1SECURE-LOGGING(7) The secure logging manual page SECURE-LOGGING(7)
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6 secure-logging - Forward integrity and confidentiality for system logs
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9 $(slog --key-file <host key file> --mac-file <MAC file> $RAWMSG)
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12 Secure logging is an extension to syslog-ng providing forward integrity
13 and confidentiality of system logs. It is implemented in form of a
14 module and is configured as a template in the syslog-ng configuration
15 file.
16 The main objective of the secure logging module is to provide tamper
18 evident logging, i.e. to adequately protect log records of an
19 information system and to provide a sensor indicating attack attempts.
20 The secure logging module achieves this by authentically encrypting
21 each log record with an individual cryptographic key used only once and
22 protecting the integrity of the whole log archive by a cryptographic
23 authentication code. Each attempt to tamper with either an individual
24 log record or the log archive itself will be immediately detected
25 during log archive verification. Therefore, an attacker can no longer
26 tamper with log records without being detected.
27 In order to use the log file created by the secure logging module for
29 analysis, the log file must first be decrypted and its integrity
30 verified. This is achieved with a command line utility that is part of
31 the secure logging module and is installed as part of the syslog-ng
32 package. This utility can easily be integrated into the import module
33 of existing analysis environments.
34 The secure logging environment uses a cryptographic key for encrypting
36 log entries. Each individual log entry is encrypted with its own key
37 which is immediately discarded after successful encryption in order to
38 provide forward integrity. An efficient algorithm generates the key for
39 the next log entry based the key used for encrypting the previous log
40 entry. The resulting chain of keys preserves forward integrity, i.e. a
41 potential attacker cannot deduce the previous key from the current key.
42 In order to ease log verification and analysis, a sequence number is
44 added to each log entry. This sequence number is also added to the
45 encryption key creating a one-to-one correspondence of the encryption
46 key with the log entry. In order to prevent truncation attacks or
47 deletion of individual log entries, a message authentication code (MAC)
48 is iteratively applied to the complete log archive and stored in a
49 separate file. It is used upon verification to check the integrity of
50 the whole log archive.
51 Here is an example of three short original log messages that will be
53 sent to a destination with secure logging enabled:
54 This is a log message
56 And here comes another log message
57 This is a log message with a longer text that is processed without any problems
58 In order to inspect the status of the secure logging environment, one
61 can check the sequence counter by querying the key file with the
62 slogkey utility like this:
63 user@host:~> slogkey --counter /etc/syslog-ng/host.key
65 counter=3
66 Counting starts at zero. This is why the counter is set to three after
69 processing three messages.
70 The output of the secure logging template for the three example
72 messages is shown below. One clearly sees the sequence number that was
73 prepended to each message. The colon indicates the end of the sequence
74 number and the start of the original message. As three message were
75 processed, the sequence counter of the key will be three.
76 AAAAAAAAAAA=:k3/dYpLsgO2tUJKSauo6dycIBzW6OTC3pyA9TP+7AnqFgEojBzgC2rcK4OPfRtr8yg==
78 AQAAAAAAAAA=:smw0ATISVgN+BYEu5d7OLBE7aQhHpK9Ro4MndmNgSVrqhcmRCBCj6DUnD6ku0Z29CKJ0N6LAJUgByX4Ev+g=
79 AgAAAAAAAAA=:5UVybnKL1EAbgC4CLfd8HpgurjREf4LEN61/yWHSD2hbXjRD4QmQdtbwguT1chzdItKSQASps9QRIvR5Jd4AHzHfqxI4aRgdUBcNbAq26nwUCg5vPWygjmbtQaxZgCJYkry8slxCigmbTVs=
80 The output of a successful verification run is shown below.
83 0000000000000000: This is a log message
85 0000000000000001: And here comes another log message
86 0000000000000002: This is a log message with a longer text that is processed without any problems
87 The original log messages have been successfully restored.
90 Additionally, the sequence counter is also prepended to the clear text
91 messages. This helps when in analyzing problems with a particular log
92 entry. As real log files will contain thousands of entries. the
93 sequence counter eases identification of erroneous entries.
94 Before the secure logging module can be used as part of an existing
96 syslog-ng installation, several preparatory activities need to be
97 performed.
98
100 In order to bootstrap the system, an initial key k0 must be created and
101 installed on the log host before secure logging environment is started
102 for the first time.
103 The newly created host key k0 has its counter set to 0 indicating that
105 it represents the initial host key k0. This host key k0 must be kept
106 secret and not be disclosed to third parties. It will be required to
107 successfully decrypt and verify log archives processed by the secure
108 logging environment. As each log entry will be encrypted with its own
109 key, a new host key will be created after successful processing of a
110 log entry and will replace the previous key. Therefore, the initial
111 host key needs to be stored in a safe place before starting the secure
112 logging environment, as it will be deleted from the log host after
113 processing of the first log entry. The following steps must be done
114 before starting the secure logging environment. Steps 1 and 2 are
115 performed with the slogkey utility. See slogkey(1) for details on how
116 to generate a master key and to derive a host key from it. Step 3 and 4
117 depend on the actual deployment in a target environment.
118 1. Create a master key
120 2. Derive an initial host key k0 from a previously created master key
122 3. Store the initial host key k0 in a safe location outside of the log
124 host
125 4. Deploy the key k0 on the log host where the secure logging module
127 will be used
128
130 Secure logging is configured by adding the corresponding statements to
131 the syslog-ng.conf file. See syslog-ng.conf(5) for information on how
132 to configure syslog-ng using the configuration file. Details can be
133 found in the The syslog-ng Administrator Guide[1].
134 Secure logging is implemented as a template and is configured
136 accordingly. Apart from the actual template configuration, no other
137 settings are required in order to activate secure logging. The secure
138 logging is activated by placing the following statement in the
139 configuration file
140 template("$(slog --key-file <host key file> --mac-file <MAC file>
142 $RAWMSG)\n");
143 where
145 slog
147 The name of the secure logging template function. This name can be
148 also be found by calling syslog-ng with the --module-registry
149 arguments and checking the template-func property of the secure
150 logging module in the corresponding output.
151 --key-file or -k
153 The host key. <host key file> is the full path of the file storing
154 the host key on the log host. If this arguments is not supplied or
155 does not point to a valid regular key file, syslog-ng will not
156 start and a display a corresponding error message.
157 --mac-file or -m
159 The MAC file. <MAC file> is the full path of the MAC file on the
160 log host. The file does not need to exist, as it will be
161 automatically created upon the initial start. If the path is not
162 correct, syslog-ng will not start and a display a corresponding
163 error message.
164 $RAWMSG
166 $RAWMSG provides access to the original log message received at the
167 source. This macro is only available if the store-raw-message flag
168 was set for the source. Otherwise, an empty string will be passed
169 to the secure logging template. If access to the original message
170 is not available, e.g. if the source does not support the
171 store-raw-message flag, then the $MSG macro can also be used. In
172 this case, however, the integrity guarantee provided by secure
173 logging is limited to the content that this macro provides and does
174 not protect the complete original message.
175 \n
177 \n is the line separator. This is important, as the secure logging
178 template expects log entries to be separated by a line separator.
179 When detecting a line separator, the log entry is regarded as
180 complete and is encrypted with the current host key. Therefore,
181 only a single line separator is allowed.
182 The secure logging template can be combined with any source or
184 destination within the following limitations:
185 • Sources must be line-oriented. Secure logging uses a line separator
187 in order to distinguish between individual log entries. Sources
188 which provide data in a different format, e.g. in the form of raw
189 data obtained directly from a database system, cannot currently be
190 used with the secure logging template, as the separation of log
191 entries is not clearly defined for this type of data.
192 • Only sources for which the store-raw-message flag is implemented
194 and set do benefit from the integrity guarantee provided by the
195 secure logging template. Secure logging aims at protecting the
196 integrity of complete log messages including all associated
197 meta-data, such as timestamps and host names. syslog-ng parses the
198 log message into its internal format and provide easy access to
199 parts of a message through macros. While this is convenient when
200 rewriting log messages, it is not helpful for secure logging.
201 syslog-ng provides the store-raw-message flag which provides access
202 to a copy of the original log message after parsing. This is the
203 log message processed and protected by the secure logging template.
204 If the source does not support the store-raw-message flag, then the
205 $MSG macro can also be used. However, in this case the integrity
206 guarantee provided by secure logging is limited to the content that
207 this macro provides.
208 • Log rotation of any kind cannot be used with destinations using
210 secure logging. The reason is that log rotate will overwrite, i.e.
211 delete previous log files. This destroys the cryptographic chain of
212 trust of the log entries making recovery impossible. In order to
213 allow for a an efficient handling of log files, the secure logging
214 environment features iterative verification. Using iterative
215 verification, a log file can be verified in steps. For this to
216 work, the log file must first be downloaded from the log host,
217 together with the corresponding host key and MAC file to a
218 verification host. After this download the log file can be safely
219 deleted from the log host. Verification is then performed on the
220 verification host using the iterative mode of the slogverify
221 utility. See slogverify(1) for details.
222 The following example configuration shows the use of the secure logging
224 template on a file destination.
225 #############################################################################
227 # Minimal syslog-ng.conf file with secure logging enabled. Encrypted log
228 # entries will be logged to a single file called /var/log/messages.slog
229 #
230 @version: 4.0
232 @include "scl.conf"
233 source s_local {
235 system();
236 internal();
237 };
238 source s_network {
240 network(
241 transport("udp")
242 port(514)
243 flags(store-raw-message)
244 );
245 };
246 # Secure logging template definition
248 template secure_logging {
249 template("$(slog --key-file /etc/syslog-ng/host.key --mac-file /etc/syslog-ng/mac.dat $RAWMSG)\n");
250 };
251 # This configures a secure logging destination
253 destination d_local {
254 file("/var/log/messages.slog" template(secure_logging));
255 };
256 log {
258 source(s_local);
259 # This source has the raw message flag set
261 source(s_network);
262 # This statement activates secure logging for this destination
264 destination(d_local);
265 };
266
269 In order to analyze the log file created in a secure logging
270 environment, the log files must be decrypted and their integrity be
271 verified. Verification requires both the initial host key k0 and the
272 corresponding MAC file and is performed with the slogverify utility. It
273 is not normally performed on the log host where the secure logging
274 environment is producing log data. In a typical deployment, log files
275 would be retrieved from the log host and transferred to a central log
276 collector where verification it performed. As verification requires the
277 use of the initial host key k0, it should only be performed in a
278 trusted environment.
279 Normal mode
281 In normal mode, a complete log archive is verified at once. In a
282 typical environment, this would mean retrieving a log file together
283 with is MAC file from a log host and retrieving the corresponding
284 initial key k0 form a safe location and supplying them to the
285 slogverify utility. A typical call sequence for verification in normal
286 mode would look like this
287 slogverify --key-file host0.key --mac-file mac.dat
289 /var/log/messages.slog /var/log/verified/messages
290 with
292 host0.key
294 The initial host key k0. Supplying the initial key k0 is enough for
295 decrypting all log entries, as the key derivation algorithm is able
296 to generate the necessary keys for all subsequent log entries based
297 on the initial key k0.
298 mac.dat
300 The MAC file from the log host.
301 /var/log/messages.slog
303 The file containing the encrypted log entries as retrieved from a
304 log host.
305 /var/log/verified/messages
307 The file receiving the plain text log after decryption.
308 Log files may become large and not fit into system memory. Verification
310 is therefore performed in chunks. Each part of the log file is
311 transferred to an internal buffer on which verification is performed.
312 After the buffer has been processed, the next chunk is fetched. An
313 optional buffer argument can be supplied to the slogverify utility in
314 order to change the default buffer size of 1000 log entries to a number
315 suitable for the system on which the verification is performed, for
316 example
317 slogverify --key-file host.key --mac-file mac.dat
319 /var/log/messages.slog /var/log/verified/messages 8000
320 See slogverify(1) for details on verification in normal mode.
322 Iterative mode
324 Verification in normal mode may not be suitable for some application
325 scenarios. Many log hosts use log rotation in order to preserve storage
326 space. In log rotation, a threshold for the maximum amount of storage
327 space and the number of generations is defined for different type of
328 log files. When either storage space is exhausted or the number of
329 generations is reached, the oldest log file will be overwritten by new
330 incoming log data. This procedure is not possible in secure logging
331 environment, as overwriting, i.e. deleting a log file would break the
332 cryptographic chain that is established between the log entries. This
333 comes as no surprise, as one of the main objectives of secure logging
334 is to protect against deletion of log entries by a potential attacker.
335 In order allow for a procedure similar to log rotation, the secure
337 logging environment features an iterative mode. In iterative mode, log
338 files can be split into different files and each of these files can be
339 verified separately. Care must be taken when performing verification in
340 iterative mode, as each of the different log files needs to be
341 accompanied by a copy of the host key and the MAC files present on the
342 system at the time of retrieval. A typical usage scenario for the
343 iterative mode would look like this:
344 1. Define a storage threshold for the secure logging file destination.
346 In this example we assume 500MB.
347 2. Let the secure logging environment produce log data that is written
349 to the destination until 500MB are reached.
350 3. Stop the secure logging environment and retrieve the log file, the
352 host key and the MAC files from the log host.
353 4. Delete the log file on the log host but leave host key and MAC file
355 untouched.
356 5. Restart the secure logging environment.
358 6. Perform verification in iterative mode with the log file, the host
360 key and the MAC just retrieved.
361 Steps 2-6 have to repeated each time the log file reaches a size of 50
363 MB. Assuming that the log file parts will be named after the iteration,
364 e.g. log.1, log.2, log.3, etc. and a similar convention is applied to
365 the host keys and MAC files, a typical call sequence for the validation
366 of a log file part in iterative mode after three iterations will look
367 like this:
368 slogverify --iterative --prev-key-file host.key.2 --prev-mac-file
370 mac.dat.2 --mac-file mac.dat /var/log/messages.slog.3
371 /var/log/verified/messages.3
372 with
374 host.key.2
376 The host key from the previous iteration. In this example, this is
377 the second iteration.
378 mac.dat.2
380 The MAC file from the previous iteration. In the example,
381 verification is performed during the third iteration, so the MAC
382 file from the second iteration is required.
383 mac.dat
385 The current MAC file from the log host.
386 /var/log/messages.slog.3
388 The file part containing the encrypted log entries as retrieved
389 from the log host during the third iteration.
390 /var/log/verified/messages.3
392 The file receiving the plain text log after decryption during the
393 third iteration.
394 In a real deployment, the above steps would typically be automated
396 using a scripting engine.
397 See slogverify(1) for details on verification in iterative mode.
399
401 /usr/bin/slogkey
402 /usr/bin/slogencrypt
404 /usr/bin/slogverify
406 /etc/syslog-ng.conf
408
410 syslog-ng.conf(5)
411 slogkey(1)
413 slogencrypt(1)
415 slogverify(1)
417 Note
419 For the detailed documentation of see The syslog-ng Administrator
420 Guide[1]
421 If you experience any problems or need help with syslog-ng, visit
423 the syslog-ng mailing list[2].
424 For news and notifications about of syslog-ng, visit the syslog-ng
426 blogs[3].
427 For specific information requests related to secure logging send a
429 mail to the Airbus Secure Logging Team <secure-logging@airbus.com>.
430
432 This manual page was written by the Airbus Secure Logging Team
433 <secure-logging@airbus.com>.
434
437 1. The syslog-ng Administrator Guide
438 https://www.balabit.com/documents/syslog-ng-ose-latest-guides/en/syslog-ng-ose-guide-admin/html/index.html
439 2. syslog-ng mailing list
441 https://lists.balabit.hu/mailman/listinfo/syslog-ng
442 3. syslog-ng blogs
444 https://syslog-ng.org/blogs/
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