[tor-dev] New revision: Proposal 295: Using ADL for relay cryptography (solving the crypto-tagging attack)
Watson Ladd
watsonbladd at gmail.com
Mon Mar 18 16:04:41 UTC 2019
Some comments: some purely editorial, some substantive.
Editorial: stuff is xored with zero, the concatenation language is not used
consistently. I found it difficult to understand the proposed scheme and
check equivalence to the paper. Maybe some more words to explain the
layering would help.
Substantive: Does it matter that it is possible to compute a message that
doesnt change the digest if you know the key?
On Fri, Mar 1, 2019 at 9:05 AM Nick Mathewson <nickm at torproject.org> wrote:
>
> Hi!
>
> I'm sending a new version of proposal 295 from Tomer Ashur, Orr
> Dunkelman, and Atul Luykx. It's an updated version of their design
> for an improved relay cell encryption scheme, to prevent tagging
> attacks.
>
> This proposal is checked into the torspec repository. I'm also
> linking to a diagram for this scheme (and its latex source) from Atul
> Luykx: https://people.torproject.org/~nickm/prop295/
>
> Finally, I have a draft python reference implementation for an older
> version of this proposal. I hope to be updating it soon and sending
> out a link next week.
>
> cheers! -- Nick
>
>
>
> Filename: 295-relay-crypto-with-adl.txt
> Title: Using ADL for relay cryptography (solving the crypto-tagging
attack)
> Author: Tomer Ashur, Orr Dunkelman, Atul Luykx
> Created: 22 Feb 2018
> Last-Modified: 1 March 2019
> Status: Open
>
>
> 0. Context
>
> Although Crypto Tagging Attacks were identified already in the
> original Tor design, it was not before the rise of the
> Procyonidae in 2012 that their severity was fully realized. In
> Proposal 202 (Two improved relay encryption protocols for Tor
> cells) Nick Mathewson discussed two approaches to stymie tagging
> attacks and generally improve Tor's cryptography. In Proposal 261
> (AEZ for relay cryptography) Mathewson puts forward a concrete
> approach which uses the tweakable wide-block cipher AEZ.
>
> This proposal suggests an alternative approach to Proposal 261
> using the notion of Release (of) Unverified Plaintext (RUP)
> security. It describes an improved algorithm for circuit
> encryption based on CTR-mode which is already used in Tor, and an
> additional component for hashing.
>
> Incidentally, and similar to Proposal 261, this proposal employs
> the ENCODE-then-ENCIPHER approach thus it improves Tor's E2E
> integrity by using (sufficient) redundancy.
>
> For more information about the scheme and a security proof for
> its RUP-security see
>
> Tomer Ashur, Orr Dunkelman, Atul Luykx: Boosting
> Authenticated Encryption Robustness with Minimal
> Modifications. CRYPTO (3) 2017: 3-33
>
> available online at https://eprint.iacr.org/2017/239 .
>
> For authentication between the OP and the edge node we use
> the PIV scheme: https://eprint.iacr.org/2013/835
>
> 2. Preliminaries
>
> 2.1 Motivation
>
> For motivation, see proposal 202.
>
> 2.2. Notation
>
> Symbol Meaning
> ------ -------
> M Plaintext
> C_I Ciphertext
> CTR Counter Mode
> N_I A de/encryption nonce (to be used in CTR-mode)
> T_I A tweak (to be used to de/encrypt the nonce)
> T'_I A running digest
> ^ XOR
> || Concatenation
> (This is more readable than a single | but must be adapted
> before integrating the proposal into tor-spec.txt)
>
> 2.3. Security parameters
>
> HASH_LEN -- The length of the hash function's output, in bytes.
>
> PAYLOAD_LEN -- The longest allowable cell payload, in bytes. (509)
>
> DIG_KEY_LEN -- The key length used to digest messages (e.g.,
> using GHASH). Since GHASH is only defined for 128-bit keys, we
> recommend DIG_KEY_LEN = 128.
>
> ENC_KEY_LEN -- The key length used for encryption (e.g., AES). We
> recommend ENC_KEY_LEN = 128.
>
> 2.4. Key derivation (replaces Section 5.2.2)
>
> For newer KDF needs, Tor uses the key derivation function HKDF
> from RFC5869, instantiated with SHA256. The generated key
> material is:
>
> K = K_1 | K_2 | K_3 | ...
>
> where, if H(x,t) denotes HMAC_SHA256 with value x and key t,
> and m_expand denotes an arbitrarily chosen value,
> and INT8(i) is an octet with the value "i", then
> K_1 = H(m_expand | INT8(1) , KEY_SEED )
> and K_(i+1) = H(K_i | m_expand | INT8(i+1) , KEY_SEED ),
> in RFC5869's vocabulary, this is HKDF-SHA256 with info ==
> m_expand, salt == t_key, and IKM == secret_input.
>
> When used in the ntor handshake a string of key material is
> generated and is used in the following way:
>
> Length Purpose Notation
> ------ ------- --------
> HASH_LEN forward digest IV DF *
> HASH_LEN backward digest IV DB *
> ENC_KEY_LEN encryption key Kf
> ENC_KEY_LEN decryption key Kb
> DIG_KEY_LEN forward digest key Khf
> DIG_KEY_LEN backward digest key Khb
> ENC_KEY_LEN forward tweak key Ktf
> ENC_KEY_LEN backward tweak key Ktb
> DIGEST_LEN nonce to use in the *
> hidden service protocol
>
> * I am not sure that we need these any longer.
>
> Excess bytes from K are discarded.
>
> 2.6. Ciphers
>
> For hashing(*) we use GHASH with a DIG_KEY_LEN-bit key. We write
> this as Digest(K,M) where K is the key and M the message to be
> hashed.
>
> We use AES with an ENC_KEY_LEN-bit key. For AES encryption
> (resp., decryption) we write E(K,X) (resp., D(K,X)) where K is an
> ENC_KEY_LEN-bit key and X the block to be encrypted (resp.,
> decrypted).
>
> For a stream cipher, unless otherwise specified, we use
> ENC_KEY_LEN-bit AES in counter mode, with a nonce that is
> generated as explained below. We write this as Encrypt(K,N,X)
> (resp., Decrypt(K,N,X)) where K is the key, N the nonce, and X
> the message to be encrypted (resp., decrypted).
>
> (*) The terms hash and digest are used interchangeably.
>
> 3. Routing relay cells
>
> 3.1. Forward Direction
>
> The forward direction is the direction that CREATE/CREATE2 cells
> are sent.
>
> 3.1.1. Routing from the Origin
>
> Let n denote the integer representing the destination node. For
> I = 1...n+1, T'_{I} is initialized to the 128-bit string consisting
> entirely of '0's. When an OP sends a relay cell, they prepare the
> cell as follows:
>
> The OP prepares the authentication part of the message:
>
> C_{n+1} = M
> T_{n+1} = Digest(Khf_n,T'_{n+1}||C_{n+1})
> N_{n+1} = T_{n+1} ^ E(Ktf_n,T_{n+1} ^ 0)
> T'_{n+1} = T_{n+1}
>
> Then, the OP prepares the multi-layered encryption:
>
> For I=n...1:
> C_I = Encrypt(Kf_I,N_{I+1},C_{I+1})
> T_I = Digest(Khf_I,T'_I||C_I)
> N_I = T_I ^ E(Ktf_I,T_I ^ N_{I+1})
> T'_I = T_I
>
> The OP sends C_1 and N_1 to node 1.
>
> 3.1.2. Relaying Forward at Onion Routers
>
> When a forward relay cell is received by OR I, it decrypts the
> payload with the stream cipher, as follows:
>
> 'Forward' relay cell:
>
> T_I = Digest(Khf_I,T'_I||C_I)
> N_{I+1} = T_I ^ D(Ktf_I,T_I ^ N_I)
> C_{I+1} = Decrypt(Kf_I,N_{I+1},C_I)
> T'_I = T_I
>
> The OR then decides whether it recognizes the relay cell as
> described below. If the OR recognizes the cell, it processes the
> contents of the relay cell. Otherwise, it passes C_{I+1}||N_{I+1}
> along the circuit if the circuit continues.
>
> For more information, see section 4 below.
>
> 3.2. Backward Direction
>
> The backward direction is the opposite direction from
> CREATE/CREATE2 cells.
>
> 3.2.1. Relaying Backward at Onion Routers
>
> When a backward relay cell is received by OR I, it encrypts the
> payload with the stream cipher, as follows:
>
> 'Backward' relay cell:
>
> T_I = Digest(Khb_I,T'_I||C_{I+1})
> N_I = T_I ^ E(Ktb_I,T_I ^ N_{I+1})
> C_I = Encrypt(Kb_I,N_I,C_{I+1})
> T'_I = T_I
>
> with C_{n+1} = M and N_{n+1}=0. Once encrypted, the node passes
> C_I and N_I along the circuit towards the OP.
>
> 3.2.2. Routing to the Origin
>
> When a relay cell arrives at an OP, the OP decrypts the payload
> with the stream cipher as follows:
>
> OP receives relay cell from node 1:
>
> For I=1...n, where n is the end node on the circuit:
> C_{I+1} = Decrypt(Kb_I,N_I,C_I)
> T_I = Digest(Khb_I,T'_I||C_{I+1})
> N_{I+1} = T_I ^ D(Ktb_I,T_I ^ N_I)
> T'_I = T_I
>
> If the payload is recognized (see Section 4.1),
> then:
>
> The sending node is I. Stop, process the
> payload and authenticate.
>
> 4. Application connections and stream management
>
> 4.1. Relay cells
>
> Within a circuit, the OP and the end node use the contents of
> RELAY packets to tunnel end-to-end commands and TCP connections
> ("Streams") across circuits. End-to-end commands can be initiated
> by either edge; streams are initiated by the OP.
>
> The payload of each unencrypted RELAY cell consists of:
>
> Relay command [1 byte]
> 'Recognized' [2 bytes]
> StreamID [2 bytes]
> Length [2 bytes]
> Data [PAYLOAD_LEN-23 bytes]
>
> The 'recognized' field is used as a simple indication that the
> cell is still encrypted. It is an optimization to avoid
> calculating expensive digests for every cell. When sending cells,
> the unencrypted 'recognized' MUST be set to zero.
>
> When receiving and decrypting cells the 'recognized' will always
> be zero if we're the endpoint that the cell is destined for. For
> cells that we should relay, the 'recognized' field will usually
> be nonzero, but will accidentally be zero with P=2^-16.
>
> If the cell is recognized, the node moves to verifying the
> authenticity of the message as follows(*):
>
> forward direction (executed by the end node):
>
> T_{n+1} = Digest(Khf_n,T'_{n+1}||C_{n+1})
> Tag = T_{n+1} ^ D(Ktf_n,T_{n+1} ^ N_{n+1})
> T'_{n+1} = T_{n+1}
>
> The message is authenticated (i.e., M = C_{n+1}) if
> and only if Tag = 0
>
> backward direction (executed by the OP):
>
> The message is authenticated (i.e., C_{n+1} = M) if
> and only if N_{n+1} = 0
>
>
> The old Digest field is removed since sufficient information for
> authentication is now included in the nonce part of the payload.
>
> (*) we should consider dropping the 'recognized' field
> altogether and always try to authenticate. Note that this is
> an optimization question and the crypto works just as well
> either way.
>
> The 'Length' field of a relay cell contains the number of bytes
> in the relay payload which contain real payload data. The
> remainder of the payload is padding bytes.
>
> 4.2. Appending the encrypted nonce and dealing with version-homogenic
> and version-heterogenic circuits
>
> When a cell is prepared to be routed from the origin (see Section
> 3.1.1) the encrypted nonce N is appended to the encrypted cell
> (occupying the last 16 bytes of the cell). If the cell is
> prepared to be sent to a node supporting the new protocol, S is
> combined with other sources to generate the layer's
> nonce. Otherwise, if the node only supports the old protocol, n
> is still appended to the encrypted cell (so that following nodes
> can still recover their nonce), but a synchronized nonce (as per
> the old protocol) is used in CTR-mode.
>
> When a cell is sent along the circuit in the 'backward'
> direction, nodes supporting the new protocol always assume that
> the last 16 bytes of the input are the nonce used by the previous
> node, which they process as per Section 3.2.1. If the previous
> node also supports the new protocol, these cells are indeed the
> nonce. If the previous node only supports the old protocol, these
> bytes are either encrypted padding bytes or encrypted data.
>
> 5. Security
>
> 5.1. Resistance to crypto-tagging attacks
>
> A crypto-tagging attack involves a circuit with two colluding
> nodes and at least one honest node between them. The attack works
> when one node makes a change to the cell (tagging) in a way that
> can be undone by the other colluding party. In between, the
> tagged cell is processed by honest nodes which do not detect the
> change. The attack is possible due to the malleability property
> of CTR-mode: a change to a ciphertext bit effects only the
> respective plaintext bit in a predicatble way. This proposal
> frustrates the crypto-tagging attack by linking the nonce to the
> encrypted message such that any change to the ciphertext results
> in a random nonce and hence, random plaintext.
>
> Let us consider the following 3-hop scenario: the entry and end
> nodes are malicious and colluding and the middle node is honest.
>
> 5.1.1. forward direction
>
> Suppose that node I tags the ciphertext part of the message
> (C'_{I+1} != C_{I+1}) then forwards it to the next node (I+1). As
> per Section 3.1.2. Node I+1 digests C'_{I+1} to generate T_{I+1}
> and N_{I+2}. Since C'_{I+2} is different than it should be, so
> are the resulting T_{I+1} and N_{I+2}. Hence, decrypting C'_{I+2}
> using these values results in a random string for C_{I+2}. Since
> C_{I+2} is now just a random string, it is decrypted into a
> random string and cannot be 'recognized' nor
> authenticated. Furthermore, since C'_{I+1} is different than what
> it should be, T'_{I+1} (i.e., the running digest of the middle
> node) is now out of sync with that of the OP, which means that
> all future cells sent through this node will decrypt into garbage
> (random strings).
>
> Likewise, suppose that instead of tagging the ciphertext, Node I
> node tags the encrypted nonce N'_{I+1} != N_{I+1}. Now, when Node
> I+1 digests the payload the tweak T_{I+1} is find, but using it
> to decrypt N'_{I+1} again results in a random nonce for
> N_{I+2}. This random nonce is used to decrypt C_{I+1} into a
> random C'_{I+2} which is not recognized by the end node. Since
> C_{I+2} is now a random string, the running digest of the end
> node is now out of sync, which prevents the end node from
> decrypting further cells.
>
> 5.1.2. Backward direction
>
> In the backward direction the tagging is done by Node I+2
> untagging by the Node I. Suppose first that Node I+2 tags the
> ciphertext C_{I+2} and sends it to Node I+1. As per Section
> 3.2.1, Node I+1 first digests C_{I+2} and uses the resulting
> T_{I+1} to generate a nonce N_{I+1}. From this it is clear that
> any change introduced by Node I+2 influences the entire payload
> and cannot be removed by Node I.
>
> Unlike in Section 5.1.1., the cell is blindly delivered by Node I
> to the OP which decrypts it. However, since the payload leaving
> the end node was modified, the message cannot be authenticated by
> the OP which can be trusted to tear down the circuit.
>
> Suppose now that tagging is done by Node I+2 to the nonce part of
> the payload, i.e., N_{I+2}. Since this value is encrypted by Node
> I+1 to generate its own nonce N_{I+1}, again, a random nonce is
> used which affects the entire keystream of CTR-mode. The cell
> again cannot be authenticated by the OP and the circuit is torn
> down.
>
> We note that the end node can modify the plain message before
> ever encrypting it and this cannot be discovered by the Tor
> protocol. This vulnerability is outside the scope of this
> proposal and users should always use TLS to make sure that their
> application data is encrypted before it enters the Tor network.
>
> 5.2. End-to-end authentication
>
> Similar to the old protocol, this proposal only offers end-to-end
> authentication rather than per-hop authentication. However,
> unlike the old protocol, the ADL-construction is non-malleable
> and hence, once a non-authentic message was processed by an
> honest node supporting the new protocol, it is effectively
> destroyed for all nodes further down the circuit. This is because
> the nonce used to de/encrypt all messages is linked to (a digest
> of) the payload data.
>
> As a result, while honest nodes cannot detect non-authentic
> messages, such nodes still destroy the message thus invalidating
> its authentication tag when it is checked by edge nodes. As a
> result, security against crypto-tagging attacks is ensured as
> long as an honest node supporting the new protocol processes the
> message between two dishonest ones.
>
> 5.3 The Running Digest
>
> Unlike the old protocol, the running digest is now computed as
> the output of a GHASH call instead of a hash function call
> (SHA256). Since GHASH does not provide the same type of security
> guarantees as SHA256, it is worth discussing why security is not
> lost from computing the running digest differently.
>
> The running digets is used to ensure that if the same payload is
> encrypted twice, then the resulting ciphertext does not remain
> the same. Therefore, all that is needed is that the digest should
> repeat with low probability. GHASH is a universal hash function,
> hence it gives such a guarantee assuming its key is chosen
> uniformly at random.
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