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split into RFCs for signed envelope / addr records

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Yusef Napora
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# RFC 0002 - Signed Envelopes
- Start Date: 2019-10-21
- Related RFC: [0003 Address Records][addr-records-rfc]
## Abstract
This RFC proposes a "signed envelope" structure that contains an arbitray byte
string payload, a signature of the payload, and the public key that can be used
to verify the signature.
This was spun out of an earlier draft of the [address records
RFC][addr-records-rfc], since it's generically useful.
## Problem Statement
Sometimes we'd like to store some data in a public location (e.g. a DHT, etc),
or make use of potentially untrustworthy intermediaries to relay information. It
would be nice to have an all-purpose data container that includes a signature of
the data, so we can verify that the data came from a specific peer and that it hasn't
been tampered with.
## Wire Format
Since we already have a [protobuf definition for public keys][peer-id-spec], we
can use protobuf for this as well and easily embed the key in the envelope:
```protobuf
message SignedEnvelope {
PublicKey publicKey = 1; // see peer id spec for definition
string purpose = 2; // arbitrary user-defined string for context
bytes cid = 3; // CIDv1 of contents
bytes contents = 4; // payload
bytes signature = 5; // signature of purpose + cid + contents
}
```
The `publicKey` field contains the public key whose secret counterpart was used
to sign the message. This MUST be consistent with the peer id of the signing
peer, as the recipient will derive the peer id of the signer from this key.
The `purpose` field is an aribitrary string that can be used to give some hint
as to the contents. For example, if `contents` contains a serialized
`AddressState` record, `purpose` might contain the string `"AddressState"`. The
contents of the ``purpose`` field are signed alongside `contents` to prevent
tampering, and may be empty if desired.
The `cid` field contains a version 1 [CID][cid] (content id) that corresponds to
the `content` field. It's used for retrieving messages from [local
storage](#local-storage-of-signed-envelopes), and the embedded multicodec also
gives a hint as to the data type of the `contents`. If the user does not specify
a multicodec when constructing the envelope, the default will be
[`raw`](https://github.com/multiformats/multicodec/blob/master/table.csv#L34)
for raw binary.
## Signature Production / Verification
When signing, a peer will prepare a buffer by concatenating the following:
- The string `"libp2p-signed-envelope:"`, encoded as UTF-8
- The `purpose` field, encoded as UTF-8
- The `cid` field
- The `contents` field
Then they will sign the buffer according to the rules in the [peer id
spec][peer-id-spec] and set the `signature` field accordingly.
To verify, a peer will "inflate" the `publicKey` into a domain object that can
verify signatures, prepare a buffer as above and verify the `signature` field
against it.
## Local Storage of Signed Envelopes
Signed envelopes can be used for ephemeral data, but we may also want to persist
them for a while and / or make previously recieved envelopes accesible to
various libp2p modules.
For example, if the envelope contains an [address record][addr-records-rfc],
those records might be used to populate a peer store with self-certified
records. Rather than requiring the peer store to persist the full envelope, we
could have a separate "envelope storage" service that keeps signed messages
around for future reference.
The peer store can then just store the `cid` alongside a flag that indicates
that the address came from a trusted source. If we're using a persistent peer
store and the process restarts, we can look up the stored `cid` in the envelope
storage and verify the signature again.
If we decide to build this, the storage service should have some kind of garbage
collection / TTL scheme to avoid unbounded growth.
[addr-records-rfc]: ./0003-address-records.md
[peer-id-spec]: ../peer-ids/peer-ids.md

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# RFC 0003 - Address Records with Metadata
- Start Date: 2019-10-04
- Related Issues:
- [libp2p/issues/47](https://github.com/libp2p/libp2p/issues/47)
- [go-libp2p/issues/436](https://github.com/libp2p/go-libp2p/issues/436)
## Abstract
This RFC proposes a method for distributing address records, which contain a
peer's publicly reachable listen addresses, as well as some metadata that can
help other peers categorize addresses and prioritize thme when dialing.
The record described here does not include a signature, but it is expected to
be serialized and wrapped in a [signed envelope][envelope-rfc], which will
prove the identity of the issuing peer. The dialer can then prioritize
self-certified addresses over addresses from an unknown origin.
## Problem Statement
All libp2p peers keep a "peer store" (called a peer book in some
implementations), which maps [peer ids][peer-id-spec] to a set of known
addresses for each peer. When the application layer wants to contact a peer, the
dialer will pull addresses from the peer store and try to initiate a connection
on one or more addresses.
Addresses for a peer can come from a variety of sources. If we have already made
a connection to a peer, the libp2p [identify protocol][identify-spec] will
inform us of other addresses that they are listening on. We may also discover
their address by querying the DHT, checking a fixed "bootstrap list", or perhaps
through a pubsub message or an application-specific protocol.
In the case of the identify protocol, we can be fairly certain that the
addresses originate from the peer we're speaking to, assuming that we're using a
secure, authenticated communication channel. However, more "ambient" discovery
methods such as DHT traversal and pubsub depend on potentially untrustworthy
third parties to relay address information.
Even in the case of receiving addresses via the identify protocol, our
confidence that the address came directly from the peer is not actionable, because
the peer store does not track the origin of an address. Once added to the peer
store, all addresses are considered equally valid, regardless of their source.
We would like to have a means of distributing _verifiable_ address records,
which we can prove originated from the addressed peer itself. We also need a way to
track the "provenance" of an address within libp2p's internal components such as
the peer store. Once those pieces are in place, we will also need a way to
prioritize addresses based on their authenticity, with the most strict strategy
being to only dial certified addresses.
### Complications
While producing a signed record is fairly trivial, there are a few aspects to
this problem that complicate things.
1. Addresses are not static. A given peer may have several addresses at any given
time, and the set of addresses can change at arbitrary times.
2. Peers may not know their own addresses. It's often impossible to automatically
infer one's own public address, and peers may need to rely on third party
peers to inform them of their observed public addresses.
3. A peer may inadvertently or maliciously sign an address that they do not
control. In other words, a signature isn't a guarantee that a given address is
valid.
4. Some addresses may be ambiguous. For example, addresses on a private subnet
are valid within that subnet but are useless on the public internet.
The first point implies that the address record should include some kind of
temporal component, so that newer records can replace older ones as the state
changes over time. This could be a timestamp and/or a simple sequence number
that each node increments whenever they publish a new record.
The second and third points highlight the limits of certifying information that
is itself uncertain. While a signature can prove that the addresses originated
from the peer, it cannot prove that the addresses are correct or useful. Given
the asymmetric nature of real-world NATs, it's often the case that a peer is
_less likely_ to have correct information about its own address than an outside
observer, at least initially.
This suggests that we should include some measure of "confidence" in our
records, so that peers can distribute addresses that they are not fully certain
are correct, while still asserting that they created the record. For example,
when requesting a dial-back via the [AutoNAT service][autonat], a peer could
send a "provisional" address record. When the AutoNAT peer confirms the address,
that address could be marked as confirmed and advertised in a new record.
Regarding the fourth point about ambiguous addresses, it would also be desirable
for the address record to include a notion of "routability," which would
indicate how "accessible" the address is likely to be. This would allow us to
mark an address as "LAN-only," if we know that it is not mapped to a publicly
reachable address but would still like to distribute it to local peers.
## Address Record Format
Here's a protobuf that might work:
```protobuf
// Routability indicates the "scope" of an address, meaning how visible
// or accessible it is. This allows us to distinguish between LAN and
// WAN addresses.
//
// Side Note: we could potentially have a GLOBAL_RELAY case, which would
// make it easy to prioritize non-relay addresses in the dialer. Bit of
// a mix of concerns though.
enum Routability {
// catch-all default / unknown scope
UNKNOWN = 1;
// another process on the same machine
LOOPBACK = 2;
// a local area network
LOCAL = 3;
// public internet
GLOBAL = 4;
// reserved for future use
INTERPLANETARY = 100;
}
// Confidence indicates how much we believe in the validity of the
// address.
enum Confidence {
// default, unknown confidence. we don't know one way or another
UNKNOWN = 1;
// INVALID means we know that this address is invalid and should be deleted
INVALID = 2;
// UNCONFIRMED means that we suspect this address is valid, but we haven't
// fully confirmed that we're reachable.
UNCONFIRMED = 3;
// CONFIRMED means that we fully believe this address is valid.
// Each node / implementation can have their own criteria for confirmation.
CONFIRMED = 4;
}
// AddressInfo is a multiaddr plus some metadata.
message AddressInfo {
bytes multiaddr = 1;
Routability routability = 2;
Confidence confidence = 3;
}
// AddressState contains the listen addresses (and their metadata)
// for a peer at a particular point in time.
//
// Although this record contains a wall-clock `issuedAt` timestamp,
// there are no guarantees about node clocks being in sync or correct.
// As such, the `issuedAt` field should be considered informational,
// and `version` should be preferred when ordering records.
message AddressState {
// the peer id of the subject of the record.
bytes subjectPeer = 1;
// `version` is an increment-only counter that can be used to
// order AddressState records chronologically. Newer records
// MUST have a higher `version` than older records, but there
// can be gaps between version numbers.
uint64 version = 2;
// The `issuedAt` timestamp stores the creation time of this record in
// seconds from the unix epoch, according to the issuer's clock. There
// are no guarantees about clock sync or correctness. SHOULD NOT be used
// to order AddressState records; use `seqno` instead.
uint64 issuedAt = 3;
// All current listen addresses and their metadata.
repeated AddressInfo addresses = 4;
}
```
The idea with the structure above is that you send some metadata along with your
addresses: your "routability", and your own confidence in the validity of the
address. This is wrapped in an `AddressInfo` struct along with the address
itself.
Then you have a big list of `AddressInfo`s, which we put in an `AddressState`.
An `AddressState` identifies the `subject` of the record,
#### Example
Here's an example. Alice has an address that she thinks is publicly reachable
but has not confirmed. She also has a LAN-local address that she knows is valid,
but not routable via the public internet:
```javascript
{
subjectPeer: "QmAlice...",
version: 23456,
issuedAt: 1570215229,
addresses: [
{
addr: "/ip4/1.2.3.4/tcp/42/p2p/QmAlice",
routability: "GLOBAL",
confidence: "UNCONFIRMED"
},
{
addr: "/ip4/10.0.1.2/tcp/42/p2p/QmAlice",
routability: "LOCAL",
confidence: "CONFIRMED"
}
]
}
```
If Alice wants to publish her address to a public shared resource like a DHT,
she should omit `LOCAL` and other unreachable addresses, and peers should
likewise filter out `LOCAL` addresses from public sources.
## Certification / Verification
This structure can be contained in a [signed envelope][envelope-rfc], which lets
us issue "self-certified" address records that are signed by the `subjectPeer`.
## Peer Store APIs
## Dialing Strategies
## TODO
Some things I'd like to cover but haven't got to or figured out yet:
- how to store signed records
- should be separate from "working set" that's optimized for retrieval
- need to store unaltered bytes
- how to surface routability and confidence via peerstore APIs
- figure out if IPLD is the way to go here. If not, what serialization format,
etc.
- extend identify protocol to include signed records?
- how are addresses prioritized when dialing?
[identify-spec]: ../identify/README.md
[peer-id-spec]: ../peer-ids/peer-ids.md
[autonat]: https://github.com/libp2p/specs/issues/180
[ipld]: https://ipld.io/
[ipld-schema-schema]: https://github.com/ipld/specs/blob/master/schemas/schema-schema.ipldsch
[envelope-rfc]: ./0002-signed-envelopes.md