restore test takes only this box's archives (R-727): an archive encrypted with another key is another box's

Red-proof RP39. Released as v0.138.0 by release-agent.sh.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0159rPz1ZhFKsS53msqPYxtS

internal/backup/runner.go

@@ -357,6 +357,16 @@ func (r *BackupRunner) PickSettledRestoreCandidateOn(ctx context.Context, target
 	if err != nil {
 		return "", time.Time{}, err
 	}
+	// R-727 (v0.138.0): on an ENCRYPTED storage, only archives written with THIS storage's key are this box's.
+	// Measured 2026-09-30 on a fresh box for a returning customer: the PBS namespace still held two archives
+	// of earlier boxes (same guest id 9201, same token), the newest settled one was an earlier box's, and the
+	// test failed `wrong key` every evaluation. The archive carries no host id; its key fingerprint is the
+	// discriminator (PVE's content `encrypted`, the storage's `encryption-key`). A lookup failure returns
+	// the error — the tier reads UNKNOWN, never "nothing to prove".
+	ownKey, err := r.storageKeyFingerprint(ctx, target)
+	if err != nil {
+		return "", time.Time{}, fmt.Errorf("reading the key fingerprint of storage %s: %w", target, err)
+	}
 	var best string
 	var bestCTime int64 = -1
 	known := map[int]bool{} // vmid → the guest exists on this node (asked once per vmid per pick)
@@ -372,6 +382,10 @@ func (r *BackupRunner) PickSettledRestoreCandidateOn(ctx context.Context, target
 			r.noteNotAGuestBackupOnce(e, why)
 			continue
 		}
+		if ownKey != "" && !strings.EqualFold(e.Encrypted, ownKey) {
+			r.noteNotAGuestBackupOnce(e, fmt.Sprintf("written by another box (key %s, this box's key %s) — not this box's proof", shortFP(e.Encrypted), shortFP(ownKey)))
+			continue
+		}
 		// R-689 (v0.136.0): … OF A GUEST THAT STILL EXISTS here. Measured on demo-hp 2026-09-27 right after
 		// v0.135.0: with the golden skipped, the pick fell to `vzdump-lxc-9100-2026_08_21…`, a leftover of a
 		// guest deleted in August — proving nothing about any guest this box runs. "Does not exist" skips the
@@ -667,3 +681,29 @@ func (r *BackupRunner) noteNotAGuestBackupOnce(e proxmox.StorageContent, why str
 			"target", r.target, "volid", e.VolID, "size_bytes", e.Size, "reason", why)
 	}
 }
+
+// storageKeyFingerprint returns the named storage's client-side encryption key fingerprint ("" when the
+// storage is not encrypted — a local dir holds only this box's own vzdumps).
+func (r *BackupRunner) storageKeyFingerprint(ctx context.Context, target string) (string, error) {
+	sts, err := r.api.ListStorage(ctx)
+	if err != nil {
+		return "", err
+	}
+	for _, st := range sts {
+		if st.Storage == target {
+			return strings.TrimSpace(st.EncryptionKey), nil
+		}
+	}
+	return "", nil
+}
+
+// shortFP is the first 8 bytes of a key fingerprint, for a log line.
+func shortFP(fp string) string {
+	if fp == "" {
+		return "none"
+	}
+	if len(fp) > 23 {
+		return fp[:23] + "…"
+	}
+	return fp
+}