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Jimmy Debe
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---
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slug: 20
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title: 20/TOY-ETH-PM
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name: Toy Ethereum Private Message
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status: draft
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tags: waku/application
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editor: Franck Royer <franck@status.im>
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contributors:
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---
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**Content Topics**:
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- Public Key Broadcast: `/eth-pm/1/public-key/proto`,
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- Private Message: `/eth-pm/1/private-message/proto`.
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This specification explains the Toy Ethereum Private Message protocol
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which enables a peer to send an encrypted message to another peer
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using the Waku v2 network, and the peer's Ethereum address.
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The main purpose of this specification is to demonstrate how Waku v2 can be used for encrypted messaging purposes,
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using Ethereum accounts for identity.
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This protocol caters for Web3 wallets restrictions, allowing it to be implemented only using standard Web3 API.
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In the current state, the protocol has privacy and features [limitations](#limitations), has not been audited
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and hence is not fit for production usage.
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We hope this can be an inspiration for developers wishing to build on top of Waku v2.
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## Goal
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Alice wants to send an encrypted message to Bob, where only Bob can decrypt the message.
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Alice only knows Bob's Ethereum Address.
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## Variables
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Here are the variables used in the protocol and their definition:
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- `B` is Bob's Ethereum address (or account),
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- `b` is the private key of `B`, and is only known by Bob.
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- `B'` is Bob's Encryption Public Key, for which `b'` is the private key.
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- `M` is the private message that Alice sends to Bob.
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## Design Requirements
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The proposed protocol MUST adhere to the following design requirements:
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1. Alice knows Bob's Ethereum address,
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2. Bob is willing to participate to Eth-PM, and publishes `B'`,
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3. Bob's ownership of `B'` MUST be verifiable,
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4. Alice wants to send message `M` to Bob,
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5. Bob SHOULD be able to get `M` using [10/WAKU2 spec](../../standards/core/10/WAKU2.md),
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6. Participants only have access to their Ethereum Wallet via the Web3 API,
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7. Carole MUST NOT be able to read `M`'s content even if she is storing it or relaying it,
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8. [Waku Message Version 1](../../standards/application/26) Asymmetric Encryption is used for encryption purposes.
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## Limitations
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Alice's details are not included in the message's structure,
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meaning that there is no programmatic way for Bob to reply to Alice
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or verify her identity.
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Private messages are sent on the same content topic for all users.
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As the recipient data is encrypted, all participants must decrypt all messages which can lead to scalability issues.
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This protocol does not guarantee Perfect Forward Secrecy nor Future Secrecy:
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If Bob's private key is compromised, past and future messages could be decrypted.
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A solution combining regular [X3DH](https://www.signal.org/docs/specifications/x3dh/)
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bundle broadcast with [Double Ratchet](https://signal.org/docs/specifications/doubleratchet/) encryption would remove these limitations;
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See the [Status secure transport spec](https://specs.status.im/spec/5) for an example of a protocol that achieves this in a peer-to-peer setting.
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Bob MUST decide to participate in the protocol before Alice can send him a message.
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This is discussed in more in details in [Consideration for a non-interactive/uncoordinated protocol](#consideration-for-a-non-interactiveuncoordinated-protocol)
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## The protocol
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### Generate Encryption KeyPair
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First, Bob needs to generate a keypair for Encryption purposes.
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Bob SHOULD get 32 bytes from a secure random source as Encryption Private Key, `b'`.
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Then Bob can compute the corresponding SECP-256k1 Public Key, `B'`.
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### Broadcast Encryption Public Key
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For Alice to encrypt messages for Bob,
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Bob SHOULD broadcast his Encryption Public Key `B'`.
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To prove that the Encryption Public Key `B'` is indeed owned by the owner of Bob's Ethereum Account `B`,
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Bob MUST sign `B'` using `B`.
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### Sign Encryption Public Key
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To prove ownership of the Encryption Public Key,
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Bob must sign it using [EIP-712](https://eips.ethereum.org/EIPS/eip-712) v3,
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meaning calling `eth_signTypedData_v3` on his Wallet's API.
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Note: While v4 also exists,
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it is not available on all wallets and the features brought by v4 is not needed for the current use case.
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The `TypedData` to be passed to `eth_signTypedData_v3` MUST be as follows, where:
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- `encryptionPublicKey` is Bob's Encryption Public Key, `B'`, in hex format, **without** `0x` prefix.
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- `bobAddress` is Bob's Ethereum address, corresponding to `B`, in hex format, **with** `0x` prefix.
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```js
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const typedData = {
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domain: {
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chainId: 1,
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name: 'Ethereum Private Message over Waku',
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version: '1',
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},
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message: {
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encryptionPublicKey: encryptionPublicKey,
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ownerAddress: bobAddress,
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},
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primaryType: 'PublishEncryptionPublicKey',
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types: {
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EIP712Domain: [
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{ name: 'name', type: 'string' },
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{ name: 'version', type: 'string' },
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{ name: 'chainId', type: 'uint256' },
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],
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PublishEncryptionPublicKey: [
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{ name: 'encryptionPublicKey', type: 'string' },
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{ name: 'ownerAddress', type: 'string' },
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],
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},
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}
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```
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### Public Key Message
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The resulting signature is then included in a `PublicKeyMessage`, where
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- `encryption_public_key` is Bob's Encryption Public Key `B'`, not compressed,
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- `eth_address` is Bob's Ethereum Address `B`,
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- `signature` is the EIP-712 as described above.
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```protobuf
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syntax = "proto3";
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message PublicKeyMessage {
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bytes encryption_public_key = 1;
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bytes eth_address = 2;
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bytes signature = 3;
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}
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```
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This MUST be wrapped in a Waku Message version 0, with the Public Key Broadcast content topic.
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Finally, Bob SHOULD publish the message on Waku v2.
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## Consideration for a non-interactive/uncoordinated protocol
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Alice has to get Bob's public Key to send a message to Bob.
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Because an Ethereum Address is part of the hash of the public key's account,
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it is not enough in itself to deduce Bob's Public Key.
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This is why the protocol dictates that Bob MUST send his Public Key first,
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and Alice MUST receive it before she can send him a message.
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Moreover, nim-waku, the reference implementation of [13/WAKU2-STORE](../../standards/core/13/store.md)),
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stores messages for a maximum period of 30 days.
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This means that Bob would need to broadcast his public key at least every 30 days to be reachable.
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Below we are reviewing possible solutions to mitigate this "sign up" step.
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### Retrieve the public key from the blockchain
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If Bob has signed at least one transaction with his account then his Public Key can be extracted from the transaction's ECDSA signature.
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The challenge with this method is that standard Web3 Wallet API does not allow Alice to specifically retrieve all/any transaction sent by Bob.
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Alice would instead need to use the `eth.getBlock` API to retrieve Ethereum blocks one by one.
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For each block, she would need to check the `from` value of each transaction until she finds a transaction sent by Bob.
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This process is resource intensive and can be slow when using services such as Infura due to rate limits in place,
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which makes it inappropriate for a browser or mobile phone environment.
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An alternative would be to either run a backend that can connect directly to an Ethereum node,
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use a centralized blockchain explorer
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or use a decentralized indexing service such as [The Graph](https://thegraph.com/).
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Note that these would resolve a UX issue only if a sender wants to proceed with _air drops_.
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Indeed, if Bob does not publish his Public Key in the first place
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then it can be an indication that he simply does not participate in this protocol and hence will not receive messages.
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However, these solutions would be helpful if the sender wants to proceed with an _air drop_ of messages:
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Send messages over Waku for users to retrieve later, once they decide to participate in this protocol.
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Bob may not want to participate first but may decide to participate at a later stage
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and would like to access previous messages.
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This could make sense in an NFT offer scenario:
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Users send offers to any NFT owner,
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NFT owner may decide at some point to participate in the protocol and retrieve previous offers.
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### Publishing the public in long term storage
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Another improvement would be for Bob not having to re-publish his public key every 30 days or less.
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Similarly to above, if Bob stops publishing his public key then it may be an indication that he does not participate in the protocol anymore.
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In any case, the protocol could be modified to store the Public Key in a more permanent storage, such as a dedicated smart contract on the blockchain.
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## Send Private Message
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Alice MAY monitor the Waku v2 to collect Ethereum Address and Encryption Public Key tuples.
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Alice SHOULD verify that the `signature`s of `PublicKeyMessage`s she receives are valid as per EIP-712.
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She SHOULD drop any message without a signature or with an invalid signature.
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Using Bob's Encryption Public Key, retrieved via [10/WAKU2](../../standards/core/10/WAKU2.md), Alice MAY now send an encrypted message to Bob.
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If she wishes to do so, Alice MUST encrypt her message `M` using Bob's Encryption Public Key `B'`,
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as per [26/WAKU-PAYLOAD Asymmetric Encryption specs](../../standards/application/26/payload.md/#asymmetric).
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Alice SHOULD now publish this message on the Private Message content topic.
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# Copyright
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Copyright and related rights waived via [CC0](https://creativecommons.org/publicdomain/zero/1.0/).
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## References
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- [10/WAKU2 spec](../../standards/core/10/WAKU2.md)
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- [Waku Message Version 1](../../standards/application/26)
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-
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