Merge feat/dummy-key: persist anti-enumeration dummy key across restarts

# Conflicts:
#	RSYNC_COMPAT.md

README.md

@@ -522,11 +522,17 @@ 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
+`user:SHA256HEX` stores are rejected. FastSync also maintains an owner-only
-is stable within one daemon lifetime but changes across restarts, so a
+(mode 0600) `<store>.dummykey` sidecar next to the store: it holds the store-wide
-restart-gated enumeration channel remains (persisting a dummy key is out of
+dummy key, is auto-created on first load, and must be preserved across daemon
-scope); and the store iteration count is observable pre-auth by design, since
+restarts so the dummy challenge for an unknown user stays stable (the key is
-the miss path must match a hit.
+never regenerated while the sidecar exists). If the sidecar cannot be created
+(process-substitution/FIFO store path such as `/dev/fd/N`, a read-only
+filesystem, or a missing directory), the daemon logs a warning and uses a
+transient key, so the cross-restart guarantee does not hold for those
+deployments. One residual is accepted: the store
+iteration count is observable pre-auth by design, since the miss path must match
+a hit.
 
 An `auth users` module accepts credentials only when one of two conditions
 holds: (a) the connection is an encrypted, verified TLS connection whose client

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. **Transport policy (hardening A7-3/S1):** an auth-required module accepts credentials only when either (a) the connection is an encrypted, verified TLS connection whose client certificate matches `--client-cn`, or (b) the connection is plaintext from a loopback TCP peer **and** the operator explicitly passed `--allow-unauthenticated`. A remote plaintext peer, and a loopback plaintext peer without that flag, are refused at the config gate before any challenge is sent; `--allow-unauthenticated` never permits remote plaintext auth (remote peers still require verified TLS). Daemon modules are a `--daemon`-only feature — the SSH `--stdio` path never loads a daemon config and is not an auth transport for them. Because the loopback allowance trusts whichever peer the kernel reports as `127.0.0.1`, it assumes nothing relays remote connections to the daemon: a local TCP forwarder or TLS-terminating proxy in front of an auth-module listener makes remote clients appear as loopback and bypasses the mutual-TLS identity check, so do not front an auth-module listener with such a relay.
+- **`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. The sidecar must be preserved across restarts for that guarantee; if it cannot be created (a process-substitution/FIFO store path such as `/dev/fd/N`, a read-only filesystem, or a missing directory), the daemon logs a warning and uses a transient per-run key, so unknown-user challenges change across restarts and the cross-restart guarantee does not hold for that deployment. One residual is accepted: the store iteration count is observable pre-auth by design, since the miss path must match a hit. **Transport policy (hardening A7-3/S1):** an auth-required module accepts credentials only when either (a) the connection is an encrypted, verified TLS connection whose client certificate matches `--client-cn`, or (b) the connection is plaintext from a loopback TCP peer **and** the operator explicitly passed `--allow-unauthenticated`. A remote plaintext peer, and a loopback plaintext peer without that flag, are refused at the config gate before any challenge is sent; `--allow-unauthenticated` never permits remote plaintext auth (remote peers still require verified TLS). Daemon modules are a `--daemon`-only feature — the SSH `--stdio` path never loads a daemon config and is not an auth transport for them. Because the loopback allowance trusts whichever peer the kernel reports as `127.0.0.1`, it assumes nothing relays remote connections to the daemon: a local TCP forwarder or TLS-terminating proxy in front of an auth-module listener makes remote clients appear as loopback and bypasses the mutual-TLS identity check, so do not front an auth-module listener with such a relay.
-- **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.
+- **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, a mode other than exactly 0600, the wrong size or the wrong type as a fatal load error (fail closed). If the sidecar cannot be created (e.g. a process-substitution store path such as `/dev/fd/N`, a read-only filesystem, or a missing directory), the daemon logs a warning and uses a transient per-run key, so the cross-restart stability guarantee does not hold there.
 - **Plaintext caveat:** an auth-required module is refused, **before any challenge is sent**, unless the connection is encrypted and verified TLS whose client certificate matches the server's `--client-cn`, or it is plaintext from a loopback TCP peer **and** the operator passed `--allow-unauthenticated`. A remote plaintext peer, and a loopback plaintext peer without that flag, never receive a challenge, and `--allow-unauthenticated` never permits remote plaintext auth (remote peers still require verified TLS). On the loopback plaintext transport that remains permitted, a local sniffer could still read the challenge and response and mount an **offline dictionary attack** against a weak password, so use `--tls` for any real deployment. `--client-cn` matches the certificate CN only (not a subjectAltName), which is acceptable for a private CA. Clients sending daemon credentials with `--password-file` to a non-loopback daemon must use `--tls`; the client rejects such a destination before any network I/O. 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). 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
+  /* Store-wide secret loaded from (or created in) the exact-mode-0600
-   * unknown/off-list user is HMAC-SHA256(dummy_key, username)[:SALT_LEN], so
+   * `<store_path>.dummykey` sidecar, so it also survives a daemon restart.  The
-   * repeated probes of the same username always see an identical challenge
+   * dummy salt handed out for an unknown/off-list user is
-   * while different usernames differ -- with no fresh-random tell. */
+   * 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"
+/* Exact-mode-0600 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. */
@@ -66,7 +73,10 @@ static bool is_comment_char(char c) {
 
 /* Open a --password-file / --early-input after verifying the EXACT inode we
  * will read: it must be owned by the effective user and grant no group/other
- * permission bit (mode 0600), mirroring the TLS private-key check.  We open by
+ * permission bit (so 0600 and stricter modes such as 0400 are accepted),
+ * mirroring the TLS private-key check.  This only rejects group/other bits,
+ * deliberately unlike the dummy-key sidecar which requires EXACT mode 0600.  We
+ * open by
  * path and then fstat the resulting fd (rather than stat()ing the path first
  * and reopening it), so the permission decision is made on the same inode that
  * is read and cannot be raced by swapping the path between check and open.
@@ -583,6 +593,311 @@ 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 exact-mode-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 & 07777) != 0600 ||
+      st.st_size != (off_t)CREDENTIAL_KEY_LEN) {
+    set_error(err, err_size,
+              "refusing to read dummy key file '%s': it must be an owned regular file with exact "
+              "mode 0600 and 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;
+}
+
+/* fsync the directory containing `path` (best effort).  After publishing the
+ * sidecar with link(2), syncing the directory makes the new name durable so a
+ * crash cannot leave a restart without the key it just started using. */
+static void fsync_containing_dir(const char* path) {
+  char* dir = str_dup(path);
+  if (!dir)
+    return;
+  char* slash = strrchr(dir, '/');
+  if (!slash) {
+    free(dir);
+    dir = str_dup(".");
+    if (!dir)
+      return;
+  } else if (slash == dir) {
+    slash[1] = '\0'; /* keep the leading '/' */
+  } else {
+    *slash = '\0';
+  }
+  int dfd = open(dir, O_RDONLY | O_DIRECTORY | O_CLOEXEC);
+  free(dir);
+  if (dfd < 0)
+    return;
+  fsync(dfd);
+  close(dfd);
+}
+
+/* Load the persistent dummy key for `store_path` from its `<store_path>.dummykey`
+ * sidecar, creating it (exact 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.
+ *
+ * Creation is ATOMIC: the key is written to a private same-directory temp file
+ * and hard-linked into place, so a concurrent starter (or reader) never observes
+ * a partial/zero sidecar that would fail the load closed.  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;
+  }
+
+  /* Readers reject a planted symlink (O_NOFOLLOW) and never block on a planted
+   * FIFO (O_NONBLOCK; fstat rejects the non-regular file before any data read).
+   * Any open error other than ENOENT fails closed. */
+  int fd = open(sidecar, O_RDONLY | O_NOFOLLOW | O_NONBLOCK | 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 (EACCES, or ELOOP
+     * from a symlink): 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;
+  }
+
+  /* Publish atomically: write a private same-directory temp file, fsync it,
+   * then hard-link it into place.  A concurrent reader therefore only ever
+   * sees a complete 32-byte sidecar (or none), never a partial/zero file.
+   *
+   * The temp name carries both the pid and a fresh random suffix, so it is not
+   * predictable.  If the name nevertheless already exists (a SIGKILL/crash
+   * leftover, pid reuse, or a planted file) the stale temp is removed and the
+   * O_EXCL create is retried once, so it can never silently defeat persistence
+   * for this pid. */
+  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;
+  }
+
+  uint8_t name_rand[8];
+  if (!credentials_random_bytes(name_rand, sizeof(name_rand))) {
+    set_error(err, err_size, "failed to generate the dummy key temp name");
+    free(sidecar);
+    credentials_burn((char*)fresh, sizeof(fresh));
+    return false;
+  }
+  char name_hex[sizeof(name_rand) * 2 + 1];
+  static const char hex_digits[] = "0123456789abcdef";
+  for (size_t i = 0; i < sizeof(name_rand); i++) {
+    name_hex[2 * i] = hex_digits[name_rand[i] >> 4];
+    name_hex[2 * i + 1] = hex_digits[name_rand[i] & 0x0f];
+  }
+  name_hex[sizeof(name_hex) - 1] = '\0';
+
+  char tmp_suffix[64];
+  int pn = snprintf(tmp_suffix, sizeof(tmp_suffix), ".tmp.%ld.%s", (long)getpid(), name_hex);
+  if (pn < 0 || (size_t)pn >= sizeof(tmp_suffix)) {
+    set_error(err, err_size, "failed to build the dummy key temp path");
+    free(sidecar);
+    credentials_burn((char*)fresh, sizeof(fresh));
+    return false;
+  }
+  size_t sidecar_len = (size_t)n;
+  size_t tmp_len = sidecar_len + (size_t)pn;
+  char* tmp = malloc(tmp_len + 1);
+  if (!tmp) {
+    set_error(err, err_size, "out of memory building the dummy key temp path");
+    free(sidecar);
+    credentials_burn((char*)fresh, sizeof(fresh));
+    return false;
+  }
+  snprintf(tmp, tmp_len + 1, "%s%s", sidecar, tmp_suffix);
+
+  /* Bounded create: at most one unlink+retry on EEXIST.  The retry keeps
+   * O_EXCL, so only a stale name is reclaimed and a live peer's temp is never
+   * truncated. */
+  int create_errno = 0;
+  for (int attempt = 0; attempt < 2; attempt++) {
+    fd = open(tmp, O_WRONLY | O_CREAT | O_EXCL | O_CLOEXEC, 0600);
+    if (fd >= 0)
+      break;
+    create_errno = errno;
+    if (create_errno != EEXIST || attempt == 1)
+      break;
+    unlink(tmp);
+  }
+  /* umask can clear owner bits from the 0600 create mode while the reader
+   * requires an exact 0600, so force the mode on the fd before publishing; a
+   * failure here is treated like any other create failure (warning + ephemeral
+   * key) so the published sidecar is always exactly 0600. */
+  if (fd >= 0 && fchmod(fd, 0600) != 0) {
+    create_errno = errno;
+    close(fd);
+    unlink(tmp);
+    fd = -1;
+  }
+  if (fd < 0) {
+    /* Creation failed (read-only filesystem, missing directory, fchmod, ...).
+     * Warn and fall back to an ephemeral key: unknown-user challenges stay
+     * deterministic within this daemon lifetime but will change on restart. */
+    char* escaped = output_escape(tmp, 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 : tmp, strerror(create_errno));
+    free(escaped);
+    memcpy(out, fresh, CREDENTIAL_KEY_LEN);
+    free(tmp);
+    free(sidecar);
+    credentials_burn((char*)fresh, sizeof(fresh));
+    return true;
+  }
+
+  size_t written = 0;
+  bool write_ok = true;
+  int write_errno = 0;
+  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;
+      write_errno = errno;
+      break;
+    }
+    if (w == 0) {
+      /* A zero-length write is not a system error; errno is stale here, so
+       * report a clear short-write instead of a bogus strerror(errno). */
+      write_ok = false;
+      write_errno = 0;
+      break;
+    }
+    written += (size_t)w;
+  }
+  if (write_ok && fsync(fd) != 0) {
+    write_ok = false;
+    write_errno = errno;
+  }
+  close(fd);
+  if (!write_ok) {
+    /* Do not leave a truncated temp file behind; fall back to an ephemeral key
+     * instead of failing closed on the next restart. */
+    unlink(tmp);
+    char* escaped = output_escape(tmp, log_get_8_bit_output());
+    const char* why = write_errno != 0 ? strerror(write_errno) : "short write";
+    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 : tmp, why);
+    free(escaped);
+    memcpy(out, fresh, CREDENTIAL_KEY_LEN);
+    free(tmp);
+    free(sidecar);
+    credentials_burn((char*)fresh, sizeof(fresh));
+    return true;
+  }
+
+  if (link(tmp, sidecar) != 0) {
+    int link_errno = errno;
+    if (link_errno == EEXIST) {
+      /* A concurrent starter published first; adopt its key.  Read it back
+       * through the same hardened path (no symlink, no block, exact mode). */
+      int rfd = open(sidecar, O_RDONLY | O_NOFOLLOW | O_NONBLOCK | O_CLOEXEC);
+      if (rfd < 0) {
+        set_error(err, err_size, "cannot open dummy key file '%s': %s", sidecar, strerror(errno));
+        unlink(tmp);
+        free(tmp);
+        free(sidecar);
+        credentials_burn((char*)fresh, sizeof(fresh));
+        return false;
+      }
+      bool ok = read_dummy_key_fd(rfd, sidecar, out, err, err_size);
+      close(rfd);
+      unlink(tmp);
+      free(tmp);
+      free(sidecar);
+      credentials_burn((char*)fresh, sizeof(fresh));
+      return ok;
+    }
+    /* Linking failed for another reason (e.g. no hard-link support on this
+     * filesystem).  Warn and fall back to an ephemeral key. */
+    unlink(tmp);
+    char* escaped = output_escape(sidecar, log_get_8_bit_output());
+    log_message(LOG_LEVEL_WARNING,
+                "cannot publish dummy key file %s: %s; using a transient dummy key so unknown-user "
+                "challenges will change across restarts",
+                escaped ? escaped : sidecar, strerror(link_errno));
+    free(escaped);
+    memcpy(out, fresh, CREDENTIAL_KEY_LEN);
+    free(tmp);
+    free(sidecar);
+    credentials_burn((char*)fresh, sizeof(fresh));
+    return true;
+  }
+
+  /* Published: make the new directory entry durable, then drop the private
+   * temp name (the sidecar keeps the inode alive). */
+  fsync_containing_dir(sidecar);
+  unlink(tmp);
+  memcpy(out, fresh, CREDENTIAL_KEY_LEN);
+  free(tmp);
+  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 +905,15 @@ 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)
+  /* Load (or create) the store-wide dummy key once for the final (possibly
-   * store.  It makes an unknown-user challenge deterministic, so fail the load
+   * merged) store.  It makes an unknown-user challenge deterministic AND
-   * if the CSPRNG is unavailable rather than degrading the anti-enumeration
+   * stable across daemon restarts, so a restart cannot be used as a
-   * property. */
+   * username-enumeration oracle.  It is persisted in an exact-mode-0600 sidecar
-  if (!credentials_random_bytes(store->dummy_key, sizeof(store->dummy_key))) {
+   * next to the credential store; a NULL store path (empty store) keeps it
-    set_error(err, err_size, "failed to generate the credential store dummy key");
+   * 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 +26,17 @@
  * 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 exact-mode-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 exact-mode-0600 regular file of exactly 32 bytes fails the load
+ * (fail closed); if it cannot be created (e.g. a read-only mount or a restrictive
+ * umask the fchmod cannot repair) the daemon warns and uses a transient per-run
+ * key instead.  NOTE: the sidecar requires EXACT 0600, whereas the store /
+ * password files only reject group/other bits (a deliberate difference).
+ *
  * 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
@@ -156,8 +167,9 @@ bool credentials_verify_response(const CredentialVerifier* v, const char* user,
 bool credentials_hash_store_line(const char* user, const char* password, uint32_t iters, char* out,
                                  size_t out_sz, char* err, size_t err_size);
 
-/* Read `user:password` lines from `path` (the same owner-only check as the
+/* Read `user:password` lines from `path` (the same no-group/other-bits check as
- * other secret files) and write one new-format store line per entry to `out`.
+ * the other secret files) and write one new-format store line per entry to
+ * `out`.
  * Blank/comment lines are skipped; a malformed line fails the whole run.
  * Returns 0 on success, -1 on error (err filled).  Used by
  * `--hash-credentials`. */

tests/test_credentials.c

@@ -3,6 +3,7 @@
 #include "test_utils.h"
 #include "utils.h"
 #include <errno.h>
+#include <glob.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -70,8 +71,43 @@ static char* make_tmp_file(const char* contents) {
 }
 
 static void rm_temp(const char* path) {
-  if (path)
+  if (!path)
-    unlink(path);
+    return;
+  unlink(path);
+  /* Every successfully loaded store auto-creates an exact-mode-0600
+   * `<store>.dummykey` sidecar; remove it too so tests leave no stray key.  The
+   * atomic-publish temps carry a random suffix, so glob them all and remove any
+   * that a failing path may have left behind. */
+  size_t n = strlen(path) + strlen(".dummykey") + 1;
+  char* sidecar = malloc(n);
+  if (sidecar) {
+    snprintf(sidecar, n, "%s.dummykey", path);
+    unlink(sidecar);
+    free(sidecar);
+  }
+  n = strlen(path) + strlen(".dummykey.tmp.*") + 1;
+  char* pattern = malloc(n);
+  if (pattern) {
+    snprintf(pattern, n, "%s.dummykey.tmp.*", path);
+    glob_t matches;
+    memset(&matches, 0, sizeof(matches));
+    if (glob(pattern, 0, NULL, &matches) == 0) {
+      for (size_t i = 0; i < matches.gl_pathc; i++)
+        unlink(matches.gl_pathv[i]);
+    }
+    globfree(&matches);
+    free(pattern);
+  }
+}
+
+/* `<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 +781,269 @@ 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 restrictive umask must not leave the freshly published sidecar with owner
+ * bits cleared: creation forces exact 0600 with fchmod (the reader requires an
+ * exact 0600), so the daemon cannot lock itself out on the next restart. */
+static void test_credentials_dummy_key_exact_mode_under_umask() {
+  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);
+
+  /* Clear every permission bit the O_CREAT mode would otherwise provide; only
+   * the explicit fchmod can restore the exact 0600 the reader demands. */
+  mode_t old_umask = umask(0777);
+  char err[512];
+  CredentialStore* store = credentials_load(path, NULL, err, sizeof(err));
+  umask(old_umask);
+  EXPECT_NOT_NULL(store);
+
+  struct stat st;
+  EXPECT_EQ_INT(stat(sidecar, &st), 0);
+  EXPECT_EQ_INT((int)(st.st_mode & 07777), 0600);
+  EXPECT_EQ_INT((int)st.st_size, CREDENTIAL_KEY_LEN);
+  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);
+
+  /* Exact-mode rule: 0400 has no group/other bits but is not 0600, so it is
+   * rejected now (the mode must be exactly owner read+write). */
+  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, sizeof(key), fp) == sizeof(key));
+  fclose(fp);
+  EXPECT_EQ_INT(chmod(sidecar, 0400), 0);
+  EXPECT_NULL(credentials_load(path, NULL, err, sizeof(err)));
+  EXPECT_TRUE(err[0] != '\0');
+  rm_temp(path);
+  free(path);
+  free(sidecar);
+}
+
+/* A pre-existing valid sidecar is adopted verbatim (no regeneration): the
+ * unknown-user dummy salt must equal HMAC-SHA256(known key, username), and a
+ * reload must yield the same salt. */
+static void test_credentials_dummy_key_existing_sidecar_adopted() {
+  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);
+
+  /* Pre-create a valid owner-only sidecar with a known key. */
+  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, 0600), 0);
+
+  char err[512];
+  CredentialStore* store = credentials_load(path, NULL, err, sizeof(err));
+  EXPECT_NOT_NULL(store);
+  CredentialVerifier v1;
+  EXPECT_TRUE(credentials_get_verifier(store, "unknown-user", NULL, 0, &v1));
+  EXPECT_FALSE(v1.found);
+  /* HMAC-SHA256(0x5a * 32, "unknown-user")[:16], computed independently. */
+  uint8_t expect[CREDENTIAL_SALT_LEN];
+  unhex("4b0d2e6b73025cc2fcb41d0a710ff469", expect, sizeof(expect));
+  EXPECT_TRUE(memcmp(v1.salt, expect, sizeof(expect)) == 0);
+  credentials_free(store);
+
+  /* The adopted sidecar still holds exactly the pre-created key (not a fresh
+   * random one). */
+  uint8_t readback[CREDENTIAL_KEY_LEN];
+  fp = fopen(sidecar, "rb");
+  EXPECT_NOT_NULL(fp);
+  EXPECT_TRUE(fread(readback, 1, sizeof(readback), fp) == sizeof(readback));
+  fclose(fp);
+  EXPECT_TRUE(memcmp(readback, key, sizeof(key)) == 0);
+
+  /* Persisted across a reload. */
+  CredentialVerifier v2;
+  store = credentials_load(path, NULL, err, sizeof(err));
+  EXPECT_NOT_NULL(store);
+  EXPECT_TRUE(credentials_get_verifier(store, "unknown-user", NULL, 0, &v2));
+  EXPECT_TRUE(memcmp(v1.salt, v2.salt, sizeof(v1.salt)) == 0);
+  credentials_free(store);
+
+  rm_temp(path);
+  free(path);
+  free(sidecar);
+}
+
+/* A symlink planted at the sidecar path must fail the load closed (O_NOFOLLOW),
+ * even when it resolves to a valid owner-only key file. */
+static void test_credentials_dummy_key_symlink_rejected() {
+  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 target[256];
+  snprintf(target, sizeof(target), "/tmp/fs_cred_key_%d_%d", (int)getpid(), g_file_counter++);
+  uint8_t key[CREDENTIAL_KEY_LEN];
+  memset(key, 0x5a, sizeof(key));
+  FILE* fp = fopen(target, "wb");
+  EXPECT_NOT_NULL(fp);
+  EXPECT_TRUE(fwrite(key, 1, sizeof(key), fp) == sizeof(key));
+  fclose(fp);
+  EXPECT_EQ_INT(chmod(target, 0600), 0);
+  EXPECT_EQ_INT(symlink(target, sidecar), 0);
+
+  char err[512];
+  EXPECT_NULL(credentials_load(path, NULL, err, sizeof(err)));
+  EXPECT_TRUE(err[0] != '\0');
+
+  unlink(sidecar); /* remove the symlink itself, not its target */
+  unlink(target);
+  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 +1076,11 @@ 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_exact_mode_under_umask();
+  test_credentials_dummy_key_rejects_bad_sidecar();
+  test_credentials_dummy_key_existing_sidecar_adopted();
+  test_credentials_dummy_key_symlink_rejected();
+  test_credentials_dummy_key_null_store_ephemeral();
   test_credentials_burn();
 }