fix(a7-auth): persist dummy key in owner-only sidecar

The store-wide dummy key was regenerated on every credentials_load, so an
unknown user's dummy salt changed across daemon restarts while a real user's
stored salt stayed stable -- a restart-gated username-enumeration oracle.

Persist the 32-byte key in a 0600 <store>.dummykey sidecar next to the
credential store.  An absent sidecar is created with O_EXCL and fsynced; a
present sidecar is read only when it is an owner-only regular file of exactly
32 bytes (otherwise the load fails closed).  If the sidecar cannot be created
(read-only mount, missing directory) fall back to a transient per-run key with
a warning.  A NULL store path keeps the key ephemeral.

README.md

@@ -522,11 +522,13 @@ deterministic per-username dummy challenge, so probing the daemon cannot
 enumerate users. Store lines are generated with
 `fastsync-server --hash-credentials <plaintext-file>` (see `RSYNC_COMPAT.md`);
 redirect that output to an owner-only (mode 0600) file, and note that legacy
-`user:SHA256HEX` stores are rejected. Two residuals are accepted: the dummy salt
-is stable within one daemon lifetime but changes across restarts, so a
-restart-gated enumeration channel remains (persisting a dummy key is out of
-scope); and the store iteration count is observable pre-auth by design, since
-the miss path must match a hit.
+`user:SHA256HEX` stores are rejected. FastSync also maintains an owner-only
+(mode 0600) `<store>.dummykey` sidecar next to the store: it holds the store-wide
+dummy key, is auto-created on first load, and must be preserved across daemon
+restarts so the dummy challenge for an unknown user stays stable (the key is
+never regenerated while the sidecar exists). One residual is accepted: the store
+iteration count is observable pre-auth by design, since the miss path must match
+a hit.
 
 TLS provides encrypted TCP transport. Supplying `--ca` enables certificate
 verification; without it, traffic is encrypted but peer identity is not

RSYNC_COMPAT.md

@@ -639,8 +639,8 @@ now transmits targets (the prior behavior was broken/partial); its status moved
 - **Module selection & confinement:** the client requests a module with an rsync-style `host::module[/path]` destination. The module name crosses the wire as a trailing string on the config frame (bumping `PROTOCOL_VERSION` 2.14.0 → 2.15.0; the bump is required because the config-frame layout changed and the strict same-version handshake is what prevents a peer from desynchronizing on the new trailing field). The daemon looks the module up in ITS OWN config and uses the module's `path` as the authorized root through the exact same `configure_authorization` confinement the standalone server applies to `--destination-root` (`file_open_secure_parent`, `has_path_traversal`, `path_is_within`); the client never supplies the root, every client-chosen-ownership/super-user request is refused unless the module declares `client owner = yes` (the daemon's per-module opt-in, see below), and the operator `--no-super` veto forces super-user activities off for every daemon connection. The client's `/path` part is relative inside the module and is rejected if absolute or if it contains `..`. Unknown modules are refused before any data moves (the run fails cleanly at the config handshake). An absolute destination and a module request against a non-daemon server are also refused.
 - **`client owner` (client-chosen-ownership opt-in):** by default a daemon module refuses every request that would let the client pick an owner or ask for super-user activities — `--numeric-ids`, `--chown`, `--usermap`/`--groupmap`, `--fake-super`, `--copy-as`, and an explicit `--super` — at the config handshake (before `STATUS_OK`), because a daemon has no per-module opt-in for client-chosen ownership and any anonymous client could otherwise force arbitrary owner ids inside the module root. `client owner = yes` opts a single module in, allowing those requests within that module's root (the standalone listener and the SSH `--stdio` server always honor them for their single operator-authorized root). Without the opt-in the daemon also forces super-user **device** activity off for that connection — char/block device-node creation (`--devices`) and `--write-devices` — even under the default `AUTO` mode, so a non-opted module can never be made to `mknod` or write a raw device; those entries are skipped (not refused) so an ordinary `-a` push still succeeds without device nodes. The opt-in does **not** lift the privilege requirement: `--copy-as` still needs a root receiver, and the operator `--no-super` veto still forces super-user activities off for every connection. The daemon logs a prominent startup warning for each `client owner = yes` module so the operator's deliberate choice is visible.
 - **`read only` safe default:** every network transfer FastSync currently supports is a push that writes under the module root, so a `read only` module refuses the connection (clear server log "module is read only"; the client exits non-zero, nothing is transferred). A future pull/list operation can be opened up when it exists; the knob is already stored.
-- **`auth users` (A7 SCRAM-SHA-256 authentication):** a module that declares `auth users` requires the client to present credentials. The config frame carries ONLY the username; the daemon answers an auth-required module with `STATUS_AUTH_CHALLENGE` (PBKDF2 iteration count, 16-byte salt, 32-byte server nonce), the client answers with `STATUS_AUTH_RESPONSE` (fresh 32-byte client nonce + a 32-byte ClientProof), and the daemon accepts only when the proof verifies **and** the username is **on the module's `auth users` list** and has a store entry, replying `STATUS_AUTH_OK` with a 32-byte ServerSignature the client verifies before proceeding. Verification is constant-time over fixed 32-byte keys (the compare runs even for a miss), username membership uses a constant-time full-length scan, and an unknown/off-list user still receives a challenge and runs the same math against a dummy verifier: a deterministic per-username salt (`HMAC-SHA256(store dummy key, username)`), the store-wide uniform iteration count and dummy keys. Re-probing the same unknown username therefore yields an identical salt and iteration count while a different username yields a different salt, so there is no user-enumeration or timing oracle. The daemon logs the username but **never the password, proof or keys**. A module WITHOUT `auth users` stays open (legitimate rsync configuration); credentials sent to such a module are ignored. Read-only is orthogonal: even a correctly authenticated push to a `read only` module is still refused (all FastSync network transfers write). Fail-closed policy: a daemon whose config declares `auth users` on any module refuses to start unless a credential store was given (`--password-file` and/or `--early-input`); a missing or empty store is never silently treated as "open". A failed handshake (missing credentials, unknown/off-list user, wrong proof or malformed data) yields a single generic `STATUS_AUTH_FAILED` and the daemon closes before any data moves. Two residuals are accepted: the dummy salt is stable within one daemon lifetime but changes across restarts, leaving a restart-gated enumeration channel (persisting the dummy key is out of scope); and the store iteration count is observable pre-auth by design, since the miss path must match a hit.
-- **Credential store format:** server `--password-file`/`--early-input` files are line-based `user:$fastsync$1$pbkdf2-sha256$<iters>$<salt_b64>$<stored_key_b64>$<server_key_b64>`, one per line (standard base64; 16-byte salt, 32-byte keys; `iters` in `[100000, 10000000]`, default 600000). Every entry in the resulting store must agree on `iters` (a store whose entries disagree, or where a layered `--early-input` disagrees with `--password-file`, is rejected). Generate lines with `fastsync-server --hash-credentials FILE [--iterations N]`; the emitted lines are secret material, so redirect them to an owner-only (mode 0600) file (the tool warns on stderr if stdout is a group/other-accessible regular file). Blank lines and lines starting with `#`/`;` are comments; the parser is strict (a malformed line fails the whole load, so a typo can never let a different set of users in). **The legacy `user:SHA256HEX` form is hard-rejected** with an actionable "legacy" error; there is no auto-upgrade, so a replayable bearer digest can never be loaded by a 2.19.0 daemon. The client `--password-file` holds `user:password` on its first meaningful line (the literal password, used only for the handshake then burned); keep both files readable only by their owner (mode 0600). Per-username wire length is bounded (256 chars) and every decoded salt/key length is validated.
+- **`auth users` (A7 SCRAM-SHA-256 authentication):** a module that declares `auth users` requires the client to present credentials. The config frame carries ONLY the username; the daemon answers an auth-required module with `STATUS_AUTH_CHALLENGE` (PBKDF2 iteration count, 16-byte salt, 32-byte server nonce), the client answers with `STATUS_AUTH_RESPONSE` (fresh 32-byte client nonce + a 32-byte ClientProof), and the daemon accepts only when the proof verifies **and** the username is **on the module's `auth users` list** and has a store entry, replying `STATUS_AUTH_OK` with a 32-byte ServerSignature the client verifies before proceeding. Verification is constant-time over fixed 32-byte keys (the compare runs even for a miss), username membership uses a constant-time full-length scan, and an unknown/off-list user still receives a challenge and runs the same math against a dummy verifier: a deterministic per-username salt (`HMAC-SHA256(store dummy key, username)`), the store-wide uniform iteration count and dummy keys. Re-probing the same unknown username therefore yields an identical salt and iteration count while a different username yields a different salt, so there is no user-enumeration or timing oracle. The daemon logs the username but **never the password, proof or keys**. A module WITHOUT `auth users` stays open (legitimate rsync configuration); credentials sent to such a module are ignored. Read-only is orthogonal: even a correctly authenticated push to a `read only` module is still refused (all FastSync network transfers write). Fail-closed policy: a daemon whose config declares `auth users` on any module refuses to start unless a credential store was given (`--password-file` and/or `--early-input`); a missing or empty store is never silently treated as "open". A failed handshake (missing credentials, unknown/off-list user, wrong proof or malformed data) yields a single generic `STATUS_AUTH_FAILED` and the daemon closes before any data moves. The dummy key is persisted in an owner-only `<store_path>.dummykey` sidecar (auto-created on first load, mode 0600) so the dummy salt stays stable across daemon restarts, closing the restart-gated enumeration channel. One residual is accepted: the store iteration count is observable pre-auth by design, since the miss path must match a hit.
+- **Credential store format:** server `--password-file`/`--early-input` files are line-based `user:$fastsync$1$pbkdf2-sha256$<iters>$<salt_b64>$<stored_key_b64>$<server_key_b64>`, one per line (standard base64; 16-byte salt, 32-byte keys; `iters` in `[100000, 10000000]`, default 600000). Every entry in the resulting store must agree on `iters` (a store whose entries disagree, or where a layered `--early-input` disagrees with `--password-file`, is rejected). Generate lines with `fastsync-server --hash-credentials FILE [--iterations N]`; the emitted lines are secret material, so redirect them to an owner-only (mode 0600) file (the tool warns on stderr if stdout is a group/other-accessible regular file). Blank lines and lines starting with `#`/`;` are comments; the parser is strict (a malformed line fails the whole load, so a typo can never let a different set of users in). **The legacy `user:SHA256HEX` form is hard-rejected** with an actionable "legacy" error; there is no auto-upgrade, so a replayable bearer digest can never be loaded by a 2.19.0 daemon. The client `--password-file` holds `user:password` on its first meaningful line (the literal password, used only for the handshake then burned); keep both files readable only by their owner (mode 0600). Per-username wire length is bounded (256 chars) and every decoded salt/key length is validated. Loading the store also maintains an owner-only `<store_path>.dummykey` sidecar (auto-created, mode 0600, exactly 32 bytes) holding the store-wide dummy key that shapes unknown-user challenges; persist it across daemon restarts so those challenges stay stable, and treat a sidecar with the wrong owner, permissions, size or type as a fatal load error (fail closed).
 - **Plaintext caveat:** over a plaintext (non-TLS) daemon a sniffer can read the challenge and response and mount an **offline dictionary attack** against a weak password, so use `--tls` for any real deployment. Unlike the old challenge-less exchange there is **no replay**: the proof is bound to the fresh per-connection server nonce, so a captured `STATUS_AUTH_RESPONSE` cannot be reused on another connection (an integration test proxies the daemon and proves this). The daemon logs a warning when an auth-required module is reached over plaintext. TLS client-CN (`--client-cn`) is an independent transport identity check and composes with password auth: both may be required on the same connection.
 - **Wire/protocol:** the config-frame auth block is now `[int present][str_redacted username]` (the old digest field is gone), and the frame stream gains the challenge/response (`STATUS_AUTH_CHALLENGE` → `STATUS_AUTH_RESPONSE` → `STATUS_AUTH_OK`/`STATUS_AUTH_FAILED`) between the config frame and the `STATUS_OK` ack. Both are wire-layout changes, so `PROTOCOL_VERSION` is bumped **2.18.0 → 2.19.0** (see the A7 note in `src/shared/config.h`); the strict same-version handshake keeps a 2.19 client and a 2.18 server from desynchronizing.
 - **Client side:** `host::module/path` selects the TCP transport and connects to `--server-port`; `host:path` stays the SSH transport; plain paths stay local TCP. The daemon username comes from `--password-file` (first `user:password` line), and `--password-file` without a `host::module/path` destination is a client error (fail fast). A `user@host::module` form is rejected with a pointer to `--password-file`. The client's plaintext password is wiped from memory (`config_burn_auth`) at transfer teardown.

src/shared/credentials.c

@@ -1,4 +1,5 @@
 #include "credentials.h"
+#include "log.h"
 #include "utils.h"
 #include <ctype.h>
 #include <errno.h>
@@ -35,16 +36,22 @@ struct CredentialStore {
    * challenged with the same count as a hit and the count itself never leaks
    * membership.  Unused (0) for an empty store. */
   uint32_t iters;
-  /* Random secret generated once at load.  The dummy salt handed out for an
-   * unknown/off-list user is HMAC-SHA256(dummy_key, username)[:SALT_LEN], so
-   * repeated probes of the same username always see an identical challenge
-   * while different usernames differ -- with no fresh-random tell. */
+  /* Store-wide secret loaded from (or created in) the owner-only
+   * `<store_path>.dummykey` sidecar, so it also survives a daemon restart.  The
+   * dummy salt handed out for an unknown/off-list user is
+   * HMAC-SHA256(dummy_key, username)[:SALT_LEN], so repeated probes of the same
+   * username always see an identical challenge while different usernames differ
+   * -- with no fresh-random tell, and cross-restart stability hides the
+   * restart-gated enumeration oracle. */
   uint8_t dummy_key[CREDENTIAL_KEY_LEN];
 };
 
 /* Exact marker prefix of the new store verifier field. */
 #define CREDENTIAL_STORE_PREFIX "$fastsync$1$pbkdf2-sha256$"
 #define CREDENTIAL_AUTH_PREFIX "FastSync-Auth-v1"
+/* Owner-only sidecar holding the persistent store-wide dummy key, placed next to
+ * the credential store (`<store_path>.dummykey`). */
+#define CREDENTIAL_DUMMY_KEY_SUFFIX ".dummykey"
 
 /* Fixed dummy keys used when a user is unknown or off the module's list.  They
  * can never authenticate because acceptance additionally requires found=true. */
@@ -583,6 +590,178 @@ static CredentialStore* load_store_file(const char* path, char* err, size_t err_
   return store;
 }
 
+/* Validate and read an already-open `<store>.dummykey` sidecar.  Fails closed on
+ * anything that is not an owner-only (0600) regular file of exactly
+ * CREDENTIAL_KEY_LEN bytes, so a loosened, swapped or truncated file can never
+ * silently change the dummy challenge. */
+static bool read_dummy_key_fd(int fd, const char* path, uint8_t out[CREDENTIAL_KEY_LEN], char* err,
+                              size_t err_size) {
+  struct stat st;
+  if (fstat(fd, &st) != 0) {
+    set_error(err, err_size, "cannot stat dummy key file '%s': %s", path, strerror(errno));
+    return false;
+  }
+  if (!S_ISREG(st.st_mode) || st.st_uid != geteuid() || (st.st_mode & (S_IRWXG | S_IRWXO)) != 0 ||
+      st.st_size != (off_t)CREDENTIAL_KEY_LEN) {
+    set_error(err, err_size,
+              "refusing to read dummy key file '%s': it must be an owner-only (0600) regular file "
+              "of exactly %d bytes",
+              path, CREDENTIAL_KEY_LEN);
+    return false;
+  }
+  size_t got = 0;
+  while (got < CREDENTIAL_KEY_LEN) {
+    ssize_t n = read(fd, out + got, CREDENTIAL_KEY_LEN - got);
+    if (n < 0) {
+      if (errno == EINTR)
+        continue;
+      set_error(err, err_size, "cannot read dummy key file '%s': %s", path, strerror(errno));
+      return false;
+    }
+    if (n == 0)
+      break;
+    got += (size_t)n;
+  }
+  if (got != CREDENTIAL_KEY_LEN) {
+    set_error(err, err_size, "dummy key file '%s' is truncated", path);
+    return false;
+  }
+  return true;
+}
+
+/* Load the persistent dummy key for `store_path` from its `<store_path>.dummykey`
+ * sidecar, creating it (mode 0600, 32 random bytes) if absent.  A NULL
+ * store_path (empty store) yields a fresh ephemeral key.  Reading an existing
+ * sidecar fails CLOSED on any validation error; only the CREATE path degrades
+ * to an ephemeral key (with a warning) when the filesystem cannot hold the
+ * sidecar (e.g. read-only mount), so a daemon still starts.  Returns false only
+ * when the CSPRNG itself fails (or a present-but-invalid sidecar is found). */
+static bool load_or_create_dummy_key(const char* store_path, uint8_t out[CREDENTIAL_KEY_LEN],
+                                     char* err, size_t err_size) {
+  if (!store_path) {
+    if (!credentials_random_bytes(out, CREDENTIAL_KEY_LEN)) {
+      set_error(err, err_size, "failed to generate the credential store dummy key");
+      return false;
+    }
+    return true;
+  }
+
+  size_t path_len = strlen(store_path);
+  size_t suffix_len = sizeof(CREDENTIAL_DUMMY_KEY_SUFFIX); /* includes the NUL */
+  if (path_len > SIZE_MAX - suffix_len) {
+    set_error(err, err_size, "credential store path is too long to build a dummy key path");
+    return false;
+  }
+  char* sidecar = malloc(path_len + suffix_len);
+  if (!sidecar) {
+    set_error(err, err_size, "out of memory building the dummy key path");
+    return false;
+  }
+  int n = snprintf(sidecar, path_len + suffix_len, "%s%s", store_path, CREDENTIAL_DUMMY_KEY_SUFFIX);
+  if (n < 0 || (size_t)n >= path_len + suffix_len) {
+    set_error(err, err_size, "credential store path is too long to build a dummy key path");
+    free(sidecar);
+    return false;
+  }
+
+  int fd = open(sidecar, O_RDONLY | O_CLOEXEC);
+  if (fd >= 0) {
+    bool ok = read_dummy_key_fd(fd, sidecar, out, err, err_size);
+    close(fd);
+    free(sidecar);
+    return ok;
+  }
+  if (errno != ENOENT) {
+    /* The sidecar exists but cannot be opened for reading (e.g. EACCES): fail
+     * closed rather than substituting a different key. */
+    set_error(err, err_size, "cannot open dummy key file '%s': %s", sidecar, strerror(errno));
+    free(sidecar);
+    return false;
+  }
+
+  uint8_t fresh[CREDENTIAL_KEY_LEN];
+  if (!credentials_random_bytes(fresh, CREDENTIAL_KEY_LEN)) {
+    set_error(err, err_size, "failed to generate the credential store dummy key");
+    free(sidecar);
+    return false;
+  }
+  fd = open(sidecar, O_WRONLY | O_CREAT | O_EXCL | O_CLOEXEC, 0600);
+  if (fd < 0) {
+    int open_errno = errno;
+    if (open_errno == EEXIST) {
+      /* A racing instance created the sidecar first; adopt its key. */
+      int rfd = open(sidecar, O_RDONLY | O_CLOEXEC);
+      if (rfd < 0) {
+        set_error(err, err_size, "cannot open dummy key file '%s': %s", sidecar, strerror(errno));
+        free(sidecar);
+        credentials_burn((char*)fresh, sizeof(fresh));
+        return false;
+      }
+      bool ok = read_dummy_key_fd(rfd, sidecar, out, err, err_size);
+      close(rfd);
+      free(sidecar);
+      credentials_burn((char*)fresh, sizeof(fresh));
+      return ok;
+    }
+    /* Creation failed for another reason (read-only filesystem, missing
+     * directory, ...).  Warn and fall back to an ephemeral key: unknown-user
+     * challenges stay deterministic within this daemon lifetime but will change
+     * on the next restart. */
+    char* escaped = output_escape(sidecar, log_get_8_bit_output());
+    log_message(LOG_LEVEL_WARNING,
+                "cannot create dummy key file %s: %s; using a transient dummy key so unknown-user "
+                "challenges will change across restarts",
+                escaped ? escaped : sidecar, strerror(open_errno));
+    free(escaped);
+    memcpy(out, fresh, CREDENTIAL_KEY_LEN);
+    free(sidecar);
+    credentials_burn((char*)fresh, sizeof(fresh));
+    return true;
+  }
+
+  size_t written = 0;
+  bool write_ok = true;
+  while (written < CREDENTIAL_KEY_LEN) {
+    ssize_t w = write(fd, fresh + written, CREDENTIAL_KEY_LEN - written);
+    if (w < 0) {
+      if (errno == EINTR)
+        continue;
+      write_ok = false;
+      break;
+    }
+    if (w == 0) {
+      write_ok = false;
+      break;
+    }
+    written += (size_t)w;
+  }
+  int write_errno = errno;
+  if (write_ok && fsync(fd) != 0) {
+    write_ok = false;
+    write_errno = errno;
+  }
+  close(fd);
+  if (!write_ok) {
+    /* Do not leave a truncated sidecar behind that would fail-closed a later
+     * restart; fall back to an ephemeral key instead. */
+    unlink(sidecar);
+    char* escaped = output_escape(sidecar, log_get_8_bit_output());
+    log_message(LOG_LEVEL_WARNING,
+                "cannot write dummy key file %s: %s; using a transient dummy key so unknown-user "
+                "challenges will change across restarts",
+                escaped ? escaped : sidecar, strerror(write_errno));
+    free(escaped);
+    memcpy(out, fresh, CREDENTIAL_KEY_LEN);
+    free(sidecar);
+    credentials_burn((char*)fresh, sizeof(fresh));
+    return true;
+  }
+  memcpy(out, fresh, CREDENTIAL_KEY_LEN);
+  free(sidecar);
+  credentials_burn((char*)fresh, sizeof(fresh));
+  return true;
+}
+
 CredentialStore* credentials_load(const char* password_file, const char* early_input_file,
                                   char* err, size_t err_size) {
   if (err && err_size)
@@ -590,12 +769,15 @@ CredentialStore* credentials_load(const char* password_file, const char* early_i
   CredentialStore* store = load_store_file(password_file, err, err_size);
   if (!store)
     return NULL;
-  /* Generate the store-wide dummy key once for the final (possibly merged)
-   * store.  It makes an unknown-user challenge deterministic, so fail the load
-   * if the CSPRNG is unavailable rather than degrading the anti-enumeration
-   * property. */
-  if (!credentials_random_bytes(store->dummy_key, sizeof(store->dummy_key))) {
-    set_error(err, err_size, "failed to generate the credential store dummy key");
+  /* Load (or create) the store-wide dummy key once for the final (possibly
+   * merged) store.  It makes an unknown-user challenge deterministic AND
+   * stable across daemon restarts, so a restart cannot be used as a
+   * username-enumeration oracle.  It is persisted in an owner-only sidecar next
+   * to the credential store; a NULL store path (empty store) keeps it
+   * ephemeral.  Fail the load if the CSPRNG is unavailable rather than
+   * degrading the anti-enumeration property. */
+  const char* store_path = password_file ? password_file : early_input_file;
+  if (!load_or_create_dummy_key(store_path, store->dummy_key, err, err_size)) {
     credentials_free(store);
     return NULL;
   }

src/shared/credentials.h

@@ -26,6 +26,15 @@
  * hard-rejected with an actionable "legacy" error; there is no auto-upgrade.
  * Use `fastsync-server --hash-credentials` to generate new-format lines.
  *
+ * Alongside the store, credentials_load maintains an owner-only (0600)
+ * `<store_path>.dummykey` sidecar holding the store-wide random dummy key.  It
+ * is auto-created on first load and MUST be preserved across restarts: it makes
+ * the dummy challenge for an unknown user stable for the life of the store, so
+ * a daemon restart cannot be used as a username-enumeration oracle.  A sidecar
+ * that is not an owner-only regular file of exactly 32 bytes fails the load
+ * (fail closed); if it cannot be created (e.g. a read-only mount) the daemon
+ * warns and uses a transient per-run key instead.
+ *
  * Client --password-file format: the FIRST meaningful (non-comment, non-blank)
  * line is `user:password`, holding the literal password.  The client keeps it
  * only for the duration of the handshake and wipes it at teardown; the file

tests/test_credentials.c

@@ -70,8 +70,28 @@ static char* make_tmp_file(const char* contents) {
 }
 
 static void rm_temp(const char* path) {
-  if (path)
-    unlink(path);
+  if (!path)
+    return;
+  unlink(path);
+  /* Every successfully loaded store auto-creates an owner-only
+   * `<store>.dummykey` sidecar; remove it too so tests leave no stray key. */
+  size_t n = strlen(path) + strlen(".dummykey") + 1;
+  char* sidecar = malloc(n);
+  if (sidecar) {
+    snprintf(sidecar, n, "%s.dummykey", path);
+    unlink(sidecar);
+    free(sidecar);
+  }
+}
+
+/* `<store>.dummykey` sidecar path (caller frees). */
+static char* dummy_sidecar_path(const char* store_path) {
+  size_t n = strlen(store_path) + strlen(".dummykey") + 1;
+  char* out = malloc(n);
+  if (!out)
+    return NULL;
+  snprintf(out, n, "%s.dummykey", store_path);
+  return out;
 }
 
 /* Build a valid new-format line for user/password at iters. */
@@ -745,6 +765,130 @@ static void test_credentials_rejects_group_or_other_accessible() {
   free(path);
 }
 
+/* Loading a store auto-creates an owner-only `<store>.dummykey` sidecar whose
+ * key is stable across reloads, so an unknown-user dummy salt is identical
+ * across two loads (the anti-restart enumeration property). */
+static void test_credentials_dummy_key_persisted() {
+  char line[CREDENTIAL_MAX_LINE];
+  EXPECT_TRUE(make_store_line("alice", KAT_PASSWORD, CREDENTIAL_MIN_ITERS, line, sizeof(line)));
+  char contents[CREDENTIAL_MAX_LINE + 2];
+  snprintf(contents, sizeof(contents), "%s\n", line);
+  char* path = make_tmp_file(contents);
+  EXPECT_NOT_NULL(path);
+  char* sidecar = dummy_sidecar_path(path);
+  EXPECT_NOT_NULL(sidecar);
+
+  char err[512];
+  CredentialStore* store = credentials_load(path, NULL, err, sizeof(err));
+  EXPECT_NOT_NULL(store);
+
+  struct stat st;
+  EXPECT_EQ_INT(stat(sidecar, &st), 0);
+  EXPECT_TRUE(S_ISREG(st.st_mode));
+  EXPECT_TRUE((st.st_mode & (S_IRWXG | S_IRWXO)) == 0);
+  EXPECT_EQ_INT((int)(st.st_mode & 07777), 0600);
+  EXPECT_EQ_INT((int)st.st_size, CREDENTIAL_KEY_LEN);
+
+  CredentialVerifier v1;
+  EXPECT_TRUE(credentials_get_verifier(store, "unknown-user", NULL, 0, &v1));
+  EXPECT_FALSE(v1.found);
+  credentials_free(store);
+
+  store = credentials_load(path, NULL, err, sizeof(err));
+  EXPECT_NOT_NULL(store);
+  CredentialVerifier v2;
+  EXPECT_TRUE(credentials_get_verifier(store, "unknown-user", NULL, 0, &v2));
+  EXPECT_FALSE(v2.found);
+  EXPECT_TRUE(memcmp(v1.salt, v2.salt, sizeof(v1.salt)) == 0);
+  credentials_free(store);
+
+  rm_temp(path);
+  free(path);
+  free(sidecar);
+}
+
+/* A sidecar that is group/other accessible, the wrong size, or not a regular
+ * file must fail the load closed. */
+static void test_credentials_dummy_key_rejects_bad_sidecar() {
+  char err[512];
+  char line[CREDENTIAL_MAX_LINE];
+  EXPECT_TRUE(make_store_line("alice", KAT_PASSWORD, CREDENTIAL_MIN_ITERS, line, sizeof(line)));
+  char contents[CREDENTIAL_MAX_LINE + 2];
+  snprintf(contents, sizeof(contents), "%s\n", line);
+
+  /* Group/other permission bits on the sidecar. */
+  char* path = make_tmp_file(contents);
+  EXPECT_NOT_NULL(path);
+  char* sidecar = dummy_sidecar_path(path);
+  EXPECT_NOT_NULL(sidecar);
+  uint8_t key[CREDENTIAL_KEY_LEN];
+  memset(key, 0x5a, sizeof(key));
+  FILE* fp = fopen(sidecar, "wb");
+  EXPECT_NOT_NULL(fp);
+  EXPECT_TRUE(fwrite(key, 1, sizeof(key), fp) == sizeof(key));
+  fclose(fp);
+  EXPECT_EQ_INT(chmod(sidecar, 0640), 0);
+  EXPECT_NULL(credentials_load(path, NULL, err, sizeof(err)));
+  EXPECT_TRUE(err[0] != '\0');
+  rm_temp(path);
+  free(path);
+  free(sidecar);
+
+  /* Wrong size (not exactly 32 bytes). */
+  path = make_tmp_file(contents);
+  EXPECT_NOT_NULL(path);
+  sidecar = dummy_sidecar_path(path);
+  EXPECT_NOT_NULL(sidecar);
+  fp = fopen(sidecar, "wb");
+  EXPECT_NOT_NULL(fp);
+  EXPECT_TRUE(fwrite(key, 1, CREDENTIAL_SALT_LEN, fp) == CREDENTIAL_SALT_LEN);
+  fclose(fp);
+  EXPECT_EQ_INT(chmod(sidecar, 0600), 0);
+  EXPECT_NULL(credentials_load(path, NULL, err, sizeof(err)));
+  EXPECT_TRUE(err[0] != '\0');
+  rm_temp(path);
+  free(path);
+  free(sidecar);
+
+  /* Non-regular file (a directory at the sidecar path). */
+  path = make_tmp_file(contents);
+  EXPECT_NOT_NULL(path);
+  sidecar = dummy_sidecar_path(path);
+  EXPECT_NOT_NULL(sidecar);
+  EXPECT_EQ_INT(mkdir(sidecar, 0700), 0);
+  EXPECT_NULL(credentials_load(path, NULL, err, sizeof(err)));
+  EXPECT_TRUE(err[0] != '\0');
+  rmdir(sidecar);
+  rm_temp(path);
+  free(path);
+  free(sidecar);
+}
+
+/* A NULL store path has nowhere to persist a key, so each load gets a fresh
+ * ephemeral key (and creates no sidecar). */
+static void test_credentials_dummy_key_null_store_ephemeral() {
+  char err[512];
+  CredentialStore* store = credentials_load(NULL, NULL, err, sizeof(err));
+  EXPECT_NOT_NULL(store);
+  CredentialVerifier v1;
+  CredentialVerifier v1b;
+  EXPECT_TRUE(credentials_get_verifier(store, "nobody", NULL, 0, &v1));
+  EXPECT_FALSE(v1.found);
+  /* Within one store the dummy challenge is still deterministic. */
+  EXPECT_TRUE(credentials_get_verifier(store, "nobody", NULL, 0, &v1b));
+  EXPECT_TRUE(memcmp(v1.salt, v1b.salt, sizeof(v1.salt)) == 0);
+  credentials_free(store);
+
+  store = credentials_load(NULL, NULL, err, sizeof(err));
+  EXPECT_NOT_NULL(store);
+  CredentialVerifier v2;
+  EXPECT_TRUE(credentials_get_verifier(store, "nobody", NULL, 0, &v2));
+  /* No persistence path, so the second load's random key differs (and with it
+   * the dummy salt). */
+  EXPECT_TRUE(memcmp(v1.salt, v2.salt, sizeof(v1.salt)) != 0);
+  credentials_free(store);
+}
+
 static void test_credentials_burn() {
   char secret[32];
   memcpy(secret, "supersecretvalue", 17);
@@ -777,5 +921,8 @@ void test_credentials(void) {
   test_credentials_read_secret_file_bad();
   test_credentials_hash_file();
   test_credentials_rejects_group_or_other_accessible();
+  test_credentials_dummy_key_persisted();
+  test_credentials_dummy_key_rejects_bad_sidecar();
+  test_credentials_dummy_key_null_store_ephemeral();
   test_credentials_burn();
 }