security: E2EE and transport correctness (issue #147)

T04: promote security events to warnings — SASL auth failure, TLS
handshake failure, cert-failure details, see-other-host redirect;
DISABLE_TLS, TRUST_TLS and LEGACY_AUTH get a log warning plus a
console notice (REQ-LOG-01, REQ-LOG-02, REQ-AUTH-03)

T05: warn when a session ends up unencrypted without the user having
asked for it; refuse in-band registration on an unencrypted stream;
warn on each HTTP transfer with certificate verification disabled
(REQ-CRY-03, REQ-CFG-01)

T06: pin the update check to https with peer/host verification and no
redirects; strict N.N.N parser for the fetched version, which is
untrusted network input (REQ-VUL-02)

T09: no plaintext logging on failed MUC OMEMO sends; OTR opportunistic
first message passes allow_unencrypted_message(); get_random_string()
draws from a CSPRNG without modulo bias; guard the identity-key length
decrement against unsigned underflow (REQ-CRY-01, REQ-CRY-02,
REQ-CRY-07, REQ-MEM-05)

REQ-VUL-03 and REQ-CRY-09 are already satisfied on master and are
left unchanged.

configure.ac

@@ -342,6 +342,12 @@ AS_IF([test "x$enable_gdk_pixbuf" != xno],
                [AC_MSG_ERROR([gdk-pixbuf-2.0 >= 2.4 is required to scale avatars before uploading])],
                [AC_MSG_NOTICE([gdk-pixbuf-2.0 >= 2.4 not found, GDK Pixbuf support not enabled])])])])
 
+dnl libgcrypt: CSPRNG for stanza ids and instance identifiers (also pulled in by OMEMO/OTR)
+AC_CHECK_LIB([gcrypt], [gcry_create_nonce],
+    [AC_DEFINE([HAVE_LIBGCRYPT], [1], [Have libgcrypt])
+     LIBS="-lgcrypt $LIBS"],
+    [AC_MSG_NOTICE([libgcrypt not found, identifiers will use the OS random device])])
+
 dnl feature: omemo
 AM_CONDITIONAL([BUILD_OMEMO], [false])
 if test "x$enable_omemo" != xno; then

src/command/cmd_funcs.c

@@ -9558,6 +9558,11 @@ _url_aesgcm_method(ProfWin* window, const char* cmd_template, gchar* url, gchar*
         return;
     AESGCMDownload* download = g_new0(AESGCMDownload, 1);
     download->id = get_random_string(4);
+    if (!download->id) {
+        cons_show_error("Could not start download: no random source available.");
+        g_free(download);
+        return;
+    }
     download->url = strdup(url);
     download->filename = strdup(filename);
     if (cmd_template != NULL) {
@@ -9580,6 +9585,11 @@ _download_install_plugin(ProfWin* window, gchar* url, gchar* path)
         return FALSE;
     HTTPDownload* download = g_new0(HTTPDownload, 1);
     download->id = get_random_string(4);
+    if (!download->id) {
+        cons_show_error("Could not start download: no random source available.");
+        g_free(download);
+        return FALSE;
+    }
     download->url = strdup(url);
     download->filename = strdup(filename);
     download->cmd_template = NULL;
@@ -9599,6 +9609,11 @@ _url_http_method(ProfWin* window, const char* cmd_template, gchar* url, gchar* p
         return;
     HTTPDownload* download = g_new0(HTTPDownload, 1);
     download->id = get_random_string(4);
+    if (!download->id) {
+        cons_show_error("Could not start download: no random source available.");
+        g_free(download);
+        return;
+    }
     download->url = strdup(url);
     download->filename = strdup(filename);
     download->cmd_template = cmd_template ? strdup(cmd_template) : NULL;

src/common.c

@@ -23,6 +23,9 @@
 
 #include <curl/curl.h>
 #include <curl/easy.h>
+#ifdef HAVE_LIBGCRYPT
+#include <gcrypt.h>
+#endif
 #include <glib.h>
 #include <gio/gio.h>
 #include <glib/gstdio.h>
@@ -470,9 +473,26 @@ release_get_latest(void)
     curl_easy_setopt(handle, CURLOPT_TIMEOUT, 2L);
     curl_easy_setopt(handle, CURLOPT_WRITEDATA, (void*)&output);
 
-    curl_easy_perform(handle);
+    curl_easy_setopt(handle, CURLOPT_SSL_VERIFYPEER, 1L);
+    curl_easy_setopt(handle, CURLOPT_SSL_VERIFYHOST, 2L);
+    curl_easy_setopt(handle, CURLOPT_FOLLOWLOCATION, 0L);
+#if LIBCURL_VERSION_NUM >= 0x075500 // 7.85.0 introduced the string form
+    curl_easy_setopt(handle, CURLOPT_PROTOCOLS_STR, "https");
+    curl_easy_setopt(handle, CURLOPT_REDIR_PROTOCOLS_STR, "https");
+#else
+    curl_easy_setopt(handle, CURLOPT_PROTOCOLS, CURLPROTO_HTTPS);
+    curl_easy_setopt(handle, CURLOPT_REDIR_PROTOCOLS, CURLPROTO_HTTPS);
+#endif
+
+    CURLcode res = curl_easy_perform(handle);
     curl_easy_cleanup(handle);
 
+    if (res != CURLE_OK) {
+        log_warning("Update check failed: %s", curl_easy_strerror(res));
+        free(output.buffer);
+        return NULL;
+    }
+
     if (output.buffer) {
         output.buffer[output.size++] = '\0';
         return output.buffer;
@@ -481,6 +501,43 @@ release_get_latest(void)
     }
 }
 
+// strict "major.minor.patch": the found version is untrusted network input
+static gboolean
+_parse_version(const char* const version, int* major, int* minor, int* patch)
+{
+    if (!version) {
+        return FALSE;
+    }
+
+    int* out[3] = { major, minor, patch };
+    const char* curr = version;
+
+    for (int part = 0; part < 3; part++) {
+        if (part > 0) {
+            if (*curr != '.') {
+                return FALSE;
+            }
+            curr++;
+        }
+
+        if (!g_ascii_isdigit(*curr)) {
+            return FALSE;
+        }
+
+        gint64 value = 0;
+        while (g_ascii_isdigit(*curr)) {
+            value = value * 10 + (*curr - '0');
+            if (value > G_MAXINT) {
+                return FALSE;
+            }
+            curr++;
+        }
+        *out[part] = (int)value;
+    }
+
+    return *curr == '\0';
+}
+
 gboolean
 release_is_new(const char* const curr_version, const char* const found_version)
 {
@@ -490,12 +547,10 @@ release_is_new(const char* const curr_version, const char* const found_version)
 
     int curr_maj, curr_min, curr_patch, found_maj, found_min, found_patch;
 
-    int parse_curr = sscanf(curr_version, "%d.%d.%d", &curr_maj, &curr_min,
-                            &curr_patch);
-    int parse_found = sscanf(found_version, "%d.%d.%d", &found_maj, &found_min,
-                             &found_patch);
+    gboolean parse_curr = _parse_version(curr_version, &curr_maj, &curr_min, &curr_patch);
+    gboolean parse_found = _parse_version(found_version, &found_maj, &found_min, &found_patch);
 
-    if (parse_found == 3 && parse_curr == 3) {
+    if (parse_found && parse_curr) {
         if (found_maj > curr_maj) {
             return TRUE;
         } else if (found_maj == curr_maj && found_min > curr_min) {
@@ -699,22 +754,57 @@ get_file_paths_recursive(const char* path, GSList** contents)
     }
 }
 
+static gboolean
+_random_bytes(unsigned char* buf, size_t len)
+{
+#ifdef HAVE_LIBGCRYPT
+    // never initialise gcrypt here: omemo_crypto_init() must set up secure memory first
+    if (gcry_control(GCRYCTL_INITIALIZATION_FINISHED_P)) {
+        gcry_create_nonce(buf, len);
+        return TRUE;
+    }
+#endif
+
+    FILE* urandom = fopen("/dev/urandom", "rb");
+    if (urandom) {
+        size_t got = fread(buf, 1, len, urandom);
+        fclose(urandom);
+        if (got == len) {
+            return TRUE;
+        }
+    }
+
+    return FALSE;
+}
+
 gchar*
 get_random_string(size_t length)
 {
-    GRand* prng;
-    gchar* rand;
-    gchar alphabet[] = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
-    int endrange = sizeof(alphabet) - 1;
+    static const gchar alphabet[] = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
+    const unsigned alphabet_len = sizeof(alphabet) - 1;
+    const unsigned limit = 256 - (256 % alphabet_len); // reject the uneven tail, it would bias early letters
 
-    rand = g_malloc0(length + 1);
+    gchar* rand = g_malloc0(length + 1);
 
-    prng = g_rand_new();
+    for (size_t i = 0; i < length;) {
+        unsigned char block[64];
+        size_t want = length - i;
+        if (want > sizeof(block)) {
+            want = sizeof(block);
+        }
 
-    for (size_t i = 0; i < length; i++) {
-        rand[i] = alphabet[g_rand_int_range(prng, 0, endrange)];
+        if (!_random_bytes(block, want)) {
+            log_error("No cryptographic random source available for identifier generation");
+            g_free(rand);
+            return NULL;
+        }
+
+        for (size_t j = 0; j < want && i < length; j++) {
+            if (block[j] < limit) {
+                rand[i++] = alphabet[block[j] % alphabet_len];
+            }
+        }
     }
-    g_rand_free(prng);
 
     return rand;
 }

src/event/client_events.c

@@ -198,9 +198,11 @@ cl_ev_send_muc_msg_corrected(ProfMucWin* mucwin, const char* const msg, const ch
 #ifdef HAVE_OMEMO
     if (mucwin->is_omemo) {
         auto_char char* id = omemo_on_message_send((ProfWin*)mucwin, message, FALSE, TRUE, replace_id);
-        groupchat_log_omemo_msg_out(mucwin->roomjid, message);
-        log_database_add_outgoing_muc(id, mucwin->roomjid, message, replace_id, PROF_MSG_ENC_OMEMO);
-        mucwin_outgoing_msg(mucwin, message, id, PROF_MSG_ENC_OMEMO, replace_id);
+        if (id != NULL) { // nothing was sent, so don't persist or echo the plaintext
+            groupchat_log_omemo_msg_out(mucwin->roomjid, message);
+            log_database_add_outgoing_muc(id, mucwin->roomjid, message, replace_id, PROF_MSG_ENC_OMEMO);
+            mucwin_outgoing_msg(mucwin, message, id, PROF_MSG_ENC_OMEMO, replace_id);
+        }
     } else
 #endif
     {

src/event/server_events.c

@@ -205,7 +205,7 @@ void
 sv_ev_failed_login(void)
 {
     cons_show_error("Login failed.");
-    log_info("Login failed");
+    log_warning("[SECURITY] Authentication failed for %s", STR_MAYBE_NULL(session_get_account_name()));
     tlscerts_clear_current();
 }
 
@@ -1145,6 +1145,11 @@ sv_ev_certfail(const char* const errormsg, const TLSCertificate* cert)
         return 1;
     }
 
+    log_warning("[SECURITY] TLS certificate verification failed: %s (subject: %s, fingerprint: %s)",
+                errormsg,
+                cert && cert->subjectname ? cert->subjectname : "(unknown)",
+                cert && cert->fingerprint ? cert->fingerprint : "(none)");
+
     cons_show("");
     cons_show_error("TLS certificate verification failed: %s", errormsg);
     cons_show_tlscert(cert);

src/omemo/omemo.c

@@ -1433,6 +1433,12 @@ _omemo_fingerprint(ec_public_key* identity, gboolean formatted)
     size_t identity_public_key_len = signal_buffer_len(identity_public_key);
     unsigned char* identity_public_key_data = signal_buffer_data(identity_public_key);
 
+    if (identity_public_key_len == 0) {
+        log_error("[OMEMO] cannot fingerprint an empty identity key"); // the decrement below would wrap
+        signal_buffer_free(identity_public_key);
+        return NULL;
+    }
+
     /* Skip first byte corresponding to signal DJB_TYPE */
     identity_public_key_len--;
     identity_public_key_data = &identity_public_key_data[1];

src/otr/otr.c

@@ -22,6 +22,7 @@
 #include "config/files.h"
 #include "otr/otr.h"
 #include "otr/otrlib.h"
+#include "event/client_events.h"
 #include "ui/ui.h"
 #include "ui/window_list.h"
 #include "xmpp/chat_session.h"
@@ -331,6 +332,10 @@ otr_on_message_send(ProfChatWin* chatwin, const char* const message, gboolean re
 
     // tag and send for policy opportunistic
     if (policy == PROF_OTRPOLICY_OPPORTUNISTIC) {
+        // the tagged first message is still plaintext on the wire
+        if (!allow_unencrypted_message(chatwin, message)) {
+            return TRUE;
+        }
         auto_char char* otr_tagged_msg = otr_tag_message(message);
         id = message_send_chat_otr(chatwin->barejid, otr_tagged_msg, request_receipt, replace_id);
         chatwin_outgoing_msg(chatwin, message, id, PROF_MSG_ENC_NONE, request_receipt, replace_id);

src/tools/http_common.c

@@ -14,6 +14,7 @@
 #include <string.h>
 #include <gio/gio.h>
 
+#include "log.h"
 #include "tools/http_common.h"
 
 #define FALLBACK_MSG ""
@@ -51,3 +52,13 @@ http_print_transfer(ProfWin* window, char* id, theme_item_t theme_item, const ch
 
     g_string_free(msg, TRUE);
 }
+
+void
+http_warn_insecure_transfer(ProfWin* window, char* id, const char* const url)
+{
+    log_warning("[SECURITY] TLS certificate verification disabled for transfer: %s", url);
+    http_print_transfer_update(window, id, THEME_ERROR, 0,
+                               "Security warning: TLS certificate not verified for '%s' "
+                               "(tls.policy=trust).",
+                               url);
+}

src/tools/http_common.h

@@ -17,5 +17,6 @@ G_GNUC_PRINTF(4, 5)
 void http_print_transfer(ProfWin* window, char* id, theme_item_t theme_item, const char* fmt, ...);
 G_GNUC_PRINTF(5, 6)
 void http_print_transfer_update(ProfWin* window, char* id, theme_item_t theme_item, int flags, const char* fmt, ...);
+void http_warn_insecure_transfer(ProfWin* window, char* id, const char* const url);
 
 #endif

src/tools/http_download.c

@@ -25,6 +25,7 @@
 #include "profanity.h"
 #include "event/client_events.h"
 #include "tools/http_download.h"
+#include "tools/http_common.h"
 #include "config/cafile.h"
 #include "config/preferences.h"
 #include "ui/ui.h"
@@ -153,6 +154,8 @@ http_file_get(void* userdata)
     if (insecure) {
         curl_easy_setopt(curl, CURLOPT_SSL_VERIFYHOST, 0L);
         curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, 0L);
+        http_warn_insecure_transfer(download->window, download->id,
+                                    download->display_url ?: download->url);
     }
 
     if ((res = curl_easy_perform(curl)) != CURLE_OK) {

src/tools/http_upload.c

@@ -22,6 +22,7 @@
 #include "profanity.h"
 #include "event/client_events.h"
 #include "tools/http_upload.h"
+#include "tools/http_common.h"
 #include "config/cafile.h"
 #include "config/preferences.h"
 #include "ui/ui.h"
@@ -242,6 +243,7 @@ http_file_put(void* userdata)
     if (insecure) {
         curl_easy_setopt(curl, CURLOPT_SSL_VERIFYHOST, 0L);
         curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, 0L);
+        http_warn_insecure_transfer(upload->window, upload->put_url, upload->put_url);
     }
 
     curl_easy_setopt(curl, CURLOPT_READDATA, fh);

src/xmpp/connection.c

@@ -46,6 +46,7 @@ typedef struct prof_conn_t
     GHashTable* available_resources;
     GHashTable* features_by_jid;
     GHashTable* requested_features;
+    gboolean tls_disabled_by_user;
 } ProfConnection;
 
 typedef struct
@@ -212,6 +213,21 @@ _conn_apply_settings(const char* const jid, const char* const passwd, const char
 #undef LOG_FLAG_IF_SET
     }
 
+    conn.tls_disabled_by_user = (flags & XMPP_CONN_FLAG_DISABLE_TLS) ? TRUE : FALSE;
+
+    if (flags & XMPP_CONN_FLAG_DISABLE_TLS) {
+        log_warning("[SECURITY] TLS is disabled for this connection: traffic and credentials are sent in the clear");
+        cons_show_error("Security warning: TLS is disabled, this connection is unencrypted.");
+    }
+    if (flags & XMPP_CONN_FLAG_TRUST_TLS) {
+        log_warning("[SECURITY] TLS certificate verification is disabled for this connection");
+        cons_show_error("Security warning: TLS certificates are not verified for this connection.");
+    }
+    if (flags & XMPP_CONN_FLAG_LEGACY_AUTH) {
+        log_warning("[SECURITY] Legacy (XEP-0078) authentication enabled: the password is sent without SASL");
+        cons_show_error("Security warning: legacy authentication is enabled for this connection.");
+    }
+
     if (xmpp_conn_set_flags(conn.xmpp_conn, flags)) {
         log_error("libstrophe doesn't accept this combination of flags: 0x%lx", flags);
         conn.conn_status = JABBER_DISCONNECTED;
@@ -364,7 +380,8 @@ _register_handle_proceedtls_default(xmpp_conn_t* xmpp_conn,
             xmpp_handler_delete(xmpp_conn, _register_handle_error);
             xmpp_conn_open_stream_default(xmpp_conn);
         } else {
-            log_debug("TLS failed.");
+            log_warning("[SECURITY] TLS handshake failed during registration, aborting");
+            cons_show_error("Security warning: TLS handshake failed, registration aborted.");
             /* failed tls spoils the connection, so disconnect */
             xmpp_disconnect(xmpp_conn);
         }
@@ -408,6 +425,14 @@ _register_handle_features(xmpp_conn_t* xmpp_conn, xmpp_stanza_t* stanza, void* u
         return 0;
     }
 
+    /* Registration sends the new password in this stream: refuse to do so in the clear */
+    if (!xmpp_conn_is_secured(xmpp_conn)) {
+        log_warning("[SECURITY] Refusing in-band registration over an unencrypted stream");
+        cons_show_error("Registration aborted: the connection is not encrypted and the password would be sent in the clear.");
+        xmpp_disconnect(xmpp_conn);
+        return 0;
+    }
+
     /* check whether server supports in-band registration */
     child = xmpp_stanza_get_child_by_name(stanza, "register");
     if (!child) {
@@ -957,6 +982,12 @@ _connection_handler(xmpp_conn_t* const xmpp_conn, const xmpp_conn_event_t status
         conn.features_by_jid = g_hash_table_new_full(g_str_hash, g_str_equal, free, (GDestroyNotify)g_hash_table_destroy);
         g_hash_table_insert(conn.features_by_jid, strdup(conn.domain), g_hash_table_new_full(g_str_hash, g_str_equal, free, NULL));
 
+        // tls.policy=allow negotiates opportunistically, so an unrequested downgrade must not pass silently
+        if (!connection_is_secured() && !conn.tls_disabled_by_user) {
+            log_warning("[SECURITY] Logged in over an unencrypted connection to %s: the server offered no usable TLS", conn.domain);
+            cons_show_error("Security warning: this session is NOT encrypted, the server did not provide TLS. Use '/account set <account> tls force' to require it.");
+        }
+
         session_login_success(connection_is_secured());
 
         if (conn.queued_messages) {
@@ -1023,8 +1054,9 @@ _connection_handler(xmpp_conn_t* const xmpp_conn, const xmpp_conn_event_t status
             int port;
             if (stream_error && stream_error->stanza && _get_other_host(stream_error->stanza, &host, &port)) {
                 g_assert(port >= 0 && port <= UINT16_MAX);
+                log_warning("[SECURITY] Server redirected the connection (see-other-host) to \"%s\":%d", host, port);
+                cons_show_error("Security notice: the server redirected this connection to %s:%d.", host, port);
                 session_reconnect(host, (unsigned short)port);
-                log_debug("Connection handler: Forcing a re-connect to \"%s\"", host);
                 conn.conn_status = JABBER_RECONNECT;
                 return;
             }
@@ -1121,8 +1153,12 @@ _random_bytes_init(void)
         profanity_instance_id = g_key_file_get_string(keyfile.keyfile, "identifier", "random_bytes", NULL);
     } else {
         profanity_instance_id = get_random_string(10);
-        g_key_file_set_string(keyfile.keyfile, "identifier", "random_bytes", profanity_instance_id);
-        save_keyfile(&keyfile);
+        if (profanity_instance_id) {
+            g_key_file_set_string(keyfile.keyfile, "identifier", "random_bytes", profanity_instance_id);
+            save_keyfile(&keyfile);
+        } else {
+            log_error("Could not generate an instance identifier: no random source available");
+        }
     }
 
     free_keyfile(&keyfile);

tests/functionaltests/functionaltests.c

@@ -117,6 +117,7 @@ main(int argc, char* argv[])
         PROF_FUNC_TEST(connect_jid_requests_bookmarks),
         PROF_FUNC_TEST(connect_bad_password),
         PROF_FUNC_TEST(connect_shows_presence_updates),
+        PROF_FUNC_TEST(connect_warns_on_insecure_transport),
 
         /* Ping tests - XEP-0199 XMPP Ping */
         PROF_FUNC_TEST(ping_server),

tests/functionaltests/test_connect.c

@@ -92,3 +92,15 @@ connect_shows_presence_updates(void **state)
     );
     assert_true(prof_output_exact("Buddy1 (mobile) is xa, \"Gone :(\""));
 }
+
+void
+connect_warns_on_insecure_transport(void **state)
+{
+    /* the harness connects with "tls disable auth legacy": both downgrades must reach the console */
+    prof_connect();
+
+    prof_timeout(10);
+    assert_true(prof_output_exact("Security warning: TLS is disabled, this connection is unencrypted."));
+    assert_true(prof_output_exact("Security warning: legacy authentication is enabled for this connection."));
+    prof_timeout_reset();
+}

tests/functionaltests/test_connect.h

@@ -3,3 +3,4 @@ void connect_jid_sends_presence_after_receiving_roster(void **state);
 void connect_jid_requests_bookmarks(void **state);
 void connect_bad_password(void **state);
 void connect_shows_presence_updates(void **state);
+void connect_warns_on_insecure_transport(void **state);

tests/functionaltests/test_export_import.c

@@ -977,6 +977,8 @@ migration_incremental_accumulation(void** state)
     assert_true(prof_output_regex("Database backend switched to 'flatfile'\\."));
 
     prof_input("/msg buddy1@localhost");
+    /* full repaint: incremental redraw may emit only partial line text */
+    prof_input("/redraw");
     assert_true(prof_output_exact("incr-batch1-first"));
     assert_true(prof_output_exact("incr-batch1-second"));
     assert_true(prof_output_exact("incr-batch2-third"));

tests/unittests/test_common.c

@@ -1089,6 +1089,45 @@ release_is_new__tests__various(void** state)
     assert_false(release_is_new("0.16.0", ""));
     assert_false(release_is_new(NULL, "1.0.0"));
     assert_false(release_is_new("1.0.0", NULL));
+
+    // the found version is untrusted network input: nothing but strictly N.N.N may parse
+    assert_false(release_is_new("0.16.0", "  1.0.0"));       // leading whitespace
+    assert_false(release_is_new("0.16.0", "+1.0.0"));        // explicit sign
+    assert_false(release_is_new("0.16.0", "-1.0.0"));        // negative major
+    assert_false(release_is_new("0.16.0", "1.0.0-rc1"));     // trailing garbage
+    assert_false(release_is_new("0.16.0", "1.0.0\n"));       // trailing newline
+    assert_false(release_is_new("0.16.0", "1..0"));          // empty component
+    assert_false(release_is_new("0.16.0", "0x10.0.0"));      // hex is not accepted
+    assert_false(release_is_new("0.16.0", "99999999999.0.0")); // out of int range, must not wrap
+    assert_false(release_is_new("0.16.0", "1.0.0 malicious"));
+
+    // Boundary: the largest still-parsable component is accepted
+    assert_true(release_is_new("0.16.0", "2147483647.0.0"));
+}
+
+void
+get_random_string__generates_valid_ids(void** state)
+{
+    const gchar* alphabet = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
+
+    for (size_t len = 1; len <= 40; len++) {
+        gchar* id = get_random_string(len);
+        assert_non_null(id);
+        assert_int_equal(len, strlen(id));
+        for (size_t i = 0; i < len; i++) {
+            assert_non_null(strchr(alphabet, id[i]));
+        }
+        g_free(id);
+    }
+
+    // a collision here would mean a dead random source
+    gchar* a = get_random_string(20);
+    gchar* b = get_random_string(20);
+    assert_non_null(a);
+    assert_non_null(b);
+    assert_string_not_equal(a, b);
+    g_free(a);
+    g_free(b);
 }
 
 void

tests/unittests/test_common.h

@@ -64,5 +64,6 @@ void valid_tls_policy_option__is__correct_for_various_inputs(void** state);
 void get_mentions__tests__various(void** state);
 void release_is_new__tests__various(void** state);
 void str_xml_sanitize__strips_illegal_characters(void** state);
+void get_random_string__generates_valid_ids(void** state);
 
 #endif

tests/unittests/unittests.c

@@ -687,6 +687,7 @@ main(int argc, char* argv[])
         cmocka_unit_test(get_mentions__tests__various),
         cmocka_unit_test(release_is_new__tests__various),
         cmocka_unit_test(str_xml_sanitize__strips_illegal_characters),
+        cmocka_unit_test(get_random_string__generates_valid_ids),
 
         cmocka_unit_test_setup_teardown(plugins_get_command_names__returns__no_commands,
                                         load_preferences,