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[tor-dev] [RFC] Proposal: "Res tokens: Anonymous Credentials for Onion Service DoS Resilience"
Hello all,
after lots of investigation on anonymous credentials, we are glad to
present you with a draft of the onion services anti-DoS proposal using
tokens.
While the basic idea of the proposal should remain reasonably solid,
there are various XXX sprinkled around the proposal and some of them
definitely need to be addressed before the proposal becomes truly
usable.
We are particularly looking forward to feedback about:
- Token issuance services
- The anonymous credential scheme chosen
- The XXXs and design decisions of the proposal
Hope you have a pleasant read!
---
```
Filename: 331-res-tokens-for-anti-dos.md
Title: Res tokens: Anonymous Credentials for Onion Service DoS Resilience
Author: George Kadianakis, Mike Perry
Created: 11-02-2021
Status: Draft
```
+--------------+ +------------------+
| Token Issuer | | Onion Service |
+--------------+ +------------------+
^ ^
| +----------+ |
Issuance | 1. | | 2. | Redemption
+------->| Alice |<-------+
| |
+----------+
# 0. Introduction
This proposal specifies a simple anonymous credential scheme based on Blind
RSA signatures designed to fight DoS abuse against onion services. We call
the scheme "Res tokens".
Res tokens are issued by third-party issuance services, and are verified by
onion services during the introduction protocol (through the INTRODUCE1
cell).
While Res tokens are used for denial of service protection in this proposal,
we demonstrate how they can have application in other Tor areas as well, like
improving the IP reputation of Tor exit nodes.
# 1. Motivation
Denial of service attacks against onion services have been explored in the past
and various defenses have been proposed:
- Tor proposal #305 specifies network-level rate-limiting mechanisms.
- Onionbalance allows operators to scale their onions horizontally.
- Tor proposal #327 increases the attacker's computational requirements (not implemented yet).
While the above proposals in tandem should provide reasonable protection
against many DoS attackers, they fundamentally work by reducing the assymetry
between the onion service and the attacker. This won't work if the attacker
is extremely powerful because the assymetry is already huge and cutting it
down does not help.
we believe that a proposal based on cryptographic guarantees -- like Res
tokens -- can offer protection against even extremely strong attackers.
# 2. Overview
In this proposal we introduce an anonymous credential scheme -- Res tokens --
that is well fitted for protecting onion services against DoS attacks. We
also introduce a system where clients can acquire such anonymous credentials
from various types of Token Issuers and then redeem them at the onion service
to gain access even when under DoS conditions.
In section [TOKEN_DESIGN], we list our requirements from an anonymous
credential scheme and provide a high-level overview of how the Res token
scheme works.
In section [PROTOCOL_SPEC], we specify the token issuance and redemption protocols,
as well as the mathematical operations that need to be conducted for these to work.
In section [TOKEN_ISSUERS], we provide a few examples and guidelines for
various token issuer services that could exist.
In section [DISCUSSION], we provide more use cases for Res tokens as well as
future improvements we can conduct to the scheme.
# 3. Design [TOKEN_DESIGN]
In this section we will go over the high-level design of the system, and on
the next section we will delve into the lower-level details of the protocol.
## 3.1. Anonymous credentials
Anonymous credentials or tokens are cryptographic identifiers that allow
their bearer to maintain an identity while also preserving anonymity.
Clients can acquire a token in a variety of ways (e.g. registering on a
third-party service, solving a CAPTCHA, completing a PoW puzzle) and then
redeem it at the onion service proving this way that work was done, but
without linking the act of token acquisition with the act of token
redemption.
## 3.2. Anonymous credential properties
The anonymous credential literature is vast and there are dozens of
credential schemes with different properties [REF_TOKEN_ZOO], in this section
we detail the properties we care about for this use case:
- Public Verifiability: Because of the distributed trust properties of the
Tor network, we need anonymous credentials that can be issued by one
party (the token issuer) and verified by a different party (in this case
the onion service).
- Perfect unlinkability: Unlinkability between token issuance and token
redemption is vital in private settings like Tor. For this reason we want
our scheme to preserve its unlinkability even if its fundamental security
assumption is broken. We want unlinkability to be protected by
information theoretic security or random oracle, and not just
computational security.
- Small token size: The tokens will be transfered to the service through the
INTRODUCE1 cell which is not flexible and has only a limited amount of
space (about 200 bytes) [REF_INTRO_SPACE]. We need tokens to be small.
- Quick Verification: Onions are already experiencing resource starvation
because of the DoS attacks so it's important that the process of
verifying a token should be as quick as possible. In section [TOKEN_PERF]
we will go deeper into this requirement.
After careful consideration of the above requirements, we have leaned towards
using Blind RSA as the primitive for our tokens, since it's the fastest
scheme by far that also allows public verifiability. See also Appendix B
[BLIND_RSA_PROOF] for a security proof sketch of Blind RSA perfect unlinkability.
## 3.3. Other security considerations
Apart from the above properties we also want:
- Double spending protection: We don't want Malory to be able to double spend
her tokens in various onion services thereby amplifying her attack. For
this reason our tokens are not global, and can only be redeemed at a
specific destination onion service.
- Metadata: We want to encode metadata/attributes in the tokens. In
particular, we want to encode the destination onion service and an
expiration date. For more information see section [DEST_DIGEST]. For
blind RSA tokens this is usually done using "partially blind signatures"
but to keep it simple we instead encode the destination directly in the
message to be blind-signed and the expiration date using a set of
rotating signing keys.
- One-show: There are anonymous credential schemes with multi-show support
where one token can be used multiple times in an unlinkable
fashion. However, that might allow an adversary to use a single token to
launch a DoS attack, since revocation solutions are complex and
inefficient in anonymous credentials. For this reason, in this work we
use one-show tokens that can only be redeemed once. That takes care of
the revocation problem but it means that a client will have to get more
tokens periodically.
## 3.4. Res tokens overview
Throughout this proposal we will be using our own token scheme, named "Res",
which is based on blind RSA signatures. In this modern cryptographic world,
not only we have the audacity of using Chaum's oldest blind signature scheme
of all times, but we are also using RSA with a modulus of 1024 bits...
The reason that Res uses only 1024-bits RSA is because we care most about
small token size and quick verification rather than the unforgeability of the
token. This means that if the attacker breaks the issuer's RSA signing key
and issues tokens for herself, this will enable the adversary to launch DoS
attacks against onion services, but it won't allow her to link users (because
of the "perfect unlinkability" property).
Furthermore, Res tokens get a short implicit expiration date by having the
issuer rapidly rotate issuance keys every few hours. This means that even if
an adversary breaks an issuance key, she will be able to forge tokens for
just a few hours before that key expires.
For more ideas on future schemes and improvements see section [FUTURE_RES].
## 3.5. Token performance requirements [TOKEN_PERF]
As discussed above, verification performance is extremely important in the
anti-DoS use case. In this section we provide some concrete numbers on what
we are looking for.
In proposal #327 [REF_POW_PERF] we measured that the total time spent by the
onion service on processing a single INTRODUCE2 cell ranges from 5 msec to 15
msecs with a mean time around 5.29 msec. This time also includes the launch
of a rendezvous circuit, but does not include the additional blocking and
time it takes to process future cells from the rendezvous point.
We also measured that the parsing and validation of INTRODUCE2 cell ("top
half") takes around 0.26 msec; that's the lightweight part before the onion
service decides to open a rendezvous circuit and do all the path selection
and networking.
This means that any defenses introduced by this proposal should add minimal
overhead to the above "top half" procedure, so as to apply access control in
the lightest way possible.
For this reason we implemented a basic version of the Res token scheme in
Rust and benchmarked the verification and issuance procedure [REF_RES_BENCH].
We measured that the verification procedure from section [RES_VERIFY] takes
about 0.104 ms, which we believe is a reasonable verification overhead for
the purposes of this proposal.
We also measured that the issuance procedure from [RES_ISSUANCE] takes about
0.614 ms.
# 4. Specification [PROTOCOL_SPEC]
+--------------+ +------------------+
| Token Issuer | | Onion Service |
+--------------+ +------------------+
^ ^
| +----------+ |
Issuance | 1. | | 2. | Redemption
+------->| Alice |<-------+
| |
+----------+
## 4.0. Notation
Let `a || b` be the concatenation of a with b.
Let `a^b` denote the exponentiation of a to the bth power.
Let `a == b` denote a check for equality between a and b.
Let FDH_N(msg) be a Full Domain Hash (FDH) of 'msg' using SHA256 and
stretching the digest to be equal to the size of an RSA modulus N.
## 4.1. Token issuer setup
The Issuer creates a set of ephemeral RSA-1024 "issuance keys" that will be
used during the issuance protocol. Issuers will be rotating these ephemeral
keys every 6 hours.
The Issuer exposes the set of active issuance public keys through a REST HTTP
API that can be accessed by visiting /issuers.keys.
Tor directory authorities periodically fetch the issuer's public keys and
vote for those keys in the consensus so that they are readily available by
clients. The keys in the current consensus are considered active, whereas the
ones that have fallen off have expired.
XXX how many issuance public keys are active each time? how does overlapping
keys work? clients and onions need to know precise expiration date for
each key. this needs to be specified and tested for robustness.
XXX every how often does the fetch work? how does the voting work? which
issuers are considered official? specify consensus method.
XXX An alternative approach: Issuer has a long-term ed25519 certification key
that creates expiring certificates for the ephemeral issuance keys. Alice
shows the certificate to the service to prove that the token comes from
an issuer. The consensus includes the long-term certification key of the
issuers to establish ground truth.
This way we avoid the synchronization between dirauths and issuers, and
the multiple overlapping active issuance keys. However, certificates
might not fit in the INTRODUCE1 cell (prop220 certs take 104 bytes on
their own). Also certificate metadata might create a vector for
linkability attacks between the issuer and the verifier.
## 4.2. Onion service signals ongoing DoS attack
When an onion service is under DoS attack it adds the following line in the
"encrypted" (inner) part of the v3 descriptor as a way to signal to its
clients that tokens are required for gaining access:
"token-required" SP token-type SP issuer-list NL
[At most once]
token-type: Is the type of token supported ("res" for this proposal)
issuer: A comma separated list of issuers which are supported by this onion service
## 4.3. Token issuance
When Alice visits an onion service with an active "token-required" line in
its descriptor it checks whether there are any tokens available for this
onion service in its token store. If not, it needs to acquire some and hence
the token issuance protocol commences.
### 4.3.1. Client preparation [DEST_DIGEST]
Alice first chooses an issuer supported by the onion service depending on her
preferences by looking at the consensus and her Tor configuration file for
the current list of active issuers.
After picking a supported issuer, she performs the following preparation
before contacting the issuer:
1) Alice extracts the issuer's public key (N,e) from the consensus
2) Alice computes a destination digest as follows:
dest_digest = FDH_N(destination || salt)
where:
- 'destination' is the 32-byte ed25519 public identity key of the destination onion
- 'salt' is a random 32-byte value,
3) Alice samples a blinding factor 'r' uniformly at random from [1, N)
4) Alice computes:
blinded_message = dest_digest * r^e (mod N)
After this phase is completed, Alice has a blinded message that is tailored
specifically for the destination onion service. Alice will send the blinded
message to the Token Issuer, but because of the blinding the Issuer does not
get to learn the dest_digest value.
XXX Is the salt needed? Reevaluate.
### 4.3.3. Token Issuance [RES_ISSUANCE]
Alice now initiates contact with the Token Issuer and spends the resources
required to get issued a token (e.g. solve a CAPTCHA or a PoW, create an
account, etc.). After that step is complete, Alice sends the blinded_message
to the issuer through a JSON-RPC API.
After the Issuer receives the blinded_message it signs it as follows:
blinded_signature = blinded_message ^ d (mod N)
where:
- 'd' is the private RSA exponent.
and returns the blinded_signature to Alice.
XXX specify API (JSON-RPC? Needs SSL + pubkey pinning.)
### 4.3.4. Unblinding step
Alice verifies the received blinded signature, and unblinds it to get the
final token as follows:
token = blinded_signature * r^{-1} (mod N)
= blinded_message ^ d * r^{-1] (mod N)
= (dest_digest * r^e) ^d * r^{-1} (mod N)
= dest_digest ^ d * r * r^{-1} (mod N)
= dest_digest ^ d (mod N)
where:
- r^{-1} is the multiplicative inverse of the blinding factor 'r'
Alice will now use the 'token' to get access to the onion service.
By verifying the received signature using the issuer keys in the consensus,
Alice ensures that a legitimate token was received and that it has not
expired (since the issuer keys are still in the consensus).
## 4.4. Token redemption
### 4.4.1. Alice sends token to onion service
Now that Alice has a valid 'token' it can request access to the onion
service. It does so by embedding the token into the INTRODUCE1 cell to the
onion service.
To do so, Alice adds an extension to the encrypted portion of the INTRODUCE1
cell by using the EXTENSIONS field (see [PROCESS_INTRO2] section in
rend-spec-v3.txt). The encrypted portion of the INTRODUCE1 cell only gets
read by the onion service and is ignored by the introduction point.
We propose a new EXT_FIELD_TYPE value:
[02] -- ANON_TOKEN
The EXT_FIELD content format is:
TOKEN_VERSION [1 byte]
ISSUER_KEY [4 bytes]
DEST_DIGEST [32 bytes]
TOKEN [128 bytes]
SALT [32 bytes]
where:
- TOKEN_VERSION is the version of the token ([0x01] for Res tokens)
- ISSUER_KEY is the public key of the chosen issuer (truncated to 4 bytes)
- DEST_DIGEST is the 'dest_digest' from above
- TOKEN is the 'token' from above
- SALT is the 32-byte 'salt' added during blinding
This will increase the INTRODUCE1 payload size by 199 bytes since the data
above is 197 bytes, the extension type and length is 2 extra bytes, and the
N_EXTENSIONS field is always present. According to ticket #33650, INTRODUCE1
cells currently have more than 200 bytes available so we should be able to
fit the above fields in the cell.
XXX maybe we don't need to pass DEST_DIGEST and we can just derive it
XXX maybe with a bit of tweaking we can even use a 1536-bit RSA signature here...
### 4.4.2. Onion service verifies token [RES_VERIFY]
Upon receiving an INTRODUCE1 cell with the above extension the service
verifies the token. It does so as follows:
1) The service checks its double spend protection cache for an element that
matches DEST_DIGEST. If one is found, verification fails.
2) The service checks: DEST_DIGEST == FDH_N(service_pubkey || SALT), where
'service_pubkey' is its own long-term identity pubkey.
3) The service finds the corresponding issuer pubkey 'e' based on ISSUER_KEY
from the consensus or its configuration file
4) The service checks: TOKEN ^ e == DEST_DIGEST
Finally the onion service adds the DEST_DIGEST to its double spend protection
cache to avoid the same token getting redeemed twice. Onion services keep a
double spend protection cache by maintaining a sorted array of truncated
DEST_DIGEST elements.
If any of the above steps fail, the verification process aborts and the
introduction request gets discarded.
If all the above verification steps have been completed successfully, the
service knows that this a valid token issued by the token issuer, and that
the token has been created for this onion service specifically. The service
considers the token valid and the rest of the onion service protocol carries
out as normal.
# 5. Token issuers [TOKEN_ISSUERS]
In this section we go over some example token issuers. While we can have
official token issuers that are supported by the Tor directory authorities,
it is also possible to have unofficial token issuers between communities that
can be embedded directly into the configuration file of the onion service and
the client.
In general, we consider the design of token issuers to be independent from
this proposal so we will touch the topic but not go too deep into it.
## 5.1. CAPTCHA token issuer
A use case resembling the setup of Cloudflare's PrivacyPass would be to have
a CAPTCHA service that issues tokens after a successful CAPTCHA solution.
Tor Project, Inc runs https://ctokens.torproject.org which serves hCaptcha
CAPTCHAs. When the user solves a CAPTCHA the server gives back a list of
tokens. The amount of tokens rewarded for each solution can be tuned based on
abuse level.
Clients reach this service via a regular Tor Exit connection, possibly via a
dedicated exit enclave-like relay that can only connect to https://ctokens.torproject.org.
Upon receiving tokens, Tor Browser delivers them to the Tor client via the
control port, which then stores the tokens into a token cache to be used when
connecting to onion services.
In terms of UX, most of the above procedure can be hidden from the user by
having Tor Browser do most of the things under the scenes and only present
the CAPTCHA to the user if/when needed (if the user doesn't have tokens
available for that destination).
XXX specify control port API between browser and tor
## 5.2. PoW token issuer
An idea that mixes the CAPTCHA issuer with proposal#327, would be to have a
token issuer that accepts PoW solutions and provides tokens as a reward.
This solution tends to be less optimal than applying proposal#327 directly
because it doesn't allow us to fine-tune the PoW difficulty based on the
attack severity; which is something we are able to do with proposal#327.
However, we can use the fact that token issuance happens over HTTP to
introduce more advanced PoW-based concepts. For example, we can design token
issuers that accept blockchain shares as a reward for tokens. For example, a
system like Monero's Primo could be used to provide DoS protection and also
incentivize the token issuer by being able to use those shares for pool
mining [REF_PRIMO].
## 5.3. Onion service self-issuing
The onion service itself can also issue tokens to its users and then use
itself as an issuer for verification. This way it can reward trusted users by
giving it tokens for the future. The tokens can be rewarded from within the
website of the onion service and passed to the Tor Client through the control
port, or they can be provided in an out-of-bands way for future use
(e.g. from a journalist to a future source using a QR code).
Unfortunately, the anonymous credential scheme specified in this proposal is
one-show, so the onion service cannot provide a single token that will work
for multiple "logins". In the future we can design multi-show credential
systems that also have revocation to further facilitate this use case (see
[FUTURE_RES] for more info).
# 6. User Experience
This proposal has user facing UX consequences.
Ideally we want this process to be invisible to the user and things to "just
work". This can be achieved with token issuers that don't require manual work
by the user (e.g. the PoW issuer, or the onion service itself), since both the
token issuance and the token redemption protocols don't require any manual work.
In the cases where manual work is needed by the user (e.g. solving a CAPTCHA)
it's ideal if the work is presented to the user right before visiting the
destination and only if it's absolutely required. An explanation about the
service being under attack should be given to the user when the CAPTCHA is
provided.
# 7. Security
In this section we analyze potential security threats of the above system:
- An evil client can hoard tokens for hours and unleash them all at once to
cause a denial of service attack. We might want to make the key rotation
even more frequent if we think that's a possible threat.
- A trusted token issuer can always DoS an onion service by forging tokens.
- Overwhelming attacks like "top half attacks" and "hybrid attacks" from
proposal#327 is valid for this proposal as well.
- A bad RNG can completely wreck the linkability properties of this proposal.
XXX Actually analyze the above if we think there is merit to listing them
# 8. Discussion [DISCUSSION]
## 8.1. Using Res tokens on Exit relays
There are more scenarios within Tor that could benefit from Res tokens
however we didn't expand on those use cases to keep the proposal short. In
the future, we might want to split this document into two proposals: one
proposal that specifies the token scheme, and another that specifies how to
use it in the context of onion servicves, so that we can then write more
proposals that use the token scheme as a primitive.
An extremely relevant use case would be to use Res tokens as a way to protect
and improve the IP reputation of Exit relays. We can introduce an exit pool
that requires tokens in exchange for circuit streams. The idea is that exits
that require tokens will see less abuse, and will not have low scores in the
various IP address reputation systems that now govern who gets access to
websites and web services on the public Internet. We hope that this way we
will see less websites blocking Tor.
## 8.2. Future improvements to this proposal [FUTURE_RES]
The Res token scheme is a pragmatic scheme that works for the space/time
constraints of this use case but it's far from ideal for the greater future
(RSA? RSA-1024?).
After Tor proposal#319 gets implemented we will be able to pack more data in
RELAY cells and that opens the door to token schemes with bigger token
sizes. For example, we could design schemes based on BBS+ that can provide
more advanced features like multi-show and complex attributes but currently
have bigger token sizes (300+ bytes). That would greatly improve UX since the
client won't have to solve multiple CAPTCHAs to gain access. Unfortunately,
another problem here is that right now pairing-based schemes have
significantly worse verification performance than RSA (e.g. in the order of
4-5 ms compared to <0.5 ms). We expect pairing-based cryptography performance
to only improve in the future and we are looking forward to these advances.
When we switch to a multi-show scheme, we will also need revocation support
otherwise a single client can abuse the service with a single multi-show
token. To achieve this we would need to use blacklisting schemes based on
accumulators (or other primitives) that can provide more flexible revocation
and blacklisting; however these come at the cost of additional verification
time which is not something we can spare at this time. We warmly welcome
research on revocation schemes that are lightweight on the verification side
but can be heavy on the proving side.
## 8.3. Other uses for tokens in Tor
There is more use cases for tokens in Tor but we think that other token
schemes with different properties would be better suited for those.
In particular we could use tokens as authentication mechanisms for logging
into services (e.g. acquiring bridges, or logging into Wikipedia). However
for those use cases we would ideally need multi-show tokens with revocation
support. We can also introduce token schemes that help us build a secure name
system for onion services.
We hope that more research will be done on how to combine various token
schemes together, and how we can maintain agility while using schemes with
different primitives and properties.
# 9. Acknowledgements
Thanks to Jeff Burdges for all the information about Blind RSA and anonymous
credentials.
Thanks to Michele Orrù for the help with the unlinkability proof and for the
discussions about anonymous credentials.
Thanks to Chelsea Komlo for pointing towards anonymous credentials in
the context of DoS defenses for onion services.
---
# Appendix A: RSA Blinding Security Proof [BLIND_RSA_PROOF]
This proof sketch was provided by Michele Orrù:
```
RSA Blind Sigs: https://en.wikipedia.org/wiki/Blind_signature#Blind_RSA_signatures
As you say, blind RSA should be perfectly blind.
I tried to look at Boneh-Shoup, Katz-Lindell, and Bellare-Goldwasser for a proof, but didn't find any :(
The basic idea is proving that:
for any message "m0" that is blinded with "r0^e" to obtain "b" (that is sent to the server), it is possible to freely choose another message "m1" that blinded with another opening "r1^e" to obtain the same "b".
As long as r1, r0 are chosen uniformly at random, you have no way of telling if what message was picked and therefore it is *perfectly* blind.
To do so:
Assume the messages ("m0" and "m1") are invertible mod N=pq (this happens at most with overwhelming probability phi(N)/N if m is uniformly distributed as a result of a hash, or you can enforce it at signing time).
Blinding happens by computing:
b = m0 * (r0^e).
However, I can also write:
b = m0 * r0^e = (m1/m1) * m0 * r0^e = m1 * (m0/m1*r0^e).
This means that r1 = (m0/m1)^d * r0 is another valid blinding factor for b, and it's distributed exactly as r0 in the group of invertibles (it's unif at random, because r0 is so).
```
---
[REF_TOKEN_ZOO]: https://tokenzoo.github.io/
[REF_INTRO_SPACE]: https://gitlab.torproject.org/legacy/trac/-/issues/33650#note_2350910
[REF_CHAUM]: https://eprint.iacr.org/2001/002.pdf
[REF_PRIMO]: https://repo.getmonero.org/selene/primo
https://www.monerooutreach.org/stories/RPC-Pay.html
[REF_POW_PERF]: https://gitlab.torproject.org/tpo/core/torspec/-/blob/master/proposals/327-pow-over-intro.txt#L1050
[REF_RES_BENCH]: https://github.com/asn-d6/res_tokens_benchmark
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