syslog Working Group J. Kelsey
Internet-Draft NIST
Intended status: Standards Track J. Callas
Expires: September 16, 2007 PGP Corporation
A. Clemm
Cisco
March 15, 2007
Signed syslog Messages
draft-ietf-syslog-sign-21.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 . . . . . . . . . . . . . . . . . . . . . . . . 13
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 . . . . 17
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. Cryptography Constraints . . . . . . . . . . . . . . . . . 25
8.2. Packet Parameters . . . . . . . . . . . . . . . . . . . . 25
8.3. Message Authenticity . . . . . . . . . . . . . . . . . . . 26
8.4. Sequenced Delivery . . . . . . . . . . . . . . . . . . . . 26
8.5. Replaying . . . . . . . . . . . . . . . . . . . . . . . . 26
8.6. Reliable Delivery . . . . . . . . . . . . . . . . . . . . 26
8.7. Sequenced Delivery . . . . . . . . . . . . . . . . . . . . 26
8.8. Message Integrity . . . . . . . . . . . . . . . . . . . . 27
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8.9. Message Observation . . . . . . . . . . . . . . . . . . . 27
8.10. Man In The Middle Attacks . . . . . . . . . . . . . . . . 27
8.11. Denial of Service . . . . . . . . . . . . . . . . . . . . 27
8.12. 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 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 sender as
the generator of each message and a single receiver as the collector
of each message, provisions need to be made to cover situations in
which messages are sent to multiple receivers. This concerns in
particular situations in which different messages are sent to
different receivers, which means that some messages are sent to some
receivers 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 sender. A
Signature Block always belongs to exactly one signature group and it
always signs messages belonging only to that signature group.
Additionally, a sender 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 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 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
message delivery mechanism and utilizes the concept of STRUCTURED-
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DATA elements defined in that document. In fact, this specification
mandates implementation of syslog protocol. Nevertheless, it is
conceivable that the concepts that underly 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
design.
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 in 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 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 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 sender and receiver implementations implementing this
specification MUST support messages of up to and including 2048
octets in length, in order to minimize the chances of truncation from
happening. While syslog sender and receiver 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 the other fields, that document describes the
concept of STRUCTURED DATA (SD). STRUCTURED DATA is defined in the
form 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, 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
that are 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 is simply 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.
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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, implementations SHOULD use the same value for APP-NAME,
PROCID, and MSGID fields for every Signature Block message, 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 MSG part of a Signature Block message SHOULD be
empty.
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. Receivers that implement syslog-
sign know to distinguish messages that are associated with syslog-
sign from the syslog messages 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
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printable ASCII, and any binary values MUST be base 64 encoded, as
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
(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 value:
"[ssign VER=xxx RSID=xxx SG=xxx SPRI=xxx GBC=xxx FMN=xxx CNT=xxx
HB=xxx SIGN=xxx]".
The fields are separated by single spaces and are described below.
4.2.1. Version
The Signature Block version field is 4 octets in length. 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).
Protocol Version - 2 octets, with the protocol version that is
described in this document referred to with "01" as the value.
Hash Algorithm - 1 octet, with the definition that in conjunction
with Protocol Version 01, a value of "1" denotes SHA1, a value of
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2 denotes SHA256, as defined in FIPS-180-2.2002 [2].
Signature Scheme - 1 octet, with the definition that in
conjunction with Protocol Version 01, a value of "1" denotes
OpenPGP DSA - RFC 2440 [5], 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. A reboot session ID is expected to increase whenever a
sender reboots in order to allow receivers to uniquely distinguish
messages and message signatures across reboots. A device needs to
hence support persisting previous reboot session ID across reboots.
In cases where a sender 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 a sender reboots, starting with a value of 1. If the value
latches at 9999999999, then manual intervention may be required to
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 identifier may take on any value from 0-3 inclusive. The SPRI
may take on 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
also referred to as 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 send syslog
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messages of Facilities 0 through 15 to one destination while those
with Facilities 16 through 23 to another. In such cases, associated
Signature Blocks should likely be sent to the corresponding syslog
servers as well, signing the syslog messages that are intended for
each destination separately. This way, each syslog destination
receives Signature Blocks for all syslog messages that it receives,
and only for those. The ability to to associate different categories
of syslog messages with different signature groups, signed in
separate Signature Blocks, provides administrators with flexibility
to that 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 appropriate 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 is used for
all Signature Blocks. Furthermore, it is RECOMMENDED to use as
that value the same value that is used for 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 a device 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
signature group per PRI value provides administrators with a
large degree of flexibility with regards to how to divide up the
processing syslog messages and their signatures after they are
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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 next 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 sender and the intended receivers 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 along 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 device 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 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.
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
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. If the value of
the global block counter latches at 9999999999 and the reboot session
ID has a value other than 0 (indicating the fact that persisting 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. In
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the case in which the reboot session ID is in fact 0 and persisting
of reboot session IDs is not supported, when the global block counter
latches, it resumes at 0 also in this case but the reboot session ID
MUST NOT be incremented and remains at 0.
Note that the global block counter crosses signature groups; it
allows us 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
sender MUST reset the global block counter to 0 after a reboot
occurs. Applications need to apply consideration to 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 and hence
jeopardizing integrity of the log.
4.2.5. First Message Number
This is a decimal value between 1 and 10 octets. 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 first 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 that it belongs to; a
message number hence does not identify the message across signature
groups.
Should the message number within the same reboot session and
signature group reach 9999999999, the message number becomes modulo
9999999999. This means it latches and restarts with 1 (not with 0,
which would be modulo 10000000000). In such event, the global block
counter will be vastly different between two occurrences of the same
message number.
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.
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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. 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 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 in
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 receiver. A Payload Block is not sent
directly, but in (one or more) fragments. Those fragments are termed
Certificate Blocks. All devices send at least one Certificate Block
at the beginning of a new reboot session, carrying public key
information in effect for 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. 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, 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
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.
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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. That is, each reboot session has only one Payload
Block, regardless of how many signature groups it supports. A
Payload Block MUST have the following fields. Each of these fields
are separated by a single space character. 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.
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 the time stamp format specified
in RFC xxxx [8] (essentially, TIMESTAMP-3339 format time stamp
format per RFC 3339 [12] with some further restrictions).
c. Key Blob Type, a one-octet 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, if any, base 64 encoded per RFC 4648 [7] and
consisting of the raw key data.
5.3. Certificate Block
The Certificate Block must get the Payload Block to the collector.
The Certificate Block itself needs to be distinguished from the
syslog message that carries it, refererred to as a Certificate Block
message.
Because certificates can legitimately be much longer than 2048
octets, 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 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
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piece of the Payload Block that a Certificate Block is to carry
SHOULD be chosen such that the length of the Certificate Block
message does not exceed 2048 octets.
5.3.1. syslog Messages Containing a Certificate Block
As with a Signature Block, each Certificate Block is carried in its
own syslog message, referred to as a Certificate Block message.
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. This
specification 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 value:
"[ssign-cert VER=xxx RSID=xxx SG=xxx SPRI=xxx TBPL=xxx INDEX=xxx
FLEN=xxx FRAG=xxx SIGN=xxx]".
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 4.2.8. The SPRI field is identical in format
and meaning to the SPRI field described there.
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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 value between 1 and 8 octets. 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.
5.3.2.7. 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 sender and receiver have in message formats:
in general, the sender 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; since the collector MUST ignore
Signature/Certificate Blocks it has already received and
authenticated, the sender can in principle change its redundancy
level for any reason, without communicating this fact to the
collector.
Although the 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 sender to the collector,
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
redundant sending.
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6.2. 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 short Signature Blocks, in
order to allow the collector 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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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 consider that the stored log files have
already been separated by sender, 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, 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 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.
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3. Skip all other Signature Blocks with the same First Message
Number.
d. 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 processes on the collector machine 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 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 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 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 last 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 last entry in the queue.
e. Whenever a Signature Block 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 unread. 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 if 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 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 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 contains signature
information of 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. Cryptography 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 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 4086 [13] and NIST 800-90 [3].
8.2. Packet Parameters
As a sender, it is advisable to avoid message lengths exceeding 2048
octets. Various problems might result if a sender were to send out
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 receivers 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.
Senders need to rigidly enforce the correctness of the message body.
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.
Receivers 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
sender. That association is established by the receiver 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 to not 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 they can compromise the private key of the true sender.
8.4. Sequenced Delivery
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 a sender. The reviewer can also identify messages that have been
replayed.
8.5. Replaying
Event messages might 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.
8.6. 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 sender but not
received by the receiver by properly reviewing the Signature Block
information. In addition, the information in subsequent Signature
Blocks allows a reviewer to determine if any Signature Block messages
were lost in transit.
8.7. 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.
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8.8. 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 sender. A damaged
Signature Block or Certificate Block is evident because the receiver
will not be able to validate that it was signed by the sender.
8.9. 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.
8.10. 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
sender 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 device. If that is the case, then
this MITM attack will not succeed.
8.11. Denial of Service
An attacker might send invalid Signature Block messages to overwhelm
the receiver'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 receiver 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.
8.12. 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 regards 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 incremented monotonically with
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decimal values, up to a maximum 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
incremented monotonically 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 scenario in which the
single octet represenation becomes a limitation, as more Hash
Algorithms can be supported by defining additional Protocol Version
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 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 incremented monotonically 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 scenario in which the single octet represenation becomes
a limitation, as more Signature Schemes can be supported by defining
additional Protocol Version 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 monotonically 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 RFC 2434 (Section 5.2). 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-sec@employees.org
The current Chairs of the Working Group can be contacted at:
Chris Lonvick
Cisco
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)", 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-19.txt (work in progress)",
November 2006.
[9] Okmianski, A., "Transmission of syslog Messages over UDP,
draft-ietf-syslog-transport-udp-08.txt (work in progress)",
November 2006.
[10] Miao, F. and M. Yuzhi, "TLS Transport Mapping for syslog,
draft-ietf-syslog-transport-tls-06.txt (work in progress)",
December 2006.
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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.
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Authors' Addresses
John Kelsey
NIST
Email: john.kelsey@nist.gov
Jon Callas
PGP Corporation
Email: jon@callas.org
Alexander Clemm
Cisco
Email: alex@cisco.com
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Full Copyright Statement
Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
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Kelsey, et al. Expires September 16, 2007 [Page 36]
