Ubuntu Manpage: secure-logging - Forward integrity and confidentiality for system logs

Provided by: syslog-ng-mod-slog_4.8.1-1_amd64

NAME

       secure-logging - Forward integrity and confidentiality for system logs

SYNOPSIS

       $(slog --key-file  <host key file> --mac-file <MAC file> $RAWMSG)

DESCRIPTION

Secure logging is an extension to syslog-ng providing forward integrity and
confidentiality of system logs. It is implemented in form of a module and is configured as
a template in the syslog-ng configuration file.

The main objective of the secure logging module is to provide tamper evident logging, i.e.
to adequately protect log records of an information system and to provide a sensor
indicating attack attempts. The secure logging module achieves this by authentically
encrypting each log record with an individual cryptographic key used only once and
protecting the integrity of the whole log archive by a cryptographic authentication code.
Each attempt to tamper with either an individual log record or the log archive itself will
be immediately detected during log archive verification. Therefore, an attacker can no
longer tamper with log records without being detected.

In order to use the log file created by the secure logging module for analysis, the log
file must first be decrypted and its integrity verified. This is achieved with a command
line utility that is part of the secure logging module and is installed as part of the
syslog-ng package. This utility can easily be integrated into the import module of
existing analysis environments.

The secure logging environment uses a cryptographic key for encrypting log entries. Each
individual log entry is encrypted with its own key which is immediately discarded after
successful encryption in order to provide forward integrity. An efficient algorithm
generates the key for the next log entry based the key used for encrypting the previous
log entry. The resulting chain of keys preserves forward integrity, i.e. a potential
attacker cannot deduce the previous key from the current key.

In order to ease log verification and analysis, a sequence number is added to each log
entry. This sequence number is also added to the encryption key creating a one-to-one
correspondence of the encryption key with the log entry. In order to prevent truncation
attacks or deletion of individual log entries, a message authentication code (MAC) is
iteratively applied to the complete log archive and stored in a separate file. It is used
upon verification to check the integrity of the whole log archive.

Here is an example of three short original log messages that will be sent to a destination
with secure logging enabled:

This is a log message
And here comes another log message
This is a log message with a longer text that is processed without any problems

In order to inspect the status of the secure logging environment, one can check the
sequence counter by querying the key file with the slogkey utility like this:

user@host:~> slogkey --counter /etc/syslog-ng/host.key
counter=3

Counting starts at zero. This is why the counter is set to three after processing three
messages.

The output of the secure logging template for the three example messages is shown below.
One clearly sees the sequence number that was prepended to each message. The colon
indicates the end of the sequence number and the start of the original message. As three
message were processed, the sequence counter of the key will be three.

AAAAAAAAAAA=:k3/dYpLsgO2tUJKSauo6dycIBzW6OTC3pyA9TP+7AnqFgEojBzgC2rcK4OPfRtr8yg==
AQAAAAAAAAA=:smw0ATISVgN+BYEu5d7OLBE7aQhHpK9Ro4MndmNgSVrqhcmRCBCj6DUnD6ku0Z29CKJ0N6LAJUgByX4Ev+g=
AgAAAAAAAAA=:5UVybnKL1EAbgC4CLfd8HpgurjREf4LEN61/yWHSD2hbXjRD4QmQdtbwguT1chzdItKSQASps9QRIvR5Jd4AHzHfqxI4aRgdUBcNbAq26nwUCg5vPWygjmbtQaxZgCJYkry8slxCigmbTVs=

The output of a successful verification run is shown below.

0000000000000000: This is a log message
0000000000000001: And here comes another log message
0000000000000002: This is a log message with a longer text that is processed without any problems

The original log messages have been successfully restored. Additionally, the sequence
counter is also prepended to the clear text messages. This helps when in analyzing
problems with a particular log entry. As real log files will contain thousands of entries.
the sequence counter eases identification of erroneous entries.

Before the secure logging module can be used as part of an existing syslog-ng
installation, several preparatory activities need to be performed.

KEY GENERATION

       In order to bootstrap the system, an initial key k0 must be created and installed on the
       log host before secure logging environment is started for the first time.

       The newly created host key k0 has its counter set to 0 indicating that it represents the
       initial host key k0. This host key k0 must be kept secret and not be disclosed to third
       parties. It will be required to successfully decrypt and verify log archives processed by
       the secure logging environment. As each log entry will be encrypted with its own key, a
       new host key will be created after successful processing of a log entry and will replace
       the previous key. Therefore, the initial host key needs to be stored in a safe place
       before starting the secure logging environment, as it will be deleted from the log host
       after processing of the first log entry. The following steps must be done before starting
       the secure logging environment. Steps 1 and 2 are performed with the slogkey utility. See
       slogkey(1) for details on how to generate a master key and to derive a host key from it.
       Step 3 and 4 depend on the actual deployment in a target environment.

        1. Create a master key

        2. Derive an initial host key k0 from a previously created master key

        3. Store the initial host key k0 in a safe location outside of the log host

        4. Deploy the key k0 on the log host where the secure logging module will be used

CONFIGURATION

Secure logging is configured by adding the corresponding statements to the syslog-ng.conf
file. See syslog-ng.conf(5) for information on how to configure syslog-ng using the
configuration file. Details can be found in the The syslog-ng Administrator Guide[1].

Secure logging is implemented as a template and is configured accordingly. Apart from the
actual template configuration, no other settings are required in order to activate secure
logging. The secure logging is activated by placing the following statement in the
configuration file

template("$(slog --key-file <host key file> --mac-file <MAC file> $RAWMSG)\n");

where

slog
The name of the secure logging template function. This name can be also be found by
calling syslog-ng with the --module-registry arguments and checking the template-func
property of the secure logging module in the corresponding output.

--key-file or -k
The host key. <host key file> is the full path of the file storing the host key on the
log host. If this arguments is not supplied or does not point to a valid regular key
file, syslog-ng will not start and a display a corresponding error message.

--mac-file or -m
The MAC file. <MAC file> is the full path of the MAC file on the log host. The file
does not need to exist, as it will be automatically created upon the initial start. If
the path is not correct, syslog-ng will not start and a display a corresponding error
message.

$RAWMSG
$RAWMSG provides access to the original log message received at the source. This macro
is only available if the store-raw-message flag was set for the source. Otherwise, an
empty string will be passed to the secure logging template. If access to the original
message is not available, e.g. if the source does not support the store-raw-message
flag, then the $MSG macro can also be used. In this case, however, the integrity
guarantee provided by secure logging is limited to the content that this macro
provides and does not protect the complete original message.

\n
\n is the line separator. This is important, as the secure logging template expects
log entries to be separated by a line separator. When detecting a line separator, the
log entry is regarded as complete and is encrypted with the current host key.
Therefore, only a single line separator is allowed.

The secure logging template can be combined with any source or destination within the
following limitations:

• Sources must be line-oriented. Secure logging uses a line separator in order to
distinguish between individual log entries. Sources which provide data in a different
format, e.g. in the form of raw data obtained directly from a database system, cannot
currently be used with the secure logging template, as the separation of log entries
is not clearly defined for this type of data.

• Only sources for which the store-raw-message flag is implemented and set do benefit
from the integrity guarantee provided by the secure logging template. Secure logging
aims at protecting the integrity of complete log messages including all associated
meta-data, such as timestamps and host names. syslog-ng parses the log message into
its internal format and provide easy access to parts of a message through macros.
While this is convenient when rewriting log messages, it is not helpful for secure
logging. syslog-ng provides the store-raw-message flag which provides access to a
copy of the original log message after parsing. This is the log message processed and
protected by the secure logging template. If the source does not support the
store-raw-message flag, then the $MSG macro can also be used. However, in this case
the integrity guarantee provided by secure logging is limited to the content that this
macro provides.

• Log rotation of any kind cannot be used with destinations using secure logging. The
reason is that log rotate will overwrite, i.e. delete previous log files. This
destroys the cryptographic chain of trust of the log entries making recovery
impossible. In order to allow for a an efficient handling of log files, the secure
logging environment features iterative verification. Using iterative verification, a
log file can be verified in steps. For this to work, the log file must first be
downloaded from the log host, together with the corresponding host key and MAC file to
a verification host. After this download the log file can be safely deleted from the
log host. Verification is then performed on the verification host using the iterative
mode of the slogverify utility. See slogverify(1) for details.

The following example configuration shows the use of the secure logging template on a file
destination.

#############################################################################
# Minimal syslog-ng.conf file with secure logging enabled. Encrypted log
# entries will be logged to a single file called /var/log/messages.slog
#

@version: 4.8
@include "scl.conf"

source s_local {
system();
internal();
};

source s_network {
network(
transport("udp")
port(514)
flags(store-raw-message)
);
};

# Secure logging template definition
template secure_logging {
template("$(slog --key-file /etc/syslog-ng/host.key --mac-file /etc/syslog-ng/mac.dat $RAWMSG)\n");
};

# This configures a secure logging destination
destination d_local {
file("/var/log/messages.slog" template(secure_logging));
};

log {
source(s_local);

# This source has the raw message flag set
source(s_network);

# This statement activates secure logging for this destination
destination(d_local);
};

LOG VERIFICATION

In order to analyze the log file created in a secure logging environment, the log files
must be decrypted and their integrity be verified. Verification requires both the initial
host key k0 and the corresponding MAC file and is performed with the slogverify utility.
It is not normally performed on the log host where the secure logging environment is
producing log data. In a typical deployment, log files would be retrieved from the log
host and transferred to a central log collector where verification it performed. As
verification requires the use of the initial host key k0, it should only be performed in a
trusted environment.

Normal mode
In normal mode, a complete log archive is verified at once. In a typical environment, this
would mean retrieving a log file together with is MAC file from a log host and retrieving
the corresponding initial key k0 form a safe location and supplying them to the slogverify
utility. A typical call sequence for verification in normal mode would look like this

slogverify --key-file host0.key --mac-file mac.dat /var/log/messages.slog
/var/log/verified/messages

with

host0.key
The initial host key k0. Supplying the initial key k0 is enough for decrypting all log
entries, as the key derivation algorithm is able to generate the necessary keys for
all subsequent log entries based on the initial key k0.

mac.dat
The MAC file from the log host.

/var/log/messages.slog
The file containing the encrypted log entries as retrieved from a log host.

/var/log/verified/messages
The file receiving the plain text log after decryption.

Log files may become large and not fit into system memory. Verification is therefore
performed in chunks. Each part of the log file is transferred to an internal buffer on
which verification is performed. After the buffer has been processed, the next chunk is
fetched. An optional buffer argument can be supplied to the slogverify utility in order to
change the default buffer size of 1000 log entries to a number suitable for the system on
which the verification is performed, for example

slogverify --key-file host.key --mac-file mac.dat /var/log/messages.slog
/var/log/verified/messages 8000

See slogverify(1) for details on verification in normal mode.

Iterative mode
Verification in normal mode may not be suitable for some application scenarios. Many log
hosts use log rotation in order to preserve storage space. In log rotation, a threshold
for the maximum amount of storage space and the number of generations is defined for
different type of log files. When either storage space is exhausted or the number of
generations is reached, the oldest log file will be overwritten by new incoming log data.
This procedure is not possible in secure logging environment, as overwriting, i.e.
deleting a log file would break the cryptographic chain that is established between the
log entries. This comes as no surprise, as one of the main objectives of secure logging is
to protect against deletion of log entries by a potential attacker.

In order allow for a procedure similar to log rotation, the secure logging environment
features an iterative mode. In iterative mode, log files can be split into different files
and each of these files can be verified separately. Care must be taken when performing
verification in iterative mode, as each of the different log files needs to be accompanied
by a copy of the host key and the MAC files present on the system at the time of
retrieval. A typical usage scenario for the iterative mode would look like this:

1. Define a storage threshold for the secure logging file destination. In this example we
assume 500MB.

2. Let the secure logging environment produce log data that is written to the destination
until 500MB are reached.

3. Stop the secure logging environment and retrieve the log file, the host key and the
MAC files from the log host.

4. Delete the log file on the log host but leave host key and MAC file untouched.

5. Restart the secure logging environment.

6. Perform verification in iterative mode with the log file, the host key and the MAC
just retrieved.

Steps 2-6 have to repeated each time the log file reaches a size of 50 MB. Assuming that
the log file parts will be named after the iteration, e.g. log.1, log.2, log.3, etc. and a
similar convention is applied to the host keys and MAC files, a typical call sequence for
the validation of a log file part in iterative mode after three iterations will look like
this:

slogverify --iterative --prev-key-file host.key.2 --prev-mac-file mac.dat.2 --mac-file
mac.dat /var/log/messages.slog.3 /var/log/verified/messages.3

with

host.key.2
The host key from the previous iteration. In this example, this is the second
iteration.

mac.dat.2
The MAC file from the previous iteration. In the example, verification is performed
during the third iteration, so the MAC file from the second iteration is required.

mac.dat
The current MAC file from the log host.

/var/log/messages.slog.3
The file part containing the encrypted log entries as retrieved from the log host
during the third iteration.

/var/log/verified/messages.3
The file receiving the plain text log after decryption during the third iteration.

In a real deployment, the above steps would typically be automated using a scripting
engine.

See slogverify(1) for details on verification in iterative mode.

FILES

       /usr/bin/slogkey

       /usr/bin/slogencrypt

       /usr/bin/slogverify

       /etc/syslog-ng.conf

AUTHOR

       This manual page was written by the Airbus Secure Logging Team
       <secure-logging@airbus.com>.

COPYRIGHT

NOTES

        1. The syslog-ng Administrator Guide
           https://www.balabit.com/documents/syslog-ng-ose-latest-guides/en/syslog-ng-ose-guide-admin/html/index.html

        2. syslog-ng mailing list
           https://lists.balabit.hu/mailman/listinfo/syslog-ng

        3. syslog-ng blogs
           https://syslog-ng.org/blogs/