RFC 0001: text Peer Id as CID

This is an RFC to modify peerid spec to alter the default string
representation from Multihash to CIDv1 in Base32 and to support
encoding/decoding text peerids as CIDs.

It is also the first RFC ever, following suggestions from
https://github.com/libp2p/specs/issues/198
and creating a template for future RFCs as a side-effect.

License: MIT
Signed-off-by: Marcin Rataj <lidel@lidel.org>

RFC/0001-text-peerid-cid.md

@@ -0,0 +1,61 @@
+- Start Date: 2019-08-15
+- Related issues: [go-ipfs/issues/5287](https://github.com/ipfs/go-ipfs/issues/5287), [multicodec/issues/130](https://github.com/multiformats/multicodec/issues/130), [go-libp2p-core/pull/41](https://github.com/libp2p/go-libp2p-core/pull/41)
+
+# RFC 0001: Text Peer Ids as CIDs
+
+## Abstract
+
+This is an RFC to modify Peer Id spec to alter the default string representation
+from Multihash to CIDv1 in Base32 and to support encoding/decoding text Peer Ids as CIDs.
+
+[ipld-cid-spec]: https://github.com/ipld/cid
+
+## Motivation
+
+1.  Current text representation of Peer Id ([multihash][multihash] in [Base58btc][base58btc]) is case-sensitive.
+    This means we can't use it in case-insensitive contexts such as domain names ([RFC1035][rfc1035] + [RFC1123][rfc1123]) or [FAT](fat) filesystems.
+2.  [CID][ipld-cid-spec] provide [multibase][multibase] support and `base32`
+    makes a [safe default][cidv1b32-move] that will work  in case-insensitive contexts,
+    enabling us to put Peer Ids  [in domains][cid-in-subdomains] or create files with Peer Ids as names.
+3.  It's much easier to upgrade wire protocols than text.
+    This RFC makes Peer Ids in text form fully self describing, making them more future-proof.
+    A dedicated [multicodec][multicodec] in text-encoded CID will indicate that [it's a hash of a libp2p public key][libp2p-key-multicodec].
+
+[rfc1035]: http://tools.ietf.org/html/rfc1035
+[rfc1123]: https://tools.ietf.org/html/rfc1123
+[multibase]: https://github.com/multiformats/multibase/
+[multicodec]: https://github.com/multiformats/multicodec
+[multihash]: https://github.com/multiformats/multihash
+[cid-in-subdomains]: https://github.com/ipfs/in-web-browsers/issues/89
+[libp2p-key-multicodec]: https://github.com/multiformats/multicodec/issues/130
+[cidv1b32-move]: https://github.com/ipfs/ipfs/issues/337
+[base58btc]: https://en.bitcoinwiki.org/wiki/Base58#Alphabet_Base58
+[fat]: https://en.wikipedia.org/wiki/Design_of_the_FAT_file_system
+
+## Detailed design
+
+1. Switch text encoding and decoding of Peer Ids from Multihash to [CID][ipld-cid-spec].
+2. The new text representation should be CIDv1 with additional requirements:
+    - MUST have [multicodec][multicodec] set to `libp2p-key` (`0x72`)
+    - SHOULD have [multibase][multibase] set to `base32` (Base32 without padding, as specified by [RFC4648][rfc4648])
+
+[rfc4648]: https://tools.ietf.org/html/rfc4648
+
+### Backward compatibility
+
+The old text representation (Multihash encoded as [`base58btc`][base58btc])
+is a valid CIDv0 and does not require any special handling.
+
+[base58btc]: https://en.bitcoinwiki.org/wiki/Base58#Alphabet_Base58
+
+## Alternatives
+
+We could just add a [multibase][multibase] prefix to multihash, but that requires more work and introduces a new format.
+This option was rejected as using CID enables reuse of existing serializers/deserializers and does not create any new standards.
+
+## Unresolved questions
+
+This RFC punts pids-as-cids on the wire down the road but that's something we can revisit if it ever becomes relevant.
+
+[go-libp2p-core-41]: https://github.com/libp2p/go-libp2p-core/pull/41
+[libp2p-specs-111]: https://github.com/libp2p/specs/issues/111