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sd-id128: add new sd_id128_get_machine_app_specific() API

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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
This commit is contained in:
Lennart Poettering 2016-11-17 17:07:46 +01:00
parent 0fe5f3c5d7
commit 70fc4f5790
7 changed files with 110 additions and 20 deletions
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5
Makefile-man.am
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@ -397,6 +397,7 @@ MANPAGES_ALIAS += \
man/sd_id128_from_string.3 \
man/sd_id128_get_boot.3 \
man/sd_id128_get_invocation.3 \
man/sd_id128_get_machine_app_specific.3 \
man/sd_id128_is_null.3 \
man/sd_id128_t.3 \
man/sd_is_mq.3 \
@ -750,6 +751,7 @@ man/sd_id128_equal.3: man/sd-id128.3
man/sd_id128_from_string.3: man/sd_id128_to_string.3
man/sd_id128_get_boot.3: man/sd_id128_get_machine.3
man/sd_id128_get_invocation.3: man/sd_id128_get_machine.3
man/sd_id128_get_machine_app_specific.3: man/sd_id128_get_machine.3
man/sd_id128_is_null.3: man/sd-id128.3
man/sd_id128_t.3: man/sd-id128.3
man/sd_is_mq.3: man/sd_is_fifo.3
@ -1531,6 +1533,9 @@ man/sd_id128_get_boot.html: man/sd_id128_get_machine.html
man/sd_id128_get_invocation.html: man/sd_id128_get_machine.html
$(html-alias)
man/sd_id128_get_machine_app_specific.html: man/sd_id128_get_machine.html
$(html-alias)
man/sd_id128_is_null.html: man/sd-id128.html
$(html-alias)

15
man/machine-id.xml
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@ -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>

65
man/sd_id128_get_machine.xml
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@ -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>

5
src/libsystemd/libsystemd.sym
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@ -511,3 +511,8 @@ global:
sd_bus_get_exit_on_disconnect;
sd_id128_get_invocation;
} LIBSYSTEMD_231;
LIBSYSTEMD_233 {
global:
sd_id128_get_machine_app_specific;
} LIBSYSTEMD_232;

32
src/libsystemd/sd-id128/sd-id128.c
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@ -27,6 +27,7 @@
#include "hexdecoct.h"
#include "id128-util.h"
#include "io-util.h"
#include "khash.h"
#include "macro.h"
#include "random-util.h"
#include "util.h"
@ -181,3 +182,34 @@ _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;
}

2
src/systemd/sd-id128.h
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@ -39,12 +39,12 @@ union sd_id128 {
#define SD_ID128_STRING_MAX 33
char *sd_id128_to_string(sd_id128_t id, char s[SD_ID128_STRING_MAX]);
int sd_id128_from_string(const char *s, sd_id128_t *ret);
int sd_id128_randomize(sd_id128_t *ret);
int sd_id128_get_machine(sd_id128_t *ret);
int sd_id128_get_machine_app_specific(sd_id128_t app_id, sd_id128_t *ret);
int sd_id128_get_boot(sd_id128_t *ret);
int sd_id128_get_invocation(sd_id128_t *ret);

6
src/test/test-id128.c
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@ -153,5 +153,11 @@ int main(int argc, char *argv[]) {
assert_se(id128_read_fd(fd, ID128_UUID, &id2) >= 0);
assert_se(sd_id128_equal(id, id2));
assert_se(sd_id128_get_machine_app_specific(SD_ID128_MAKE(f0,3d,aa,eb,1c,33,4b,43,a7,32,17,29,44,bf,77,2e), &id) >= 0);
assert_se(sd_id128_get_machine_app_specific(SD_ID128_MAKE(f0,3d,aa,eb,1c,33,4b,43,a7,32,17,29,44,bf,77,2e), &id2) >= 0);
assert_se(sd_id128_equal(id, id2));
assert_se(sd_id128_get_machine_app_specific(SD_ID128_MAKE(51,df,0b,4b,c3,b0,4c,97,80,e2,99,b9,8c,a3,73,b8), &id2) >= 0);
assert_se(!sd_id128_equal(id, id2));
return 0;
}