fix(vaults): harden the KID window check and clear a stale flag on licence

docs/guide/vaults.md

@@ -34,11 +34,11 @@ unshackle pushes keys from every source, not only a local CDM. A run against a r
 
 ### Wrong keys from a poisoned vault
 
-A public vault can return the right KID with a wrong content key. The decrypters accept a wrong content key and write noise. unshackle therefore trusts a content key from a vault only after FFmpeg decodes the decrypted track with strict error detection. For a fragmented MP4, unshackle reads from the encrypted file which fragments each KID encrypts, and FFmpeg decodes up to three windows of each KID: from its first, middle and last fragment on through the next fragments of that KID, for up to 10 seconds. A window never runs into a fragment of another KID or a clear fragment. A clear lead or a lead under another KID therefore cannot hide a wrong content key, and only the KID that fails is flagged. A KID that is not in this map, and every KID of any other file, gets one check of the whole file. FFmpeg decodes the first seconds and the last seconds. A failure there flags each of those KIDs. Content keys from a licence skip this check. A file this FFmpeg build cannot judge counts as a pass, never as a wrong content key: a codec it has no decoder for, or a header it cannot read. Without FFmpeg there is no check at all.
+A public vault can return the right KID with a wrong content key. The decrypters accept a wrong content key and write noise. unshackle therefore trusts a content key from a vault only after FFmpeg decodes the decrypted track with strict error detection. For a fragmented MP4, unshackle reads from the encrypted file which fragments each KID encrypts, and FFmpeg decodes up to three windows of each KID: from its first, middle and last fragment on through the next fragments of that KID, for up to 4 seconds. A window never runs into a fragment of another KID or a clear fragment. A clear lead or a lead under another KID therefore cannot hide a wrong content key, and only the KID that fails is flagged. A KID that is not in this map, and every KID of any other file, gets one check of the whole file. FFmpeg decodes the first seconds and the last seconds. A failure there flags each of those KIDs. When a mapped KID fails, the KIDs that are not in the map get no verdict from that pass; the retry with the replaced key judges them. Content keys from a licence skip this check. A file this FFmpeg build cannot judge counts as a pass, never as a wrong content key: a codec it has no decoder for, or a header it cannot read. Without FFmpeg there is no check at all.
 
 When the decode fails, unshackle:
 
-1. Flags the KID:KEY in the `bad_keys` table of every local SQLite vault, with the service and the name of the vault it came from. The pair also goes out of the service table, except in a vault marked `no_push`, where only the flag is written. A flagged pair cannot go back into a local vault, from a later run or from `kv copy`. An `API` vault that served the pair also gets a report; `HTTP` and `MySQL` vaults have no report route. A pair from a local vault goes to no remote vault, even when a remote vault holds the same pair.
+1. Flags the KID:KEY in the `bad_keys` table of every local SQLite vault, with the service and the name of the vault it came from. The pair also goes out of the service table, except in a vault marked `no_push`, where only the flag is written. A flagged pair cannot go back into a local vault from another vault, a later run, or `kv copy`. A licence that returns the pair clears the flag and stores it. An `API` vault that served the pair also gets a report; `HTTP` and `MySQL` vaults have no report route. A pair from a local vault goes to no remote vault, even when a remote vault holds the same pair.
 2. Restores the ciphertext from a hard link it made before decryption, then asks for the content key again. Every vault lookup skips a flagged pair, so the request reaches the remaining vaults and then the CDM. A remote answer that arrived after the winning one is reused instead of queried again.
 3. Removes the flag when the CDM returns the same content key, because then the decode check was wrong, not the content key. A content key from another vault is checked in its turn, and flagged as well if it also fails.