proposal 121: incorporated comments by Nick posted to or-dev on 10-Oct-2007

Change history:

  26-Sep-2007  Initial proposal for or-dev

  08-Dec-2007  Incorporated comments by Nick posted to or-dev on 10-Oct-2007

Overview:

Details:

  [XXX Restructure this section in separate patch:

   A) The general mechanisms to perform authentication at three

      authentication points (directory, service, introduction point)

   B) A specific authentication protocol based on secret cookies. -KL]

  [XXX Describe use of descriptor cookie as "/0/ Client authentication at

   directory". Optional encryption/decryption using a descriptor cookie is

   understood since proposal 114, but not used by servers and clients. -KL]

  /1/ Client authentication at the hidden service

  In proposal 114 a client (Alice) who has a valid secret cookie, which may

  Authentication data will be transmitted via the RELAY_INTRODUCE1 cell

  from Alice to Bob that is forwarded by the IPo. For this message several

  format versions are specified in the rend-spec in section 1.8. We will

  use the format version 3. This specification already contains the fields

  use the format version 3, which is specified, but not implemented by

  December 2007. This specification already contains the fields

  "AUTHT" (to specify the authentication method), "AUTHL" (length of the

  authentication data), and "AUTHD" (the authentication data) that will be

  used to store authentication data. Since these fields are encrypted with

  the circuit to the rendezvous point if the provided authentication data

  is valid, otherwise he will drop the cell. This will improve security due

  to preventing communication between Bob and Alice if she is an attacker.

  Especially, it prevents the attack described by Øverlier and Syverson in

  their paper "Locating Hidden Servers", even without the need for guards.

  As a positive side effect it reduces network traffic because it avoids

  Bob from building unnecessary circuits to the rendezvous points.

  Authentication at the HS should be the last gatekeeper and the number of

  (1) Alice creates a password x and sends the password digest h(x) to Bob

      out of band.

      [XXX Don't distinguish between x and h(x), so that both Alice and Bob

       can be the initiator of the password exchange. -KL]

  (2) Alice sends h(x) to Bob, encrypted with Bob's fresh service key (not

      subject to this proposal, see proposal 114).

  (3) Bob decrypts Alice's message using his private service key (see

  the SHA-1 secure hash (see tor-spec) of the shared secret (password).

  The premise to use password authentication is that Bob must send the

  password to Alice outside Tor. If at the same time the secret cookie is

  password to Alice -- or the other way around -- outside Tor.

  If at the same time the secret cookie is

  transmitted and the message is intercepted the attacker can gain access

  to the service. Therefore, a secure way to exchange this information must

  be established.

  The second authentication mechanism is public-key authentication. The

  well-known RSA implementation will be used as cipher (see tor-spec). 

  Authentication data will be the hash of the rendezvous cookie, signed

  with the private key (SK).

  When Alice wants to use this authentication method she sets "AUTHT" to

  "2" and "AUTHL" to "128" which is the size of the encrypted data. Since

  the rendezvous cookie changes each time Alice connects, replay attacks

  can be easily prevented.

  (1) Alice creates a private key e and sends the corresponding public key

      d to Bob out of band.

  (2) Alice generates a random rendezvous cookie r, computes PKSign(e, r),

      encrypts it with Bob's fresh service key (see proposal 114), and

      sends the result to Bob.

  (3) Bob decrypts Alice's message using his private service key (see

      proposal 114) and verifies PKSign(e, r) with d.

  The premise for public-key authentication is that Alice must send the

  generated public key to Bob outside Tor. If an attacker is able to swap

  that key, the attacker could perform a man-in-the-middle attack, if he

  managed to serve as an IPo for Bob. Therefore a secure exchange channel

  must be established.

  Depending on what authentication data Bob knows from Alice (password

  and/or public key, or other data that is added later) there are several

  choices for Alice to authenticate to the service.

  [Removed public-key authentication protocol. -KL]

  After validating the provided "AUTHD" Bob builds a circuit to the

  rendezvous point and starts interacting with Alice. If Bob cannot

  identify the client he must refuse the request by not connecting to the

  rendezvous point.

  [XXX Bob should discard an IPo after a certain number of cells containing

   bad auth data. But any denouncement by other IPos or clients, e.g. by

   replaying cells, must be inhibited. Maybe Bob should keep a history of

   connection attempts within a certain time and discard an IPo after a

   specific threshold. And maybe authentication to the service should be

   based on a nonce, so that the service can differentiate between a replay

   attack by an introduction point and regular reconnection attempts. More

   thoughts needed here. -KL]

  It will also still be possible to establish v2 hidden services without

  authentication. Therefore the "AUTHT" field must be set to "0". "AUTHL"

      out of band.

  (2) Bob creates a random value y, computes h(h(x)|y), and sends the

      result to the introduction point.

      [XXX There should be a separate y for each introduction point, so

       that none of them may impersonate Alice to any of the other

       introduction points. -KL]

  (3) Bob encrypts y with a secret cookie (see proposal 114) and writes it

      to a rendezvous service descriptor.

  (4) Alice fetches Bob's rendezvous service descriptor, decrypts y using

  (5) The introduction point decrypts h(h(x)|y) from Alice's message and

      compares it to the value it knows from Bob (from step 2).

  If Alice wants to use public-key authentication to authenticate herself

  at Bob's HS, the challenge-response authentication protocol is slightly

  different.

  The IPo's are provided with a list of random value hashes h(r) with an

  entry for each user via the RELAY_ESTABLISH_INTRO cell. For public-key

  authentication Alice uses an RSA public/private-key pair (as specified in

  tor-spec). The public key is made known to Bob out of band. The IPo's

  will now be sent a new ESTABLISH_INTRO cell with an additional random

  value hash for Alice and a new descriptor is uploaded to the responsible

  directories. The public-key authentication part of the service descriptor

  holds a blank separated list of key-value pairs with one pair for every

  authorized user. The hash of the public key of a user serves as a key,

  while the PK-encrypted r represents the value. Authorized users can now

  find their respective key-value pair and decrypt the value of h(r). This

  result serves as an authorization token at the IPo in the same way as

  with password authentication. The IPo does not know which authentication

  method was used since the tokens always have the same format.

  (1) Alice creates a private key e and sends the corresponding public key

      d to Bob out of band.

  (2) Bob creates a random value y and sends it to the introduction point.

  (3) Bob computes PKEncrypt(d, y), encrypts the result with a secret

      cookie (see proposal 114), and writes it to a rendezvous service

      descriptor.

  (4) Alice fetches Bob's rendezvous service descriptor, decrypts

      PKEncrypt(d, y) using the secret cookie (see proposal 114), decrypts

      y from it using her private key e, and sends it to the introduction

      point.

  (5) The introduction point compares y with the value it knows from Bob

      (from step 2).

  [Removed public-key authentication protocol. -KL]

  To remove a user from a group, Bob needs to update the random value list

  at the IPo's.

  client and used by Alice to determine her entry in the list. The

  remaining 128 octets contain the PK-encrypted token needed to

  authenticate to the IPo.

  [XXX Handle space limitation problem, either by using fewer space, by

   sending multiple cells, or by finding a protocol that is

   space-independent here. -KL]

  The part of the RELAY_INTRODUCE1 cell that can be read by the IPo has the

  following fields added:

  AUTHT  The auth type that is supported   [1 octet]

  AUTHL  Length of auth data               [1 octets]

  AUTHD  Auth data                         [variable]

  [XXX Insert a version field, so that we won't be facing the same problems

   again when specifying the next version of INTRODUCE1 cells. -KL]

  The AUTHT and AUTHL fields are provided to allow extensions of the

  protocol. Currently, we set AUTHT to 1 for password/public-key

  authentication and AUTHL to 20 for the length of the authorization token.

  [XXX Insert file format containing auth data here. -KL]

Security implications:

  In addition to the security features proposed in 114-distributed-storage

  When using our authentication for hidden services the implementation of

  IPo's needs to be extended. Therefore we use version information provided

  in router descriptors to be sure that we only send modified

  RELAY_ESTABLISH_INTRO cells to routers that can handle them. Clients of

  v2 hidden services will have to update their Tor installation if they

  RELAY_ESTABLISH_INTRO cells to routers that can handle them. Clients and

  service providers will have to update their Tor installation if they

  want to be able to use the service.