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+---
+rfc:
+start_date: 2018-02-12
+pr:
+status: draft
+---
+
+# Entry Invalidation
+
+## Summary
+
+This RFC proposes a way to invalidate an entry so previous changes considered
+mistakes can be removed from the computed list of records.
+
+
+## Motivation
+
+Currently, the append-only nature of Registers doesn't cope well with
+mistakes. A situation that has arisen more than once is getting two records
+about the same thing with different identifiers because a human mistake.
+Another situation is having a change in the history of a record that is wrong.
+
+We need a way to mark entries as invalid so we can keep compatibility with
+other copies of the Register (consumers, replicas, etc) and allow computing
+the correct(ed) list of records.
+
+
+## Explanation
+
+[TODO: Currently two alternatives are explored]
+
+
+### Alternative A: Special entry
+
+The proposal introduces a new type of entry that allows listing a set of entry
+identifiers. This mechanism would allow invalidating discrete entries and by
+extension invalidating the full history for a key when all entries for that
+key are flagged as invalid. It also introduces a new RSF command to avoid
+overloading the `append-entry` command.
+
+[TODO: entry number or entry hash? Entry number is aligned with existing ways
+to refer to entries. Entry hash invalidates the entry by its content]
+
+#### RSF
+
+```
+invalidate-entry	user	2018-02-12T10:11:12Z	3;234;355
+```
+
+#### JSON
+
+```json
+{
+  "entry-number": "356",
+  "revoked-entries": ["3", "234", "355"],
+  "entry-timestamp": "2018-02-12T10:11:12Z"
+}
+```
+
+#### Properties
+
+* New entry type without key. Breaking change diverging from previous designs.
+* Records are unaffected.
+* Revoked entries require a mechanism/documentation to explain how to apply
+  them when computing the list of records.
+* Entry proofs are kept intact and invalidations are part of the tree.
+
+
+### Alternative B: Reserved entry key + new item shape
+
+The proposal introduces a reserved key and a new type of item where entry
+identifiers are listed. This mechanism allows invalidating discrete entries
+and by extension invalidating the full history for a key.
+
+
+[TODO: Review if "chore:revoked-entries" is the right reserved key]
+
+
+#### RSF
+
+```
+add-item	{"id":"chore:revoked-entries","entry-numbers":["3","234","355"]}
+append-entry	user	chore:revoked-entries	2018-02-12T10:11:12Z	sha-256:0000000000000000000000000000000000000000000000000000000000000000
+```
+
+#### JSON
+
+**Entry 356:**
+
+```json
+{
+  "index-entry-number": "356",
+  "entry-number": "356",
+  "key": "chore:revoked-entries",
+  "item-hash": [ "sha-256:0000000000000000000000000000000000000000000000000000000000000000"],
+  "entry-timestamp": "2018-02-12T10:11:12Z"
+}
+```
+
+**Item sha-256:0000000000000000000000000000000000000000000000000000000000000000:**
+
+```json
+{
+  "id": "chore:revoked-entries",
+  "entry-numbers": ["3", "234", "355"]
+}
+```
+
+#### CSV
+
+**Entry 356:**
+
+```csv
+index-entry-number,entry-number,entry-timestamp,key,item-hash
+356,356,2018-02-12T10:11:12Z,chore:revoked-entries,"sha-256:0000000000000000000000000000000000000000000000000000000000000000"
+```
+
+**Item sha-256:0000000000000000000000000000000000000000000000000000000000000000:**
+
+```csv
+id,entry-numbers
+chore:revoked-entries,3;234;355;
+```
+
+#### Properties
+
+* Entries are consumed regularly (no breaking changes).
+* This approach uses reserved key in the user space. Requires avoiding clashes
+  with user data.
+* Knowledge of the reserved key is required to filter it out when collecting
+  list data.
+* Records don't list reserved keys. You don't get the same information if you
+  get it from `/records` or from `/entries`. Emphasises the dichotomy between
+  porcelain and plumbing.
+* Revoked entries require a mechanism/documentation to explain how to apply
+  them when creating the list of records.
+* Entry proofs are kept intact and invalidations are part of the tree.
+
+