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rfc:rfc5848

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

Kelsey, et al. Standards Track [Page 2] RFC 5848 Signed Syslog Messages May 2010

   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

Kelsey, et al. Standards Track [Page 3] RFC 5848 Signed Syslog Messages May 2010

 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

Kelsey, et al. Standards Track [Page 4] RFC 5848 Signed Syslog Messages May 2010

 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

Kelsey, et al. Standards Track [Page 5] RFC 5848 Signed Syslog Messages May 2010

 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.

Kelsey, et al. Standards Track [Page 6] RFC 5848 Signed Syslog Messages May 2010

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".

Kelsey, et al. Standards Track [Page 7] RFC 5848 Signed Syslog Messages May 2010

 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.

Kelsey, et al. Standards Track [Page 8] RFC 5848 Signed Syslog Messages May 2010

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.

Kelsey, et al. Standards Track [Page 9] RFC 5848 Signed Syslog Messages May 2010

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.

Kelsey, et al. Standards Track [Page 10] RFC 5848 Signed Syslog Messages May 2010

 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

Kelsey, et al. Standards Track [Page 11] RFC 5848 Signed Syslog Messages May 2010

     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

Kelsey, et al. Standards Track [Page 12] RFC 5848 Signed Syslog Messages May 2010

 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.

Kelsey, et al. Standards Track [Page 13] RFC 5848 Signed Syslog Messages May 2010

 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

Kelsey, et al. Standards Track [Page 14] RFC 5848 Signed Syslog Messages May 2010

 [" 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.

Kelsey, et al. Standards Track [Page 15] RFC 5848 Signed Syslog Messages May 2010

 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).

Kelsey, et al. Standards Track [Page 16] RFC 5848 Signed Syslog Messages May 2010

 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].

Kelsey, et al. Standards Track [Page 17] RFC 5848 Signed Syslog Messages May 2010

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

Kelsey, et al. Standards Track [Page 18] RFC 5848 Signed Syslog Messages May 2010

     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.

Kelsey, et al. Standards Track [Page 19] RFC 5848 Signed Syslog Messages May 2010

 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].

Kelsey, et al. Standards Track [Page 20] RFC 5848 Signed Syslog Messages May 2010

     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.

Kelsey, et al. Standards Track [Page 21] RFC 5848 Signed Syslog Messages May 2010

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

Kelsey, et al. Standards Track [Page 22] RFC 5848 Signed Syslog Messages May 2010

 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
 NCV2NUAwe4RAeAnSQuvv2KS51SnHFAaWJNU2XVDYvW1LjmJgg4vKvQPo3HEOD+2hEkt1z
 cXADe03u5pmHoWy5FGiyCbglYxJkUJJrQqlTSS6vID9yhsmEnh07w3pOsxmb4qYo0uWQr
 AAenBweVMlBgV3ZA5IMA8xq8l+i8wCgkWJjCjfLar7s+0X3HVrRroyARv8EAIYoxofh9m
 N8n821BTTuQnz5hp40d6Z3UudKePu2di5Mx3GFelwnV0Qh5mSs0YkuHJg0mcXyUAoeYry
 5X6482fUxbm+gOHVmYSDtBmZEB8PTEt8Os8aedWgKEt/E4dT+Hmod4omECLteLXxtScTM
 gDXyC+bSBMjRRCaeWhHrYYdYBACCWMdTc12hRLJTn8LX99kv1I7qwgieyna8GCJv/rEgC
 ssS9E1qARM+h19KovIUOhl4VzBw3rK7v8Dlw/CJyYDd5kwSvCwjhO21LiReeS90VPYuZF
 RC1B82Sub152zOqIcAWsgd4myCCiZbWBsuJ8P0gtarFIpleNacCc6OV3i2Rg=="
 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.

Kelsey, et al. Standards Track [Page 23] RFC 5848 Signed Syslog Messages May 2010

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:

Kelsey, et al. Standards Track [Page 24] RFC 5848 Signed Syslog Messages May 2010

    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

Kelsey, et al. Standards Track [Page 25] RFC 5848 Signed Syslog Messages May 2010

 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

Kelsey, et al. Standards Track [Page 26] RFC 5848 Signed Syslog Messages May 2010

 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.

Kelsey, et al. Standards Track [Page 27] RFC 5848 Signed Syslog Messages May 2010

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.

Kelsey, et al. Standards Track [Page 28] RFC 5848 Signed Syslog Messages May 2010

         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

Kelsey, et al. Standards Track [Page 29] RFC 5848 Signed Syslog Messages May 2010

     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

Kelsey, et al. Standards Track [Page 30] RFC 5848 Signed Syslog Messages May 2010

     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

Kelsey, et al. Standards Track [Page 31] RFC 5848 Signed Syslog Messages May 2010

     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.

Kelsey, et al. Standards Track [Page 32] RFC 5848 Signed Syslog Messages May 2010

 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

Kelsey, et al. Standards Track [Page 33] RFC 5848 Signed Syslog Messages May 2010

 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] RFC 5848 Signed Syslog Messages May 2010

 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] RFC 5848 Signed Syslog Messages May 2010

        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] RFC 5848 Signed Syslog Messages May 2010

 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.

Kelsey, et al. Standards Track [Page 37] RFC 5848 Signed Syslog Messages May 2010

 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

Kelsey, et al. Standards Track [Page 38] RFC 5848 Signed Syslog Messages May 2010

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]

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