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[tor-dev] Propsal 263 Quantum-safe Hybrid handshake for Tor, updated feature request



Hi list,

Thanks for all your valuable comments.
We have updated the feature request following your comments.
Please see the attachment for the updated feature request, and see
https://github.com/zhenfeizhang/ntru-tor/commit/0354fe1d61b2b79615301ff387e8c03230235f12
for the modification over last version.

Thanks for your time, and please let us know if you have any further comments/suggestions.

Cheers,
Zhenfei
Title: Request to change key exchange protocol for handshake v1.1
Author: John SCHANCK, William WHYTE and Zhenfei ZHANG
Created: 9 Jan 2016

1. Introduction

  Recognized handshake types are:
    0x0000  TAP         --  the original Tor handshake;
    0x0001  reserved
    0x0002  ntor        --  the ntor+curve25519+sha256 handshake;

  Request for a new (set of) handshake type:
    0x010X  ntor+qsh    --  the hybrid of ntor+curve25519+sha3 handshake
                            and a quantum-safe key encapsulation mechanism

  where
    0X0101  ntor+qsh    --  refers this modular design, no specific KEM is
                            assigned.

    0X0101  ntor+ntru   --  the quantum safe KEM is based on NTRUEncrypt, with
                            parameter ntrueess443ep1

    0X0102  ntor+ntru   --  the quantum safe KEM is based on NTRUEncrypt, with
                            parameter ntrueess743ep1

    0X0103  ntor+rlwe   --  the quantum safe KEM is based on ring learning with
                            error encryption scheme; parameter not specified

	DEPENDENCY:
	  Proposal 249: Allow CREATE cells with >505 bytes of handshake data

  1.1 Motivation: Quantum-safe forward-secure key agreement

    We are trying to add Quantum-safe forward-secrecy to the key agreement in 
    tor handshake. (Classical) forward-secrecy means that if the long-term key 
    is compromised, the communication prior to this compromise still stays 
    secure. Similarly, Quantum-safe forward-secrecy implies if the long-term 
    key is compromised due to attackers with quantum-computing capabilities, the
    prior communication still remains secure.
        
    Current approaches for handling key agreement, for instance, the ntor 
    handshake protocol, do not have this feature. ntor uses ECC, which will be 
    broken when quantum computers become available. This allows the simple yet 
    very effective harvest-then-decrypt attack, where an adversary with 
    significant storage capabilities harvests Tor handshakes now and decrypt 
    them in the future. 
    
    The proposed handshake protocol achieves quantum-safe forward-secrecy and 
    stops those attacks by introducing a secondary short-term pre-master secret 
    that is transported via a quantum-safe method. In the case where long-term
    key is compromised via quantum algorithm, the attacker still need to recover
    the second pre-master secret to be able to decrypt the communication.    

  1.2 Motivation: Allowing plug & play for quantum-safe encryption algorithms

    We would like to be conservative on the selection of quantum-safe encryption
    algorithm. For this purpose, we propose a modular design that allows any
    quantum-safe encryption algorithm to be included in this handshake
    framework. We will illustrate the proposal with NTRUEncrypt encryption
    algorithm.

2. Proposal

  2.1 Overview

    In Tor, authentication is one-way in the authenticated key-exchange
    protocol. This proposed new handshake protocol is consistent with that
    approach.

    We aim to provide quantum-safe forward-secrecy and modular design to the Tor
    handshake, with the minimum impact on the current version. We aim to use
    as many existing mechanisms as possible.

    For purposes of comparison, proposed modifications are indicated with * at
    the beginning of the corresponding line, the original approaches in ntor
    are marked with # when applicable.

    In order to enable variant quantum-safe algorithms for Tor handshake, we
    propose a modular approach that allows any quantum-safe encryption algorithm
    to be adopted in this framework. Our approach is a hybridization of ntor
    protocol and a KEM. We instantiate this framework with NTRUEncrypt, a
    lattice-based encryption scheme that is believed to be quantum resistant.
    This framework is expandable to other quantum-safe encryptions such as Ring 
    Learning with Error (R-LWE) based schemes.

    2.1.1 Achieved Property:

      1)  The proposed key exchange method is quantum-safe forward-secure: two 
      secrets are exchanged, one protected by ECC, one protected by NTRUEncrypt,
      and then put through the native Tor Key Derivation Function (KDF) to 
      derive the encryption and authentication keys. Both secrets are protected 
      with one-time keys for their respective public key algorithms.

      2)  The proposed key exchange method provides one-way authentication: The
      server is authenticated, while the client remains anonymous.

      3)  The protocol is almost backward compatible with its previous
      version: ntor. By omitting a single field, one obtains the exact ntor
      protocol. That is, the protocol is at least as secure as ntor.

    2.1.2 General idea:

      When a client wishes to establish a one-way authenticated key K with a
      server, through following steps a session key is established:
      1)  Establish a common secret E (classical cryptography, i.e., ECC) using
      a one-way authenticated key exchange protocol.
      #ntor currently uses this approach#;
      2)  Establish a common "parallel" secret P using a key encapsulation
      mechanism similar to TLS_RSA. In this feature request we use NTRUEncrypt
      as an example.
      3)  Establish a new session key k = KDF(E|P, info, i), where KDF is a Key
      Derivation Function.

    2.1.3 Building Blocks

      1)  ntor: ECDH-type key agreement protocol with one-way authentication;
      ##existing approach: See 5.1.4 tor-spec.txt##

      2)  A quantum-safe encryption algorithm: we use QSE to refer to the
      quantum-safe encryption algorithm, and use NTRUEncrypt as our example;
      **new approach**

      3)  HMAC-based Extract-and-Expand Key Derivation Function - KDF-RFC5869;
      ##existing approach: See 5.2.2 tor-spec.txt## 
      Note: a new hash function, SHA3 as in FIPS 202, will be used, rather than
      SHA256 as in ntor.

  2.2 The protocol

    2.2.1 Initialization

      H(x,t) as HMAC_SHA3 with message x and key t.
      H_LENGTH      = 32
      ID_LENGTH     = 20
      G_LENGTH      = 32

*     QSPK_LENGTH   = XXX           length of QSE public key
*     QSC_LENGTH    = XXX           length of QSE cipher

*     PROTOID       = "ntor-curve25519-sha3-1-[qseid]"
#pre  PORTOID       = "ntor-curve25519-sha256-1"

      t_mac         = PROTOID | ":mac"
      t_key         = PROTOID | ":key_extract"
      t_verify      = PROTOID | ":verify"

      These three variables define three different cryptographic hash functions:
      hash1         = HMAC(*, t_mac);
      hash2         = HMAC(*, t_key);
      hash3         = HMAC(*, t_verify);

      MULT(A,b)     = the multiplication of the curve25519 point 'A' by the
                      scalar 'b'.
      G             = The preferred base point for curve25519
      KEYGEN()      = The curve25519 key generation algorithm,
                      returning a private/public keypair.
      m_expand      = PROTOID | ":key_expand"

      curve25519
        b, B        = KEYGEN();

*     QSH
*       QSSK,QSPK   = QSKEYGEN();
*       cipher      = QSENCRYPT (*, PK);
*       message     = QSDECRYPT (*, SK);

    2.2.2 Handshake

      To perform the handshake, the client needs to know an identity key digest
      for the server, and an ntor onion key (a curve25519 public key) for that
      server. Call the ntor onion key "B".

      The client generates a temporary key pair:
        x, X        = KEYGEN();

      an QSE temporary key pair:
*       QSSK, QSPK  = QSKEYGEN();

================================================================================
      and generates a client-side handshake with contents:
        NODEID      Server identity digest  [ID_LENGTH   bytes]
        KEYID       KEYID(B)                [H_LENGTH    bytes]
        CLIENT_PK   X                       [G_LENGTH    bytes]
*       QSPK        QSPK                    [QSPK_LENGTH bytes]
================================================================================

      The server generates an ephemeral curve25519 keypair:
        y, Y        = KEYGEN();

      a ephemeral "parallel" secret for encryption with QSE:
*       PAR_SEC     P                       [H_LENGTH    bytes]

      and computes:
*       C           = ENCRYPT( P | B, QSPK);

      Then it uses its ntor private key 'b' to compute an ECC secret
        E           = EXP(X,y) | EXP(X,b) | B | X | Y

      and computes:

*       secret_input    = E | P | QSPK | ID | PROTOID
#pre    secret_input    = E | ID | PROTOID

        KEY_SEED        = H(secret_input, t_key)
        verify          = H(secret_input, t_verify)
*       auth_input      = verify | B | Y | X | C | QSPK
                          | ID | PROTOID | "Server"
#pre    auth_input      = verify | B | Y | X | ID | PROTOID | "Server"

================================================================================
      The server's handshake reply is:
        SERVER_PK       Y                       [G_LENGTH     bytes]
        AUTH            H(auth_input, t_mac)    [H_LENGTH     bytes]
*       QSCIPHER        C                       [QSPK_LENGTH  bytes]
================================================================================
      The client then checks Y is in G^*, and computes

        E               = EXP(Y,x) | EXP(B,x) | B | X | Y
*       P'              = DECRYPT(C, QSSK)

      extract P,B from P' (P' = P|B), verifies B, and computes

*       secret_input    = E | P | QSPK | ID | PROTOID
#pre    secret_input    = E | ID | PROTOID

        KEY_SEED        = H(secret_input, t_key)
        verify          = H(secret_input, t_verify)
*       auth_input      = verify | B | Y | X | C | ID | PROTOID | "Server"
#pre    auth_input      = verify | B | Y | X | ID | PROTOID | "Server"

      The client verifies that AUTH == H(auth_input, t_mac).

      Both parties now have a shared value for KEY_SEED. This value will be used
      during Key Derivation Function - KDF-RFC5869 (see 5.2.2 tor-spec.txt)

  2.3 Instantiation with NTRUEncrypt

    The example uses the NTRU parameter set NTRU_EESS443EP1. This has keys
    and ciphertexts of length 610 bytes. This parameter set delivers 128 bits
    classical security and quantum security. For 256 bits classcial and quantum 
    security, use NTRU_EESS743EP1.

    We adjust the following parameters:

    handshake type:
    0X0101  ntor+ntru       the quantum safe KEM is based on NTRUEncrypt, with
                            parameter ntrueess443ep1
    PROTOID       = "ntor-curve25519-sha3-1-ntrueess443ep1"
    QSPK_LENGTH   = 610     length of NTRU_EESS439EP1 public key
    QSC_LENGTH    = 610     length of NTRU_EESS439EP1 cipher

    NTRUEncrypt can be adopted in our framework without further modification.

3. Security Concerns

  The proof of security can be found at https://eprint.iacr.org/2015/287
  We highlight some desired features.

  3.1 One-way Authentication
    The one-way authentication feature is inherent from the ntor protocol.

  3.2 Multiple Encryption
    The technique to combine two encryption schemes used in 2.2.4 is named
    Multiple Encryption. Discussion of appropriate security models can be
    found in [DK05]. Proof that the proposed handshake is secure under this
    model can be found at https://eprint.iacr.org/2015/287.

  3.3 Cryptographic hash function
    The default hash function HMAC_SHA256 from Tor. This is suffice to provide
    desired security for the present day. However, to be more future proof, we
    propose to use HMAC_SHA3 when Tor starts to migrant to SHA3.

  3.4 Key Encapsulation Mechanism
    The KEM in our protocol can be proved to be KEM-CCA-2 secure.

  3.5 Quantum-safe Forward Secrecy
    The quantum-safe forward secrecy is achieved.

  3.6 Quantum-safe authentication
    The proposed protocol is secure only until a quantum computer is developed 
    that is capable of breaking the onion keys in real time. Such a computer can
    compromise the authentication of ntor online; the security of this approach 
    depends on the authentication being secure at the time the handshake is 
    executed. This approach is intended to provide security against the 
    harvest-then-decrypt attack while an acceptable quantum-safe approach with 
    security against an active attacker is developed.

4. Candidate quantum-safe encryption algorithms

  We do not propose any quantum-safe encryption algorithms in this proposal. 
  This documents focus on the hybrid design. The implementer should modularize 
  the protocol with appropriate interfaces that allows any quantum-safe 
  encryption algorithms to be used in this setting.
  
  Candidate quantum-safe encryption algorithms will be included in another 
  proposal. This document will refer to the proposal when both proposals are 
  mature. 


5. Bibliography

[DK05] Y. Dodis, J. Katz, "Chosen-Ciphertext Security of Mulitple Encryption",
    Theory of Cryptography Conference, 2005.
    http://link.springer.com/chapter/10.1007%2F978-3-540-30576-7_11
    (conference version) or http://cs.nyu.edu/~dodis/ps/2enc.pdf (preprint)

















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