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.