ipns: document backward compatibility

We have to document legacy signature, because it is the only reason for
copying 'data' fields into the main protobuf.

Added section about backward compatibility so our approach is more clear.

IPNS.md

@@ -25,6 +25,7 @@ IPNS records provide cryptographically verifiable, mutable pointers to objects.
 - [IPNS Record](#ipns-record)
   - [Record Serialization Format](#record-serialization-format)
   - [Record Size Limit](#record-size-limit)
+  - [Backward Compatibility](#backward-compatibility)
 - [Protocol](#protocol)
   - [Overview](#overview)
   - [Record Creation](#record-creation)
@@ -175,7 +176,7 @@ message IpnsEntry {
  // deserialized copy of data[value]
  optional bytes value = 1;
 
- // unused legacy field, use 'signatureV2' instead
+ // legacy field, verify 'signatureV2' instead
  optional bytes signatureV1 = 2;
 
  // deserialized copies of data[validityType] and data[validity]
@@ -216,6 +217,24 @@ Records over the limit MAY be ignored. Handling records larger than the
 limit is not recommended so as to keep compatibility with implementations and
 transports that follow this specification.
 
+### Backward Compatibility
+
+Implementations that want to interop with the public IPFS swarm MUST maintain
+backward compatibility for legacy consumers of IPNS records:
+
+
+- A legacy publisher MUST always be able to update to the latest implementation
+  of this specification without breaking record resolution for legacy consumers.
+- A legacy consumer MUST always be able to resolve IPNS name, even when publisher
+  updated to the latest implementation of this specification.
+
+This means, for example, that changes made to the `IpnsEntry` protobuf, or
+validation logic should always be additive.
+
+Future changes to this spec should include design decisions that allow legacy
+nodes to gracefully ignore new fields and verify compatible records using
+legacy logic.
+
 ## Protocol
 
 ### Overview
@@ -243,8 +262,7 @@ Finally, the network nodes may also republish their records, so that the records
 
 ### Record Creation
 
-IPNS record MUST be serialized as `IpnsEntry` protobuf and `IpfsEntry.data` MUST be signed using the private key.
-
+IPNS record MUST be serialized as `IpnsEntry` protobuf, and `IpfsEntry.data` MUST be signed using the private key.
 Creating a new IPNS record MUST follow the below steps:
 
 1. Create `IpnsEntry` and set `value`, `validity`, `validityType`, `sequence`, and `ttl`
@@ -256,9 +274,13 @@ Creating a new IPNS record MUST follow the below steps:
    - The order of fields impacts signature verification. If you are using an alternative CBOR implementation, make sure the CBOR field order follows [RFC7049](https://www.rfc-editor.org/rfc/rfc7049) sorting rules: length and then bytewise. The order of fields impacts signature verification.
 4. If your public key can't be inlined inside the IPNS Name, include a serialized copy in `IpnsEntry.pubKey`
    - This step SHOULD be skipped for Ed25519, and any other key types that are inlined inside of [IPNS Name](#ipns-name) itself.
-6. Create bytes for signing by concatenating `ipns-signature:` prefix (bytes in hex: `69706e732d7369676e61747572653a`) with raw CBOR bytes from `IpnsEntry.data`
-7. Sign concatenated bytes from the previous step using the private key, and store the signature in `IpnsEntry.signatureV2`
-8. Confirm that the serialized `IpnsEntry` bytes sum to less than or equal to [the size limit](#record-size-limit).
+5. Create `IpnsEntry.signatureV2`
+   - Create bytes for signing by concatenating `ipns-signature:` prefix (bytes in hex: `69706e732d7369676e61747572653a`) with raw CBOR bytes from `IpnsEntry.data`
+   - Sign concatenated bytes from the previous step using the private key, and store the signature in `IpnsEntry.signatureV2`
+6. Create `IpnsEntry.signatureV1` (backward compatibility, for legacy software)
+   - Create bytes for signing by concatenating `IpnsEntry.value` + `IpnsEntry.validity` + `string(IpnsEntry.validityType)`
+   - Sign concatenated bytes from the previous step using the private key, and store the legacy signature in `IpnsEntry.signatureV1`
+7. Confirm that the serialized `IpnsEntry` bytes sum to less than or equal to [the size limit](#record-size-limit).
 
 ### Record Verification