[Author Prev][Author Next][Thread Prev][Thread Next][Author Index][Thread Index]
Re: [tor-dev] Proposal 332: Ntor protocol with extra data, version 3.
Hi Nick, you might look at the Noise framework:
http://noiseprotocol.org/noise.html
Noise has a naming scheme for "handshake patterns". Ntor matches what
we call NK1. Your new scheme I think matches NK (the 1 in NK1
indicates a "deferred" pattern where the DH operation that
authenticates the server is performed prior to the 2nd message rather
than the first).
NK1:
<- s
...
-> e
<- e, ee, es
(read as: initiator has pre-knowledge of server's static public key s;
initiator sends their ephemeral in first message; responder sends
their ephemeral in second message, then performs the two 2 DH
operations, hashing them together and using the result for encrypting
future data).
In NK, the ephemeral-static DH is performed earlier so that the first
message's handshake payload can be encrypted:
NK:
<- s
...
-> e, es
<- e, ee
You also wanted to add an (optional) pre-shared key, which Noise supports:
NKpsk0:
<- s
...
-> psk, e, es
<- e, ee
Some advantages of Noise are that you could reuse existing libraries
and Noise's growing body of security analysis.
Also, we're working on KEM extensions for post-quantum, signatures,
and other things, so Noise might make it easier to evolve the protocol
(eg NK1 vs NK vs NKpsk0).
Trevor
On Mon, Jul 12, 2021 at 9:02 AM Nick Mathewson <nickm@xxxxxxxxxxxxxx> wrote:
>
> ```
> Filename: 332-ntor-v3-with-extra-data.md
> Title: Ntor protocol with extra data, version 3.
> Author: Nick Mathewson
> Created: 12 July 2021
> Status: Open
> ```
>
> # Overview
>
> The ntor handshake is our current protocol for circuit
> establishment.
>
> So far we have two variants of the ntor handshake in use: the "ntor
> v1" that we use for everyday circuit extension (see `tor-spec.txt`)
> and the "hs-ntor" that we use for v3 onion service handshake (see
> `rend-spec-v3.txt`). This document defines a third version of ntor,
> adapting the improvements from hs-ntor for use in regular circuit
> establishment.
>
> These improvements include:
>
> * Support for sending additional encrypted and authenticated
> protocol-setup handshake data as part of the ntor handshake. (The
> information sent from the client to the relay does not receive
> forward secrecy.)
>
> * Support for using an external shared secret that both parties must
> know in order to complete the handshake. (In the HS handshake, this
> is the subcredential. We don't use it for circuit extension, but in
> theory we could.)
>
> * Providing a single specification that can, in the future, be used
> both for circuit extension _and_ HS introduction.
>
> # The improved protocol: an abstract view
>
> Given a client "C" that wants to construct a circuit to a
> relay "S":
>
> The client knows:
> * B: a public "onion key" for S
> * ID: an identity for S, represented as a fixed-length
> byte string.
> * CM: a message that it wants to send to S as part of the
> handshake.
> * An optional "verification" string.
>
> The relay knows:
> * A set of [(b,B)...] "onion key" keypairs. One of them is
> "current", the others are outdated, but still valid.
> * ID: Its own identity.
> * A function for computing a server message SM, based on a given
> client message.
> * An optional "verification" string. This must match the "verification"
> string from the client.
>
> Both parties have a strong source of randomness.
>
> Given this information, the client computes a "client handshake"
> and sends it to the relay.
>
> The relay then uses its information plus the client handshake to see
> if the incoming message is valid; if it is, then it computes a
> "server handshake" to send in reply.
>
> The client processes the server handshake, and either succeeds or fails.
>
> At this point, the client and the relay both have access to:
> * CM (the message the client sent)
> * SM (the message the relay sent)
> * KS (a shared byte stream of arbitrary length, used to compute
> keys to be used elsewhere in the protocol).
>
> Additionally, the client knows that CM was sent _only_ to the relay
> whose public onion key is B, and that KS is shared _only_ with that
> relay.
>
> The relay does not know which client participated in the handshake,
> but it does know that CM came from the same client that generated
> the key X, and that SM and KS were shared _only_ with that client.
>
> Both parties know that CM, SM, and KS were shared correctly, or not
> at all.
>
> Both parties know that they used the same verification string; if
> they did not, they do not learn what the verification string was.
> (This feature is required for HS handshakes.)
>
> # The handshake in detail
>
> ## Notation
>
> We use the following notation:
>
> * `|` -- concatenation
> * `"..."` -- a byte string, with no terminating NUL.
> * `ENCAP(s)` -- an encapsulation function. We define this
> as `htonll(len(s)) | s`. (Note that `len(ENCAP(s)) = len(s) + 8`).
> * `PARTITION(s, n1, n2, n3, ...)` -- a function that partitions a
> bytestring `s` into chunks of length `n1`, `n2`, `n3`, and so
> on. Extra data is put into a final chunk. If `s` is not long
> enough, the function fails.
>
> We require the following crypto operations:
>
> * `KDF(s,t)` -- a tweakable key derivation function, returning a
> keystream of arbitrary length.
> * `H(s,t)` -- a tweakable hash function of output length
> `DIGEST_LEN`.
> * `MAC(k, msg, t)` -- a tweakable message-authentication-code function,
> of output length `MAC_LEN`.
> * `EXP(pk,sk)` -- our Diffie Hellman group operation, taking a
> public key of length `PUB_KEY_LEN`.
> * `KEYGEN()` -- our Diffie-Hellman keypair generation algorithm,
> returning a (secret-key,public-key) pair.
> * `ENC(k, m)` -- a stream cipher with key of length `ENC_KEY_LEN`.
> `DEC(k, m)` is its inverse.
>
> Parameters:
>
> * `PROTOID` -- a short protocol identifier
> * `t_*` -- a set of "tweak" strings, used to derive distinct
> hashes from a single hash function.
> * `ID_LEN` -- the length of an identity key that uniquely identifies
> a relay.
>
> Given our cryptographic operations and a set of tweak strings, we
> define:
>
> ```
> H_foo(s) = H(s, t_foo)
> MAC_foo(k, msg) = MAC(k, msg, t_foo)
> KDF_foo(s) = KDF(s, t_foo)
> ```
>
> See Appendix A.1 below for a set of instantiations for these operations
> and constants.
>
> ## Client operation, phase 1
>
> The client knows:
> B, ID -- the onion key and ID of the relay it wants to use.
> CM -- the message that it wants to send as part of its
> handshake.
> VER -- a verification string.
>
> First, the client generates a single-use keypair:
>
> x,X = KEYGEN()
>
> and computes:
>
> Bx = EXP(B,x)
> secret_input_phase1 = Bx | ID | X | B | PROTOID | ENCAP(VER)
> phase1_keys = KDF_msgkdf(secret_input_phase1)
> (ENC_K1, MAC_K1) = PARTITION(phase1_keys, ENC_KEY_LEN, MAC_KEY_LEN)
>
> encrypted_msg = ENC(ENC_K1, CM)
> msg_mac = MAC_msgmac(MAC_K1, ID | B | X | encrypted_msg)
>
> and sends:
>
> NODEID ID [ID_LEN bytes]
> KEYID B [PUB_KEY_LEN bytes]
> CLIENT_PK X [PUB_KEY_LEN bytes]
> MSG encrypted_msg [len(CM) bytes]
> MAC msg_mac [last MAC_LEN bytes of message]
>
> The client remembers x, X, B, ID, Bx, and msg_mac.
>
> ## Server operation
>
> The relay checks whether NODEID is as expected, and looks up
> the (b,B) keypair corresponding to KEYID. If the keypair is
> missing or the NODEID is wrong, the handshake fails.
>
> Now the relay uses `X=CLIENT_PK` to compute:
>
> Xb = EXP(X,b)
> secret_input_phase1 = Xb | ID | X | B | PROTOID | ENCAP(VER)
> phase1_keys = KDF_msgkdf(secret_input_phase1)
> (ENC_K1, MAC_K1) = PARTITION(phase1_keys, ENC_KEY_LEN, MAC_KEY_LEN)
>
> expected_mac = MAC_msgmac(MAC_K1, ID | B | X | MSG)
>
> If `expected_mac` is not `MAC`, the handshake fails. Otherwise
> the relay computes `CM` as:
>
> CM = DEC(MSG, ENC_K1)
>
> The relay then checks whether `CM` is well-formed, and in response
> composes `SM`, the reply that it wants to send as part of the
> handshake. It then generates a new ephemeral keypair:
>
> y,Y = KEYGEN()
>
> and computes the rest of the handshake:
>
> Xy = EXP(X,y)
> secret_input = Xy | Xb | ID | B | X | Y | PROTOID | ENCAP(VER)
> ntor_key_seed = H_key_seed(secret_input)
> verify = H_verify(secret_input)
>
> RAW_KEYSTREAM = KDF_final(ntor_key_seed)
> (ENC_KEY, KEYSTREAM) = PARTITION(RAW_KEYSTREAM, ENC_KEY_LKEN, ...)
>
> encrypted_msg = ENC(ENC_KEY, SM)
>
> auth_input = verify | ID | B | Y | X | MAC | ENCAP(encrypted_msg) |
> PROTOID | "Server"
> AUTH = H_auth(auth_input)
>
> The relay then sends:
>
> Y Y [PUB_KEY_LEN bytes]
> AUTH AUTH [DIGEST_LEN bytes]
> MSG encrypted_msg [len(SM) bytes, up to end of the message]
>
> The relay uses KEYSTREAM to generate the shared secrets for the
> newly created circuit.
>
> ## Client operation, phase 2
>
> The client computes:
>
> Yx = EXP(Y, x)
> secret_input = Yx | Bx | ID | B | X | Y | PROTOID | ENCAP(VER)
> ntor_key_seed = H_key_seed(secret_input)
> verify = H_verify(secret_input)
>
> auth_input = verify | ID | B | Y | X | MAC | ENCAP(MSG) |
> PROTOID | "Server"
> AUTH_expected = H_auth(auth_input)
>
> If AUTH_expected is equal to AUTH, then the handshake has
> succeeded. The client can then calculate:
>
> RAW_KEYSTREAM = KDF_final(ntor_key_seed)
> (ENC_KEY, KEYSTREAM) = PARTITION(RAW_KEYSTREAM, ENC_KEY_LKEN, ...)
>
> SM = DEC(ENC_KEY, MSG)
>
> SM is the message from the relay, and the client uses KEYSTREAM to
> generate the shared secrets for the newly created circuit.
>
> # Security notes
>
> Whenever comparing bytestrings, implementations SHOULD use
> constant-time comparison function to avoid side-channel attacks.
>
> To avoid small-subgroup attacks against the Diffie-Hellman function,
> implementations SHOULD either:
>
> * Make sure that all incoming group members are in fact in the DH
> group.
> * Validate all outputs from the EXP function to make sure that
> they are not degenerate.
>
>
> # Notes on usage
>
> We don't specify what should actually be done with the resulting
> keystreams; that depends on the usage for which this handshake is
> employed. Typically, they'll be divided up into a series of tags
> and symmetric keys.
>
> The keystreams generated here are (conceptually) unlimited. In
> practice, the usage will determine the amount of key material
> actually needed: that's the amount that clients and relays will
> actually generate.
>
> The PROTOID parameter should be changed not only if the
> cryptographic operations change here, but also if the usage changes
> at all, or if the meaning of any parameters changes. (For example,
> if the encoding of CM and SM changed, or if ID were a different
> length or represented a different type of key, then we should start
> using a new PROTOID.)
>
>
> # A.1 Instantiation
>
> Here are a set of functions based on SHA3, SHAKE128, Curve25519, and
> AES256:
>
> ```
> H(s, t) = SHA3_256(ENCAP(t) | s)
> MAC(k, msg, t) = SHA3_256(ENCAP(t) | ENCAP(k) | s)
> KDF(s, t) = SHAKE_128(ENCAP(t) | s)
> ENC(k, m) = AES_256_CTR(k, m)
>
> EXP(pk,sk), KEYGEN: defined as in curve25519
>
> DIGEST_LEN = MAC_LEN = ENC_KEY_LEN = PUB_KEY_LEN = 32
>
> ID_LEN = 32 (representing an ed25519 identity key)
> ```
>
> Notes on selected operations: SHA3 can be pretty slow, and AES256 is
> likely overkill. I'm choosing them anyway because they are what we
> use in hs-ntor, and in my preliminary experiments they don't account
> for even 1% of the time spent on this handshake.
>
> ```
> t_msgkdf = PROTOID | ":kdf_phase1"
> t_msgmac = PROTOID | ":msg_mac"
> t_key_seed = PROTOID | ":key_seed"
> t_verify = PROTOID | ":verify"
> t_final = PROTOID | ":kdf_final"
> t_auth = PROTOID | ":auth_final"
> ```
>
> # A.2 Encoding for use with Tor circuit extension
>
> Here we give a concrete instantiation of ntor-v3 for use with
> circuit extension in Tor, and the parameters in A.1 above.
>
> If in use, this is a new CREATE2 type. Clients should not use it
> unless the relay advertises support by including an appropriate
> version of the `Relay=X` subprotocol in its protocols list.
>
> When the encoding and methods of this section, along with the
> instantiations from the previous section, are in use, we specify:
>
> PROTOID = "ntor3-curve25519-sha3_256-1"
>
> The key material is extracted as follows, unless modified by the
> handshake (see below). See tor-spec.txt for more info on the
> specific values:
>
> Df Digest authentication, forwards [20 bytes]
> Db Digest authentication, backwards [20 bytes]
> Kf Encryption key, forwards [16 bytes]
> Kb Encryption key, backwards [16 bytes]
> KH Onion service nonce [20 bytes]
>
> We use the following meta-encoding for the contents of client and
> server messages.
>
> [Any number of times]:
> TYPE [one byte]
> LEN [one byte]
> BODY [LEN bytes]
>
> We do not specify specific TYPE semantics here; we leave those for
> other proposals.
>
> All parties MUST reject messages that are not well-formed per the
> rules above.
>
> To avoid partitioning, clients MUST reject messages with TYPEs that
> they do not recognize. (Therefore, whenever we specify a new server
> message TYPE, we must say that it can only be included if the client
> signals that it understands it.)
>
> # A.3 How much space is available?
>
> We start with a 498-byte payload in each relay cell.
>
> The header of the EXTEND2 cell, including link specifiers and other
> headers, comes to 89 bytes.
>
> The client handshake requires 128 bytes (excluding CM).
>
> That leaves 281 bytes, "which should be plenty".
>
> # X.1 Negotiating proposal-324 circuit windows
>
> (We should move this section into prop324 when this proposal is
> finished.)
>
> We define a type value, CIRCWINDOW_INC.
>
> We define a triplet of consensus parameters: `circwindow_inc_min`,
> `cincwindow_inc_max`, and `circwindow_inc_dflt`. These all have
> range (1,65535).
>
> When the authority operators want to experiment with different
> values for `circwindow_inc_dflt`, they set `circwindow_inc_min` and
> `circwindow_inc_max` to the range in which they want to experiment,
> making sure that the existing `circwindow_inc_dflt` is within that
> range.
>
> vWhen a client sees that a relay supports the ntor3 handshake type
> (subprotocol `Relay=X`), and also supports the flow control
> algorithms of proposal 324 (subprotocol `FlowCtrl=X`), then the
> client sends a message, with type `CIRCWINDOW_INC`, containing a
> two-byte integer equal to `circwindow_inc_dflt`.
>
> The relay rejects the message if the value given is outside of the
> [`circwindow_inc_min`, `circwindow_inc_max`] range. Otherwise, it
> accepts it, and replies with the same message that the client sent.
> _______________________________________________
> tor-dev mailing list
> tor-dev@xxxxxxxxxxxxxxxxxxxx
> https://lists.torproject.org/cgi-bin/mailman/listinfo/tor-dev
_______________________________________________
tor-dev mailing list
tor-dev@xxxxxxxxxxxxxxxxxxxx
https://lists.torproject.org/cgi-bin/mailman/listinfo/tor-dev