From 6dc59c5a07efa36c754296029d60155ae237b505 Mon Sep 17 00:00:00 2001 From: mfahampshire Date: Thu, 5 Dec 2024 22:56:17 +0100 Subject: [PATCH 1/2] revert credit to ticket & ticketbook --- .../network/cryptography/zk-nym/rerandomise.mdx | 13 +++++-------- .../cryptography/zk-nym/unlinkability.mdx | 8 +++----- .../cryptography/zk-nym/zk-nym-overview.mdx | 16 ++++++++-------- 3 files changed, 16 insertions(+), 21 deletions(-) diff --git a/documentation/docs/pages/network/cryptography/zk-nym/rerandomise.mdx b/documentation/docs/pages/network/cryptography/zk-nym/rerandomise.mdx index 271fc17bf05..3b4a296eff6 100644 --- a/documentation/docs/pages/network/cryptography/zk-nym/rerandomise.mdx +++ b/documentation/docs/pages/network/cryptography/zk-nym/rerandomise.mdx @@ -2,23 +2,20 @@ import { Callout } from 'nextra/components' # Rerandomisation & Incremental Spend -Each zk-nym credit will not be valid for the entire amount of data that the credential aggregated from the PSCs is; if the aggregated credential is worth (e.g.) 10GB of Mixnet data, each credit will be worth far less (e.g. 100MB). This amount will be globally uniform in order to avoid situations where differently sized credits allow for patterns to emerge. +Each ticket will not be valid for the entire amount of data that the ticketbook aggregated from the PSCs is; if the aggregated ticketbook is worth (e.g.) 10GB of Mixnet data, each ticket will be worth far less (e.g. 100MB). This amount will be globally uniform in order to avoid situations where differently sized tickets allow for patterns to emerge. The functionality included in the following code block examples were added to the [nym-cli tool](../tools/nym-cli.md) for illustrative purposes only: this is not necessarily how credentials will be accessed in the future. - **Furthermore, the `nym-cli` uses the words 'tickets' in place of 'credits' and 'ticketbook' in place of 'aggregated credential': this is wording that we are moving away from now in place of the simpler credential/credit.** This will be updated in the `nym-cli` tool in a future release. - - The numbers used in this high level overview are for illustration purposes only. The figures used in production will potentially vary. Note that individual zkNym sizes will be uniform across the Network. + The numbers used in this high level overview are for illustration purposes only. The figures used in production will potentially vary. Note that individual ticket sizes will be uniform across the Network. - -## Why not spend the entire credential at once? +## Why a 'ticketbook', not individual 'tickets', and why not spend them all at once? This is to account for the need for a client to change their ingress Gateway, either because the Gateway itself has gone down / is not offering the required bandwidth, or because a user might simply want to split their traffic across multiple Gateways for extra privacy. -This means that clients are not tied to particular Gateways they have 'spent' their entire subscription amount with; if the ingress Gateway goes down, or the client simply wishes to use another ingress Gateway, the user has multiple other zk-nym credits they can use that account for their remaining purchased bandwidth. +This means that clients are not tied to particular Gateways they have 'spent' their entire subscription amount with; if the ingress Gateway goes down, or the client simply wishes to use another ingress Gateway, the user has multiple other tickets they can use that account for their remaining purchased bandwidth. -Going back to the `nym-cli` tool to illustrate this; we can generate multiple unlinkable credits ('tickets' on this command output) from a single aggregated credential ('ticketbook' below): +Going back to the `nym-cli` tool to illustrate this; we can generate multiple unlinkable tickets from a single ticketbook aggregated from PSCs: ```sh ❯ ./nym-cli ecash generate-ticket --credential-storage storage.db --provider 6qidVK21zpHD298jdDa1RRpbRozP29ENVyqcSbm6hQrG --full diff --git a/documentation/docs/pages/network/cryptography/zk-nym/unlinkability.mdx b/documentation/docs/pages/network/cryptography/zk-nym/unlinkability.mdx index 90db0096135..e6554299b6d 100644 --- a/documentation/docs/pages/network/cryptography/zk-nym/unlinkability.mdx +++ b/documentation/docs/pages/network/cryptography/zk-nym/unlinkability.mdx @@ -1,14 +1,12 @@ import { Callout } from 'nextra/components' # Unlinkability -Each time a credential is requested by an ingress Gateway to prove that a client has purchased data to send through the Mixnet, instead of sending that credential, the Requester's device will produce a zk-nym credit. This is a rereandomised value that is able to be verified as being legitimate (in that it was created by a valid root credential) but **not linked to any other credits**, either previously generated or to be generated in the future. This feature also allows for a credential to be split across multiple ingress Gateways / connections and [incrementally spent](./rerandomise.md) over time. +Each time a credential is requested by an ingress Gateway to prove that a client has purchased data to send through the Mixnet the Requester's device will produce a ticket. This is a rereandomised value that is able to be verified as being legitimate (in that it was created by a valid root ticketbook) but **not linked to any other tickets**, either previously generated or to be generated in the future. This feature also allows for a single ticketbook to allow access to be split across multiple ingress Gateways / connections and [incrementally spent](./rerandomise.md) over time. The functionality included in the following code block examples were added to the [nym-cli tool](../tools/nym-cli.md) for illustrative purposes only: this is not necessarily how credentials will be accessed in the future. -**Furthermore, the `nym-cli` uses the words 'tickets' in place of 'credits' and 'ticketbook' in place of 'aggregated credential': this is wording that we are moving away from now in place of the simpler credential/credit.** This will be updated in the `nym-cli` tool in a future release. - The numbers used in this high level overview are for illustration purposes only. The figures used in production will potentially vary. Note that individual zkNym sizes will be uniform across the Network. @@ -24,7 +22,7 @@ PAYMENT FOR TICKET 3: 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 ``` -Now lets generate another credit to spend either topping up once the previous one's data allowance has been used, or with another Gateway. Notice that the `ticket-index` is the same: this is generated from the same aggregated credential as the one above! +Now lets generate another ticket to spend either topping up once the previous one's data allowance has been used, or with another Gateway. Notice that the `ticket-index` is the same: this is generated from the same aggregated credential as the one above! ```sh ❯ ./nym-cli ecash generate-ticket --credential-storage storage.db --provider 6qidVK21zpHD298jdDa1RRpbRozP29ENVyqcSbm6hQrG --ticket-index=3 @@ -37,4 +35,4 @@ PAYMENT FOR TICKET 3: 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 ``` -These are both generated by the _same_ underlying credential and used in a way that they cannot be tied to each other. An ingress Gateway might (for instance) get 100 connection requests from 100 Nym clients, each validated with a zk-nym credi t. It has no way of knowing whether these are all from the same single subscription, or 100 different ones. +These are both generated by the _same_ underlying ticketbook and used in a way that they cannot be tied to each other. An ingress Gateway might (for instance) get 100 connection requests from 100 Nym clients, each validated with a ticket. It has no way of knowing whether these are all from the same single subscription, or 100 different ones. diff --git a/documentation/docs/pages/network/cryptography/zk-nym/zk-nym-overview.mdx b/documentation/docs/pages/network/cryptography/zk-nym/zk-nym-overview.mdx index ec019a3266d..14cc7488c64 100644 --- a/documentation/docs/pages/network/cryptography/zk-nym/zk-nym-overview.mdx +++ b/documentation/docs/pages/network/cryptography/zk-nym/zk-nym-overview.mdx @@ -14,7 +14,7 @@ import { Callout } from 'nextra/components' Generation of zk-nyms involves the following actors / pieces of infrastructure: -- **Requester needing a zk-nym** for example a single user using the NymVPN app, or a company purchasing zk-nyms to distribute to their app users, in the instance of an app integrating a Mixnet client via one of the SDKs. The Requester is represented by a Bech32 address on the Nyx blockchain. +- **Requester needing a zk-nym** for example a single user using the NymVPN app, or a company purchasing zk-nyms to distribute to their app users, in the instance of an app integrating a Mixnet client via one of the SDKs. The Requester is represented by a Bech32 address on the Nyx blockchain. - [NymAPI](../../architecture/nyx/nym-api) instances working together on signature generation and spent credential validation, referred to as the **NymAPI Quorum**. Members of the Quorum are a subset of the Nyx chain Validator set (other tasks they perform include a multisig used for triggering reward payouts to the Network Infrastructure Node Operators and maintaining the global Bloom Filter for double-spend protection). - **OrderAPI**: an API creating crypto/fiat to `NYM` swaps and then depositing the NYM tokens in a smart contract managed by the NymAPI Quroum for payment verification. Implementation details of the API will be released in the coming months. @@ -26,8 +26,8 @@ Generation happens in 3 distinct stages: From the perspective of the Requester most of this happens under the hood, but results in the creation and usage of an **unlinkable, rerandomisable anonymous proof-of-payment credential** - a zk-nym - with which to access the Mixnet without fear of doxxing themselves via linking app usage and payment information. The user experience is further enhanced by the fact that a single credential can be split into multiple small zk-nyms, meaning that a Requester may buy a large chunk of bandwidth but 'spend' this in the form of multiple zk-nyms with different ingress Gateways. Whilst this happens under the hood, what it affords the Requester is an ease of experience in that they have to 'top up' their bandwidth less and are able to chop and change ingress points to the Nym Network as they see fit, akin to the UX of most modern day VPNs and dVPNs. ## Key Generation & Payment -- First, a Cosmos [Bech32 address](https://docs.cosmos.network/main/build/spec/addresses/bech32) is created for the Requester. This is used to identify themselves when interacting with the OrderAPI via signed authentication tokens. This is the only identity that the OrderAPI is able to see, and is not able to link this to the zk-nyms that will be generated. This identity never leaves the Requester’s device and there is no email or any personal details needed for signup. If a Requester is simply 'topping up' their subscription, the creation of the address is skipped as it already exists. -- The Requester also generates an ed25519 keypair: this is used to identify and authenticate them in the case of using zk-nyms across several devices as an individual user. However, this is never used in the clear: these keys are used as private attribute values within generated credentials which are verified via zero-knowledge. +- First, a Cosmos [Bech32 address](https://docs.cosmos.network/main/build/spec/addresses/bech32) is created for the Requester. This is used to identify themselves when interacting with the OrderAPI via signed authentication tokens. **This is the only identity that the OrderAPI is able to see, and is not able to link this to the zk-nyms that will be generated.** This identity never leaves the Requester’s device and there is no email or any personal details needed for signup. If a Requester is simply 'topping up' their subscription, the creation of the address is skipped as it already exists. +- The Requester also generates an ed25519 keypair: this is used to identify and authenticate them in the case of using zk-nyms across several devices as an individual user. However, **this is never used in the clear**: these keys are used as private attribute values within generated credentials which are verified via zero-knowledge and not publicly exposed. - The Requester can then interact with various payment backends to pay for their zk-nyms with crypto, fiat options, or natively with NYM tokens. - Payment options will trigger the OrderAPI. This will: - Create a swap for `` to `NYM` tokens. @@ -35,24 +35,24 @@ From the perspective of the Requester most of this happens under the hood, but r - The Requester sends a request to each member of the Quorum requesting a zk-nym credential. This request is signed with their private key and includes the transaction hash of the NYM deposit into the deposit contract, performed either by themselves or the OrderAPI. ## Issue zk-nym -At this point, NYM tokens have been deposited into the smart contract controlled by the Quorum's multisig and a zk-nym credential has been requested. Next, each member of the Quorum who responds to the Requester's request for a zk-nym checks the validity and returns a PSC signed with part of the master key (since this is a threshold cryptsystem, not all members of the Quroum must respond to create a zk-nym, only enough to pass the threshold). The process looks like this: +At this point, NYM tokens have been deposited into the smart contract controlled by the Quorum's multisig and a zk-nym has been requested. Next, each member of the Quorum who responds to the Requester's request for a zk-nym checks the validity and returns a partial blinded signature - a 'partial signed credential' ('PSC') - signed with part of the master key (since this is a threshold cryptsystem, not all members of the Quroum must respond to create a zk-nym, only enough to pass the threshold). The process looks like this: - Members of the Quroum performs several checks to verify the request is valid: - They verify the signature sent as part of the request is valid and that the request was made in the last 48 hours. - They verify that the amount requested matches the amount deposited in the transation, the hash of which was signed by the Requester's ed25519 key and sent as part of the request. -- Members then create a partial blinded signature - a 'partial signed credential' ('PSC') - from their fragment of the master key generated and split amongst them at the beginning of the Quroum in the initial DKG ceremony. +- Members then create a PSC from their fragment of the master key generated and split amongst them at the beginning of the Quroum in the initial DKG ceremony. - The member also creates a `key:value` entry in their local cache with the transaction hash as the key, and the PSC + encrypted signature as the value. This is used later for zk-nym validation and is cleaned after a predefined timeout. - These PSCs are given back to the Requester after setting up a secure channel via DH key exchange, with each replying Quorum member also sending their public key for verification that the returned PSC was signed by them. -Once the Requester has received over the threshold number of PSCs they can assemble them into a credential signed by the master key. The Requester never learns this master key (it is a private attribute) but the credential can be verified by the Quroum as being valid by checking for a proof that the credential's private attribute - the value of the master key - is valid. +Once the Requester has received over the threshold number of PSCs they can assemble them into a 'ticketbook' of 'tickets' - spendable credentials - signed by the master key. The Requester never learns this master key (it is a private attribute) but the credential can be verified by the Quroum as being valid by checking for a proof that the credential's private attribute - the value of the master key - is valid. ![](/images/network/deposit-generate.png) ## Spend zk-nym to Access Mixnet -- Once the credential has been aggregated from the PSCs returned from > threshold of Quorum members, smaller 'zk-nym credits' can be generated from it, accounting for smaller chunks of bandwidth which can be 'spent' with ingress Gateways. This occurs entirely offline, on the device of the zk-nym Requester. See pages on the scheme's [unlinkability](unlinkability.md) and [rerandomisation and incremental spending](./rerandomise.md) features for further information on this. -- This zk-nym credit is later presented to the Quorum by the Gateway that collected it, which is used to calculate reward percentages given to Nym Network infrastructure operators by the Quorum, with payouts triggered by their multisig wallet. Both ingress Gateways and the Quorum use spent zk-nym credits when engaging in [double spending protection](./double-spend-prot.md). +- Once the ticketbook has been aggregated from the PSCs returned from > threshold of Quorum members, smaller 'ticket' credentials can be generated from it, accounting for smaller chunks of bandwidth which can be 'spent' with ingress Gateways. This occurs entirely offline, on the device of the zk-nym Requester. See pages on the scheme's [unlinkability](unlinkability.md) and [rerandomisation and incremental spending](./rerandomise.md) features for further information on this. +- This ticket is later presented to the Quorum by the Gateway that collected it, which is used to calculate reward percentages given to Nym Network infrastructure operators by the Quorum, with payouts triggered by their multisig wallet. Both ingress Gateways and the Quorum use spent tickets when engaging in [double spending protection](./double-spend-prot.md). ![](/images/network/use-zknym.png) From 4dcc3ff0370d82f150c04de0b6978b375660a4a5 Mon Sep 17 00:00:00 2001 From: mfahampshire Date: Thu, 5 Dec 2024 23:23:56 +0100 Subject: [PATCH 2/2] revert credit to ticket & ticketbook and actually get all the instances to replace --- .../pages/network/cryptography/zk-nym/double-spend-prot.mdx | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/documentation/docs/pages/network/cryptography/zk-nym/double-spend-prot.mdx b/documentation/docs/pages/network/cryptography/zk-nym/double-spend-prot.mdx index 05a5c86423d..6da843967be 100644 --- a/documentation/docs/pages/network/cryptography/zk-nym/double-spend-prot.mdx +++ b/documentation/docs/pages/network/cryptography/zk-nym/double-spend-prot.mdx @@ -17,13 +17,13 @@ However, the offline approach introduces certain limitations. - Any potential repercussions against double spenders can only be implemented once the user requests a new credential for their zk-nym Generator (aka they have to 'top up' and buy more bandwidth allowance), assuming they haven't altered their identifier (the Bech32 address). An exploitable scenario arises from these limitations: -- A malicious user purchases bandwidth and aggregates a valid zk-nym credential in the standard way, worth $10 of crypto/fiat. Subsequently, the malicious user proceeds to sell the credential to 100 users for $1 each, allowing each user to generate zk-nym credits of 100MB from this **valid** credential. Under the offline approach, entry nodes forego double-spending checks; so long as the clients all used different ingress Gateways, all 100 users could access the network without obtaining a subscription. As bandwidth consumption is tracked locally between client and ingress node, and each zk-nym credit is rerandomised, there is no way that ingress Gateways would know that the zk-credential used by the client has been shared with other parties. This loophole highlights the need for stringent measures to counter such potential abuses within the system, without creating either speed bottlenecks (in the case of the Online model) or impacting the anonymity of the system. We can, however, mitigate this problem without doing either of these things. +- A malicious user purchases bandwidth and aggregates a valid zk-nym credential in the standard way, worth $10 of crypto/fiat. Subsequently, the malicious user proceeds to sell the credential to 100 users for $1 each, allowing each user to generate zk-nym tickets of 100MB from this **valid** credential. Under the offline approach, entry nodes forego double-spending checks; so long as the clients all used different ingress Gateways, all 100 users could access the network without obtaining a subscription. As bandwidth consumption is tracked locally between client and ingress node, and each zk-nym ticket is rerandomised, there is no way that ingress Gateways would know that the zk-credential used by the client has been shared with other parties. This loophole highlights the need for stringent measures to counter such potential abuses within the system, without creating either speed bottlenecks (in the case of the Online model) or impacting the anonymity of the system. We can, however, mitigate this problem without doing either of these things. ## Solution to Offline Double Spending To efficiently prevent the fraudulent use of tickets within the Nym network, a two-tiered solution is in place that combines (1) the immediate detection of double-spending attempts at the level of individuals ingress Gateways and (2) subsequent identification and blacklisting of offending clients at the Quorum level. ### Entry Node Implementation: Real-Time Ticket Unspending Validation -Each spent zk-nym credit contains as an attribute a unique serial number, which is revealed in plaintext to the respective ingress Gateway. Each Gateway has a copy of a [Bloom Filter](https://www.geeksforgeeks.org/bloom-filters-introduction-and-python-implementation/) - on receiving a credit, it will check against its copy of a local database to check whether this serial number has already been seen. If so, it rejects the credit as being double-spent and the client's connection request is rejected. If not, it will add the serial number to its local DB. +Each spent zk-nym ticket contains as an attribute a unique serial number, which is revealed in plaintext to the respective ingress Gateway. Each Gateway has a copy of a [Bloom Filter](https://www.geeksforgeeks.org/bloom-filters-introduction-and-python-implementation/) - on receiving a ticket, it will check against its copy of a local database to check whether this serial number has already been seen. If so, it rejects the ticket as being double-spent and the client's connection request is rejected. If not, it will add the serial number to its local DB. > Since each time a zk-nym credential is rerandomised its serial number is changed, the serial number being shared in no way identifies a client or user. @@ -32,6 +32,6 @@ Each Gateway will periodically share their serial numbers with the Quorum and re > Crucially, ingress Gateways refrain from extensive computations to identify the original ticket owner, and avoids broadcasting information about the double-spending attempt to other ingress Gateways. The entry node is also not involved in any global blacklisting process of the clients. The sole purpose of this check is to swiftly identify any attempts at double-spending and add the seen ticket's serial number to the local DB cache. ### Nym-API Implementation: Blacklisting and Penalties for Double-Spenders -All Gateways periodically forward the collected credits to the Quorum, enabling them to pinpoint and blacklist any clients who double spend. Upon receiving the credits, the Quorum appends all the incoming serial numbers to the global list of spend zk-nym serial numbers and proceed with the identification process for any malicious users engaging in double-spending. +All Gateways periodically forward the collected tickets to the Quorum, enabling them to pinpoint and blacklist any clients who double spend. Upon receiving the tickets, the Quorum appends all the incoming serial numbers to the global list of spend zk-nym serial numbers and proceed with the identification process for any malicious users engaging in double-spending. This identification phase involves looking for instances of double spending, identifying the id of the double-spending client, and blacklisting this client by its id. Subsequently, when this client requests a new credential, their plaintext public identifier is included in the request. The Quorum then checks if this identifier is blacklisted. If it is, a new credential is not issued. Furthermore, since the PSCs are only attainable after depositing NYM as payment, the Quorum has the authority to withhold the deposited NYMs as a punitive measure for any detected instances of double-spending.