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Internet Engineering Task Force (IETF) J. Kelsey
Request for Comments: 5848 NIST
Category: Standards Track J. Callas
ISSN: 2070-1721 PGP Corporation
A. Clemm
Cisco Systems
May 2010
Signed Syslog Messages
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 5424, "The Syslog Protocol".
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5848.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Kelsey, et al. Standards Track [Page 1]
RFC 5848 Signed Syslog Messages May 2010
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . 5
3. Syslog Message Format . . . . . . . . . . . . . . . . . . . . 5
4. Signature Blocks . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Syslog Messages Containing a Signature Block . . . . . . . 7
4.2. Signature Block Format and Fields . . . . . . . . . . . . 7
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 . . . . . . . . . . . . . . . . . 13
4.2.5. First Message Number . . . . . . . . . . . . . . . . . 13
4.2.6. Count . . . . . . . . . . . . . . . . . . . . . . . . 14
4.2.7. Hash Block . . . . . . . . . . . . . . . . . . . . . . 14
4.2.8. Signature . . . . . . . . . . . . . . . . . . . . . . 14
4.2.9. Example . . . . . . . . . . . . . . . . . . . . . . . 15
5. Payload and Certificate Blocks . . . . . . . . . . . . . . . . 15
5.1. Preliminaries: Key Management and Distribution Issues . . 15
5.2. Payload Block . . . . . . . . . . . . . . . . . . . . . . 16
5.2.1. Block Format and Fields . . . . . . . . . . . . . . . 16
5.2.2. Signer Authentication and Authorization . . . . . . . 18
5.3. Certificate Block . . . . . . . . . . . . . . . . . . . . 19
5.3.1. Syslog Messages Containing a Certificate Block . . . . 19
5.3.2. Certificate Block Format and Fields . . . . . . . . . 20
6. Redundancy and Flexibility . . . . . . . . . . . . . . . . . . 24
6.1. Configuration Parameters . . . . . . . . . . . . . . . . . 24
6.1.1. Configuration Parameters for Certificate Blocks . . . 24
6.1.2. Configuration Parameters for Signature Blocks . . . . 26
6.2. Overlapping Signature Blocks . . . . . . . . . . . . . . . 27
7. Efficient Verification of Logs . . . . . . . . . . . . . . . . 27
7.1. Offline Review of Logs . . . . . . . . . . . . . . . . . . 28
7.2. Online Review of Logs . . . . . . . . . . . . . . . . . . 29
8. Security Considerations . . . . . . . . . . . . . . . . . . . 32
8.1. Cryptographic Constraints . . . . . . . . . . . . . . . . 32
8.2. Packet Parameters . . . . . . . . . . . . . . . . . . . . 33
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8.3. Message Authenticity . . . . . . . . . . . . . . . . . . . 33
8.4. Replaying . . . . . . . . . . . . . . . . . . . . . . . . 33
8.5. Reliable Delivery . . . . . . . . . . . . . . . . . . . . 34
8.6. Sequenced Delivery . . . . . . . . . . . . . . . . . . . . 34
8.7. Message Integrity . . . . . . . . . . . . . . . . . . . . 34
8.8. Message Observation . . . . . . . . . . . . . . . . . . . 34
8.9. Man-in-the-Middle Attacks . . . . . . . . . . . . . . . . 34
8.10. Denial of Service . . . . . . . . . . . . . . . . . . . . 35
8.11. Covert Channels . . . . . . . . . . . . . . . . . . . . . 35
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35
9.1. Structured Data and Syslog Messages . . . . . . . . . . . 35
9.2. Version Field . . . . . . . . . . . . . . . . . . . . . . 36
9.3. SG Field . . . . . . . . . . . . . . . . . . . . . . . . . 38
9.4. Key Blob Type . . . . . . . . . . . . . . . . . . . . . . 38
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 39
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 39
11.1. Normative References . . . . . . . . . . . . . . . . . . . 39
11.2. Informative References . . . . . . . . . . . . . . . . . . 40
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 content 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. The originator of syslog-sign messages is simply
referred to as a "signer". The signer can be the same originator as
the originator whose messages it signs, or it can be a separate
originator.
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
from the same originator 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
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kept together for signing purposes by the signer. A Signature Block
always belongs to exactly one Signature Group and always signs
messages belonging only to that Signature Group.
Additionally, a signer sends Certificate Blocks to provide key
management information between the signer and the collector. A
Certificate Block has a field to denote the type of key material
which may be such things as a Public Key Infrastructure using X.509
(PKIX) certificate, an OpenPGP (Pretty Good Privacy) certificate, or
even an indication that a key had been pre-distributed. In the cases
of certificates being sent, the certificates may have to be split
across multiple Certificate Blocks carried in separate messages.
It is possible that the same host contains multiple signers that each
use their own keys to sign syslog messages. In this case, each
signer sends its own Certificate Block and Signature Blocks.
Furthermore, each signer defines its own Signature Groups. Each
signer on a given host needs to use a distinct combination of APP-
NAME, and PROCID for its Signature Block and Certificate Block
message. (This implies that the combination of HOSTNAME, APP-NAME,
and PROCID uniquely distinguishes originators of syslog-sign messages
across hosts, provided that the signers use a unique HOSTNAME.)
The collector may verify that the hash of each received message
matches the signed hash contained in the corresponding 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 process of signing works as long as the collector accepts the
syslog messages, the Certificate Blocks and the Signature Blocks.
Once that is done, the process is complete. After that, anyone can
go back, find the key material, and validate the received messages
using the information in the Signature Blocks. Finding the key
material is very easily done with Key Blob Types C, P, and K (see
Section 4.2) since the public key is in the Payload Block. If Key
Blob Types N or U are used, some poking around may be required to
find the key material. The only way to have a vendor-specific
implementation is through N or U; however, also in that case, the key
material will have to be available in some form which could be used
by implementations of other vendors.
Because the mechanism that is described in this specification uses
the concept of STRUCTURED-DATA elements defined in [RFC5424],
compliant implementations of this specification MUST also implement
[RFC5424]. It is conceivable that the concepts underlying this
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specification 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.
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Syslog Message Format
This specification is intended to be used in conjunction with the
syslog protocol as defined in [RFC5424]. The syslog protocol
therefore MUST be supported by implementations of this specification.
Because the originator generating the Signature Block message, also
simply referred to as "signer", 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. One of the
effects of such behavior, including message alteration by relays,
would be to render any signing invalid and hence make the mechanism
useless. Likewise, any truncation of messages that occurs between
sending and receiving renders the mechanism useless. For this
reason, syslog signer 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 signer and collector implementations MAY support
messages with a length longer than 2048 octets, implementers need to
be aware that any message truncations that occur render the mechanism
useless. In such cases, it is up to the operator to ensure that the
syslog messages can be received properly and can be validated.
[RFC5426] recommends using the Transport Layer Security (TLS)
transport and deliberately constrains the use of UDP. UDP is NOT
RECOMMENDED for use with signed syslog because its recommended
payload size of 480 octets is too restrictive for the purposes of
syslog-sign. A 480-octet Signature Block could sign only 9 normal
messages, meaning that at a significant proportion of messages would
be Signature Block messages. The 480-octet limitation is primarily
geared towards small embedded systems with significant resource
constraints that, because of those constraints, would not implement
syslog-sign in the first place. In addition, the use of UDP is
geared towards syslog messages that are primarily intended for
troubleshooting, a very different purpose from the application
targeted by syslog-sign. Where syslog UDP transport is used, it is
the responsibility of operators to ensure that network paths are
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configured in a way that messages of sufficient length (up to and
including 2048 octets) can be properly delivered.
This specification uses the syslog message format described in
[RFC5424]. 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 data that is
associated with Signature Blocks and Certificate Blocks as Structured
Data. For this purpose, the special syslog messages defined in this
document include 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 [RFC5424], they identify the machine that
originates the syslog message in the HOSTNAME field. Therefore, the
Signature Block and Certificate Block data do not need to include any
additional parameter to identify the machine that originates the
message.
In addition, several signers MAY sign messages on a single host
independently of each other, each using their own Signature Groups.
In that case, each unique signer is distinguished by the combination
of APP-NAME and PROCID. (By the same token, the same message might
be signed by multiple signers.) Each unique signer MUST have a
unique APP-NAME and PROCID on each host. (This implies that the
combination of HOSTNAME, APP-NAME and PROCID uniquely distinguishes
the originator of syslog-sign messages, provided that the signers use
a unique HOSTNAME.) A Signature Block message MUST use the same
combination of HOSTNAME, APP-NAME, and PROC-ID that was used to send
the corresponding Certificate Block messages containing the Payload
Block.
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.
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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 [RFC5424]. This specification does
not mandate particular values for these fields; however, for
consistency, a signer MUST use the same values for APP-NAME, PROCID,
and MSGID fields for every Signature Block message that is sent,
whichever values are chosen. It MUST also use the same value for its
HOSTNAME field. To allow for the possibility of multiple signers per
host, the combination of APP-NAME and PROCID MUST be unique for each
such signer on any given host. If a signer daemon is restarted, it
MAY use a new PROCID for what is otherwise the same signer but MUST
continue to use the same APP-NAME. If it uses a new PROCID, it MUST
send a new Payload Block using Certificate Block messages that use
the same new PROCID (and the same APP-NAME). It is RECOMMENDED (but
not required) to use 110 as value for the PRI field, corresponding to
facility 13 (log audit) 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 MAY 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) MUST NOT 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. The intent of syslog-sign is to sign a stream of syslog
messages, not to alter it.
4.2. Signature Block Format and Fields
The content of a Signature Block message is the Signature Block
itself. The Signature Block MUST be encoded as an SD ELEMENT, as
defined in [RFC5424].
The SD-ID MUST have the value of "ssign".
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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 base64 encoded, as
defined in [RFC4648].
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
(base64 encoded binary)
Signature SIGN variable
(base64 encoded binary)
The fields MUST be provided in the order listed. Each SD parameter
MUST occur once and only once in the Signature Block. New SD
parameters MUST NOT be added unless a new Version of the protocol is
defined. (Implementations that wish to add proprietary extensions
will need to define a separate SD ELEMENT.) 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 [RFC5424]. The fields are separated by
single spaces and are described in the subsequent subsections.
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4.2.1. Version
The Version field is an alphanumeric value that has a length of 4
octets, which may include leading zeroes. The first 2 octets and the
last octet contain a decimal character in the range of "0" to "9",
whereas the third octet contains an alphanumeric character in the
range of "0" to "9", "a" to "z", or "A" to "Z". 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 "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]. (This is the octet that
can have a value of not just "0" to "9" but also "a" to "z" and
"A" to "Z".)
Signature Scheme - 1 octet, where, in conjunction with Protocol
Version 01, a value of "1" denotes OpenPGP DSA, defined in
[RFC4880] and [FIPS.186-2.2000].
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.
As long as the sender and receiver are both adhering to [RFC5424],
the prerequisites are in place so that signed messages can be
received by the receiver and validated with a Signature Block. To
ensure immediate validation of received messages, all implementations
MUST support SHA1, and SHA256 SHOULD be supported.
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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 strictly monotonically increase
(i.e., to never repeat or decrease) whenever a signer reboots in
order to allow collectors to distinguish messages and message
signatures across reboots. There are several ways in which this may
be accomplished. In one way, the Reboot Session ID may increase by
1, starting with a value of 1. Note that in this case, a signer is
required to retain the previous Reboot Session ID across reboots. In
another way, a value of the Unix time (number of seconds since 1
January 1970) may be used. Implementers of this method need to
beware of the possibility of multiple reboots occurring within a
single second. Implementers need to also beware of the year 2038
problem, which will cause the 32-bit representation of Unix time to
wrap in the year 2038. In yet another way, implementations where the
Simple Network Management Protocol (SNMP) engine and the signer
always reboot at the same time might consider using the
snmpEngineBoots value as a source for this counter as defined in
[RFC3414].
In cases where a signer is not able to guarantee that the Reboot
Session ID is always increased after a reboot, the Reboot Session ID
MUST always be set to a value of 0. If the value can no longer be
increased (e.g., because it reaches 9999999999), it SHOULD be reset
to a value of 1. Implementations SHOULD ensure that such a reset
does not go undetected, for example, by requesting operator
acknowledgment when a reset is performed upon reboot. (Operator
acknowledgment may not be possible in all situations, e.g., in the
case of embedded devices.)
If a reboot of a signer takes place, Signature Block messages MAY use
a new PROCID. However, Signature Block messages of the same signer
MUST continue to use the same HOSTNAME, APP-NAME, and MSGID.
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
also called Signature Groups. A Signature Block contains only hashes
of those syslog messages that are part of the same Signature Group.
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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 contain
signatures for 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 a signer 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 regard to how to divide up the
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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 1 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 also
provides administrators with the flexibility to group syslog
messages into Signature Groups according to criteria that are not
tied to the PRI value. Note that this option is not intended for
deployments that lack such an arrangement, as in those cases a
collector could misinterpret the intended meaning of the
Signature Group. A collector that receives Signature Block
messages of a Signature Group of whose scheme it is not aware
SHOULD bring this fact to the attention of the system
administrator. The particular mechanism used for that is
implementation-specific and outside the scope of this
specification.
One reasonable way to configure some installations is to have only
one Signature Group, indicated with SG=0, and have the signer 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
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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
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
signer 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.
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Should the message number reach 9999999999 within the same reboot
session and Signature Group, the message number subsequently restarts
at 1. In such an 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.
4.2.7. Hash Block
The hash block is a block of hashes, each separately encoded in
base64. 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 space
character is used to separate the hashes. Note, the hash block
constitutes a single SD-PARAM; a Signature Block message MUST include
all its hashes in a single hash block and MUST NOT spread its hashes
across several hash blocks.
The "entire syslog message" refers to what is described as the syslog
message excluding transport parts that are described in [RFC5425] and
[RFC5426], 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 base64 encoded as per [RFC4648].
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 base64 per [RFC4648]. The
signature is calculated over the completely formatted Signature Block
message (starting from the first octet of PRI and continuing to the
last octet of MSG, or STRUCTURED-DATA if MSG is not present), before
the SIGN parameter (SD Parameter Name and the space before it
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[" SIGN"], "=", and the corresponding value) is added. (In other
words, the digital signature is calculated over the whole message,
with the "SIGN=value" portion removed.) For the OpenPGP DSA
signature scheme, the value of the signature field contains the DSA
values r and s, encoded as two multiprecision integers (see
[RFC4880], Sections 5.2.2 and 3.2), concatenated, and then encoded in
base64 [RFC4648].
4.2.9. Example
An example of a Signature Block message is depicted below, broken
into lines to fit publication rules. There is a space at the end of
each line, with the exception of the last line, which ends with "]".
<110>1 2009-05-03T14:00:39.529966+02:00 host.example.org syslogd
2138 - [ssign VER="0111" RSID="1" SG="0" SPRI="0" GBC="2" FMN="1"
CNT="7" HB="K6wzcombEvKJ+UTMcn9bPryAeaU= zrkDcIeaDluypaPCY8WWzwHpPok=
zgrWOdpx16ADc7UmckyIFY53icE= XfopJ+S8/hODapiBBCgVQaLqBKg=
J67gKMFl/OauTC20ibbydwIlJC8= M5GziVgB6KPY3ERU1HXdSi2vtdw=
Wxd/lU7uG/ipEYT9xeqnsfohyH0="
SIGN="AKBbX4J7QkrwuwdbV7Taujk2lvOf8gCgC62We1QYfnrNHz7FzAvdySuMyfM="]
The message is of syslog-sign protocol version "01". It uses SHA1 as
hash algorithm and an OpenPGP DSA signature scheme. Its reboot
session ID is 1. Its Signature Group is 0, which means that all
syslog messages go to the same destination; its Signature Priority
(which can effectively be ignored because all syslog messages will be
signed regardless of their PRI value) is 0. Its Global Block Counter
is 2. The first message number is 1; the message contains 7 message
hashes.
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, signers send at least one Certificate
Block at the beginning of a new reboot session.
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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 signer 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
signer and collector (or offline log viewer) both have some key
material (such as a root public key or pre-distributed 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.
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. A signer creates a new Payload Block after each
reboot. The Payload Block is used until the next reboot.
5.2.1. Block Format and Fields
A Payload Block MUST have the following fields:
a. Full local timestamp for the signer at the time the reboot
session started. This must be in the timestamp format specified
in [RFC5424] (essentially, timestamp format per [RFC3339] with
some further restrictions).
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b. Key Blob Type, a one-octet field containing one of five values:
1. 'C' -- a PKIX certificate (per [RFC5280]).
2. 'P' -- an OpenPGP KeyID and OpenPGP certificate (a
Transferable Public Key as defined in [RFC4880], Section
11.1). The first 8 octets of the key blob field contain the
OpenPGP KeyID (identifying which key or subkey inside the
OpenPGP certificate is used), followed by the OpenPGP
certificate itself.
3. 'K' -- the public key whose corresponding private key is
being used to sign these messages. For the OpenPGP DSA
signature scheme, the key blob field contains the DSA prime
p, DSA group order q, DSA group generator g, and DSA public-
key value y, encoded as 4 multiprecision integers (see
[RFC4880], Sections 5.5.2 and 3.2).
4. 'N' -- no key information sent; key is pre-distributed.
5. 'U' -- installation-specific key exchange information.
c. The key blob, if any, base64 encoded per [RFC4648] 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.
To ensure that the sender and receiver have at least one common Key
Blob Type, for immediate validation of received messages, all
implementations MUST support Key Blob Type "C" (PKIX certificate).
When a PKIX certificate is used ("C" Key Blob Type), it is the
certificate specified in [RFC5280]. Per [RFC5425], syslog messages
may be transported over the TLS protocol, even where there is no PKI.
If that transport is used, then the device will already have a PKIX
certificate, and it MAY use the private key associated with that
certificate to sign messages. In the case where there is no PKI, the
chain of trust of a PKIX certificate must still be established to
meet conventional security requirements. The methods for doing this
are described in [RFC5425].
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5.2.2. Signer Authentication and Authorization
When the collector receives a Payload Block, it needs to determine
whether the signatures are to be trusted. The following methods are
in scope of this specification:
a. X.509 certification path validation: The collector is configured
with one or more trust anchors (typically root Certification
Authority (CA) certificates), which allow it to verify a binding
between the subject name and the public key. Certification path
validation is performed as specified in [RFC5280].
If the HOSTNAME contains a Fully-Qualified Domain Name (FQDN) or
an IP address, it is then compared against the certificate as
described in [RFC5425], Section 5.2. Comparing other forms of
HOSTNAMEs is beyond the scope of this specification.
Collectors SHOULD support this method. Note that due to message
size restrictions, syslog-sign sends only the end-entity
certificate in the Payload Block. Depending on the PKI
deployment, the collector may need to obtain intermediate
certificates by other means (for example, from a directory).
b. X.509 end-entity certificate matching: The collector is
configured with information necessary to identify the valid end-
entity certificates of its valid peers, and for each peer, the
HOSTNAME(s) it is authorized to use.
To ensure interoperability, collectors MUST support fingerprints
of X.509 certificates as described below. Other methods MAY be
supported.
Collectors MUST support Key Blob Type 'C', and configuring the
list of valid peers using certificate fingerprints. The
fingerprint is calculated and formatted as specified in
[RFC5425], Section 4.2.2.
For each peer, the collector MUST support configuring a list of
HOSTNAMEs that this peer is allowed to use either as FQDNs or IP
addresses. Other forms of HOSTNAMEs are beyond the scope of this
specification.
If the locally configured FQDN is an internationalized domain
name, conforming implementations MUST convert it to the ASCII
Compatible Encoding (ACE) format for performing comparisons as
specified in Section 7 of [RFC5280]. An exact case-insensitive
string match MUST be supported, but the implementation MAY also
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support wildcards of any type ("*", regular expressions, etc.) in
locally configured names.
Signer implementations MUST provide a means to generate a key
pair and self-signed certificate in the case that a key pair and
certificate are not available through another mechanism, and MUST
make the certificate fingerprint available through a management
interface.
c. OpenPGP V4 fingerprints: Like X.509 fingerprints, except Key Blob
Type 'P' is used, and the fingerprint is calculated as specified
in [RFC4880], Section 12.2. When the fingerprint value is
displayed or configured, each byte is represented in hexadecimal
(using two uppercase ASCII characters), and space is added after
every second byte. For example: "0830 2A52 2CD1 D712 6E76 6EEC
32A5 CAE1 03C8 4F6E".
Signers and collectors MAY support this method.
Other methods, such as "web of trust", are beyond the scope of this
document.
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 a Certificate Block
message. In case separate collectors are associated with different
Signature Groups, Certificate Block messages need to be sent to each
collector.
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 signer 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 one octet. The
length SHOULD be chosen such that the length of the Certificate Block
message does not exceed 2048 octets.
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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, a signer MUST use the same value for APP-NAME, PROCID,
and MSGID fields for every Certificate Block message, whichever
values are chosen. It MUST also use the same value for its HOSTNAME
field. To allow for the possibility of multiple signers per host,
the combination of APP-NAME and PROCID MUST be unique for each such
originator. If a signer daemon is restarted, it MAY use a new PROCID
for what is otherwise the same signer. The combination of APP-NAME
and PROCID MUST be the same that is used for Signature Block messages
of the same signer; however, a different MSGID MAY be used for
Signature Block and Certificate Block messages. It is RECOMMENDED to
use 110 as the value for the PRI field, corresponding to facility 13
(log audit) and severity 6 (informational). The Certificate Block is
carried as Structured Data within the Certificate Block message. A
Certificate Block message MAY 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 base64
encoded per [RFC4648].
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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
(base64 encoded binary)
Signature SIGN variable
(base64 encoded binary)
The fields MUST be provided in the order listed. New SD parameters
MUST NOT be added unless a new Version of the protocol is defined.
(Implementations that wish to add proprietary extensions will need to
define a separate SD ELEMENT.) 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" TPBL="xxx"
INDEX="xxx" FLEN="xxx" FRAG="xxx" SIGN="xxx"]
Values of the fields constitute SD parameter values and are hence
enclosed in quotes, per [RFC5424]. The fields are separated by
single spaces and are described below. Each SD parameter MUST occur
once and only once.
5.3.2.1. Version
The 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.
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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
and meaning to the SPRI field described there.
A signer SHOULD send separate Certificate Block messages for each
Signature Group. This ensures that each collector that is associated
with a Signature Group will receive the necessary key material in the
case that messages of different Signature Groups are sent to
different collectors. Note that the signer needs to get the same
Payload Block to each collector, as for any given signer there is a
one-to-one relationship between Payload Block and Reboot Session
across all Signature Groups. Deployments that wish to associate
different key material (and hence different Payload Blocks) with
different Signature Groups can use separate signers for that purpose,
each distinguished by its own combination of HOSTNAME, APP-NAME, and
PROCID.
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 1
to 8 octets with leading zeroes omitted.
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". (Note, it is not numbered "0".)
5.3.2.6. Fragment Length
The total length of this fragment expressed as a decimal integer with
1 to 4 octets with leading zeroes omitted. 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.
Its length must match the indicated fragment length.
5.3.2.8. Signature
This is a digital signature, encoded in base64, as per [RFC4648].
The Version field effectively specifies the original encoding of the
signature. The signature is calculated over the completely formatted
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Certificate Block message, before the SIGN parameter is added (see
Section 4.2.8). For the OpenPGP DSA signature scheme, the value of
the signature field contains the DSA values r and s, encoded as 2
multiprecision integers (see [RFC4880], Sections 5.2.2 and 3.2),
concatenated, and then encoded in base64 [RFC4648].
5.3.2.9. Example
An example of a Certificate Block message is depicted below, broken
into lines to fit publication rules. There are no spaces at the end
of the lines that contain the key blob and the signature.
<110>1 2009-05-03T14:00:39.519307+02:00 host.example.org syslogd
2138 - [ssign-cert VER="0111" RSID="1" SG="0" SPRI="0" TPBL="587"
INDEX="1" FLEN="587" FRAG="2009-05-03T14:00:39.519005+02:00 K BACsLMZ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=="
SIGN="AKAQEUiQptgpd0lKcXbuggGXH/dCdQCgdysrTBLUlbeGAQ4vwrnLOqSL7+c="]
The message is of syslog-sign protocol version "01". It uses SHA1 as
hash algorithm and an OpenPGP DSA signature scheme. Its reboot
session ID is 1. Its Signature Group is 0; its Signature Priority is
0. The Total Payload Block Length is 587 octets. The index into the
payload block is 1 (meaning this is the first fragment). The length
of the fragment is 587 (meaning that the Certificate Block message
contains the entire Payload Block). The Payload Block has the
timestamp 2009-05-03T14:00:39.519005+02:00. The Key Blob Type is
'K', meaning that it contains a public key whose corresponding
private key is being used to sign these messages.
Note that the Certificate Block message in this example has a
timestamp that is very close to the timestamp in the Payload Block.
The fact that the timestamps are so close implies that this is the
first Certificate Block message sent in this reboot session;
additional Certificate Block messages can be sent later with a later
timestamp, which will carry the same Payload Block that will still
contain the same timestamp.
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6. Redundancy and Flexibility
As described in Section 8.5 of [RFC5424], a transport sender may
discard syslog messages. Likewise, when syslog messages are sent
over unreliable transport, 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. The signer is allowed to
send Signature and Certificate Blocks multiple times. Sending
Signature and Certificate Blocks multiple times provides redundancy
with the intent to ensure that the collector or relay does get the
Signature Blocks and in particular the Payload Block at some point in
time. In the meantime, any online review of logs as described in
Section 7.2 is delayed until the needed blocks are received. The
collector MUST ignore duplicates of Signature Blocks and Certificate
Blocks that it has already received and authenticated. In principle,
the signer can change its redundancy level for any reason, without
communicating this fact to the collector.
A signer that is also the originator of messages that it signs does
not need to queue up other messages while sending redundant
Certificate Block and Signature Block messages. It MAY send
redundant Certificate Block messages even after Signature Block
messages and regular syslog messages have been sent. By the same
token, it MAY send redundant Signature Block messages even after
newer syslog messages that are signed by a subsequent Signature Block
have been sent, or even after a subsequent Signature Block message.
In addition, the signer has flexibility in how many hashes to include
within a Signature Block. It is legitimate for an originator to send
short Signature Blocks to allow the collector to verify messages with
minimal delay.
6.1. Configuration Parameters
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 that may be useful in
controlling redundant transmission from the transport sender to the
transport receiver and that may be useful for administrators to
configure.
6.1.1. Configuration Parameters for Certificate Blocks
Certificate Blocks are always sent at the beginning of a new reboot
session. One technique to ensure reliable delivery (see Section 8.5)
is to send multiple copies. This can be controlled by a
"certInitialRepeat" parameter:
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certInitialRepeat = number of times each Certificate Block should
be sent before the first message is sent.
It is also useful to resend Certificate Blocks every now and then for
long-lived reboot sessions. This can be controlled by the
certResendDelay and certResendCount parameters:
certResendDelay = maximum time delay in seconds until resending
the Certificate Block.
certResendCount = maximum number of other syslog messages to send
until resending the Certificate Block.
In some cases, it may be desirable to allow for configuration of the
transport sender such that Certificate Blocks are not sent at all
after the first normal syslog message has been sent. This could be
expressed by setting both certResendDelay and certResendCount to "0".
However, configuring the transport sender to send redundant
Certificate Blocks even after the first message, in particular when
the UDP transport [RFC5426] is used, is RECOMMENDED.
In one set of circumstances, the receiver may receive a Certificate
Block, some group of syslog messages, and some corresponding
Signature Blocks. If the receiver reboots after that, then the
conditions of recovery will vary depending upon the transport. For
UDP [RFC5426], the receiver SHOULD continue to use the cached
Certificate Block, but MUST validate the RSID value to make sure that
it has the most current one. If the receiver cannot validate that it
has the most current Certificate Block, then it MUST wait for a
retransmission of the Certificate Block, which may be controlled by
the certResendDelay and certResendCount parameters. It is up to the
operators to ensure that Certificate Blocks are sent frequently
enough to meet this set of circumstances.
For TLS transport [RFC5425], the sender MUST send a fresh Certificate
Block when a session is established. This will keep the sender and
receiver synchronized with the most current Certificate Block.
Implementations that support sending syslog messages of different
Signature Groups to different collectors and which wish to offer very
granular controls MAY allow the above parameters to be configured on
a per Signature Group basis.
The choice of reasonable values in a given deployment depends on
several factors, including the acceptable delay that may be incurred
from the receipt of a syslog message until the corresponding
Signature Block is received, whether UDP or TLS transport is used,
and the available management bandwidth. The following might be a
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reasonable choice for a deployment in which reliability of underlying
transport and of collector implementation are of little concern:
certInitialRepeat=1, certResendDelay=1800 seconds,
certResendCount=10000
The following might be a reasonable choice for a deployment in which
reliability of transmission over UDP transport could be an issue:
certInitialRepeat=2, certResendDelay=300 seconds,
certResendCount=1000
6.1.2. Configuration Parameters for Signature Blocks
Verification of log messages involves a certain delay of time that is
caused by the lag in time between the sending of the message itself
and the corresponding Signature Block. The following configuration
parameter can be useful to limit the time lag that will be incurred
(note that the maximum message length may also force generating a
Signature Block; see Sections 4.2.6 and 4.2.7):
sigMaxDelay = generate a new Signature Block if this many seconds
have elapsed since the message with the First Message Number of
the Signature Block was sent.
Retransmissions of Signature Blocks are not sent immediately after
the original transmission, but slightly later. The following
parameters control when those retransmissions are done:
sigNumberResends = number of times a Signature Block is resent.
(It is recommended to select a value of greater than "0" in
particular when the UDP transport [RFC5426] is used.)
sigResendDelay = send the next retransmission when this many
seconds have elapsed since the previous sending of this Signature
Block.
sigResendCount = send the next retransmission when this many other
syslog messages have been sent since the previous sending of this
Signature Block.
The choice of reasonable values in a given deployment depends on
several factors, including the acceptable delay that may be incurred
from the receipt of a syslog message until the corresponding
Signature Block is received so that the syslog message can be
verified, the reliability of the underlying transport, and the
available management bandwidth. The following might be a reasonable
choice for a deployment where reliability of transport and collector
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are of little concern and where there is a need to have syslog
messages generally signed within 5 minutes:
sigMaxDelay=300 seconds, sigNumberResends=2, sigResendDelay=300
seconds, sigResendCount=500
The following would be a reasonable choice for a deployment that
needs to validate syslog messages typically within 60 seconds, but no
more than 3 minutes after receipt:
sigMaxDelay=30 seconds, sigNumberResends=5, sigResendDelay=30
seconds, sigResendCount=100
6.2. Overlapping Signature Blocks
Notwithstanding the fact that the signer is not constrained in
whether it decides to send redundant Signature Block messages,
Signature Blocks SHOULD NOT overlap. This facilitates their
processing by the receiving collector. This means that an originator
of Signature Block messages, after having sent a first message with
some First Message Number and a Count, SHOULD NOT send a second
message with the same First Message Number but a different Count. It
also means that an originator of Signature Block messages SHOULD NOT
send a second message whose First Message Number is greater than the
First Message Number, but smaller than the First Message Number plus
the Count indicated in the first message.
That said, the possibility of Signature Blocks that overlap does
provide additional flexibility with regard to redundancy; it provides
an additional option that may be desirable in some deployments.
Therefore, collectors MUST be designed in a way that they can cope
with overlapping Signature Blocks when confronted with them. The
collector MUST ignore hashes of messages that it has already received
and validated.
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. This section
outlines a method for online and a method for offline verification of
logs that implementations MAY choose to implement to verify logs
efficiently. Implementations MAY also choose to implement a
different method; it is ultimately up to each implementation how to
process the messages that it receives.
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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.
To do so, we first go through the stored log file. Each message in
the log file is classified as a normal message, a Signature Block
message, or a Certificate Block message. Signature Blocks and
Certificate Blocks are then separated by signer (as identified by
HOSTNAME, APP-NAME, PROCID), Reboot Session ID, and Signature Group,
and stored in their own files. Normal messages are stored in a keyed
file, indexed on their hash values. They are not separated by
signer, as their (HOSTNAME, APP-NAME, PROCID) identifies the
application that generated the message. The application that
generated the message does not have to coincide with the signer.
For each signer, Reboot Session ID, and Signature Group, we then:
a. 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.
b. 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. We
initialize a cursor for the last message number processed with
the number 0. For each Signature Block in the file, we do the
following:
1. Verify the signature on the Signature Block.
2. If the value of the First Message Number of the Signature
Block is less than or equal to the last message number
processed, skip the first (last message number processed
minus First Message Number plus 1) hashes.
3. For each remaining hashed message in the Signature Block:
a. Look up the hash value in the keyed message file.
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b. If the message is found, write (message number, message
text) to the authenticated log file.
4. Set the last message number processed to the value of the
First Message Number plus the Count of the Signature Block
minus 1.
5. Skip all other Signature Blocks with the same First Message
Number unless one with a larger Count is encountered.
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 that have been
authenticated. We will assume that we are handling only one
signer, Signature Group, and Reboot Session ID at any given time.
(For the concurrent support of multiple signers, Signature
Groups, and Reboot Session IDs, the same procedure is applied
analogously to each. Signature Block messages and Certificate
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Block messages clearly indicate their respective signer,
Signature Group, and Reboot Session ID.)
b. We have two data structures: A "Waiting for Signature" queue in
which (arrival sequence, hash of message) pairs are kept in
sorted order, and a "Waiting for Message" queue in which (message
number, hash of message) pairs are kept in sorted order. In
addition, we have 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.
Note: The "Waiting for Signature" queue gets used in the normal
case, when the signature arrives after the message itself. It
holds messages that have been received but whose signature has
yet to arrive. The "Waiting for Message" queue gets used in the
case that messages are lost or misordered (either in the network
or in relays). It holds signatures that have been received but
whose corresponding messages have yet to arrive. Since a single
Signature Block can cover only a limited number of messages (due
to size restrictions), and massive reordering/delaying is rare,
it is expected that both queues would be relatively small.
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 additional Certificate Block
message that arrives is discarded. Any syslog messages or
Signature Block messages that arrive before all Certificate
Blocks have been received need to be buffered. Once all
Certificate Blocks have been received, the messages in the buffer
can be retrieved and processed as if they were just arriving.
d. Whenever a normal message arrives, we first check if its hash
value is found in the "Waiting for Message" queue. If it is, we
write the message number (from the "Waiting for Message" queue)
and the message into the authenticated message file and remove
the entry from the queue.
Otherwise, we add (arrival sequence, hash of message) to the
"Waiting for Signature" queue. If our hash table already 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 "Waiting for Signature" message queue is full, we remove
the oldest message from the queue. That message could not be
validated close enough to real time. In order to update the hash
table accordingly, we use that entry's hash to index the hash
table. If that entry has count 1, we delete the entry from the
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hash table; otherwise, we decrement its count. By removing the
message from the "Waiting for Signature" message queue without
having actually received the message's signature, we make it
impossible to authenticate the message should its signature
arrive later. Implementers therefore need to ensure that queues
are dimensioned sufficiently large to not expose the collector
against Denial-of-Service (DoS) attacks that attempt to flood the
collector with unsigned messages.
e. Whenever a Signature Block message arrives, we check its
originator, (i.e., the signer) by way of HOSTNAME, APP-NAME, and
PROCID, as well as its Signature Group and Reboot Session ID to
ensure it matches our Certificate Blocks. We then 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 and the same Count or a greater Count has already been
received. If so, we discard the Signature Block. We then check
the signature. Again, we discard the Signature Block if the
signature is not valid.
Otherwise, we proceed with processing the hashes in the Signature
Block. A Signature Block contains a sequence of hashes, each of
which is associated with a message number, starting with the
First Message Number for the first hash and incrementing by one
for each subsequent hash. For each hash, we first check to see
whether the message hash is in the hash table. If this is the
case, it means that we have received the signature for a message
that was received earlier, and we do the following:
1. We check if a message with the same message number is already
in the authenticated message file. If that is the case, the
signed hash is a duplicate and we discard it.
2. Otherwise (the signed hash is not a duplicate), we write the
(message number, message text) into the authenticated message
file. We also update the hash table accordingly, using that
entry's hash to index the hash table. If that entry has
Count 1, we delete the entry from the hash table; otherwise,
we decrement its count.
Otherwise (the message hash is not in the hash table), we write
the (message number, message hash) to the "Waiting for Message"
queue.
If the "Waiting for Message" queue is full, we remove the oldest
entry. In that case, a message that was signed by the signer
could not be validated by the receiver, either because the
message was lost or because the signature arrived way ahead of
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the actual message. By removing the entry from the "Waiting for
Message" queue without having actually received the message, we
make it impossible to authenticate the a legitimate message
should that message still arrive later. Implementers need to
ensure queues are dimensioned sufficiently large so that the
chances of such a scenario actually occurring is minimized.
f. The result of this is a sequence of messages in the authenticated
message file. Each message in the message file has been
authenticated. The sequence is labeled with numbers showing the
order in which the messages were originally transmitted.
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.
8. Security Considerations
Normal syslog event messages are unsigned and have most of the
security attributes described in Section 8 of [RFC5424]. 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
[RFC5424].
The model for syslog-sign is a direct trust system where the
certificate transferred is its own trust anchor. If a transport
sender sends a stream of syslog messages that is signed using a
certificate, the operator or application will transfer to the
transport receiver the certificate that was used when signing. There
is no need for a certificate chain.
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.
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Certain operations in this specification involve the use of random
numbers. An appropriate entropy source SHOULD be used to generate
these numbers. See [RFC4086] and [NIST800.90].
8.2. Packet Parameters
As a signer, it is advisable to avoid message lengths exceeding 2048
octets. Various problems might result if a signer 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.
Signers need to rigidly adhere to the RFC 5424 format when sending
messages. If a collector receives a message that is not formatted
properly, then it might drop it, or it may modify it while receiving
it. (See Appendix A.2 of [RFC5424].) If that were to happen, the
hash of the sent message would not match the hash of the received
message.
Collectors are not to malfunction in the case that 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.
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 he or she 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
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been replayed. Using a method for the verification of logs such as
the one outlined in Section 7, a replayed message can be detected by
checking prior to writing a message to the authenticated log file
whether the message is already contained in it.
8.5. Reliable Delivery
Event messages sent over UDP might be lost in transit. [RFC5425] can
be used for the reliable delivery of syslog messages; however, it
does not protect against loss of syslog messages at the application
layer, for example, if the TCP connection or TLS session has been
closed by the transport receiver for some reason. A reviewer can
identify 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 signer.
8.8. Message Observation
Unless TLS is used as a secure transport [RFC5425], 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
Kelsey, et al. Standards Track [Page 34]
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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 signer. If that is the case, then
this MITM attack will not succeed. Methods for establishing a chain
of trust are also described in [RFC5425].
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. Implementers 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.
9. IANA Considerations
9.1. Structured Data and Syslog Messages
With regard to [RFC5424], IANA has added the following values (with
each parameter listed as mandatory) to the registry titled "syslog
Structured Data ID Values":
Kelsey, et al. Standards Track [Page 35]
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Structured Data ID Structured Data Parameter
------------------ -------------------------
ssign
VER
RSID
SG
SPRI
GBC
FMN
CNT
HB
SIGN
ssign-cert
VER
RSID
SG
SPRI
TPBL
INDEX
FLEN
FRAG
SIGN
In addition, several fields are controlled by the IANA in both the
Signature Block and the Certificate Block, as outlined in the
following sections.
9.2. Version Field
IANA has created three registries, each associated with a different
subfield of the Version field of Signature Blocks and Certificate
Blocks, described in Sections 4.2.1 and 5.3.2.1, respectively.
The first registry that IANA has created is titled "syslog-sign
Protocol Version Values". It is for the values of the Protocol
Version subfield. The Protocol Version subfield constitutes the
first two octets in the Version field. New values shall be assigned
by the IANA using the "IETF Review" policy defined in [RFC5226].
Assigned numbers are to be increased by 1, 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.
Kelsey, et al. Standards Track [Page 36]
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IANA has registered the Protocol Version values shown below.
Value Protocol Version
----- ----------------
00 Reserved
01 Defined in RFC 5848
The second registry that IANA has created is titled "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 Review" policy
defined in [RFC5226]. Assigned values are to be increased
sequentially, first up to a maximum value of "9", then from "a" to
"z", then from "A" to "Z". 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 has registered 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 has created is titled "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
Review" policy defined in [RFC5226]. 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
concurrently.
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IANA has registered the Signature Scheme values shown below.
Value Protocol Version Signature Scheme
----- ---------------- ----------------
0 01 Reserved
1 01 OpenPGP DSA
9.3. SG Field
IANA has created a registry titled "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 Review" policy
defined in [RFC5226]. 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 has registered the SG Field values shown below.
Value Meaning
----- -------
0 There is only one Signature Group.
1 Each PRI value is associated with its own Signature
Group.
2 Each Signature Group contains a range of PRI
values.
3 Signature Groups are not assigned with any of the
above relationships to PRI values of the syslog
messages they sign.
9.4. Key Blob Type
IANA has created a registry titled "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 Review" policy defined in [RFC5226]. Uppercase
letters may be assigned as values. Lowercase letters are left as
vendor specific and shall not be assigned by the IANA.
IANA has registered 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 pre-distributed
U installation-specific key exchange information
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10. Acknowledgements
The authors wish to thank the current Chairs of the Syslog Working
Group, David Harrington and Chris Lonvick, and the other members of
the Working Group, in particular Alex Brown, Chris Calabrese, Steve
Chang, Pasi Eronen, Carson Gaspar, Rainer Gerhards, Drew Gross,
Albert Mietus, Darrin New, Marshall Rose, Andrew Ross, Martin
Schuette, Holt Sorenson, Rodney Thayer, and the many Counterpane
Internet Security engineering and operations people who commented on
various versions of this proposal.
11. References
11.1. Normative References
[FIPS.186-2.2000] National Institute of Standards and Technology,
"Digital Signature Standard", FIPS PUB 186-2,
January 2000, <http://csrc.nist.gov/publications/
fips/archive/fips186-2/fips186-2.pdf>.
[FIPS.180-2.2002] 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>.
[RFC2119] Bradner, S., "Key words for use in RFCs to
Indicate Requirement Levels", BCP 14, RFC 2119,
March 1997.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64
Data Encodings", RFC 4648, October 2006.
[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D.,
and R. Thayer, "OpenPGP Message Format", RFC 4880,
November 2007.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for
Writing an IANA Considerations Section in RFCs",
BCP 26, RFC 5226, May 2008.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen,
S., Housley, R., and W. Polk, "Internet X.509
Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile",
RFC 5280, May 2008.
[RFC5424] Gerhards, R., "The syslog Protocol", RFC 5424,
March 2009.
Kelsey, et al. Standards Track [Page 39]
RFC 5848 Signed Syslog Messages May 2010
[RFC5425] Miao, F., Yuzhi, M., and J. Salowey, "TLS
Transport Mapping for syslog", RFC 5425,
March 2009.
[RFC5426] Okmianski, A., "Transmission of syslog Messages
over UDP", RFC 5426, March 2009.
11.2. Informative References
[NIST800.90] 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/
SP800-90revised_March2007.pdf>.
[RFC3339] Klyne, G. and C. Newman, "Date and Time on the
Internet: Timestamps", RFC 3339, July 2002.
[RFC3414] Blumenthal, U. and B. Wijnen, "User-based Security
Model (USM) for version 3 of the Simple Network
Management Protocol (SNMPv3)", RFC 3414,
December 2002.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker,
"Randomness Recommendations for Security",
RFC 4086, June 2005.
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. Standards Track [Page 40]