secure-logging - Man Page

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

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.

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

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

\n

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);
};

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.

/usr/bin/slogkey

/usr/bin/slogencrypt

/usr/bin/slogverify

/etc/syslog-ng.conf

syslog-ng.conf(5)

slogkey(1)

slogencrypt(1)

slogverify(1)

Note

For the detailed documentation of see The syslog-ng Administrator Guide^[1]

If you experience any problems or need help with syslog-ng, visit the syslog-ng mailing list^[2].

For news and notifications about of syslog-ng, visit the syslog-ng blogs^[3].

For specific information requests related to secure logging send a mail to the Airbus Secure Logging Team <secure-logging@airbus.com>.

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

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/

slogencrypt(1), slogkey(1), slogverify(1).