commit 317d09bf14ef32382e450b7d131f56853e2ffef6 Author: Nick Mathewson nickm@torproject.org Date: Sat Sep 14 20:19:23 2019 -0400
Add Counter Galois Onion as proposal 308 --- proposals/000-index.txt | 2 + proposals/308-counter-galois-onion.txt | 550 +++++++++++++++++++++++++++++++++ 2 files changed, 552 insertions(+)
diff --git a/proposals/000-index.txt b/proposals/000-index.txt index 69ae099..f65f50e 100644 --- a/proposals/000-index.txt +++ b/proposals/000-index.txt @@ -228,6 +228,7 @@ Proposals by number: 305 ESTABLISH_INTRO Cell DoS Defense Extension [DRAFT] 306 A Tor Implementation of IPv6 Happy Eyeballs [OPEN] 307 Onion Balance Support for Onion Service v3 [DRAFT] +308 Counter Galois Onion: A New Proposal for Forward-Secure Relay Cryptography [DRAFT]
Proposals by status: @@ -240,6 +241,7 @@ Proposals by status: 303 When and how to remove support for protocol versions 305 ESTABLISH_INTRO Cell DoS Defense Extension 307 Onion Balance Support for Onion Service v3 + 308 Counter Galois Onion: A New Proposal for Forward-Secure Relay Cryptography NEEDS-REVISION: 212 Increase Acceptable Consensus Age [for 0.2.4.x+] 219 Support for full DNS and DNSSEC resolution in Tor [for 0.2.5.x] diff --git a/proposals/308-counter-galois-onion.txt b/proposals/308-counter-galois-onion.txt new file mode 100644 index 0000000..ead0e81 --- /dev/null +++ b/proposals/308-counter-galois-onion.txt @@ -0,0 +1,550 @@ +Filename: 308-counter-galois-onion.txt +Title: Counter Galois Onion: A New Proposal for Forward-Secure Relay Cryptography +Authors: Jean Paul Degabriele, Alessandro Melloni, Martijn Stam +Created: 13 Sep 2019 +Last-Modified: 13 Sep 2019 +Status: Draft + + + +1. Background and Motivation + + In Proposal 202, Mathewson expressed the need to update Tor's Relay + cryptography and protect against tagging attacks. Towards this goal he + outlined two possible approaches for constructing an onion encryption + scheme that should be able to withstand tagging attacks. Later, in + Proposal 261, Mathewson proposed a concrete scheme based on the + tweakable wide-block cipher AEZ. The security of Proposal 261 was + analysed in [DS18]. An alternative scheme was suggested in Proposal 295 + which combines an instantiation of the PIV construction from [ST14] and + a variant of the GCM-RUP construction from [ADL17]. In this document we + propose yet another scheme, Counter Galois Onion (CGO) + which improves over proposals 261 and 295 in a number of ways. CGO has + a minimalistic design requiring only a block cipher in counter-mode and + a universal hash function. To take advantage of Intel's AES-NI and + PCLMULQDQ instructions we recommend using AES and POLYVAL [GLL18]. In + terms of security, it protects against tagging attacks while + simultaneously providing forward security with respect to end-to-end + authenticity and confidentiality. Furthermore CGO performs better than + proposal 295 in terms of efficiency and its support of "leaky pipes". + + +1.2 Design Overview + + CGO makes due with a universal hash function while simultaneously + satisfying forward security. It employs two distinct types of + encryption, a dynamic encryption scheme DEnc and a static encryption + scheme SEnc. DEnc is used for end-to-end encryption (layer n) and SEnc + is used for the intermediate layers (n-1 to 1). DEnc is a Forward- + Secure Authenticated Encryption scheme for securing end-to-end + communication and SEnc provides the non-malleability for protecting + against tagging attacks. In order to provide forward security, the key + material in DEnc is updated with every encryption whereas in SEnc the + key material is static. To support leaky pipes, in the forward + direction each OR first attempts a partial decryption using DEnc and + if it fails it reverts to decrypting using SEnc. The rest of the + document describes the scheme's operation in terms of the low-level + primitives and we make no further mention of DEnc and SEnc. However, + on an intuitive level it can be helpful to think of: + + a) the combinations of E(KSf_I, *) and PH(HSf_I, *) as well as + E(KDf_I, *) and PH(HDf_I, *) as two instances of a tweakable block + cipher, + + b) the operation E(Sf_I, <0>) | E(Sf_I, <1>) | E(Sf_I, <2>) | ... as a + PRG with seed Sf_I, + + c) and E(JSf_I, <IV>) | E(JSf_I, <IV+1>) | ... | E(JSf_I, <IV+31>) as + counter-mode encryption with <IV> as the initial vector. + + +2. Preliminaries + +2.1. Notation + + Symbol Meaning + ------ ------- + M Plaintext + Sf_I PRG Seed, forward direction, layer I + Sb_I PRG Seed, backward direction, layer I + Cf_I Ciphertext, forward direction, layer I + Cb_I Ciphertext, backward direction, layer I + Tf_I Tag, forward direction, layer I + LTf_I Last Tag, forward direction, layer I + Tb_I Tag, backward direction, layer I + LTb_I Last Tag, backward direction, layer I + Nf_I Nonce, forward direction, layer I + LNf_I Last Nonce, forward direction, layer I + Nb_I Nonce, backward direction, layer I + LNb_I Last Nonce, backward direction, layer I + JSf_I Static Block Cipher Key, forward direction, layer I + JSb_I Static Block Cipher Key, backward direction, layer I + KSf_I Static Block Cipher Key, forward direction, layer I + KSb_I Static Block Cipher Key, backward direction, layer I + KDf_I Dynamic Block Cipher Key, forward direction, layer I + KDb_I Dynamic Block Cipher Key, backward direction, layer I + HSf_I Static Poly-Hash Key, forward direction, layer I + HSb_I Static Poly-Hash Key, backward direction, layer I + HDf_I Dynamic Poly-Hash Key, forward direction, layer I + HDb_I Dynamic Poly-Hash Key, backward direction, layer I + ^ Bitwise XOR operator + | Concatenation + && Logical AND operator + Z[a, b] For a string Z, the substring from byte a to byte b + (indexing starts at 1) + INT(X) Translate string X into an unsigned integer + +2.2. Security parameters %%%REVISE + + POLY_HASH_LEN -- The length of the polynomial hash function's output, + in bytes. For POLYVAL, POLY_HASH_LEN = 16. + + PAYLOAD_LEN -- The longest allowable cell payload, in bytes (509). + + HASH_KEY_LEN -- The key length used to digest messages in bytes. + For POLYVAL, DIG_KEY_LEN = 16. + + BC_KEY_LEN -- The key length, in bytes, of the block cipher used. For + AES we recommend ENC_KEY_LEN = 16. + + BC_BLOCK_LEN -- The block length, in bytes, of the block cipher used. + For AES, BC_BLOCK_LEN = 16. + +2.3. Primitives + + The polynomial hash function is POLYVAL with a HASH_KEY_LEN-bit key. We + write this as PH(H, M) where H is the key and M the message to be hashed. + + We use AES with a BC_KEY_LEN-bit key. For AES encryption (resp., + decryption) we write E(K, X) (resp., D(K, X)) where K is a BC_KEY_LEN-bit + key and X the block to be encrypted (resp., decrypted). For an integer + j, we use <j> to denote the string of length BC_BLOCK_LEN representing + that integer. + +2.4 Key derivation and initialisation (replaces Section 5.2.2) + + For newer KDF needs, Tor uses the key derivation function HKDF from + RFC5869, instantiated with SHA256. (This is due to a construction + from Krawczyk.) The generated key material is: + + K = K_1 | K_2 | K_3 | ... + + Where H(x, t) is HMAC_SHA256 with value x and key t + and K_1 = H(m_expand | INT8(1) , KEY_SEED ) + and K_(i+1) = H(K_i | m_expand | INT8(i+1) , KEY_SEED ) + and m_expand is an arbitrarily chosen value, + and INT8(i) is an octet with the value "i". + + In RFC5869's vocabulary, this is HKDF-SHA256 with info == m_expand, + salt == t_key, and IKM == secret_input. + +2.4.1. Key derivation using the KDF + + When used in the ntor handshake, for each layer I, the key material is + split into the following sequence of contiguous values: + + Length Purpose Notation + ------ ------- -------- + BC_KEY_LEN forward Seed Sf_I + BC_KEY_LEN backward Seed Sb_I + + if (I < n) in addition derive the following static keys: + + BC_KEY_LEN forward BC Key KSf_I + BC_KEY_LEN backward BC Key KSb_I + BC_KEY_LEN forward CTR Key JSf_I + BC_KEY_LEN backward CTR Key JSb_I + HASH_KEY_LEN forward poly hash key HSf_I + HASH_KEY_LEN backward poly hash key HSb_I + + Excess bytes from K are discarded. + +2.4.2. Initialisation from Seed + + For each layer I compute E(Sf_I, <0>) | E(Sf_I, <1>) | E(Sf_I, <2>) | ... + and parse the output as: + + Length Purpose Notation + ------ ------- -------- + BC_BLOCK_LEN forward Nonce Nf_I + BC_KEY_LEN forward BC Key KDf_I + HASH_KEY_LEN forward poly hash key HDf_I + BC_KEY_LEN new forward Seed Sf'_I + + Discard excess bytes, replace Sf_I with Sf'_I, and set LNf_n and LTf_I + to the zero string. + + Similarly for the backward direction, compute E(Sb_I, <0>) | E(Sb_I, <1>) + | E(Sb_I, <2>) | ... and parse the output as: + + Length Purpose Notation + ------ ------- -------- + BC_BLOCK_LEN backward Nonce Nb_I + BC_KEY_LEN forward BC Key KDb_I + HASH_KEY_LEN forward poly hash key HDb_I + BC_KEY_LEN new backward Seed Sb'_I + + Discard excess bytes, replace Sb_I with Sb'_I, and set LNb_n and LTb_I + to the zero string. + + NOTE: For layers n-1 to 1 the values Nf_I, KDf_I, HDf_I, Sf_I and their + backward counterparts are only required in order to support leaky + pipes. If leaky pipes is not required these values can be safely + omitted. + + +3. Routing relay cells + + Let n denote the number of nodes in the circuit. Then encryption layer n + corresponds to the encryption between the OP and the exit/destination + node. + + +3.1. Forward Direction + + The forward direction is the direction that CREATE/CREATE2 cells + are sent. + + +3.1.1. Routing From the Origin + + When an OP sends a relay cell, the cell is produced as follows: + + The OP computes E(Sf_n, <0>) | E(Sf_n, <1>) | E(Sf_n, <2>) | ... + and parses the output as + + Length Purpose Notation + ------ ------- -------- + 509 encryption pad Z + BC_BLOCK_LEN backward Nonce Nf'_I + BC_KEY_LEN forward BC Key KDf'_I + HASH_KEY_LEN forward poly hash key HDf'_I + BC_KEY_LEN new forward Seed Sf'_I + + Excess bytes are discarded. It then computes the n'th layer ciphertext + (Tf_n, Cf_n) as follows: + + Cf_n = M ^ Z + X_n = PH(HDf_n, (LNf_n | Cf_n)) + Y_n = Nf_n ^ X_n + Tf_n = E(KDf_n, Y_n) ^ X_n) + + and updates its state by overwriting the old variables with the new + ones. + + LNf_n = Nf_n + Nf_n = Nf'_n + KDf_n = KDf'_n + HDf_n = HDf'_n + Sf_n = Sf'_n + + It then applies the remaining n-1 layers of encryption to (Tf_n, Cf_n) + as follows: + + For I = n-1 to 1: + IV = INT(Tf_{I+1}) + Z = E(JSf_I, <IV>) | E(JSf_I, <IV+1>) | ... | E(JSf_I, <IV+31>) + % BC_BLOCK_LEN = 16 + Cf_I = Cf_{I+1} ^ Z[1, 509] + X_I = PH(HSf_n, (LTf_{I+1} | Cf_I)) + Y_I = Tf_I ^ X_I + Tf_I = E(KSf_I, Y_I) ^ X_I + LTf_{I+1} = Tf_{I+1} + + Upon completion the OP sends (Tf_1, Cf_1) to node 1. + + +3.1.2. Relaying Forward at Onion Routers + + When a forward relay cell (Tf_I, Cf_I) is received by OR I, it decrypts + it performs the following set of steps: + + 'Forward' relay cell: + + X_I = PH(HDf_n, (LNf_I | Cf_I)) + Y_I = Tf_I ^ X_I + if (Nf_I == D(KDf_I, Y_I) ^ X_I) % cell recognized and authenticated + compute E(Sf_I, <0>) | E(Sf_I, <1>) | E(Sf_I, <2>) | ... and parse the + output as Z, Nf'_I, KDf'_I, HDf'_I, Sf'_I + + M = Cf_n ^ Z + LNf_I = Nf_I + Nf_I = Nf'_I + KDf_I = KDf'_I + HDf_I = HDf'_I + Sf_I = Sf'_I + + return M + + else if (I == n) % last node, decryption has failed + send DESTROY cell to tear down the circuit + + else % decrypt and forward cell + X_I = PH(HSf_I, (LTf_{I+1} | Cf_I)) + Y_I = Tf_I ^ X_I + Tf_{I+1} = D(KSf_I, Y_I) ^ X_I + IV = INT(Tf_{I+1}) + Z = E(JSf_I, <IV>) | E(JSf_I, <IV+1>) | ... | E(JSf_I, <IV+31>) + % BC_BLOCK_LEN = 16 + Cf_{I+1} = Cf_I ^ Z[1, 509] + + forward (Tf_{I+1}, Cf_{I+1}) to OR I+1 + +3.2. Backward Direction + + The backward direction is the opposite direction from + CREATE/CREATE2 cells. + +3.2.1. Routing From the Exit Node + + At OR n encryption proceeds as follows: + + It computes E(Sb_n, <0>) | E(Sb_n, <1>) | E(Sb_n, <2>) | ... + and parses the output as + + Length Purpose Notation + ------ ------- -------- + 509 encryption pad Z + BC_BLOCK_LEN backward Nonce Nb'_I + BC_KEY_LEN forward BC Key KDb'_I + HASH_KEY_LEN forward poly hash key HDb'_I + BC_KEY_LEN new forward Seed Sb'_I + + Excess bytes are discarded. It then computes the ciphertext + (Tf_n, Cf_n) as follows: + + Cb_n = M ^ Z + X_n = PH(HDb_n, (LNb_n | Cb_n)) + Y_n = Nb_n ^ X_n + Tb_n = E(KDb_n, Y_n) ^ X_n) + + and updates its state by overwriting the old variables with the new + ones. + + LNb_n = Nb_n + Nb_n = Nb'_n + KDb_n = KDb'_n + HDb_n = HDb'_n + Sb_n = Sb'_n + + +3.2.2. Relaying Backward at the Onion Routers + + At OR I (for I < n) when a ciphertext (Tb_I, Cb_I) in the backward + direction is received it is processed as follows: + + X_I = PH(HSb_n, (LTb_{I-1} | Cb_I)) + Y_I = Tb_I ^ X_I + Tb_{I-1} = D(KSb_I, Y_I) ^ X_I + IV = INT(Tb_{I-1}) + Z = E(JSb_I, <IV>) | E(JSb_I, <IV+1>) | ... | E(JSb_I, <IV+31>) + % BC_BLOCK_LEN = 16 + Cb_{I-1} = Cb_I ^ Z[1, 509] + + The ciphertext (Tb_I, Cb_I) is then passed along the circuit towards + the OP. + + +3.2.2. Routing to the Origin + + When a ciphertext (Tb_1, Cb_1) arrives at an OP, the OP decrypts it in + two stages. It first reverses the layers from 1 to n-1 as follows: + + For I = 1 to n-1: + X_I = PH(HSb_I, (LTb_{I+1} | Cb_I)) + Y_I = Tb_I ^ X_I + Tb_{I+1} = E(KSb_I, Y_I) ^ X_I + IV = INT(Tb_{I+1}) + Z = E(JSb_I, <IV>) | E(JSb_I, <IV+1>) | ... | E(JSb_I, <IV+31>) + % BC_BLOCK_LEN = 16 + Cb_{I+1} = Cb_I ^ Z[1, 509] + + Upon completion the n'th layer of encryption is removed as follows: + + X_n = PH(HDb_n, (LNb_n | Cb_n)) + Y_n = Tb_n ^ X_n + if (Nb_n = D(KDb_n, Y_n) ^ X_n) % authentication is successful + compute E(Sb_n, <0>) | E(Sb_n, <1>) | E(Sb_n, <2>) | and parse the + output as Z, Nb'_n, KDb'_n, HDb'_n, Sb'_n + + M = Cb_n ^ Z + LNb_n = Nb_n + Nb_n = Nb'_n + KDb_n = KDb'_n + HDb_n = HDb'_n + Sb_n = Sb'_n + + return M + + else + send DESTROY cell to tear down the circuit + + +4. Application connections and stream management + +4.1. Amendments to the Relay Cell Format + + 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] + StreamID [2 bytes] + Length [2 bytes] + Data [PAYLOAD_LEN-21 bytes] + + The old Digest field is removed since sufficient information for + authentication is now included in the nonce part of the payload. + + The old 'Recognized' field is removed. Instead a cell is recognized + via a partial decryption using the node's dynamic keys - namely the + following steps (already included in Section 3): + + Forward direction: + + X_I = PH(HDf_n, (LNf_I | Cf_I)) + Y_I = Tf_I ^ X_I + if (Nf_I == D(KDf_I, Y_I) ^ X_I) % cell is recognized and authenticated + + Backward direction (executed by the OP): + + If the OP is aware of the number of layers present in the cell there + is no need to attempt to recognize the cell. Otherwise the OP can, for + each layer, first attempt a partial decryption using the dynamic keys + for that layer as follows: + + X_I = PH(HDb_I, (LNb_I | Cb_I)) + Y_I = Tb_I ^ X_I + if (Nb_I = D(KDb_I, Y_I) ^ X_I) % cell is recognized and authenticated + + 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 and Design Rationale + + We are currently working on a security proof to better substantiate our + security claims. Below is a short informal summary on the security of + CGO and its design rationale. + +5.1. Resistance to crypto-tagging attacks + + Protection against crypto-tagging attacks is provided by layers n-1 to + 1. This part of the scheme is based on the paradigm from [ADL17] which + has the property that if any single bit of the OR's input is changed + then all of the OR's output will be randomised. Specifically, if + (Tf_I, Cf_I) is travelling in the forward direction and is processed by + an honest node I, a single bit flip to either Tf_I or Cf_I will result + in both Tf_{I+1} and Cf_{I+1} being completely randomised. In addition, + the processing of (Tf_I, Cf_I) includes LTf_{I+1} so that any + modification to (Tf_I, Cf_I) at time j will in turn randomise the value + (Tf_{I+1}, Cf_{I+1}) at any time >= j . Thus once a circuit is tampered + with it is not possible to recover from it at a later stage. This helps + to protect against the standard crypto-tagging attack and variations + thereof (Section 5.2 in [DS18]). A similar argument holds in the + backward direction. + + +5.2. End-to-end authenticated encryption + + Layer n provides end-to-end authenticated encryption. Similar to the + old protocol, this proposal only offers end-to-end authentication + rather than per-hop authentication. However, CGO provides 128-bit + authentication as opposed to the 32-bit authentication provided by the + old protocol. A main observation underpinning the design of CGO is + that the n'th layer does not need to be secure against the release of + unverified plaintext (RUP). RUP security is only needed to protect + against tagging attacks and the n'th layer does not help in that regard + (but the layers below do). Consequently we employ a different scheme at + the n'th layer which is designed to provide forward-secure + authenticated encryption. + + +5.3 Forward Security + + As mentioned in the previous section CGO provides end-to-end + authenticated encryption that is also forward secure. Our notion of + forward security follows the definitions of Bellare and Yee [BY03] for + both confidentiality and authenticity. Forward-secure confidentiality + says that upon corrupting either the sender (or the receiver), the + secrecy of the messages that have already been sent (or received) is + still guaranteed. As for forward-secure authentication, upon corrupting + the sender the authenticity of previously authenticated messages is + still guaranteed (even if they have not yet been received). In order to + achieve forward-secure authenticated encryption, CGO updates the key + material of the n'th layer encryption with every cell that is + processed. In order to support leaky pipes the lower layers also need + to maintain a set of dynamic keys that are used to recognize cells that + are intended for them. This key material is only used for partial + processing, i.e. recognizing the cell, and is only updated if + verification is successful. If the cell is not recognized, the node + reverts to processing the cell with the static key material. If support + for leaky-pipes is not required this extra processing can be omitted. + + +6. Efficiency Considerations + + Although we have not carried out any experiments to verify this, we + expect CGO to perform relatively well in terms of efficiency. Firstly, + it manages to achieve forward security with just a universal hash as + opposed to other proposals which suggested the use of SHA2 or SHA3. In + this respect we recommend using POLYVAL [GLL18], a variant of GHASH + that is more compatible with Intel's PCMULQDQ instruction. Furthermore + CGO admits a certain degree of parallelisability. Supporting leaky + pipes requires an OR to first verify the cell using the the dynamic key + material and if the cell is unrecognised it goes on to process the cell + with the static key material. The important thing to note (see for + instance Section 3.1.2) is that the initial processing of the cell + using the static key material is almost identical to the verification + using the dynamic key material, and the two computations are + independent of each other. As such, although in Section 3 these were + described as being evaluated sequentially, they can in fact be computed + in parallel. In particular the two polynomial hashes could be computed + in parallel by using the new vectorised VPCMULQDQ instruction. + + We are currently looking into further optimisations of the scheme as + presented here. One such optimisation is the possibility of removing + KDf_I and KDb_I while retaining forward security. This would further + improve the efficiency of the scheme by reducing the amount of dynamic + key material that needs to be updated with every cell that is processed. + + +References + +[ADL17] Tomer Ashur, Orr Dunkelman, Atul Luykx, "Boosting Authenticated +Encryption Robustness with Minimal Modifications", CRYPTO 2017. + +[BY03] Mihir Bellare, Bennett Yee, "Forward-Security in Private-Key +Cryptography", CT-RSA 2003. + +[DS18] Jean Paul Degabriele, Martijn Stam, "Untagging Tor: A Formal +Treatment of Onion Encryption", EUROCRYPT 2018. + +[GLL18] Shay Gueron, Adam Langley, Yehuda Lindell, "AES-GCM-SIV: Nonce +Misuse-Resistant Authenticated Encryption", RFC 8452, April 2019. + +[ST13] Thomas Shrimpton, R. Seth Terashima, "A Modular Framework for +Building Variable-Input Length Tweakable Ciphers", ASIACRYPT 2013.