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

2 files changed, 61 insertions, 19 deletions

diff --git a/man/machine-id.xml b/man/machine-id.xml
index a722649de4..3c261bffcc 100644
--- a/man/machine-id.xml
+++ b/man/machine-id.xml
@@ -71,13 +71,14 @@
     <para>This machine ID adheres to the same format and logic as the
     D-Bus machine ID.</para>
 
-    <para>This ID uniquely identifies the host. It should be considered "confidential", and must not
-    be exposed in untrusted environments, in particular on the network. If a stable unique
-    identifier that is tied to the machine is needed for some application, the machine ID or any
-    part of it must not be used directly. Instead the machine ID should be hashed with a
-    cryptographic, keyed hash function, using a fixed, application-specific key. That way the ID
-    will be properly unique, and derived in a constant way from the machine ID but there will be no
-    way to retrieve the original machine ID from the application-specific one.</para>
+    <para>This ID uniquely identifies the host. It should be considered "confidential", and must not be exposed in
+    untrusted environments, in particular on the network. If a stable unique identifier that is tied to the machine is
+    needed for some application, the machine ID or any part of it must not be used directly. Instead the machine ID
+    should be hashed with a cryptographic, keyed hash function, using a fixed, application-specific key. That way the
+    ID will be properly unique, and derived in a constant way from the machine ID but there will be no way to retrieve
+    the original machine ID from the application-specific one. The
+    <citerefentry><refentrytitle>sd_id128_get_machine_app_specific</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+    API provides an implementation of such an algorithm.</para>
 
     <para>The
     <citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry>
diff --git a/man/sd_id128_get_machine.xml b/man/sd_id128_get_machine.xml
index 9a86c24aed..3938c6d836 100644
--- 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>
@@ -59,6 +60,12 @@
       </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>
       </funcprototype>
@@ -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>
@@ -117,13 +135,36 @@
   <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>
 
     <para>