syslog Working Group J. Kelsey
Internet-Draft NIST
Intended status: Standards Track J. Callas
Expires: March 31, 2008 PGP Corporation
A. Clemm
Cisco Systems
September 28, 2007
Signed syslog Messages
draft-ietf-syslog-sign-23.txt
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Copyright Notice
Copyright (C) The IETF Trust (2007).
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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.
This specification is intended to be used in conjunction with the
work defined in RFC xxxx, "The syslog Protocol".
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions Used in this Document . . . . . . . . . . . . . . 6
3. syslog Message Format . . . . . . . . . . . . . . . . . . . . 7
4. Signature Blocks . . . . . . . . . . . . . . . . . . . . . . . 8
4.1. syslog Messages Containing a Signature Block . . . . . . . 8
4.2. Signature Block Format and Fields . . . . . . . . . . . . 8
4.2.1. Version . . . . . . . . . . . . . . . . . . . . . . . 9
4.2.2. Reboot Session ID . . . . . . . . . . . . . . . . . . 10
4.2.3. Signature Group and Signature Priority . . . . . . . . 10
4.2.4. Global Block Counter . . . . . . . . . . . . . . . . . 12
4.2.5. First Message Number . . . . . . . . . . . . . . . . . 13
4.2.6. Count . . . . . . . . . . . . . . . . . . . . . . . . 14
4.2.7. Hash Block . . . . . . . . . . . . . . . . . . . . . . 14
4.2.8. Signature . . . . . . . . . . . . . . . . . . . . . . 14
5. Payload and Certificate Blocks . . . . . . . . . . . . . . . . 15
5.1. Preliminaries: Key Management and Distribution Issues . . 15
5.2. Payload Block . . . . . . . . . . . . . . . . . . . . . . 16
5.3. Certificate Block . . . . . . . . . . . . . . . . . . . . 16
5.3.1. syslog Messages Containing a Certificate Block . . . . 16
5.3.2. Certificate Block Format and Fields . . . . . . . . . 17
6. Redundancy and Flexibility . . . . . . . . . . . . . . . . . . 20
6.1. Redundancy . . . . . . . . . . . . . . . . . . . . . . . . 20
6.1.1. Configuration Parameters for Certificate Blocks . . . 20
6.1.2. Configuration Parameters for Signature Blocks . . . . 20
6.2. Flexibility . . . . . . . . . . . . . . . . . . . . . . . 21
7. Efficient Verification of Logs . . . . . . . . . . . . . . . . 22
7.1. Offline Review of Logs . . . . . . . . . . . . . . . . . . 22
7.2. Online Review of Logs . . . . . . . . . . . . . . . . . . 23
8. Security Considerations . . . . . . . . . . . . . . . . . . . 25
8.1. Cryptographic Constraints . . . . . . . . . . . . . . . . 25
8.2. Packet Parameters . . . . . . . . . . . . . . . . . . . . 25
8.3. Message Authenticity . . . . . . . . . . . . . . . . . . . 26
8.4. Replaying . . . . . . . . . . . . . . . . . . . . . . . . 26
8.5. Reliable Delivery . . . . . . . . . . . . . . . . . . . . 26
8.6. Sequenced Delivery . . . . . . . . . . . . . . . . . . . . 26
8.7. Message Integrity . . . . . . . . . . . . . . . . . . . . 26
8.8. Message Observation . . . . . . . . . . . . . . . . . . . 27
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8.9. Man In The Middle Attacks . . . . . . . . . . . . . . . . 27
8.10. Denial of Service . . . . . . . . . . . . . . . . . . . . 27
8.11. Covert Channels . . . . . . . . . . . . . . . . . . . . . 27
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
9.1. Structured Data and syslog messages . . . . . . . . . . . 28
9.2. Version Field . . . . . . . . . . . . . . . . . . . . . . 28
9.3. SG Field . . . . . . . . . . . . . . . . . . . . . . . . . 30
9.4. Key Blob Type . . . . . . . . . . . . . . . . . . . . . . 30
10. Working Group . . . . . . . . . . . . . . . . . . . . . . . . 31
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 32
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 33
12.1. Normative References . . . . . . . . . . . . . . . . . . . 33
12.2. Informative References . . . . . . . . . . . . . . . . . . 34
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 35
Intellectual Property and Copyright Statements . . . . . . . . . . 36
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1. Introduction
This document describes a mechanism, called syslog-sign in this
document, that adds origin authentication, message integrity, replay
resistance, message sequencing, and detection of missing messages to
syslog. Essentially, this is accomplished by sending a special
syslog message. The contents of this syslog message is called a
Signature Block. Each Signature Block contains, in effect, a
detached signature on some number of previously sent messages. It is
cryptographically signed and contains the hashes of previously sent
syslog messages.
While most implementations of syslog involve only a single originator
and a single collector of each message, provisions need to be made to
cover situations in which messages are sent to multiple collectors.
This concerns, in particular, situations in which different messages
are sent to different collectors, which means that some messages are
sent to some collectors but not to others. The required
differentiation of messages is generally performed based on 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 originator. A Signature Block always belongs to exactly one
signature group and always signs messages belonging only to that
signature group.
Additionally, a originator sends a Certificate Block to provide key
management information between the originator and the collector.
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 the cases of certificates being sent, the
certificates may have to be split across multiple packets.
The collector of the previous messages may verify that the hash of
each received message matches the signed hash 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.
The mechanism described in this specification is intended to be used
in conjunction with the syslog protocol as defined in RFC xxxx [8] as
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its message delivery mechanism and uses the concept of STRUCTURED-
DATA elements defined in that document. In fact, this specification
mandates implementation of syslog protocol. Nevertheless, it is
conceivable that the concepts underlying this mechanism could also be
used in conjunction with other message delivery mechanisms.
Designers of other efforts to define event notification mechanisms
are therefore encouraged to consider this specification in their
designs.
NOTE to RFC editor: replace xxxx with the actual RFC number assigned
to [8], replace zzzz with the actual RFC number assigned to [9],
replace wwww with the actual RFC number assigned to [10], replace
yyyy with the actual RFC number assigned to this document, and remove
this note.
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2. Conventions Used in this Document
The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [11].
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3. syslog Message Format
This specification is intended to be used in conjunction with the
syslog protocol as defined in RFC xxxx [8]. The syslog protocol
therefore MUST be supported by implementations of this specification.
Because the originator generating the Signature Block message signs
each message in its entirety, the messages MUST NOT be changed in
transit. By the same token, the syslog-sign messages MUST NOT be
changed in transit. Specifically, a relay as described in RFC xxxx
[8] MAY make changes to a syslog packet. If this occurs, the
mechanism described in this document is rendered useless. Likewise,
any truncation of messages that occurs between sending and receiving
renders the mechanism useless. For this reason, syslog originator
and collector implementations implementing this specification MUST
support messages of up to and including 2048 octets in length, in
order to minimize the chance of truncation. While syslog originator
and collector implementations MAY support messages with a length
larger than 2048 octets, implementors need to be aware that any
message truncations that occur render the mechanism useless.
This specification uses the syslog message format described in RFC
xxxx [8]. Along with other fields, that document describes the
concept of Structured Data (SD). Structured Data is defined in terms
of SD ELEMENTS (SDEs). An SDE consists of a name and a set of
parameter name - value pairs. The SDE name is referred to as SD-ID.
The name-value pairs are referred to as SD-PARAM, or SD Parameters,
with the name constituting the SD-PARAM-NAME, and the value
constituting the SD-PARAM-VALUE.
The syslog messages defined in this document carry the signature and
certificate data as Structured Data. The special syslog messages
defined in this document include for this purpose definitions of SDEs
to convey parameters that relate to the signing of syslog messages.
The MSG part of the syslog messages defined in this document SHOULD
simply be empty -- the content of the messages is not intended for
interpretation by humans but by applications that use those messages
to build an authenticated log.
Because the syslog messages defined in this document adhere to the
format described in RFC xxxx [8], they identify the machine that
originates the syslog message in the HOSTNAME field. Therefore, the
signature and certificate data do not need to include an additional
parameter to identify the machine that orginates the message.
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4. Signature Blocks
This section describes the format of the Signature Block and the
fields used within the Signature Block, as well as the syslog
messages used to carry the Signature Block.
4.1. syslog Messages Containing a Signature Block
There is a need to distinguish the Signature Block itself from the
syslog message that is used to carry a Signature Block. Signature
Blocks MUST be encompassed within completely formed syslog messages.
Syslog messages that contain a Signature Block are also referred to
as Signature Block messages.
A Signature Block message is identified by the presence of an SD
ELEMENT with an SD-ID with the value "ssign". In addition, a
Signature Block message MUST contain valid APP-NAME, PROCID, and
MSGID fields to be compliant with RFC xxxx [8]. This specification
does not mandate particular values for these fields; however, for
consistency, originators SHOULD use the same values for APP-NAME,
PROCID, and MSGID fields for every Signature Block message that is
sent, whichever values are chosen. It is RECOMMENDED (but not
required) to use 110 as value for the PRI field, corresponding to
facility 13 and severity 6 (informational). The Signature Block is
carried as Structured Data within the Signature Block message, per
the definitions that follow in the next section. A Signature Block
message SHOULD NOT carry other Structured Data besides the Structured
Data of the Signature Block itself.
The syslog messages defined as part of syslog-sign themselves
(Signature Block messages and Certificate Block messages) do not need
to be signed by a Signature Block. Collectors that implement syslog-
sign know to distinguish syslog messages that are associated with
syslog-sign from those that are subjected to signing and process them
differently.
4.2. Signature Block Format and Fields
The content of a Signature Block message is the Signature Block. The
Signature Block MUST be encoded as an SD ELEMENT, as defined in RFC
xxxx [8].
The SD-ID MUST have the value of "ssign".
The SDE contains the fields of the Signature Block encoded as SD
Parameters, as specified in the following. The Signature Block is
composed of the following fields. The value of each field MUST be
printable ASCII, and any binary values MUST be base 64 encoded, as
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defined in RFC 4648 [7].
Field SD-PARAM-NAME Size in octets
----- ------------- ---- -- ------
Version VER 4
Reboot Session ID RSID 1-10
Signature Group SG 1
Signature Priority SPRI 1-3
Global Block Counter GBC 1-10
First Message Number FMN 1-10
Count CNT 1-2
Hash Block HB variable, size of hash
times the number of hashes
(base 64 encoded binary)
Signature SIGN variable
(base 64 encoded binary)
A Signature Block is accordingly encoded as follows, where xxx
denotes a placeholder for the particular values:
[ssign VER="xxx" RSID="xxx" SG="xxx" SPRI="xxx" GBC="xxx" FMN="xxx"
CNT="xxx" HB="xxx" SIGN="xxx"]
Values of the fields constitute SD parameter values and are hence
enclosed in quotes, per RFC xxxx [8]. The fields are separated by
single spaces and are described below.
4.2.1. Version
The Signature Block Version field is a decimal value that has a
length of 4 octets, which may include leading zeroes. Each octet
contains a decimal character in the range of "0" to "9". 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 octets), the hash
algorithm (1 octet) and the signature scheme (1 octet).
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Protocol Version - 2 octets, with "01" as the value for the
protocol version that is described in this document.
Hash Algorithm - 1 octet, where, in conjunction with Protocol
Version 01, a value of "1" denotes SHA1 and a value of "2" denotes
SHA256, as defined in FIPS-180-2.2002 [2].
Signature Scheme - 1 octet, where, in conjunction with Protocol
Version 01, a value of "1" denotes OpenPGP DSA, defined in RFC
2440 [5] and FIPS.186-2.2000 [1].
The version, hash algorithm and signature scheme defined in this
document would accordingly be represented as "0111" (if SHA1 is used
as Hash Algorithm) and "0121" (if SHA256 is used as Hash Algorithm),
respectively (without the quotation marks).
The values of the Hash Algorithm and Signature Scheme are defined
relative to the Protocol Version. If the single-octet representation
of the values for Hash Algorithm and Signature Scheme were to ever
represent a limitation, this limitation could be overcome by defining
a new Protocol Version with additional Hash Algorithms and/or
Signature Schemes, and having implementations support both Protocol
Versions concurrently.
4.2.2. Reboot Session ID
The Reboot Session ID is a decimal value that has a length between 1
and 10 octets. The acceptable values for this are between 0 and
9999999999. Leading zeroes MUST be omitted. A Reboot Session ID is
expected to increase whenever an originator reboots in order to allow
collectors to distinguish messages and message signatures across
reboots. Hence, an originator needs to retain the previous Reboot
Session ID across reboots. In cases where an originator does not
support this capability, the Reboot Session ID MUST always be set to
a value of 0, which indicates that this capability is not supported.
Otherwise, it MUST increase whenever an originator reboots, starting
with a value of 1. If the value reaches 9999999999, then manual
intervention may be required to subsequently reset it to 1.
Implementors MAY wish to consider using the snmpEngineBoots value as
a source for this counter as defined in RFC 3414 [6].
4.2.3. Signature Group and Signature Priority
The SG parameter may take any value from 0-3 inclusive. The SPRI
parameter may take any value from 0-191 inclusive. These fields
taken together allow network administrators to associate groupings of
syslog messages with appropriate Signature Blocks and Certificate
Blocks. Groupings of syslog messages that are signed together are
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also called Signature Groups. A Signature Block contains only hashes
of those syslog messages that are part of the same Signature Group.
For example, in some cases, network administrators might have
originators send syslog messages of Facilities 0 through 15 to one
collector and those with Facilities 16 through 23 to another. In
such cases, associated Signature Blocks should likely be sent to the
corresponding collectors as well, signing the syslog messages that
are intended for each collector separately. This way, each collector
receives Signature Blocks for all syslog messages that it receives,
and only for those. The ability to associate different categories of
syslog messages with different Signature Groups, signed in separate
Signature Blocks, provides administrators with flexibility in this
regard.
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 corresponding syslog messages. In all cases,
no more than 192 distinct Signature Groups (0-191) are permitted.
The Signature Group to which a Signature Block pertains is indicated
by the Signature Priority (SPRI) field. The Signature Group (SG)
field indicates how to interpret the Signature Priority field. (Note
that the SG field does not indicate the Signature Group itself, as
its name might suggest.) The SG field can have one of the following
values:
a. "0" -- There is only one Signature Group. 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. Signature Blocks sign
all messages regardless of their PRI value. This means that, in
effect, the Signature Block's SPRI value can be ignored.
However, it is RECOMMENDED that a single SPRI value be used for
all Signature Blocks. Furthermore, it is RECOMMENDED to set that
value to the same value as the PRI field of the Signature Block
message. This way, the PRI of the Signature Block message
matches the SPRI of the Signature Block that it contains.
b. "1" -- Each PRI value is associated with its own Signature Group.
Signature Blocks for a given Signature Group have SPRI = PRI for
that Signature Group. In other words, the SPRI of the Signature
Block matches the PRI value of the syslog messages that are part
of the Signature Group and hence signed by the Signature Block.
An SG value of 1 can, for example, be used when the administrator
of an originator does not know where any of the syslog messages
will ultimately go but anticipates that messages with different
PRI values will be collected and processed separately. Having a
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Signature Group per PRI value provides administrators with a
large degree of flexibility with regard to how to divide up the
processing of syslog messages and their signatures after they are
received, at the same time allowing Signature Blocks to follow
the corresponding syslog messages to their eventual destination.
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 of syslog messages in that Signature Group.
The lowest PRI value of syslog messages in that Signature Group
will be one larger than the SPRI value of the previous Signature
Group or "0" in case there is no other Signature Group with a
lower SPRI value. The specific Signature Groups and ranges they
are associated with are subject to configuration by a system
administrator.
d. "3" -- Signature Groups are not assigned with any of the above
relationships to PRI values of the syslog messages they sign.
Instead, another scheme is used, which is outside the scope of
this specification. There has to be some predefined arrangement
between the originator and the intended collectors as to which
syslog messages are to be included in which Signature Group,
requiring configuration by a system administrator. This provides
administrators also with the flexibility to group syslog messages
into Signature Groups according to criteria that are not tied to
the PRI value.
One reasonable way to configure some installations is to have only
one Signature Group, indicated with SG=0, and have the originator
send a copy of each Signature Block to each collector. In that case,
collectors that are not configured to receive every syslog message
will still receive signatures for every message, even ones they are
not supposed to receive. While the collector will not be able to
detect gaps in the messages (because the presence of a signature of a
message that is missing does not tell the collector whether or not
the corresponding message would be of the collector's concern), it
does allow all messages that do arrive at each collector to be put
into the right order and to be verified. It also allows each
collector to detect duplicates. Likewise, configuring only one
Signature Group can be a reasonable way to configure installations
that involve relay chains, where one or more interim relays may or
may not relay all messages to the same destination.
4.2.4. Global Block Counter
The Global Block Counter is a decimal value representing the number
of Signature Blocks sent by syslog-sign before the current one, in
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this reboot session. This takes at least 1 octet and at most 10
octets displayed as a decimal counter. The acceptable values for
this are between 0 and 9999999999, starting with 0. Leading zeroes
MUST be omitted. If the value of the Global Block Counter has
reached 9999999999 and the Reboot Session ID has a value other than 0
(indicating the fact that persistence of the Reboot Session ID is
supported), then the Reboot Session ID MUST be incremented by 1 and
the Global Block Counter resumes at 0. When the Reboot Session ID is
0 (i.e., persistent Reboot Session IDs are not supported) and the
Global Block Counter reaches its maximum value, then the Global Block
Counter is reset to 0 and the Reboot Session ID MUST remain at 0.
Note that the Global Block Counter crosses Signature Groups; it
allows one to roughly synchronize when two messages were sent, even
though they went to different collectors and are part of different
Signature Groups.
Because a reboot results in the start of a new reboot session, the
originator MUST reset the Global Block Counter to 0 after a reboot
occurs. Applications need to take into account the possibility that
a reboot occurred when authenticating a log, and situations in which
reboots occur frequently may result in losing the ability to verify
the proper sequence in which messages were sent, hence jeopardizing
the integrity of the log.
4.2.5. First Message Number
This is a decimal value between 1 and 10 octets, with leading zeroes
omitted. 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 is numbered "1". This
implies that when the Reboot Session ID increases, the message number
is reset to 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. The message
number is relative to the Signature Group to which it belongs; hence,
a message number does not identify a message beyond its Signature
Group.
Should the message number reach 9999999999 within the same reboot
session and Signature Group, the message number subsequently restarts
at 1. In such event, the Global Block Counter will be vastly
different between two occurrences of the same message number.
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4.2.6. Count
The count is a 1 or 2 octet field that indicates the number of
message hashes to follow. The valid values for this field are 1
through 99. The number of hashes included in the Signature Block
MUST be chosen such that the length of the resulting syslog message
does not exceed the maximum permissible syslog message length.
4.2.7. 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, independent of the underlying
transport. Hashes are ordered from left to right in the order of
occurrence of the syslog messages that they represent.
The "entire syslog message" refers to what is described as the syslog
message excluding transport parts that are described in RFC zzzz [9]
and RFC wwww [10], and excluding other parts that may be defined in
future transports. The hash value will be the result of the hashing
algorithm run across the syslog message, starting with the "<" of the
PRI portion of the header part of the message. The hash algorithm
used and indicated by the Version field determines the size of each
hash, but the size MUST NOT be shorter than 160 bits without the use
of padding. It is base 64 encoded as per RFC 4648 [7].
The number of hashes in a hash block SHOULD be chosen such that the
resulting Signature Block message does not exceed a length of 2048
octets in order to avoid the possibility that truncation occurs.
When more hashes need to be sent than fit inside a Signature Block
message, it is advisable to start a new Signature Block.
4.2.8. Signature
This is a digital signature, encoded in base 64 per RFC 4648 [7].
The signature is calculated over the completely formatted syslog-
message, including all of the PRI, HEADER, and hashes in the hash
block, excluding spaces between fields, and also excluding the
signature field (SD Parameter Name "SIGN", "=", and corresponding
value).
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5. Payload and Certificate Blocks
Certificate Blocks and Payload Blocks provide key management for
syslog-sign. Their purpose is to support key management that uses
public key cryptosystems.
5.1. Preliminaries: Key Management and Distribution Issues
A Payload Block contains public key certificate information that is
to be conveyed to the collector. A Payload Block is sent at the
beginning of a new reboot session, carrying public key information in
effect for the reboot session. However, a Payload Block is not sent
directly, but in (one or more) fragments. Those fragments are termed
Certificate Blocks. Therefore, originators send at least one
Certificate Block at the beginning of a new 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. There is exactly one Payload Block per
reboot session.
b. The Certificate Blocks are digitally signed. The originator 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, because some other
fields of the Payload Block are not otherwise verified.) In
practice, most installations keep the same public key over long
periods of time, so that most of the time, it is 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
originator and collector (or offline log viewer) both have 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.
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5.2. Payload Block
The Payload Block is built when a new reboot session is started.
There is a one-to-one correspondence between reboot sessions and
Payload Blocks. An originator creates a new Payload Block after each
reboot. The Payload Block is used until the next reboot. A Payload
Block MUST have the following fields:
a. Full local time stamp for the originator at the time the reboot
session started. This must be in the time stamp format specified
in RFC xxxx [8] (essentially, time stamp format per RFC 3339 [12]
with some further restrictions).
b. Key Blob Type, a one-octet field containing 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
c. The key blob, if any, base 64 encoded per RFC 4648 [7] and
consisting of the raw key data.
The fields are separated by single space characters. Because a
Payload Block is not carried in a syslog message directly, only the
corresponding Certificate Blocks, it does not need to be encoded as
an SD ELEMENT. The Payload Block does not contain a field that
identifies the reboot session; instead, the reboot session can be
inferred from the Reboot Session ID parameter of the Certificate
Blocks that are used to carry the Payload Block.
5.3. Certificate Block
This section describes the format of the Certificate Block and the
fields used within the Certificate Block, as well as the syslog
messages used to carry Certificate Blocks.
5.3.1. syslog Messages Containing a Certificate Block
Certificate Blocks are used to get the Payload Block to the
collector. As with a Signature Block, each Certificate Block is
carried in its own syslog message, called Certificate Block message.
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Because certificates can legitimately be much longer than 2048
octets, the Payload Block can be split up into several pieces, with
each Certificate Block carrying a piece of the Payload Block. Note
that the originator MAY make the Certificate Blocks of any legal
length (that is, any length that keeps the entire Certificate Block
message within 2048 octets) that holds all the required fields.
Software that processes Certificate Blocks MUST deal correctly with
blocks of any legal length. The length of the fragment of the
Payload Block that a Certificate Block carries MUST be at least 1
octet. The length SHOULD be chosen such that the length of the
Certificate Block message does not exceed 2048 octets.
A Certificate Block message is identified by the presence of an SD
ELEMENT with an SD-ID with the value "ssign-cert". In addition, a
Certificate Block message MUST contain valid APP-NAME, PROCID, and
MSGID fields to be compliant with syslog protocol. Syslog-sign does
not mandate particular values for these fields; however, for
consistency, implementations SHOULD use the same value for APP-NAME,
PROCID, and MSGID fields for every Certificate Block message,
whichever values are chosen. It is RECOMMENDED to use 110 as value
for the PRI field, corresponding to facility 13 and severity 6
(informational). The Certificate Block is carried as Structured Data
within the Certificate Block message. A Certificate Block message
SHOULD NOT carry other Structured Data besides the Structured Data of
the Certificate Block itself. The MSG part of a Certificate Block
message SHOULD be empty.
5.3.2. Certificate Block Format and Fields
The contents of a Certificate Block message is the Certificate Block
itself. Like a Signature Block, the Certificate Block is encoded as
an SD ELEMENT. The SD-ID of the Certificate Block is "ssign-cert".
The Certificate Block is composed of the following fields, each of
which is encoded as an SD Parameter with parameter name as indicated.
Each field must be printable ASCII, and any binary values are base 64
encoded per RFC 4648 [7].
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Field SD-PARAM-NAME Size in octets
----- ------------- ---- -- ------
Version VER 4
Reboot Session ID RSID 1-10
Signature Group SG 1
Signature Priority SPRI 1-3
Total Payload Block Length TPBL 1-8
Index into Payload Block INDEX 1-8
Fragment Length FLEN 1-4
Payload Block Fragment FRAG variable
(base 64 encoded binary)
Signature SIGN variable
(base 64 encoded binary)
A Certificate Block is accordingly encoded as follows, where xxx
denotes a placeholder for the particular values:
[ssign-cert VER="xxx" RSID="xxx" SG="xxx" SPRI="xxx" TBPL="xxx"
INDEX="xxx" FLEN="xxx" FRAG="xxx" SIGN="xxx"]
Values of the fields constitute SD parameter values and are hence
enclosed in quotes, per RFC xxxx [8]. The fields are separated by
single spaces and are described below.
5.3.2.1. Version
The Signature Group version field is 4 octets in length. This field
is identical in format and meaning to the Version field described in
Section 4.2.1.
5.3.2.2. Reboot Session ID
The Reboot Session ID is identical in format and meaning to the RSID
field described in Section 4.2.2.
5.3.2.3. Signature Group and Signature Priority
The SIG field is identical in format and meaning to the SIG field
described in Section 4.2.3. The SPRI field is identical in format
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and meaning to the SPRI field described there.
5.3.2.4. Total Payload Block Length
The Total Payload Block Length is a value representing the total
length of the Payload Block in octets, expressed as a decimal with
one to eight octets.
5.3.2.5. Index into Payload Block
This is a decimal value between 1 and 8 octets, with leading zeroes
omitted. It contains the number of octets into the Payload Block at
which this fragment starts. The first octet of the first fragment is
numbered "1".
5.3.2.6. Fragment Length
The total length of this fragment expressed as a decimal integer with
one to four octets. The fragment length must be at least 1.
5.3.2.7. Payload Block Fragment
The Payload Block Fragment contains a fragment of the payload block,
encoded in base 64, as per RFC 4648 [7]. Its length must match the
indicated fragment length.
5.3.2.8. Signature
This is a digital signature, encoded in base 64, as per RFC 4648 [7].
The Version field effectively specifies the original encoding of the
signature. The signature is calculated over the completely formatted
syslog message, including all of the PRI, HEADER, and certificate
block, excluding spaces between fields, and also excluding the
signature field itself (SD Parameter Name "SIGN", "=", and
corresponding value).
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6. Redundancy and Flexibility
There is a general rule that determines how redundancy works and what
level of flexibility the originator and collector have in message
formats: in general, the originator 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.
6.1. Redundancy
Syslog messages are in general sent over unreliable transport, which
means that they can be lost in transit. However, if a collector does
not receive Signature and Certificate Blocks, many messages may not
be able to be verified. Sending Signature and Certificate Blocks
multiple times provides redundancy; because the collector MUST ignore
Signature/Certificate Blocks it has already received and
authenticated, the originator can in principle change its redundancy
level for any reason, without communicating this fact to the
collector.
Although the transport sender is not 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,
we define some redundancy parameters below which may be useful in
controlling redundant transmission from the transport sender to the
transport receiver, and which may be useful for administrators to
configure.
6.1.1. Configuration Parameters for 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.
6.1.2. Configuration Parameters for 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
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redundant sending.
6.2. Flexibility
An originator 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 an originator to send short Signature Blocks to
allow the collector to verify messages quickly.
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7. Efficient Verification of Logs
The logs secured with syslog-sign may be reviewed either online or
offline. Online review is somewhat more complicated and
computationally expensive, but not prohibitively so.
7.1. Offline Review of Logs
When the collector stores logs to be reviewed later, they can be
authenticated offline just before they are reviewed. Reviewing these
logs offline is simple and relatively inexpensive in terms of
resources used, so long as there is enough space available on the
reviewing machine. Here, we presume that the stored log files have
already been separated by originator, Reboot Session ID, and
Signature Group. This can be done 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 message, or a Certificate Block
message. Signature Blocks and Certificate Blocks are then 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 whether 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 have
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 First Message Number. 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.
Skip all other Signature Blocks with the same First Message
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Number.
The resulting authenticated log file contains all messages that
have been authenticated. In addition, it implicitly indicates
all gaps in the authenticated messages (specifically in the case
when all messages of the same Signature Group are sent to the
same collector), because their message numbers are missing.
One can 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 have not discussed
error-recovery, which may be necessary for the Certificate Blocks.
In practice, a simple error-recovery strategy is probably enough: if
the Payload Block is not valid, then we can just try alternate
instances of each Certificate Block, if such are available, until we
get the Payload Block right.
It is easy for an attacker to flood us with plausible-looking
messages, Signature Blocks, and Certificate Blocks.
7.2. Online Review of Logs
Some collector implementations may need to monitor log messages in
close to real-time. This can be done with syslog-sign, though it is
somewhat more complex than offline verification. This is done as
follows:
a. We have an authenticated message file, into which we write
(message number, message text) pairs which have been
authenticated. Again, we will assume that we are handling only
one Signature Group and only one Reboot Session ID at any given
time.
b. We have three data structures: A queue in which (message number,
hash of message) pairs are kept in sorted order, a queue in which
(arrival sequence, hash of message) pairs are kept in sorted
order, and a hash table that stores (message text, count) pairs
indexed by hash value. In the hash table, count may be any
number greater than zero; when count is zero, the entry in the
hash table is cleared.
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c. We must receive all the Certificate Blocks before any other
processing can really be done. (This is why they are sent
first.) Once that is done, any Certificate Block message 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. If the message
queue is full, we roll the oldest messages off the queue by
taking the oldest entry in the queue, and using it to index the
hash table. If that entry has count 1, we delete the entry from
the hash table; otherwise, we decrement its count. We then
delete the oldest entry in the queue.
e. Whenever a Signature Block message arrives, we first check to see
whether the First Message Number value is too old to still be of
interest, or if another Signature Block with that First Message
Number has already been received. If so, we discard the
Signature Block. Otherwise, we check its signature and discard
it if the signature is not valid. A Signature Block contains a
sequence of (message number, message hash) pairs. For each pair,
we first check to see whether the message hash is in the hash
table. If so, we write the (message number, message text) into
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 looked up
in the hash table. If a matching entry is found, the (message
number, message text) pair is written to the authenticated
message file. In either case, the oldest entry is then
discarded.
f. The result of this is a sequence of messages in the authenticated
message file, each of which has been authenticated, and which are
labeled with numbers showing their order of original
transmission.
One can 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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8. Security Considerations
Normal syslog event messages are unsigned and have most of the
security attributes described in Section 8 of RFC xxxx [8]. This
document also describes Certificate Blocks and Signature Blocks,
which are signed syslog messages. The Signature Blocks contain
signature information for previously sent syslog event messages. All
of this information can be used to authenticate syslog messages and
to minimize or obviate many of the security concerns described in RFC
xxxx [8].
8.1. Cryptographic Constraints
As with any technology involving cryptography, it is advisable to
check the current literature to determine whether any algorithms used
here have been found to be vulnerable to attack.
This specification uses Public Key Cryptography technologies. The
proper party or parties have to control the private key portion of a
public-private key pair. Any party that controls a private key can
sign anything it pleases.
Certain operations in this specification involve the use of random
numbers. An appropriate entropy source SHOULD be used to generate
these numbers. See RFC 4086 [13] and NIST SP 800-90 [3].
8.2. Packet Parameters
As an originator, it is advisable to avoid message lengths exceeding
2048 octets. Various problems might result if an originator were to
send messages with a length greater than 2048 octets, because relays
MAY truncate messages with lengths greater than 2048 octets which
would make it impossible for collectors to validate a hash of the
packet. To increase the chance of interoperability, it tends to be
best to be conservative with what you send but liberal in what you
are able to receive.
Originators need to rigidly enforce the correctness of message
bodies. Problems may arise if the collector does not fully accept
the syslog packets sent from an originator, or if it has problems
with the format of the Certificate Block or Signature Block messages.
Collectors are not to malfunction in case they receive malformed
syslog messages or messages containing characters other than those
specified in this document. In other words, they are to ignore such
messages and continue working.
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8.3. Message Authenticity
Syslog does not strongly associate the message with the message
originator. That association is established by the collector upon
verification of the Signature Block. Before a Signature Block is
used to ascertain the authenticity of an event message, it might be
received, stored, and reviewed by a person or automated parser. It
is advisable not to assume a message is authentic until after a
message has been validated by checking the contents of the Signature
Block.
With the Signature Block checking, an attacker may only forge
messages if it can compromise the private key of the true originator.
8.4. Replaying
Event messages might be recorded and replayed by an attacker. Using
the information contained in the Signature Blocks, a reviewer can
determine whether the received messages are the ones originally sent
by an originator. The reviewer can also identify messages that have
been replayed.
8.5. Reliable Delivery
RFC wwww [10] can be used for the reliable delivery of syslog
messages. Event messages sent over UDP might be lost in transit. A
reviewer can pinpoint any messages sent by the originator but not
received by the collector by reviewing the Signature Block
information. In addition, the information in subsequent Signature
Blocks allows a reviewer to determine whether any Signature Block
messages were lost in transit.
8.6. Sequenced Delivery
Syslog messages delivered over UDP might not only be lost, but also
arrive out of sequence. A reviewer can determine the original order
of syslog messages and identify which messages were delivered out of
order by examining the information in the Signature Block along with
any timestamp information in the message.
8.7. Message Integrity
Syslog messages might be damaged in transit. A review of the
information in the Signature Block determines whether the received
message was the intended message sent by the originator. A damaged
Signature Block or Certificate Block is evident because the collector
will not be able to validate that it was signed by the originator.
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8.8. Message Observation
Event messages, Certificate Blocks, and Signature Blocks are all sent
in plaintext. This allows 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.
8.9. Man In The Middle Attacks
It is conceivable that an attacker might intercept Certificate Block
messages and insert its own Certificate information. In that case,
the attacker would be able to receive event messages from the actual
originator and then relay modified messages, insert new messages, or
delete messages. It would then be able to construct a Signature
Block and sign it with its own private key. Network administrators
need to verify that the key contained in the Payload Block is indeed
the key being used on the actual originator. If that is the case,
then this MITM attack will not succeed.
8.10. Denial of Service
An attacker might send invalid Signature Block messages to overwhelm
the collector's processing capability and consume all available
resources. For this reason, it can be appropriate to simply receive
the Signature Block messages and process them only as time permits.
An attacker might also just overwhelm a collector by sending more
messages to it than it can handle. Implementors are advised to
consider features that minimize this threat, such as only accepting
syslog messages from known IP addresses.
8.11. Covert Channels
Nothing in this protocol attempts to eliminate covert channels. In
fact, just about every aspect of syslog messages lends itself to the
conveyance of covert signals. For example, a collusionist could send
odd and even PRI values to indicate Morse Code dashes and dots.
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9. IANA Considerations
9.1. Structured Data and syslog messages
With regard to RFC xxxx [8], IANA is requested to add the following
values to the registry entitled "syslog Structured Data id values":
SD-ID PARAM_NAME
----- ----------
ssign
VER
RSID
SG
SPRI
GBC
FMN
CNT
HB
SIGN
ssign-cert
VER
RSID
SG
SPRI
TBPL
INDEX
FLEN
FRAG
SIGN
In addition, several fields need to be controlled by the IANA in both
the Signature Block and the Certificate Block, as outlined in the
following sections.
9.2. Version Field
IANA is requested to create three registries, each associated with a
different subfield of the Version field of Signature Blocks and
Certificate Blocks, described in Section 4.2.1 and Section 5.3.2.1,
respectively.
The first registry that IANA is requested to create is entitled
"syslog-sign protocol version values". It is for the values of the
Protocol Version subfield. The Protocol Version subfield constitutes
the first 2 octets in the Version field. New values shall be
assigned by the IANA using the "IETF Consensus" policy defined in RFC
2434 [4]. Assigned numbers are to be increased by 1, up to a maximum
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value of "50". Protocol Version numbers of "51" through "99" are
vendor-specific; values in this range are not to be assigned by the
IANA.
IANA is requested to register the Protocol Version values shown
below.
VALUE PROTOCOL VERSION
----- ----------------
00 Reserved
01 Defined in RFC yyyy
The second registry that IANA is requested to create is entitled
"syslog-sign hash algorithm values". It is for the values of the
Hash Algorithm subfield. The Hash Algorithm subfield constitutes the
third octet in the Version field Signature Blocks and Certificate
Blocks. New values shall be assigned by the IANA using the "IETF
Consensus" policy defined in RFC 2434 [4]. Assigned values are to be
increased by 1, up to a maximum value of "9". The values are
registered relative to the Protocol Version. This means that the
same Hash Algorithm value can be reserved for different Protocol
Versions, possibly referring to a different hash algorithm each time.
This makes it possible to deal with future scenarios in which the
single octet representation becomes a limitation, as more Hash
Algorithms can be supported by defining additional Protocol Versions
that implementations might support concurrently.
IANA is requested to register the Hash Algorithm values shown below.
VALUE PROTOCOL VERSION HASH ALGORITHM
----- ---------------- --------------
0 01 Reserved
1 01 SHA1
2 01 SHA256
The third registry that IANA is requested to create is entitled
"syslog-sign signature scheme values". It is for the values of the
Signature Scheme subfield. The Signature Scheme subfield constitutes
the fourth octet in the Version field of Signature Blocks and
Certificate Blocks. New values shall be assigned by the IANA using
the "IETF Consensus" policy defined in RFC 2434 [4]. Assigned values
are to be increased by 1, up to a maximum value of "9". This means
that the same Signature Scheme value can be reserved for different
Protocol Versions, possibly in each case referring to a different
Signature Scheme each time. This makes it possible to deal with
future scenarios in which the single octet representation becomes a
limitation, as more Signature Schemes can be supported by defining
additional Protocol Versions that implementations might support
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concurrently.
IANA is requested to register the Signature Scheme values shown
below.
VALUE PROTOCOL VERSION SIGNATURE SCHEME
----- ---------------- ----------------
0 01 Reserved
1 01 OpenPGP DSA
9.3. SG Field
IANA is requested to create a registry entitled "syslog-sign sg field
values". It is for values of the SG Field as defined in
Section 4.2.3. New values shall be assigned by the IANA using the
"IETF Consensus" policy defined in RFC 2434 [4]. Assigned values are
to be incremented by 1, up to a maximum value of "7". Values "8" and
"9" shall be left as vendor specific and shall not be assigned by the
IANA.
IANA is requested to register the SG Field values shown below.
VALUE MEANING
----- -------
0 per RFC yyyy
1 per RFC yyyy
2 per RFC yyyy
3 per RFC yyyy
9.4. Key Blob Type
IANA is requested to create a registry entitled "syslog-sign key blob
type values". It is to register one-character identifiers for the
key blob type, per Section 5.2. New values shall be assigned by the
IANA using the "IETF Consensus" policy defined in RFC 2434 [4].
Uppercase letters may be assigned as values. Lowercase letters are
left as vendor specific and shall not be assigned by the IANA.
IANA is requested to register the key blob type values shown below.
VALUE KEY BLOB TYPE
----- ------------
'C' a PKIX certificate
'P' an OpenPGP certificate
'K' the public key whose corresponding private key is used to sign the messages
'N' no key information sent, key is predistributed
'U' installation-specific key exchange information
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10. Working Group
The working group can be contacted via the mailing list:
syslog@ietf.org
The current Chairs of the Working Group can be contacted at:
Chris Lonvick
Cisco Systems
Email: clonvick@cisco.com
David Harrington
Huawei Technologies (USA)
Email: ietfdbh@comcast.net
dharrington@huawei.com
Tel: +1-603-436-8634
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11. Acknowledgements
The authors wish to thank Alex Brown, Chris Calabrese, Steve Chang,
Carson Gaspar, Drew Gross, David Harrington, 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.
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12. References
12.1. Normative References
[1] National Institute of Standards and Technology, "Digital
Signature Standard", FIPS PUB 186-2, January 2000, <http://
csrc.nist.gov/publications/fips/fips186-2/
fips186-2-change1.pdf>.
[2] National Institute of Standards and Technology, "Secure Hash
Standard", FIPS PUB 180-2, August 2002, <http://csrc.nist.gov/
publications/fips/fips180-2/fips180-2.pdf>.
[3] National Institute of Standards and Technology, "NIST Special
Publication 800-90: Recommendation for Random Number Generation
using Deterministic Random Bit Generators", June 2006, <http://
csrc.nist.gov/publications/nistpubs/800-90/
SP800-90_DRBG-June2006-final.pdf>.
[4] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[5] Callas, J., Donnerhacke, L., Finney, H., and R. Thayer,
"OpenPGP Message Format", RFC 2440, November 1998.
[6] Blumenthal, U. and B. Wijnen, "User-based Security Model (USM)
for version 3 of the Simple Network Management Protocol
(SNMPv3)", RFC 3414, December 2002.
[7] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings",
RFC 4648, October 2006.
[8] Gerhards, R., "The syslog Protocol,
draft-ietf-syslog-protocol-23.txt (work in progress)",
September 2007.
[9] Okmianski, A., "Transmission of syslog Messages over UDP,
draft-ietf-syslog-transport-udp-12.txt (work in progress)",
September 2007.
[10] Miao, F. and M. Yuzhi, "TLS Transport Mapping for syslog,
draft-ietf-syslog-transport-tls-10.txt (work in progress)",
May 2007.
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12.2. Informative References
[11] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[12] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, July 2002.
[13] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Recommendations for Security", RFC 4086, June 2005.
Kelsey, et al. Expires March 31, 2008 [Page 34]
Internet-Draft Signed syslog Messages September 2007
Authors' Addresses
John Kelsey
NIST
Email: john.kelsey@nist.gov
Jon Callas
PGP Corporation
Email: jon@callas.org
Alexander Clemm
Cisco Systems
Email: alex@cisco.com
Kelsey, et al. Expires March 31, 2008 [Page 35]
Internet-Draft Signed syslog Messages September 2007
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Kelsey, et al. Expires March 31, 2008 [Page 36]
