syslog Working Group J. Kelsey
Internet-Draft
Expires: November 23, 2005 J. Callas
PGP Corporation
May 23, 2005
Signed syslog Messages
draft-ietf-syslog-sign-16.txt
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document describes a mechanism to add origin authentication,
message integrity, replay-resistance, message sequencing, and
detection of missing messages to the transmitted syslog messages.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. syslog Message Format . . . . . . . . . . . . . . . . . . . . 6
3. Signature Block Format and Fields . . . . . . . . . . . . . . 7
3.1 syslog Packets Containing a Signature Block . . . . . . . 7
3.2 Cookie . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.3 Version . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.4 Reboot Session ID . . . . . . . . . . . . . . . . . . . . 8
3.5 Signature Group and Signature Priority . . . . . . . . . . 8
3.6 Global Block Counter . . . . . . . . . . . . . . . . . . . 10
3.7 First Message Number . . . . . . . . . . . . . . . . . . . 11
3.8 Count . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.9 Hash Block . . . . . . . . . . . . . . . . . . . . . . . . 11
3.10 Signature . . . . . . . . . . . . . . . . . . . . . . . . 11
4. Payload and Certificate Blocks . . . . . . . . . . . . . . . . 12
4.1 Preliminaries: Key Management and Distribution Issues . . 12
4.2 Building the Payload Block . . . . . . . . . . . . . . . . 12
4.3 Building the Certificate Block . . . . . . . . . . . . . . 13
4.3.1 Cookie . . . . . . . . . . . . . . . . . . . . . . . . 14
4.3.2 Version . . . . . . . . . . . . . . . . . . . . . . . 14
4.3.3 Reboot Session ID . . . . . . . . . . . . . . . . . . 14
4.3.4 Signature Group and Signature Priority . . . . . . . . 15
4.3.5 Total Payload Block Length . . . . . . . . . . . . . . 15
4.3.6 Index into Payload Block . . . . . . . . . . . . . . . 15
4.3.7 Fragment Length . . . . . . . . . . . . . . . . . . . 15
4.3.8 Signature . . . . . . . . . . . . . . . . . . . . . . 15
5. Redundancy and Flexibility . . . . . . . . . . . . . . . . . . 16
5.1 Redundancy . . . . . . . . . . . . . . . . . . . . . . . . 16
5.1.1 Certificate Blocks . . . . . . . . . . . . . . . . . . 16
5.1.2 Signature Blocks . . . . . . . . . . . . . . . . . . . 16
5.2 Flexibility . . . . . . . . . . . . . . . . . . . . . . . 17
6. Efficient Verification of Logs . . . . . . . . . . . . . . . . 18
6.1 Offline Review of Logs . . . . . . . . . . . . . . . . . . 18
6.2 Online Review of Logs . . . . . . . . . . . . . . . . . . 19
7. Security Considerations . . . . . . . . . . . . . . . . . . . 21
7.1 Cryptography Constraints . . . . . . . . . . . . . . . . . 21
7.2 Packet Parameters . . . . . . . . . . . . . . . . . . . . 21
7.3 Message Authenticity . . . . . . . . . . . . . . . . . . . 22
7.4 Sequenced Delivery . . . . . . . . . . . . . . . . . . . . 22
7.5 Replaying . . . . . . . . . . . . . . . . . . . . . . . . 22
7.6 Reliable Delivery . . . . . . . . . . . . . . . . . . . . 22
7.7 Sequenced Delivery . . . . . . . . . . . . . . . . . . . . 22
7.8 Message Integrity . . . . . . . . . . . . . . . . . . . . 23
7.9 Message Observation . . . . . . . . . . . . . . . . . . . 23
7.10 Man In The Middle . . . . . . . . . . . . . . . . . . . . 23
7.11 Denial of Service . . . . . . . . . . . . . . . . . . . . 23
7.12 Covert Channels . . . . . . . . . . . . . . . . . . . . . 23
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8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
8.1 Version Field . . . . . . . . . . . . . . . . . . . . . . 25
8.2 SIG Field . . . . . . . . . . . . . . . . . . . . . . . . 27
8.3 Key Blob Type . . . . . . . . . . . . . . . . . . . . . . 27
9. Authors and Working Group Chair . . . . . . . . . . . . . . . 28
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 29
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 30
Intellectual Property and Copyright Statements . . . . . . . . 31
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1. Introduction
This document describes a mechanism that adds origin authentication,
message integrity, replay resistance, message sequencing, and
detection of missing messages to syslog. Essentially, this is
accomplished by sending a cryptographically signed syslog message
containing the signatures of previously sent syslog messages. The
contents of this message is called a Signature Block. Each Signature
Block contains, in effect, a detached signature on some number of
previously sent messages. While most implementations of syslog
involve only a single device as the generator of each message and a
single receiver as the collector of each message, provisions need to
be made to cover messages being sent to multiple receivers. This is
generally performed based upon the Priority value of the individual
messages. For example, messages from any Facility with a Severity
value of 3, 2, 1 or 0 may be sent to one collector while all messages
of Facilities 4, 10, 13, and 14 may be sent to another collector.
Appropriate syslog-sign messages must be kept with their proper
syslog messages. To address this, syslog-sign uses a signature-
group. A signature group identifies a group of messages that are all
kept together for signing purposes by the device. A Signature Block
always belongs to exactly one signature group and it always signs
messages belonging only to that signature group.
Additionally, a device will send a Certificate Block to provide key
management information between the sender and the receiver. This
Certificate Block has a field to denote the type of key material
which may be such things as a PKIX certificate, an OpenPGP
certificate, or even an indication that a key had been
predistributed. In all cases, these messages still use the syslog
packet format described in this document. In the cases of
certificates being sent, the certificates may have to be split across
multiple packets.
The receiver of the previous messages may verify that the digital
signature of each received message matches the signature contained in
the Signature Block. A collector may process these Signature Blocks
as they arrive, building an authenticated log file. Alternatively,
it may store all the log messages in the order they were received.
This allows a network operator to authenticate the log file at the
time the logs are reviewed.
This specification is independent of the actual transport protocol
selected. It may be used with syslog packets over traditional UDP
[5] as described in RFC 3164 [20]. It may be used with other event
notification protocols, and it may be used with the Reliable Delivery
of syslog as described in RFC 3195 [21]. Other efforts to define
event notification messages should consider this specification in
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their design.
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2. syslog Message Format
This specification does not rely upon any specific syslog message
format. It MAY be transported over a traditional syslog message
format such as that defined in the informational RFC 3164 [20], or it
MAY be used over the Reliable Delivery of syslog Messages as defined
in RFC 3195 [21]. Care must be taken when choosing a transport for
this mechanism, however. Since the device generating the Signature
Block message signs each message in its entirety, it is imperative
that the messages MUST NOT be changed in transit. It is equally
imperative that the syslog-sign messages MUST NOT be changed in
transit. Specifically, a relay, as described in RFC 3164 MAY make
changes to a syslog packet if specific fields are not found. If this
occurs, the entire mechanism is rendered useless.
For convenience, this document will use the syslog message format in
the terms described in RFC 3164. That document describes the 3 parts
of a syslog message; the PRI, HEADER, and MSG parts. The MSG part is
composed of TAG and CONTENT parts. Space characters separate each of
the fields.
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3. Signature Block Format and Fields
This section describes the Signature Block format and the fields used
within the Signature Block.
3.1 syslog Packets Containing a Signature Block
Signature Block messages MUST be encompassed within completely formed
syslog messages. It SHOULD also contain a valid TAG field. It is
RECOMMENDED that the TAG field have the value of "syslog " (without
the double quotes) to signify that this message was generated by the
syslog process. The CONTENT field of the syslog Signature Block
messages MUST have the following fields. Each of these fields are
separated by a single space character.
The Signature Block is composed of the following fields. Each field
must be printable ASCII, and any binary values are base-64 encoded.
Field Designation Size in bytes
----- ----------- ---- -- -----
Cookie Cookie 8
Version Ver 4
Reboot Session ID RSID 1-10
Signature Group SIG 1
Signature Priority SPRI 1-3
Global Block Counter GBC 1-10
First Message Number FMN 1-10
Count Count 1-2
Hash Block Hash Block variable, size of hash
(base-64 encoded binary)
Signature Signature variable
(base-64 encoded binary)
These fields are described below.
3.2 Cookie
The cookie is a eight-byte sequence to signal that this is a
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Signature Block. This sequence is "@#sigSIG" (without the double
quotes). As noted, a space character follows this, and all other
fields.
3.3 Version
The signature group version field is 4 characters in length and is
terminated with a space character. The value in this field specifies
the version of the syslog-sign protocol. This is extensible to allow
for different hash algorithms and signature schemes to be used in the
future. The value of this field is the grouping of the protocol
version (2 bytes), the hash algorithm (1 byte) and the signature
scheme (1 byte).
Protocol Version - 2 bytes with the first version as described in
this document being value of 01 to denote Version 1.
Hash Algorithm - 1 byte with the definition that 1 denotes SHA1 as
defined in FIPS-180-1.1995 [2].
Signature Scheme - 1 byte with the definition that 1 denotes
OpenPGP DSA - RFC 2440 [18], FIPS.186-1.1998 [1].
As such, the version, hash algorithm and signature scheme defined in
this document may be represented as "0111" (without the quote marks).
3.4 Reboot Session ID
The reboot session ID is a value between 1 and 10 bytes, which is
required to never repeat or decrease. The acceptable values for this
are between 0 and 9999999999. If the value latches at 9999999999,
then manual intervention may be required to reset it to 0.
Implementors MAY wish to consider using the snmpEngineBoots value as
a source for this counter as defined in RFC 2574 [19].
3.5 Signature Group and Signature Priority
The SIG identifier as described above may take on any value from 0-3
inclusive. The SPRI may take any value from 0-191. Each of these
fields are followed by a space character. These fields taken
together allows network administrators to associate groupings of
syslog messages with appropriate Signature Blocks and Certificate
Blocks. For example, in some cases, network administrators may send
syslog messages of Facilities 0 through 15 to one destination while
sending messages with Facilities 16 through 23 to another.
Associated Signature Blocks should be sent to these different syslog
servers as well.
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In some cases, an administrator may wish the Signature Blocks to go
to the same destination as the syslog messages themselves. This may
be to different syslog servers if the destinations of syslog messages
is being controlled by the Facilities or the Severities of the
messages. In other cases, administrators may wish to send the
Signature Blocks to an altogether different destination.
Syslog-sign provides four options for handling signature groups,
linking them with PRI values so they may be routed to the destination
commensurate with the appropriate syslog messages. In all cases, no
more than 192 signature groups (0-191) are permitted.
a. '0' -- There is only one signature group. All Signature Block
messages use a single PRI value which is the same SPRI value. In
this case, the administrators want all Signature Blocks to be
sent to a single destination. In all likelihood, all of the
syslog messages will also be going to that same destination. As
one example, if SIG=0, then PRI and SPRI may be 46 to indicate
that they are informational messages from the syslog daemon. If
the device is configured to send all messages with the local5
Facility (21), then the PRI and SPRI may be 174 to indicate that
they are also from the local5 Facility with a Severity of 6.
b. '1' -- Each PRI value has its own signature group. Signature
Blocks for a given signature group have SPRI = PRI for that
signature group. In this case, the administrator of a device may
not know where any of the syslog messages will ultimately go.
This use ensures that a Signature Block follows each of the
syslog messages to each destination. This may be seen to be
inefficient if groups of syslog messages are actually going to
the same syslog server. Examine an example of a device being
configured to have a SIG value of 1, which generates 16 syslog
messages with
4 from PRI=132 (Facility 16, Severity 4),
4 from PRI=148 (Facility 18, Severity 4),
4 from PRI=164, (Facility 20, Severity 4), and
4 from PRI=180 (Facility 22, Severity 4).
In actuality, the messages from Facilities local0 and local2 go
to one syslog server and messages from Facilities local4 and
local6 go to a different one. Then, the first syslog server
receives 2 Signature Blocks, the first with PRI=134 and the
second from PRI=150 - the PRI values matching the SPRI values.
The second syslog server would also receive two Signature Block
messages, the first from PRI=164 and the second from PRI=180. In
each of those Signature Blocks, the SPRI values matches their
respective PRI values. In each of these cases, the Signature
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Blocks going to each respective syslog server could have been
combined. One way to do this more efficiently is explained using
SIG=2.
c. '2' -- Each signature group contains a range of PRI values.
Signature groups are assigned sequentially. A Signature Block
for a given signature group has its own SPRI value denoting the
highest PRI value in that signature group. For flexibility, the
PRI does not have to be that upper-boundary SPRI value.
Continuing the above example, the administrator of the device may
configure SIG=2 with upper-bound SPRIs of 151 and 191. The lower
group contains all PRIs between 0 and 151, and the second group
contains all PRIs between 152 and 191. The administrator may
then wish to configure the lower group to send all of the lower
group Signature Blocks using PRI=150 (Facility 18, Severity 6),
and the upper group using PRI=182 (Facility 22, Severity 6). The
receiving syslog servers then each receive a single Signature
Block describing the 8 syslog messages sent to it.
d. '3' -- Signature groups are not assigned with any simple
relationship to PRI values. This has to be some predefined
arrangement between the sender and the intended receivers. In
this case, the administrators of the devices and syslog servers
may, as an example, use SIG=3 with a SPRI of 1 to denote that all
Warning and above syslog messages from all Facilities are sent
using a PRI of 46 (Facility 5, Severity 6).
One reasonable way to configure some installations is to have only
one signature group with SIG=0. The devices send messages to many
collectors and also send a copy of each Signature Block to each
collector. This won't allow any collector to detect gaps in the
messages, but it allows all messages that arrive at each collector to
be put into the right order, and to be verified. It also allows each
collector to detect duplicates and any messages that are not
associated with a Signature Block.
3.6 Global Block Counter
The global block counter is a value representing the number of
Signature Blocks sent out by syslog-sign before this one, in this
reboot session. This takes at least 1 byte and at most 10 bytes
displayed as a decimal counter and the acceptable values for this are
between 0 and 9999999999. If the value latches at 9999999999, then
the reboot session counter must be incremented by 1 and the global
block counter resumes at 0. Note that this counter crosses signature
groups; it allows us to roughly synchronize when two messages were
sent, even though they went to different collectors.
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3.7 First Message Number
This is a value between 1 and 10 bytes. It contains the unique
message number within this signature group of the first message whose
hash appears in this block. The very first message of the reboot
session will be numbered "1".
For example, if this signature group has processed 1000 messages so
far and message number 1001 is the first message whose hash appears
in this Signature Block, then this field contains 1001.
3.8 Count
The count is a 1 or 2 byte field displaying the number of message
hashes to follow. The valid values for this field are between 1 and
99.
3.9 Hash Block
The hash block is a block of hashes, each separately encoded in
base-64. Each hash in the hash block is the hash of the entire
syslog message represented by the hash. The hashing algorithm used
effectively specified by the Version field determines the size of
each hash, but the size MUST NOT be shorter than 160 bits. It is
base-64 encoded as per RFC 2045.
3.10 Signature
This is a digital signature, encoded in base-64, as per RFC 2045.
The signature is calculated over all fields but excludes the space
characters between them. The Version field effectively specifies the
original encoding of the signature. The signature is a signature
over the entire data, including all of the PRI, HEADER, and hashes in
the hash block. To reiterate, the signature is calculated over the
completely formatted syslog-message, excluding spaces between fields,
and also excluding this signature field.
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4. Payload and Certificate Blocks
Certificate Blocks and Payload Blocks provide key management in
syslog-sign.
4.1 Preliminaries: Key Management and Distribution Issues
The purpose of Certificate Blocks is to support key management using
public key cryptosystems. All devices send at least one Certificate
Block at the beginning of a new reboot session, carrying useful
information about the reboot session.
There are three key points to understand about Certificate Blocks:
a. They handle a variable-sized payload, fragmenting it if necessary
and transmitting the fragments as legal syslog messages. This
payload is built (as described below) at the beginning of a
reboot session and is transmitted in pieces with each Certificate
Block carrying a piece. Note that there is exactly one Payload
Block per reboot session.
b. The Certificate Blocks are digitally signed. The device does not
sign the Payload Block, but the signatures on the Certificate
Blocks ensure its authenticity. Note that it may not even be
possible to verify the signature on the Certificate Blocks
without the information in the Payload Block; in this case the
Payload Block is reconstructed, the key is extracted, and then
the Certificate Blocks are verified. (This is necessary even
when the Payload Block carries a certificate, since some other
fields of the Payload Block aren't otherwise verified.) In
practice, most installations keep the same public key over long
periods of time, so that most of the time, it's easy to verify
the signatures on the Certificate Blocks, and use the Payload
Block to provide other useful per-session information.
c. The kind of Payload Block that is expected is determined by what
kind of key material is on the collector that receives it. The
device and collector (or offline log viewer) has both some key
material (such as a root public key, or predistributed public
key), and an acceptable value for the Key Blob Type in the
Payload Block, below. The collector or offline log viewer MUST
NOT accept a Payload Block of the wrong type.
4.2 Building the Payload Block
The Payload Block is built when a new reboot session is started.
There is a one-to-one correspondence of reboot sessions to Payload
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Blocks. That is, each reboot session has only one Payload Block,
regardless of how many signature groups it may support. Like syslog
packets containing the Signature Block, Payload Block messages MUST
be completely formed syslog messages. It is RECOMMENDED that the TAG
field have the value of "syslog " (without the double quotes) to
signify that this message was generated by the syslog process. The
CONTENT field of the syslog Payload Block messages MUST have the
following fields. Each of these fields are separated by a single
space character.
a. Unique identifier of sender; by default, the sender's IP address
in dotted-decimal (IPv4) or colon-separated (IPv6) notation.
b. Full local time stamp for the device at the time the reboot
session started. This must be in TIMESTAMP-3339 format.
c. Key Blob Type, a one-byte field which holds one of five values:
1. 'C' -- a PKIX certificate.
2. 'P' -- an OpenPGP certificate.
3. 'K' -- the public key whose corresponding private key is
being used to sign these messages.
4. 'N' -- no key information sent; key is predistributed.
5. 'U' -- installation-specific key exchange information
d. The key blob, consisting of the raw key data, if any, base-64
encoded.
4.3 Building the Certificate Block
The Certificate Block must get the Payload Block to the collector.
Since certificates can legitimately be much longer than 1024 bytes,
each Certificate Block carries a piece of the Payload Block. Note
that the device MAY make the Certificate Blocks of any legal length
(that is, any length less than 1024 bytes) which holds all the
required fields. Software that processes Certificate Blocks MUST
deal correctly with blocks of any legal length.
The Certificate Block is composed of the following fields. Each
field must be printable ASCII, and any binary values are base-64
encoded.
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Field Designation Size in bytes
----- ----------- ---- -- -----
Cookie Cookie 8
Version Ver 4
Reboot Session ID RSID 1-10
Signature Group SIG 1
Signature Priority SPRI 1-3
Total Payload Block Length TPBL 1-8
Index into Payload Block Index 1-8
Fragment Length FragLen 1-3
Payload Block Fragment Fragment variable
(base-64 encoded binary)
Signature Signature variable
(base-64 encoded binary)
4.3.1 Cookie
The cookie is a eight-byte sequence to signal that this is a
Signature Block. This sequence is "@#sigCER" (without the double
quotes). As noted, a space character follows this, and all other
fields.
4.3.2 Version
The signature group version field is 4 characters in length and is
terminated with a space character. This field is identical in nature
to the Version field described in Section 3.3. As such, the version,
hash algorithm and signature scheme defined in this document may be
represented as "0111" (without the quote marks).
4.3.3 Reboot Session ID
The Reboot Session ID is identical in characteristics to the RSID
field described in Section 3.4.
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4.3.4 Signature Group and Signature Priority
The SIG field is identical in characteristics to the SIG field
described in Section 3.10. Also, the SPRI field is identical to the
SPRI field described there.
4.3.5 Total Payload Block Length
The Total Payload Block Length is a value representing the total
length of the Payload Block in bytes in decimal. This will be one to
eight bytes.
4.3.6 Index into Payload Block
This is a value between 1 and 8 bytes. It contains the number of
bytes into the Payload Block where this fragment starts. The first
byte of the first fragment is numbered "1".
4.3.7 Fragment Length
The total length of this fragment expressed as a decimal integer.
This will be one to three bytes.
4.3.8 Signature
This is a digital signature, encoded in base-64, as per RFC 2045.
The signature is calculated over all fields but excludes the space
characters between them. The Version field effectively specifies the
original encoding of the signature. The signature is a signature
over the entire data, including all of the PRI, HEADER, and hashes in
the hash block. This is consistent with the method of calculating
the signature as specified in Section 3.10. To reiterate, the
signature is calculated over the completely formatted syslog-message,
excluding spaces between fields, and also excluding this signature
field.
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5. Redundancy and Flexibility
There is a general rule that determines how redundancy works and what
level of flexibility the device and collector have in message
formats: in general, the device is allowed to send Signature and
Certificate Blocks multiple times, to send Signature and Certificate
Blocks of any legal length, to include fewer hashes in hash blocks,
etc.
5.1 Redundancy
Syslog messages are sent over unreliable transport, which means that
they can be lost in transit. However, the collector must receive
Signature and Certificate Blocks or many messages may not be able to
be verified. Sending Signature and Certificate Blocks multiple times
provides redundancy; since the collector MUST ignore Signature/
Certificate Blocks it has already received and authenticated, the
device can in principle change its redundancy level for any reason,
without communicating this fact to the collector.
Although the device isn't constrained in how it decides to send
redundant Signature and Certificate Blocks, or even in whether it
decides to send along multiple copies of normal syslog messages, here
we define some redundancy parameters below which may be useful in
controlling redundant transmission from the device to the collector.
5.1.1 Certificate Blocks
certInitialRepeat = number of times each Certificate Block should be
sent before the first message is sent.
certResendDelay = maximum time delay in seconds to delay before next
redundant sending.
certResendCount = maximum number of sent messages to delay before
next redundant sending.
5.1.2 Signature Blocks
sigNumberResends = number of times a Signature Block is resent.
sigResendDelay = maximum time delay in seconds from original
sending to next redundant sending.
sigResendCount = maximum number of sent messages to delay before
next redundant sending.
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5.2 Flexibility
The device may change many things about the makeup of Signature and
Certificate Blocks in a given reboot session. The things it cannot
change are:
* The version
* The number or arrangements of signature groups
It is legitimate for a device to send out short Signature Blocks, in
order to keep the collector able to verify messages quickly. In
general, unless something verified by the Payload Block or
Certificate Blocks is changed within the reboot session ID, any
change is allowed to the Signature or Certificate Blocks during the
session.
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6. Efficient Verification of Logs
The logs secured with syslog-sign may either be reviewed online or
offline. Online review is somewhat more complicated and
computationally expensive, but not prohibitively so.
6.1 Offline Review of Logs
When the collector stores logs and reviewed later, they can be
authenticated offline just before they are reviewed. Reviewing these
logs offline is simple and relatively cheap in terms of resources
used, so long as there is enough space available on the reviewing
machine. Here, we consider that the stored log files have already
been separated by sender, reboot session ID, and signature group.
This can be done very easily with a script file. We then do the
following:
a. First, we go through the raw log file, and split its contents
into three files. Each message in the raw log file is classified
as a normal message, a Signature Block, or a Certificate Block.
Certificate Blocks and Signature Blocks are stored in their own
files. Normal messages are stored in a keyed file, indexed on
their hash values.
b. We sort the Certificate Block file by index value, and check to
see if we have a set of Certificate Blocks that can reconstruct
the Payload Block. If so, we reconstruct the Payload Block,
verify any key-identifying information, and then use this to
verify the signatures on the Certificate Blocks we've received.
When this is done, we have verified the reboot session and key
used for the rest of the process.
c. We sort the Signature Block file by firstMessageNumber. We now
create an authenticated log file, which consists of some header
information, and then a sequence of message number, message text
pairs. We next go through the Signature Block file. For each
Signature Block in the file, we do the following:
1. Verify the signature on the Block.
2. For each hashed message in the Block:
a. Look up the hash value in the keyed message file.
b. If the message is found, write (message number, message
text) to the authenticated log file.
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3. Skip all other Signature Blocks with the same
firstMessageNumber.
d. The resulting authenticated log file contains all messages that
have been authenticated, and implicitly indicates (by missing
message numbers) all gaps in the authenticated messages.
It's pretty easy to see that, assuming sufficient space for building
the keyed file, this whole process is linear in the number of
messages (generally two seeks, one to write and the other to read,
per normal message received), and O(N lg N) in the number of
Signature Blocks. This estimate comes with two caveats: first, the
Signature Blocks arrive very nearly in sorted order, and so can
probably be sorted more cheaply on average than O(N lg N) steps.
Second, the signature verification on each Signature Block almost
certainly is more expensive than the sorting step in practice. We
haven't discussed error-recovery, which may be necessary for the
Certificate Blocks. In practice, a very simple error-recovery
strategy is probably good enough -- if the Payload Block doesn't come
out as valid, then we can just try an alternate instance of each
Certificate Block, if such are available, until we get the Payload
Block right.
It's easy for an attacker to flood us with plausible-looking
messages, Signature Blocks, and Certificate Blocks.
6.2 Online Review of Logs
Some processes on the collector machine may need to monitor log
messages in something very close to real-time. This can be done with
syslog-sign, though it is somewhat more complex than the offline
analysis. This is done as follows:
a. We have an output queue, into which we write (message number,
message text) pairs which have been authenticated. Again, we'll
assume we're handling only one signature group, and only one
reboot session ID, at any given time.
b. We have three data structures: A queue into which (message
number, hash of message) pairs is kept in sorted order, a queue
into which (arrival sequence, hash of message) is kept in sorted
order, and a hash table which stores (message text, count)
indexed by hash value. In this file, count may be any number
greater than zero; when count is zero, the entry in the hash
table is cleared.
c. We must receive all the Certificate Blocks before any other
processing can really be done. (This is why they're sent first.)
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Once that's done, any Certificate Block that arrives is
discarded.
d. Whenever a normal message arrives, we add (arrival sequence, hash
of message) to our message queue. If our hash table has an entry
for the message's hash value, we increment its count by one;
otherwise, we create a new entry with count = 1. When the
message queue is full, we roll the oldest messages off the queue
by taking the last entry in the queue, and using it to index the
hash table. If that entry has count is 1, we delete the entry in
the hash table; otherwise, we decrement its count. We then
delete the last entry in the queue.
e. Whenever a Signature Block arrives, we first check to see if the
firstMessageNumber value is too old, or if another Signature
Block with that firstMessageNumber has already been received. If
so, we discard the Signature Block unread. Otherwise, we check
its signature, and discard it if the signature isn't valid. A
Signature Block contains a sequence of (message number, message
hash) pairs. For each pair, we first check to see if the message
hash is in the hash table. If so, we write out the (message
number, message text) in the authenticated message queue.
Otherwise, we write the (message number, message hash) to the
message number queue. This generally involves rolling the oldest
entry out of this queue: before this is done, that entry's hash
value is again searched for in the hash table. If a matching
entry is found, the (message number, message text) pair is
written out to the authenticated message queue. In either case,
the oldest entry is then discarded.
f. The result of this is a sequence of messages in the authenticated
message queue, each of which has been authenticated, and which
are combined with numbers showing their order of original
transmission.
It's not too hard to see that this whole process is roughly linear in
the number of messages, and also in the number of Signature Blocks
received. The process is susceptible to flooding attacks; an
attacker can send enough normal messages that the messages roll off
their queue before their Signature Blocks can be processed.
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7. Security Considerations
Normal syslog event messages are unsigned and have most of the
security attributes described in Section 6 of RFC 3164. This
document also describes Certificate Blocks and Signature Blocks which
are signed syslog messages. The Signature Blocks contains signature
information of previously sent syslog event messages. All of this
information may be used to authenticate syslog messages and to
minimize or obviate many of the security concerns described in RFC
3164.
7.1 Cryptography Constraints
As with any technology involving cryptography, you should check the
current literature to determine if any algorithms used here have been
found to be vulnerable to attack.
This specification uses Public Key Cryptography technologies. The
proper party or parties must control the private key portion of a
public-private key pair. Any party that controls a private key may
sign anything they please.
Certain operations in this specification involve the use of random
numbers. An appropriate entropy source should be used to generate
these numbers. See RFC 1750 [8].
7.2 Packet Parameters
The message length must not exceed 1024 bytes. Various problems may
result if a device sends out messages with a length greater than 1024
bytes. As seen in RFC 3164, relays MAY truncate messages with
lengths greater than 1024 bytes which would result in a problem for
receivers trying to validate a hash of the packet. In this case, as
with all others, it is best to be conservative with what you send but
liberal in what you receive, and accept more than 1024 bytes.
Similarly, senders must rigidly enforce the correctness of the
message body. This document specifies an enhancement to the syslog
protocol but does not stipulate any specific syslog message format.
Nonetheless, problems may arise if the receiver does not fully accept
the syslog packets sent from a device, or if it has problems with the
format of the Certificate Block or Signature Block messages.
Finally, receivers must not malfunction if they receive syslog
messages containing characters other than those specified in this
document.
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7.3 Message Authenticity
Event messages being sent through syslog do not strongly associate
the message with the message sender. That fact is established by the
receiver upon verification of the Signature Block as described above.
Before a Signature Block is used to ascertain the authenticity of an
event message, it may be received, stored and reviewed by a person or
automated parser. Both of these should maintain doubt about the
authenticity of the message until after it has been validated by
checking the contents of the Signature Block.
With the Signature Block checking, an attacker may only forge
messages if they can compromise the private key of the true sender.
7.4 Sequenced Delivery
Event messages may be recorded and replayed by an attacker. However
the information contained in the Signature Blocks allows a reviewer
to determine if the received messages are the ones originally sent by
a device. This process also alerts the reviewer to replayed
messages.
7.5 Replaying
Event messages may be recorded and replayed by an attacker. However
the information contained in the Signature Blocks will allow a
reviewer to determine if the received messages are the ones
originally sent by a device. This process will also alert the
reviewer to replayed messages.
7.6 Reliable Delivery
RFC 3195 may be used for the reliable delivery of all syslog
messages. This document acknowledges that event messages sent over
UDP may be lost in transit. A proper review of the Signature Block
information may pinpoint any messages sent by the sender but not
received by the receiver. The overlap of information in subsequent
Signature Block information allows a reviewer to determine if any
Signature Block messages were also lost in transit.
7.7 Sequenced Delivery
Related to the above, syslog messages delivered over UDP not only may
be lost, but they may arrive out of sequence. The information
contained in the Signature Block allows a receiver to correctly order
the event messages. Beyond that, the timestamp information contained
in the packet may help the reviewer to visually order received
messages even if they are received out of order.
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7.8 Message Integrity
syslog messages may be damaged in transit. A review of the
information in the Signature Block determines if the received message
was the intended message sent by the sender. A damaged Signature
Block or Certificate Block will be evident since the receiver will
not be able to validate that it was signed by the sender.
7.9 Message Observation
Event messages, Certificate Blocks and Signature Blocks are all sent
in plaintext. Generally this has had the benefit of allowing network
administrators to read the message when sniffing the wire. However,
this also allows an attacker to see the contents of event messages
and perhaps to use that information for malicious purposes.
7.10 Man In The Middle
It is conceivable that an attacker may intercept Certificate Blocks
and insert their own Certificate information. In that case, the
attacker would be able to receive event messages from the actual
sender and then relay modified messages, insert new messages, or
deleted messages. They would then be able to construct a Signature
Block and sign it with their own private key. The network
administrators should verify that the key contained in the
Certificate Block is indeed the key being used on the actual device.
If that is indeed the case, then this MITM attack will not succeed.
7.11 Denial of Service
An attacker may be able to overwhelm a receiver by sending it invalid
Signature Block messages. If the receiver is attempting to process
these messages online, it may consume all available resources. For
this reason, it may be appropriate to just receive the Signature
Block messages and process them as time permits.
As with any system, an attacker may also just overwhelm a receiver by
sending more messages to it than can be handled by the infrastructure
or the device itself. Implementors should attempt to provide
features that minimize this threat. Such as only receiving syslog
messages from known IP addresses.
7.12 Covert Channels
Nothing in this protocol attempts to eliminate covert channels.
Indeed, the unformatted message syntax in the packets could be very
amenable to sending embedded secret messages. In fact, just about
every aspect of syslog messages lends itself to the conveyance of
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covert signals. For example, a collusionist could send odd and even
PRI values to indicate Morse Code dashes and dots.
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8. IANA Considerations
Two syslog packet types are specified in this document; the Signature
Block and the Certificate Block. Each of these has several fields
specified that should be controlled by the IANA. Essentially these
packet types may be differentiated based upon the value in the Cookie
field. The Signature Block packet may be identified by a value of
"@#sigSIG" in the Cookie field. The Certificate Block packet may be
identified by a value of "@#sigCER" in the Cookie field. Each of
these packet types share fields that should be consistent;
specifically, the Certificate Block packet types may be considered to
be an announcement of capabilities and the Signature Block packets
SHOULD have the same values in the fields described in this section.
This document allows that there may be some really fine reason for
the values to be different between the two packet types but the
authors and contributors can't see any valid reason for that at this
time.
This document also upholds the Facilities and Severities listed in
RFC 3164 [20]. Those values range from 0 to 191. This document also
instructs the IANA to reserve all other possible values of the
Severities and Facilities above the value of 191 and to distribute
them via the consensus process as defined in RFC 2434 [17].
The following fields are to be controlled by the IANA in both the
Signature Block packets and the Certificate Block packets.
8.1 Version Field
The Version field (Ver) is a 4 byte field. The first two bytes of
this field define the version of the Signature Block packets and the
Certificate Block Packets. This allows for future efforts to
redefine the subsequent fields in the Signature Block packets and
Certificate Block packets. A value of "00" is reserved and not used.
This document describes the fields for the version value of "01". It
is expected that this value be incremented monotonically with decimal
values up through "50" for IANA assigned values. Values "02" through
"50" will be assigned by the IANA using the "IETF Consensus" policy
defined in RFC 2434 [17]. It is not anticipated that these values
will be reused. Values of "51" through "99" will be vendor-specific,
and values in this range are not to be assigned by the IANA.
In the case of vendor-specific assigned Version numbers, all
subsequent values defined in the packet will then have vendor-
specific meaning. They may, or may not, align with the values
assigned by the IANA for these fields. For example, a vendor may
choose to define their own Version of "51" still containing values of
"1" for the Hash Algorithm and Signature Scheme which aligns with the
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IANA assigned values as defined in this document. However, they may
then choose to define a value of "5" for the Signature Group for
their own reasons.
The third byte of the Ver field defines the Hash Algorithm. It is
envisioned that this will also be a monotonically increasing value
with a maximum value of "9". The value of "1" is defined in this
document as the first assigned value and is SHA1 FIPS-180-1.1995 [2].
Subsequent values will be assigned by the IANA using the "IETF
Consensus" policy defined in RFC 2434 [17].
The forth and final byte of the Ver field defines the Signature
Scheme. It is envisioned that this too will be a monotonically
increasing value with a maximum value of "9". The value of "1" is
defined in this document as OpenPGP DSA - RFC 2440 [18], FIPS.186-
1.1998 [1]. Subsequent values will be assigned by the IANA using the
"IETF Consensus" policy defined in RFC 2434 [17]. The fields, values
assigned in this document and ranges are illustrated in the following
table.
Field Value Defined IANA Assigned Vendor Specific
in this Document Range Range
----- ---------------- ------------- ---------------
Ver
ver 01 01-50 50-99
hash 1 0-9 -none-
sig 1 0-9 -none-
If either the Hash Algorithm field or the Signature Scheme field is
needed to go beyond "9" within the current version (first two bytes),
the IANA should increment the first two bytes of this 4 byte field to
be the next value with the definition that all of the subsequent
values of fields described in this section are reset to "0" while
retaining the latest definitions given by the IANA. For example,
consider the case that the first two characters are "23" and the
latest Signature Algorithm is 4. Let's say that the latest Hash
Algorithm value is "9" but a better Hash Algorithm is defined. In
that case, the IANA will increment the first two bytes to become
"24", retain the current Hash Algorithm to be "0", define the new
Hash Algorithm to be "1" in this scheme, and define the current
Signature Scheme to also be "0". This example is illustrated in the
following table.
Current New - Equivalent New with Later
to "Current" Algorithms
------- -------------- ---------------
ver = 23 ver = 24 ver = 24
hash = 9 hash = 0 hash = 1
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sig = 4 sig = 0 sig = 0
8.2 SIG Field
The SIG field values are numbers as defined in Section 3.5. Values
"0" through "3" are assigned in this document. The IANA shall assign
values "4" through "7" using the "IETF Consensus" policy defined in
RFC 2434 [17]. Values "8" and "9" shall be left as vendor specific
and shall not be assigned by the IANA.
8.3 Key Blob Type
Section Section 4.2 defines five, one character identifiers for the
key blob type. These are the uppercase letters, "C", "P", "K", "N",
and "U". All other uppercase letters shall be assigned by the IANA
using the "IETF Consensus" policy defined in RFC 2434 [17].
Lowercase letters are left as vendor specific and shall not be
assigned by the IANA.
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9. Authors and Working Group Chair
The working group can be contacted via the mailing list:
syslog-sec@employees.org
The current Chair of the Working Group may be contacted at:
Chris Lonvick
Cisco Systems
Email: clonvick@cisco.com
The authors of this draft are:
John Kelsey
Email: kelsey.j@ix.netcom.com
Jon Callas
Email: jon@callas.org
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10. Acknowledgements
The authors wish to thank Alex Brown, Chris Calabrese, Carson Gaspar,
Drew Gross, Chris Lonvick, Darrin New, Marshall Rose, Holt Sorenson,
Rodney Thayer, Andrew Ross, Rainer Gerhards, Albert Mietus, and the
many Counterpane Internet Security engineering and operations people
who commented on various versions of this proposal.
11. References
[1] National Institute of Standards and Technology, "Digital
Signature Standard", FIPS PUB 186-1, December 1998,
<http://csrc.nist.gov/fips/fips1861.pdf>.
[2] National Institute of Standards and Technology, "Secure Hash
Standard", FIPS PUB 180-1, April 1995,
<http://www.itl.nist.gov/fipspubs/fip180-1.htm>.
[3] American National Standards Institute, "USA Code for
Information Interchange", ANSI X3.4, 1968.
[4] Menezes, A., van Oorschot, P., and S. Vanstone, ""Handbook of
Applied Cryptography", CRC Press", 1996.
[5] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[6] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[7] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[8] Eastlake, D., Crocker, S., and J. Schiller, "Randomness
Recommendations for Security", RFC 1750, December 1994.
[9] Malkin, G., "Internet Users' Glossary", RFC 1983, August 1996.
[10] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies",
RFC 2045, November 1996.
[11] Oehler, M. and R. Glenn, "HMAC-MD5 IP Authentication with
Replay Prevention", RFC 2085, February 1997.
[12] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing
for Message Authentication", RFC 2104, February 1997.
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[13] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[14] Yergeau, F., "UTF-8, a transformation format of ISO 10646",
RFC 2279, January 1998.
[15] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[16] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 2373, July 1998.
[17] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[18] Callas, J., Donnerhacke, L., Finney, H., and R. Thayer,
"OpenPGP Message Format", RFC 2440, November 1998.
[19] Blumenthal, U. and B. Wijnen, "User-based Security Model (USM)
for version 3 of the Simple Network Management Protocol
(SNMPv3)", RFC 2574, April 1999.
[20] Lonvick, C., "The BSD Syslog Protocol", RFC 3164, August 2001.
[21] New, D. and M. Rose, "Reliable Delivery for syslog", RFC 3195,
November 2001.
[22] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, July 2002.
[23] Schneier, B., "Applied Cryptography Second Edition: protocols,
algorithms, and source code in C", 1996.
Authors' Addresses
John Kelsey
Email: kelsey.j@ix.netcom.com
Jon Callas
PGP Corporation
Email: jon@callas.org
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