sd-id128: add new sd_id128_get_machine_app_specific() API

This adds an API for retrieving an app-specific machine ID to sd-id128.
Internally it calculates HMAC-SHA256 with an 128bit app-specific ID as payload
and the machine ID as key.

(An alternative would have been to use siphash for this, which is also
cryptographically strong. However, as it only generates 64bit hashes it's not
an obvious choice for generating 128bit IDs.)

Fixes: #4667

diff --git a/man/sd_id128_get_machine.xml b/man/sd_id128_get_machine.xml
index cd6c39032c497b8995bd705dda38cf4b0b07f243..e7326422b5c9e6c4718dcbff428300d4f7e485c2 100644 (file)
--- a/man/sd_id128_get_machine.xml
+++ b/man/sd_id128_get_machine.xml
@@ -44,6 +44,7 @@
 
   <refnamediv>
     <refname>sd_id128_get_machine</refname>
+    <refname>sd_id128_get_machine_app_specific</refname>
     <refname>sd_id128_get_boot</refname>
     <refname>sd_id128_get_invocation</refname>
     <refpurpose>Retrieve 128-bit IDs</refpurpose>
@@ -58,6 +59,12 @@
         <paramdef>sd_id128_t *<parameter>ret</parameter></paramdef>
       </funcprototype>
 
+      <funcprototype>
+        <funcdef>int <function>sd_id128_get_machine_app_specific</function></funcdef>
+        <paramdef>sd_id128_t <parameter>app_id</parameter></paramdef>
+        <paramdef>sd_id128_t *<parameter>ret</parameter></paramdef>
+      </funcprototype>
+
       <funcprototype>
         <funcdef>int <function>sd_id128_get_boot</function></funcdef>
         <paramdef>sd_id128_t *<parameter>ret</parameter></paramdef>
@@ -74,11 +81,22 @@
   <refsect1>
     <title>Description</title>
 
-    <para><function>sd_id128_get_machine()</function> returns the
-    machine ID of the executing host. This reads and parses the
-    <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>
-    file. This function caches the machine ID internally to make
-    retrieving the machine ID a cheap operation.</para>
+    <para><function>sd_id128_get_machine()</function> returns the machine ID of the executing host. This reads and
+    parses the <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+    file. This function caches the machine ID internally to make retrieving the machine ID a cheap operation. This ID
+    may be used wherever a unique identifier for the local system is needed. However, it is recommended to use this ID
+    as-is only in trusted environments. In untrusted environments it is recommended to derive an application specific
+    ID from this machine ID, in an irreversable (cryptographically secure) way. To make this easy
+    <function>sd_id128_get_machine_app_specific()</function> is provided, see below.</para>
+
+    <para><function>sd_id128_get_machine_app_specific()</function> is similar to
+    <function>sd_id128_get_machine()</function>, but retrieves a machine ID that is specific to the application that is
+    identified by the indicated application ID. It is recommended to use this function instead of
+    <function>sd_id128_get_machine()</function> when passing an ID to untrusted environments, in order to make sure
+    that the original machine ID may not be determined externally. The application-specific ID should be generated via
+    a tool like <command>journalctl --new-id128</command>, and may be compiled into the application. This function will
+    return the same application-specific ID for each combination of machine ID and application ID. Internally, this
+    function calculates HMAC-SHA256 of the application ID, keyed by the machine ID.</para>
 
     <para><function>sd_id128_get_boot()</function> returns the boot ID
     of the executing kernel. This reads and parses the
@@ -95,10 +113,10 @@
     <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for details. The
     ID is cached internally. In future a different mechanism to determine the invocation ID may be added.</para>
 
-    <para>Note that <function>sd_id128_get_boot()</function> and <function>sd_id128_get_invocation()</function> always
-    return UUID v4 compatible IDs.  <function>sd_id128_get_machine()</function> will also return a UUID v4-compatible
-    ID on new installations but might not on older.  It is possible to convert the machine ID into a UUID v4-compatible
-    one. For more information, see
+    <para>Note that <function>sd_id128_get_machine_app_specific()</function>, <function>sd_id128_get_boot()</function>
+    and <function>sd_id128_get_invocation()</function> always return UUID v4 compatible IDs.
+    <function>sd_id128_get_machine()</function> will also return a UUID v4-compatible ID on new installations but might
+    not on older.  It is possible to convert the machine ID into a UUID v4-compatible one. For more information, see
     <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
 
     <para>For more information about the <literal>sd_id128_t</literal>
@@ -117,12 +135,35 @@
   <refsect1>
     <title>Notes</title>
 
-    <para>The <function>sd_id128_get_machine()</function>, <function>sd_id128_get_boot()</function> and
-    <function>sd_id128_get_invocation()</function> interfaces are available as a shared library, which can be compiled
-    and linked to with the <literal>libsystemd</literal> <citerefentry
+    <para>The <function>sd_id128_get_machine()</function>, <function>sd_id128_get_machine_app_specific()</function>
+    <function>sd_id128_get_boot()</function> and <function>sd_id128_get_invocation()</function> interfaces are
+    available as a shared library, which can be compiled and linked to with the
+    <literal>libsystemd</literal> <citerefentry
     project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry> file.</para>
   </refsect1>
 
+  <refsect1>
+    <title>Examples</title>
+
+    <example>
+      <title>Application-specific machine ID</title>
+
+      <para>Here's a simple example for an application specific machine ID:</para>
+
+      <programlisting>#include &lt;systemd/sd-id128.h&gt;
+#include &lt;stdio.h&gt;
+
+#define OUR_APPLICATION_ID SD_ID128_MAKE(c2,73,27,73,23,db,45,4e,a6,3b,b9,6e,79,b5,3e,97)
+
+int main(int argc, char *argv[]) {
+        sd_id128_t id;
+        sd_id128_get_machine_app_specific(OUR_APPLICATION_ID, &amp;id);
+        printf("Our application ID: " SD_ID128_FORMAT_STR "\n", SD_ID128_FORMAT_VAL(id));
+        return 0;
+}</programlisting>
+    </example>
+  </refsect1>
+
   <refsect1>
     <title>See Also</title>
 

diff --git a/src/libelogind/libelogind.sym b/src/libelogind/libelogind.sym
index e10e2b4a7da8742cf5f8969dc6004773ea65e022..4f24666698af74195b2ce60263bec11a87985123 100644 (file)
--- a/src/libelogind/libelogind.sym
+++ b/src/libelogind/libelogind.sym
@@ -523,3 +523,8 @@ global:
         /* sd_bus_get_exit_on_disconnect; */
         sd_id128_get_invocation;
 } LIBSYSTEMD_231;
+
+LIBSYSTEMD_233 {
+global:
+        sd_id128_get_machine_app_specific;
+} LIBSYSTEMD_232;

diff --git a/src/libelogind/sd-id128/sd-id128.c b/src/libelogind/sd-id128/sd-id128.c
index d4450c70a003c357b0dafce92a542e2f67f87459..43cc86f43f5a25664892eb55e53d71ed43661448 100644 (file)
--- a/src/libelogind/sd-id128/sd-id128.c
+++ b/src/libelogind/sd-id128/sd-id128.c
@@ -181,3 +181,34 @@ _public_ int sd_id128_randomize(sd_id128_t *ret) {
         *ret = make_v4_uuid(t);
         return 0;
 }
+
+_public_ int sd_id128_get_machine_app_specific(sd_id128_t app_id, sd_id128_t *ret) {
+        _cleanup_(khash_unrefp) khash *h = NULL;
+        sd_id128_t m, result;
+        const void *p;
+        int r;
+
+        assert_return(ret, -EINVAL);
+
+        r = sd_id128_get_machine(&m);
+        if (r < 0)
+                return r;
+
+        r = khash_new_with_key(&h, "hmac(sha256)", &m, sizeof(m));
+        if (r < 0)
+                return r;
+
+        r = khash_put(h, &app_id, sizeof(app_id));
+        if (r < 0)
+                return r;
+
+        r = khash_digest_data(h, &p);
+        if (r < 0)
+                return r;
+
+        /* We chop off the trailing 16 bytes */
+        memcpy(&result, p, MIN(khash_get_size(h), sizeof(result)));
+
+        *ret = make_v4_uuid(result);
+        return 0;
+}