasonably well-accepted assumptions.
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+
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+Tor's current circuit protocol consists of the CREATE, CREATED, RELAY,
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+DESTROY, CREATE_FAST, CREATED_FAST, and RELAY_EARLY cells (including
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+all subtypes of RELAY and RELAY_EARLY cells). Tor currently has two
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+circuit-extension handshake protocols: one consists of the CREATE and
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+CREATED cells; the other, used only over the TLS connection to the
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+first node in a circuit, consists of the CREATE_FAST and CREATED_FAST
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+cells.
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+
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+
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+
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+1. Every circuit-extension handshake protocol must provide forward
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+secrecy -- the protocol must allow both the client and the relay to
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+destroy, immediately after a circuit is closed, enough key material
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+that no attacker who can eavesdrop on all handshake and circuit cells
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+and who can seize and inspect the client and relay after the circuit
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+is closed will be able to decrypt any non-handshake data sent along
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+the circuit.
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+
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+In particular, the protocol must not require that a key which can be
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+used to decrypt non-handshake data be stored for a predetermined
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+period of time, as such a key must be written to persistent storage.
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+
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+
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+
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+2. Every circuit-extension handshake protocol must specify what key
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+material must be used only once in order to allow unlinkability of
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+circuit-extension handshakes.
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+
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+
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+
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+3. Every circuit-extension handshake protocol must authenticate the relay
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+to the client -- an attacker who can eavesdrop on all handshake and
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+circuit cells and who can participate in handshakes with the client
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+must not be able to determine a symmetric session key that a circuit
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+will use without either knowing a secret key corresponding to a
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+handshake-authentication public key published by the relay or breaking
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+a cryptosystem for which the relay published a
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+handshake-authentication public key.
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+
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+
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+
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+4. Every circuit-extension handshake protocol must ensure that neither
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+the client nor the relay can cause the handshake to result in a
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+predetermined symmetric session key.
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+
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+
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+
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+5. Every circuit-extension handshake protocol should ensure that an
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+attacker who can predict the relay's ephemeral secret input to the
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+handshake and can eavesdrop on all handshake and circuit cells, but
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+does not know a secret key corresponding to the
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+handshake-authentication public key used in the handshake, cannot
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+break the handshake-authentication public key's cryptosystem, and
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+cannot predict the client's ephemeral secret input to the handshake,
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+cannot predict the symmetric session keys used for the resulting
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+circuit.
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+
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+
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+
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+6. The circuit protocol must specify an end-to-end flow-control
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+mechanism, and must allow for the addition of new mechanisms.
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+
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+
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+
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+7. The circuit protocol should specify the statistics to be exchanged
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+between circuit endpoints in order to support end-to-end flow control,
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+and should specify how such statistics can be verified.
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+
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+
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+
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+8. The circuit protocol should allow an endpoint to verify that the other
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+endpoint is participating in an end-to-end flow-control protocol
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+honestly.
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+
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