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syslog Working Group                                           J. Kelsey
Internet-Draft                                                      NIST
Intended status: Standards Track                               J. Callas
Expires: November 28, 2009                               PGP Corporation
                                                                A. Clemm
                                                           Cisco Systems
                                                            May 27, 2009


                         Signed syslog Messages
                     draft-ietf-syslog-sign-26.txt

Status of this Memo

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Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the



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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
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Abstract

   This document describes a mechanism to add origin authentication,
   message integrity, replay resistance, message sequencing, and
   detection of missing messages to the transmitted syslog messages.
   This specification is intended to be used in conjunction with the
   work defined in [RFC5424], "The syslog Protocol".


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Conventions Used in this Document  . . . . . . . . . . . . . .  7
   3.  syslog Message Format  . . . . . . . . . . . . . . . . . . . .  8
   4.  Signature Blocks . . . . . . . . . . . . . . . . . . . . . . . 10
     4.1.  syslog Messages Containing a Signature Block . . . . . . . 10
     4.2.  Signature Block Format and Fields  . . . . . . . . . . . . 11
       4.2.1.  Version  . . . . . . . . . . . . . . . . . . . . . . . 12
       4.2.2.  Reboot Session ID  . . . . . . . . . . . . . . . . . . 12
       4.2.3.  Signature Group and Signature Priority . . . . . . . . 13
       4.2.4.  Global Block Counter . . . . . . . . . . . . . . . . . 15
       4.2.5.  First Message Number . . . . . . . . . . . . . . . . . 16
       4.2.6.  Count  . . . . . . . . . . . . . . . . . . . . . . . . 16
       4.2.7.  Hash Block . . . . . . . . . . . . . . . . . . . . . . 16
       4.2.8.  Signature  . . . . . . . . . . . . . . . . . . . . . . 17
       4.2.9.  Example  . . . . . . . . . . . . . . . . . . . . . . . 17
   5.  Payload and Certificate Blocks . . . . . . . . . . . . . . . . 19
     5.1.  Preliminaries: Key Management and Distribution Issues  . . 19
     5.2.  Payload Block  . . . . . . . . . . . . . . . . . . . . . . 20
       5.2.1.  Block Format and Fields  . . . . . . . . . . . . . . . 20
       5.2.2.  Originator Authentication and Authorization  . . . . . 21
     5.3.  Certificate Block  . . . . . . . . . . . . . . . . . . . . 22
       5.3.1.  syslog Messages Containing a Certificate Block . . . . 22
       5.3.2.  Certificate Block Format and Fields  . . . . . . . . . 23
   6.  Redundancy and Flexibility . . . . . . . . . . . . . . . . . . 27
     6.1.  Configuration parameters . . . . . . . . . . . . . . . . . 27
       6.1.1.  Configuration Parameters for Certificate Blocks  . . . 27
       6.1.2.  Configuration Parameters for Signature Blocks  . . . . 28
     6.2.  Overlapping Signature Blocks . . . . . . . . . . . . . . . 29
   7.  Efficient Verification of Logs . . . . . . . . . . . . . . . . 30
     7.1.  Offline Review of Logs . . . . . . . . . . . . . . . . . . 30
     7.2.  Online Review of Logs  . . . . . . . . . . . . . . . . . . 31
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 34
     8.1.  Cryptographic Constraints  . . . . . . . . . . . . . . . . 34
     8.2.  Packet Parameters  . . . . . . . . . . . . . . . . . . . . 34
     8.3.  Message Authenticity . . . . . . . . . . . . . . . . . . . 35
     8.4.  Replaying  . . . . . . . . . . . . . . . . . . . . . . . . 35
     8.5.  Reliable Delivery  . . . . . . . . . . . . . . . . . . . . 35



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     8.6.  Sequenced Delivery . . . . . . . . . . . . . . . . . . . . 35
     8.7.  Message Integrity  . . . . . . . . . . . . . . . . . . . . 36
     8.8.  Message Observation  . . . . . . . . . . . . . . . . . . . 36
     8.9.  Man In The Middle Attacks  . . . . . . . . . . . . . . . . 36
     8.10. Denial of Service  . . . . . . . . . . . . . . . . . . . . 36
     8.11. Covert Channels  . . . . . . . . . . . . . . . . . . . . . 36
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 37
     9.1.  Structured Data and syslog messages  . . . . . . . . . . . 37
     9.2.  Version Field  . . . . . . . . . . . . . . . . . . . . . . 37
     9.3.  SG Field . . . . . . . . . . . . . . . . . . . . . . . . . 39
     9.4.  Key Blob Type  . . . . . . . . . . . . . . . . . . . . . . 39
   10. Working Group  . . . . . . . . . . . . . . . . . . . . . . . . 41
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 42
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 43
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 43
     12.2. Informative References . . . . . . . . . . . . . . . . . . 43
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 45


































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1.  Introduction

   This document describes a mechanism, called syslog-sign in this
   document, that adds origin authentication, message integrity, replay
   resistance, message sequencing, and detection of missing messages to
   syslog.  Essentially, this is accomplished by sending a special
   syslog message.  The contents of this syslog message is called a
   Signature Block.  Each Signature Block contains, in effect, a
   detached signature on some number of previously sent messages.  It is
   cryptographically signed and contains the hashes of previously sent
   syslog messages.

   While most implementations of syslog involve only a single originator
   and a single collector of each message, provisions need to be made to
   cover situations in which messages are sent to multiple collectors.
   This concerns, in particular, situations in which different messages
   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
   kept together for signing purposes by the originator.  A Signature
   Block always belongs to exactly one Signature Group and always signs
   messages belonging only to that Signature Group.

   Additionally, an originator sends a Certificate Block to provide key
   management information between the originator and the collector.
   This Certificate Block has a field to denote the type of key material
   which may be such things as a PKIX certificate, an OpenPGP
   certificate, or even an indication that a key had been pre-
   distributed.  In the cases of certificates being sent, the
   certificates may have to be split across multiple packets.

   It is possible that the same host contains multiple originators that
   each use their own keys to sign syslog messages.  In this case, each
   originator sends its own Certificate Block and Signature Blocks.
   Furthermore, each originator defines its own Signature Groups.  Each
   originator on a given host needs to use a a distinct combination of
   APP-NAME and PROCID for its Signature Block and Certificate Block
   message.  (This implies that, provided the originators include in
   their messages a HOSTNAME that uniquely identifies a host, not a
   NILVALUE, the combination of HOSTNAME, APP-NAME and PROCID uniquely
   distinguishes originators of syslog-sign messages across hosts.)



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   The collector of the previous messages may verify that the hash of
   each received message matches the signed hash contained in the
   Signature Block.  A collector may process these Signature Blocks as
   they arrive, building an authenticated log file.  Alternatively, it
   may store all the log messages in the order they were received.  This
   allows a network operator to authenticate the log file at the time
   the logs are reviewed.

   The mechanism described in this specification is intended to be used
   in conjunction with the syslog protocol as defined in [RFC5424] as
   its message delivery mechanism and uses the concept of STRUCTURED-
   DATA elements defined in that document.  In fact, this specification
   mandates implementation of syslog protocol.  Nevertheless, it is
   conceivable that the concepts underlying this mechanism could also be
   used in conjunction with other message delivery mechanisms.
   Designers of other efforts to define event notification mechanisms
   are therefore encouraged to consider this specification in their
   designs.

































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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].














































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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 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.  [RFC3164] describes relay behavior in which
   syslog messages are altered.  If such behavior were to occur in
   conjunction with syslog-sign, it would 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 originator and collector
   implementations implementing this specification MUST support messages
   of up to and including 2048 octets in length, in order to minimize
   the chance of truncation.  While syslog originator and collector
   implementations MAY support messages with a length larger than 2048
   octets, implementers need to be aware that any message truncations
   that occur render the mechanism useless.

   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 signature and
   certificate data as Structured Data.  The special syslog messages
   defined in this document include for this purpose definitions of SDEs
   to convey parameters that relate to the signing of syslog messages.
   The MSG part of the syslog messages defined in this document SHOULD
   simply be empty -- the content of the messages is not intended for
   interpretation by humans but by applications that use those messages
   to build an authenticated log.

   Because the syslog messages defined in this document adhere to the
   format described in [RFC5424], they identify the machine that
   originates the syslog message in the HOSTNAME field.  Therefore, the
   signature and certificate data do not need to include any additional
   parameter to identify the machine that orginates the message.

   In addition, the same host MAY host several originators that each
   sign syslog messages independently of each other, each using their



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   own Signature Groups.  In the context of syslog-sign, different such
   originators on the same host are distinguished by the combination of
   APP-NAME and PROCID fields used in their messages, hence the
   combination of APP-NAME and PROCID MUST be unique for an originator
   on a given host.  (This implies that, provided the message contains a
   HOSTNAME that uniquely identifies a host, not a NILVALUE, the
   combination of HOSTNAME, APP-NAME and PROCID uniquely identifies an
   originator across hosts.)  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.








































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4.  Signature Blocks

   This section describes the format of the Signature Block and the
   fields used within the Signature Block, as well as the syslog
   messages used to carry the Signature Block.

4.1.  syslog Messages Containing a Signature Block

   There is a need to distinguish the Signature Block itself from the
   syslog message that is used to carry a Signature Block.  Signature
   Blocks MUST be encompassed within completely formed syslog messages.
   Syslog messages that contain a Signature Block are also referred to
   as Signature Block messages.

   A Signature Block message is identified by the presence of an SD
   ELEMENT with an SD-ID with the value "ssign".  In addition, a
   Signature Block message MUST contain valid APP-NAME, PROCID, and
   MSGID fields to be compliant with [RFC5424].  This specification does
   not mandate particular values for these fields; however, for
   consistency, an originator 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
   originators per host, the combination of APP-NAME and PROCID MUST be
   unique for each such originator on any given host.  If an originator
   daemon is restarted, it MAY use a new PROCID for what is otherwise
   the same originator 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 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.







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4.2.  Signature Block Format and Fields

   The content of a Signature Block message is the Signature Block.  The
   Signature Block MUST be encoded as an SD ELEMENT, as defined in
   [RFC5424].

   The SD-ID MUST have the value of "ssign".

   The SDE contains the fields of the Signature Block encoded as SD
   Parameters, as specified in the following.  The Signature Block is
   composed of the following fields.  The value of each field MUST be
   printable ASCII, and any binary values MUST be base 64 encoded, as
   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
                                               (base 64 encoded binary)

      Signature                       SIGN             variable
                                               (base 64 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"]



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   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.

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 two 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.

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.



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   A Reboot Session ID is expected to strictly monotonically increase
   (i.e., to never repeat or decrease) whenever an originator 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, an originator
   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, implementers wish to
   consider using the snmpEngineBoots value as a source for this counter
   as defined in [RFC3414].

   In cases where an originator 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), then manual
   intervention may be required to subsequently reset it.

   If a reboot of an originator takes place, Signature Block messages
   MAY use a new PROCID.  However, Signature Block messages of the same
   originator 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.

   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.



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   Syslog-sign provides four options for handling Signature Groups,
   linking them with PRI values so they may be routed to the destination
   commensurate with the corresponding syslog messages.  In all cases,
   no more than 192 distinct Signature Groups (0-191) are permitted.

   The Signature Group to which a Signature Block pertains is indicated
   by the Signature Priority (SPRI) field.  The Signature Group (SG)
   field indicates how to interpret the Signature Priority field.  (Note
   that the SG field does not indicate the Signature Group itself, as
   its name might suggest.)  The SG field can have one of the following
   values:

   a.  "0" -- There is only one Signature Group.  In this case, the
       administrators want all Signature Blocks to be sent to a single
       destination; in all likelihood, all of the syslog messages will
       also be going to that same destination.  Signature Blocks sign
       all messages regardless of their PRI value.  This means that, in
       effect, the Signature Block's SPRI value can be ignored.
       However, it is RECOMMENDED that a single SPRI value be used for
       all Signature Blocks.  Furthermore, it is RECOMMENDED to set that
       value to the same value as the PRI field of the Signature Block
       message.  This way, the PRI of the Signature Block message
       matches the SPRI of the Signature Block that it contains.

   b.  "1" -- Each PRI value is associated with its own Signature Group.
       Signature Blocks for a given Signature Group have SPRI = PRI for
       that Signature Group.  In other words, the SPRI of the Signature
       Block matches the PRI value of the syslog messages that are part
       of the Signature Group and hence signed by the Signature Block.
       An SG value of 1 can, for example, be used when the administrator
       of an originator does not know where any of the syslog messages
       will ultimately go but anticipates that messages with different
       PRI values will be collected and processed separately.  Having a
       Signature Group per PRI value provides administrators with a
       large degree of flexibility with regard to how to divide up the
       processing of syslog messages and their signatures after they are
       received, at the same time allowing Signature Blocks to follow
       the corresponding syslog messages to their eventual destination.

   c.  "2" -- Each Signature Group contains a range of PRI values.
       Signature Groups are assigned sequentially.  A Signature Block
       for a given Signature Group has its own SPRI value denoting the
       highest PRI value of syslog messages in that Signature Group.
       The lowest PRI value of syslog messages in that Signature Group
       will be one larger than the SPRI value of the previous Signature
       Group or "0" in case there is no other Signature Group with a
       lower SPRI value.  The specific Signature Groups and ranges they
       are associated with are subject to configuration by a system



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       administrator.

   d.  "3" -- Signature Groups are not assigned with any of the above
       relationships to PRI values of the syslog messages they sign.
       Instead, another scheme is used, which is outside the scope of
       this specification.  There has to be some predefined arrangement
       between the originator and the intended collectors as to which
       syslog messages are to be included in which Signature Group,
       requiring configuration by a system administrator.  This provides
       administrators also with the flexibility to group syslog messages
       into Signature Groups according to criteria that are not tied to
       the PRI value.

   One reasonable way to configure some installations is to have only
   one Signature Group, indicated with SG=0, and have the originator
   send a copy of each Signature Block to each collector.  In that case,
   collectors that are not configured to receive every syslog message
   will still receive signatures for every message, even ones they are
   not supposed to receive.  While the collector will not be able to
   detect gaps in the messages (because the presence of a signature of a
   message that is missing does not tell the collector whether or not
   the corresponding message would be of the collector's concern), it
   does allow all messages that do arrive at each collector to be put
   into the right order and to be verified.  It also allows each
   collector to detect duplicates.  Likewise, configuring only one
   Signature Group can be a reasonable way to configure installations
   that involve relay chains, where one or more interim relays may or
   may not relay all messages to the same destination.

4.2.4.  Global Block Counter

   The Global Block Counter is a decimal value representing the number
   of Signature Blocks sent by syslog-sign before the current one, in
   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



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   Signature Groups.

   Because a reboot results in the start of a new reboot session, the
   originator MUST reset the Global Block Counter to 0 after a reboot
   occurs.  Applications need to take into account the possibility that
   a reboot occurred when authenticating a log, and situations in which
   reboots occur frequently may result in losing the ability to verify
   the proper sequence in which messages were sent, hence jeopardizing
   the integrity of the log.

4.2.5.  First Message Number

   This is a decimal value between 1 and 10 octets, with leading zeroes
   omitted.  It contains the unique message number within this Signature
   Group of the first message whose hash appears in this block.  The
   very first message of the reboot session is numbered "1".  This
   implies that when the Reboot Session ID increases, the message number
   is reset to 1.

   For example, if this Signature Group has processed 1000 messages so
   far and message number 1001 is the first message whose hash appears
   in this Signature Block, then this field contains 1001.  The message
   number is relative to the Signature Group to which it belongs; hence,
   a message number does not identify a message beyond its Signature
   Group.

   Should the message number reach 9999999999 within the same reboot
   session and Signature Group, the message number subsequently restarts
   at 1.  In such event, the Global Block Counter will be vastly
   different between two occurrences of the same message number.

4.2.6.  Count

   The count is a 1 or 2 octet field that indicates the number of
   message hashes to follow.  The valid values for this field are 1
   through 99.  The number of hashes included in the Signature Block
   MUST be chosen such that the length of the resulting syslog message
   does not exceed the maximum permissible syslog message length.

4.2.7.  Hash Block

   The hash block is a block of hashes, each separately encoded in base
   64.  Each hash in the hash block is the hash of the entire syslog
   message represented by the hash, independent of the underlying
   transport.  Hashes are ordered from left to right in the order of
   occurrence of the syslog messages that they represent.  The space
   character is used to separate the hashes.  Note, the hash block
   constitutes a single SD-Param; a Signature Block message MUST include



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   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 base 64 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 base 64 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 ["
   SIGN"], "=", and the corresponding value) is added.  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
   base 64 [RFC4648].

4.2.9.  Example

   An example of a Signature Block message is depicted below, broken
   into lines to fit internet-draft publication rules.  There is a space
   at the end of each line, with the exception of the last line which
   ends with "]" and the second-to-last line which ends with "ld6hg".


   <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="]




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   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.











































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5.  Payload and Certificate Blocks

   Certificate Blocks and Payload Blocks provide key management for
   syslog-sign.  Their purpose is to support key management that uses
   public key cryptosystems.

5.1.  Preliminaries: Key Management and Distribution Issues

   A Payload Block contains public key certificate information that is
   to be conveyed to the collector.  A Payload Block is sent at the
   beginning of a new reboot session, carrying public key information in
   effect for the reboot session.  However, a Payload Block is not sent
   directly, but in (one or more) fragments.  Those fragments are termed
   Certificate Blocks.  Therefore, originators send at least one
   Certificate Block at the beginning of a new reboot session.

   There are three key points to understand about Certificate Blocks:

   a.  They handle a variable-sized payload, fragmenting it if necessary
       and transmitting the fragments as legal syslog messages.  This
       payload is built (as described below) at the beginning of a
       reboot session and is transmitted in pieces with each Certificate
       Block carrying a piece.  There is exactly one Payload Block per
       reboot session.

   b.  The Certificate Blocks are digitally signed.  The originator does
       not sign the Payload Block, but the signatures on the Certificate
       Blocks ensure its authenticity.  Note that it may not even be
       possible to verify the signature on the Certificate Blocks
       without the information in the Payload Block; in this case the
       Payload Block is reconstructed, the key is extracted, and then
       the Certificate Blocks are verified.  (This is necessary even
       when the Payload Block carries a certificate, because some other
       fields of the Payload Block are not otherwise verified.)  In
       practice, most installations keep the same public key over long
       periods of time, so that most of the time, it is easy to verify
       the signatures on the Certificate Blocks, and use the Payload
       Block to provide other useful per-session information.

   c.  The kind of Payload Block that is expected is determined by what
       kind of key material is on the collector that receives it.  The
       originator and collector (or offline log viewer) both have some
       key material (such as a root public key or 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.





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5.2.  Payload Block

   The Payload Block is built when a new reboot session is started.
   There is a one-to-one correspondence between reboot sessions and
   Payload Blocks.  An originator creates a new Payload Block after each
   reboot.  The Payload Block is used until the next reboot.

5.2.1.  Block Format and Fields

   A Payload Block MUST have the following fields:

   a.  Full local time stamp for the originator at the time the reboot
       session started.  This must be in the time stamp format specified
       in [RFC5424] (essentially, time stamp format per [RFC3339] with
       some further restrictions).

   b.  Key Blob Type, a one-octet field containing one of five values:

       1.  'C' -- a PKIX certificate.

       2.  'P' -- an OpenPGP certificate (a Transferable Public Key as
           defined in [RFC4880], Section 11.1).

       3.  'K' -- the public key whose corresponding private key is
           being used to sign these messages.  For the OpenPGP DSA
           signature scheme, this field contains the DSA prime p, DSA
           group order q, DSA group generator g, and DSA public-key
           value y, encoded as four 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, base 64 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.

   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.



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   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].

5.2.2.  Originator 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 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 an 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, implementations MUST support
       fingerprints of X.509 certificates as described below.  Other
       methods MAY be supported.

       Collectors MUST support key blob type 'C', and specifying 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 specifying a list of
       HOSTNAME(s) this peer is allowed to use either as FQDNs or IP



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       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
       support wildcards of any type ("*", regular expressions, etc.) in
       locally configured names.

       Originator 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
       display 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".

       Originators 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 Certificate Block message.

   Because certificates can legitimately be much longer than 2048
   octets, the Payload Block can be split up into several pieces, with
   each Certificate Block carrying a piece of the Payload Block.  Note
   that the originator MAY make the Certificate Blocks of any legal
   length (that is, any length that keeps the entire Certificate Block
   message within 2048 octets) that holds all the required fields.



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   Software that processes Certificate Blocks MUST deal correctly with
   blocks of any legal length.  The length of the fragment of the
   Payload Block that a Certificate Block carries MUST be at least 1
   octet.  The length SHOULD be chosen such that the length of the
   Certificate Block message does not exceed 2048 octets.

   A Certificate Block message is identified by the presence of an SD
   ELEMENT with an SD-ID with the value "ssign-cert".  In addition, a
   Certificate Block message MUST contain valid APP-NAME, PROCID, and
   MSGID fields to be compliant with syslog protocol.  Syslog-sign does
   not mandate particular values for these fields; however, for
   consistency, an originator 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 originators
   per host, the combination of APP-NAME and PROCID MUST be unique for
   each such originator.  If an originator daemon is restarted, it MAY
   use a new PROCID for what is otherwise the same originator.  The
   combination of APP-NAME and PROCID MUST be the same that is used for
   Signature Block messages of the same originator; however, a different
   MSGID MAY be used for Signature Block and Certificate Block messages.
   It is RECOMMENDED to use 110 as value for the PRI field,
   corresponding to facility 13 and severity 6 (informational).  The
   Certificate Block is carried as Structured Data within the
   Certificate Block message.  It is also RECOMMENDED (but not required)
   that a Certificate Block message carry no other Structured Data
   besides the Structured Data of the Certificate Block itself.  The MSG
   part of a Certificate Block message SHOULD be empty.

5.3.2.  Certificate Block Format and Fields

   The contents of a Certificate Block message is the Certificate Block
   itself.  Like a Signature Block, the Certificate Block is encoded as
   an SD ELEMENT.  The SD-ID of the Certificate Block is "ssign-cert".
   The Certificate Block is composed of the following fields, each of
   which is encoded as an SD Parameter with parameter name as indicated.
   Each field must be printable ASCII, and any binary values are base 64
   encoded per [RFC4648].













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       Field                       SD-PARAM-NAME      Size in octets
       -----                       -------------      ---- -- ------

       Version                          VER                 4

       Reboot Session ID               RSID                1-10

       Signature Group                   SG                 1

       Signature Priority              SPRI                1-3

       Total Payload Block Length      TPBL                1-8

       Index into Payload Block       INDEX                1-8

       Fragment Length                 FLEN                1-4

       Payload Block Fragment          FRAG              variable
                                                (base 64 encoded binary)

       Signature                       SIGN             variable
                                                (base 64 encoded binary)

   The fields MUST be provided in the order listed.  New SD parameters
   MUST NOT be added unless a new Version of the protocol is defined.
   (Implementations that wish to add proprietary extensions will need to
   define a separate SD ELEMENT.)  A Certificate Block is accordingly
   encoded as follows, where xxx denotes a placeholder for the
   particular values:

   [ssign-cert VER="xxx" RSID="xxx" SG="xxx" SPRI="xxx" TPBL="xxx"
   INDEX="xxx" FLEN="xxx" FRAG="xxx" SIGN="xxx"]

   Values of the fields constitute SD parameter values and are hence
   enclosed in quotes, per [RFC5424].  The fields are separated by
   single spaces and are described below.  Each SD parameter MUST occur
   once and only once.

5.3.2.1.  Version

   The Version field is 4 octets in length.  This field is identical in
   format and meaning to the Version field described in Section 4.2.1.

5.3.2.2.  Reboot Session ID

   The Reboot Session ID is identical in format and meaning to the RSID
   field described in Section 4.2.2.




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5.3.2.3.  Signature Group and Signature Priority

   The SIG field is identical in format and meaning to the SIG field
   described in Section 4.2.3.  The SPRI field is identical in format
   and meaning to the SPRI field described there.

5.3.2.4.  Total Payload Block Length

   The Total Payload Block Length is a value representing the total
   length of the Payload Block in octets, expressed as a decimal with
   one to eight octets 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
   one to four 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 base 64, as per [RFC4648].
   The Version field effectively specifies the original encoding of the
   signature.  The signature is calculated over the completely formatted
   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 two
   multiprecision integers (see [RFC4880], Sections 5.2.2 and 3.2),
   concatenated, and then encoded in base 64 [RFC4648].

5.3.2.9.  Example

   An example of a Certificate Block message is is depicted below,
   broken into lines to fit internet-draft publication rules.  There are
   no spaces at the end of the lines that contain the key blob and the
   signature.




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   <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 BACsLMZNCV2NUAwe4RA
   eAnSQuvv2KS51SnHFAaWJNU2XVDYvW1LjmJgg4vKvQPo3HEOD+2hEkt1zcXADe03u5pmHoW
   y5FGiyCbglYxJkUJJrQqlTSS6vID9yhsmEnh07w3pOsxmb4qYo0uWQrAAenBweVMlBgV3ZA
   5IMA8xq8l+i8wCgkWJjCjfLar7s+0X3HVrRroyARv8EAIYoxofh9mN8n821BTTuQnz5hp40
   d6Z3UudKePu2di5Mx3GFelwnV0Qh5mSs0YkuHJg0mcXyUAoeYry5X6482fUxbm+gOHVmYSD
   tBmZEB8PTEt8Os8aedWgKEt/E4dT+Hmod4omECLteLXxtScTMgDXyC+bSBMjRRCaeWhHrYY
   dYBACCWMdTc12hRLJTn8LX99kv1I7qwgieyna8GCJv/rEgCssS9E1qARM+h19KovIUOhl4V
   zBw3rK7v8Dlw/CJyYDd5kwSvCwjhO21LiReeS90VPYuZFRC1B82Sub152zOqIcAWsgd4myC
   CiZbWBsuJ8P0gtarFIpleNacCc6OV3i2Rg=="
   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.  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 time
   stamp 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 the same
   time stamp as the Payload Block.  This 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 time stamp,
   which will carry the same Payload Block that will still contain the
   same time stamp.





















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6.  Redundancy and Flexibility

   As described in Section 8.5 of [RFC5424], a transport sender may
   discard syslog messages.  Likewise, when syslog messages are sent
   over unreliable transport, they can be lost in transit.  However, if
   a collector does not receive Signature and Certificate Blocks, many
   messages may not be able to be verified.  The originator 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 Signature Blocks and Certificate Blocks it has
   already received and authenticated.  The originator can in principle
   change its redundancy level for any reason, without communicating
   this fact to the collector.

   The originator 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 originator 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 which may be useful in
   controlling redundant transmission from the transport sender to the
   transport receiver, and which may be useful for administrators to
   configure.

6.1.1.  Configuration Parameters for Certificate Blocks

   Certificate Blocks are always sent at the beginning of a new reboot
   session.  One technique to ensure reliable delivery (see Section 8.5)
   is to send multiple copies.  This can be controlled by a
   "certInitialRepeat" parameter:




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      certInitialRepeat = number of times each Certificate Block should
      be sent before the first message is sent.

   It is also useful to resend Certificate Blocks every now and then for
   long-lived reboot sessions.  This can be controlled by the
   certResendDelay and certResendCount parameters:

      certResendDelay = maximum time delay in seconds until resending
      the Certificate Block.

      certResendCount = maximum number of other syslog messages to send
      until resending the Certificate Block.

   In some cases, it may be desirable to allow for configuration of the
   transport sender such that Certificate Blocks are not sent at all
   after the first normal syslog message has been sent.  This could be
   expressed by setting both certResendDelay and certResendCount to "0".
   However, it is RECOMMENDED to configure the transport sender to send
   redundant Certificate Blocks even after the first message is sent
   when the UDP transport [RFC5426] is used.

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 Section 4.2.6 and Section 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 so 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



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      Signature Block.

6.2.  Overlapping Signature Blocks

   Notwithstanding the fact that the originator 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 regards 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.




























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7.  Efficient Verification of Logs

   The logs secured with syslog-sign may be reviewed either online or
   offline.  Online review is somewhat more complicated and
   computationally expensive, but not prohibitively so.

7.1.  Offline Review of Logs

   When the collector stores logs to be reviewed later, they can be
   authenticated offline just before they are reviewed.  Reviewing these
   logs offline is simple and relatively inexpensive in terms of
   resources used, so long as there is enough space available on the
   reviewing machine.  Here, we presume that the stored log files
   containing Certificate Block and Signature Block messages have
   already been separated by originator (as identified by HOSTNAME, APP-
   NAME, PROCID), Reboot Session ID, and Signature Group.  This can be
   done easily with a script file.  We then do the following:

   a.  First, we go through the raw log file and split its contents into
       three files.  Each message in the raw log file is classified as a
       normal message, a Signature Block message, or a Certificate Block
       message.  Signature Blocks and Certificate Blocks are then stored
       in their own files.  Normal messages are stored in a keyed file,
       indexed on their hash values.

   b.  We sort the Certificate Block file by INDEX value, and check to
       see whether we have a set of Certificate Blocks that can
       reconstruct the Payload Block.  If so, we reconstruct the Payload
       Block, verify any key-identifying information, and then use this
       to verify the signatures on the Certificate Blocks we have
       received.  When this is done, we have verified the reboot session
       and key used for the rest of the process.

   c.  We sort the Signature Block file by First Message Number.  We now
       create an authenticated log file, which consists of some header
       information and then a sequence of message number, message text
       pairs.  We next go through the Signature Block file.  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.




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       3.  For each remaining hashed message in the Signature Block:

           a.  Look up the hash value in the keyed message file.

           b.  If the message is found, write (message number, message
               text) to the authenticated log file.

       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 which have been
       authenticated.  Again, we will assume that we are handling only



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       one originator, Signature Group, and Reboot Session ID at any
       given time.

   b.  We have three data structures: A queue in which (message number,
       hash of message) pairs are kept in sorted order, a queue in which
       (arrival sequence, hash of message) pairs are kept in sorted
       order, and a hash table that stores (message text, count) pairs
       indexed by hash value.  In the hash table, count may be any
       number greater than zero; when count is zero, the entry in the
       hash table is cleared.

   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 add (arrival sequence, hash
       of message) to our message queue.  If our hash table has an entry
       for the message's hash value, we increment its count by one;
       otherwise, we create a new entry with count = 1.  If the message
       queue is full, we roll the oldest messages off the queue by
       taking the oldest entry in the queue, and using it to index the
       hash table.  If that entry has count 1, we delete the entry from
       the hash table; otherwise, we decrement its count.  We then
       delete the oldest entry in the queue.

   e.  Whenever a Signature Block message arrives, we check its
       originator (by way of HOSTNAME, APP-NAME, PROCID), 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.  Otherwise, we check its signature and discard
       it if the signature is not valid.  A Signature Block contains a
       sequence 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, we do the following:

       A.  We check if a message with the same message number is already
           in the authenticated message queue.




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       B.  If that is not the case, we write the (message number,
           message text) into the authenticated message queue, otherwise
           the signed hash is a duplicate and we discard it.

       Otherwise (the message hash is not in the hash table), we write
       the (message number, message hash) to the message number queue.
       This generally involves rolling the oldest entry out of this
       queue: before this is done, that entry's hash value is again
       looked up in the hash table.  If a matching entry is found, a
       check is made if the authenticated message file already contains
       an entry with the same message number and if that is not the
       case, the (message number, message text) pair is written to the
       authenticated message.  In either case, the oldest entry is then
       discarded.

   f.  The result of this is a sequence of messages in the authenticated
       message file, each of which has been authenticated, and which are
       labeled with numbers showing their order of original
       transmission.

   One can see that this whole process is roughly linear in the number
   of messages, and also in the number of Signature Blocks received.
   The process is susceptible to flooding attacks; an attacker can send
   enough normal messages that the messages roll off their queue before
   their Signature Blocks can be processed.


























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8.  Security Considerations

   Normal syslog event messages are unsigned and have most of the
   security attributes described in Section 8 of [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.

   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 an originator, it is advisable to avoid message lengths exceeding
   2048 octets.  Various problems might result if an originator were to
   send messages with a length greater than 2048 octets, because relays
   MAY truncate messages with lengths greater than 2048 octets which
   would make it impossible for collectors to validate a hash of the
   packet.  To increase the chance of interoperability, it tends to be
   best to be conservative with what you send but liberal in what you
   are able to receive.

   Originators need to rigidly enforce the correctness of message
   bodies.  Problems may arise if the collector does not fully accept
   the syslog packets sent from an originator, or if it has problems
   with the format of the Certificate Block or Signature Block messages.



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   Collectors are not to malfunction in case they receive malformed
   syslog messages or messages containing characters other than those
   specified in this document.  In other words, they are to ignore such
   messages and continue working.

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
   been replayed.

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
   pinpoint any messages sent by the originator but not received by the
   collector by reviewing the Signature Block information.  In addition,
   the information in subsequent Signature Blocks allows a reviewer to
   determine whether any Signature Block messages were lost in transit.

8.6.  Sequenced Delivery

   Syslog messages delivered over UDP might not only be lost, but also
   arrive out of sequence.  A reviewer can determine the original order
   of syslog messages and identify which messages were delivered out of
   order by examining the information in the Signature Block along with
   any timestamp information in the message.




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8.7.  Message Integrity

   Syslog messages might be damaged in transit.  A review of the
   information in the Signature Block determines whether the received
   message was the intended message sent by the originator.  A damaged
   Signature Block or Certificate Block is evident because the collector
   will not be able to validate that it was signed by the originator.

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
   delete messages.  It would then be able to construct a Signature
   Block and sign it with its own private key.  Network administrators
   need to verify that the key contained in the Payload Block is indeed
   the key being used on the actual originator.  If that is the case,
   then this MITM attack will not succeed.  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.



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9.  IANA Considerations

9.1.  Structured Data and syslog messages

   With regard to [RFC5424], IANA is requested to add the following
   values to the registry entitled "syslog Structured Data id values":

          SD-ID         PARAM_NAME
          -----         ----------
          ssign
                        VER
                        RSID
                        SG
                        SPRI
                        GBC
                        FMN
                        CNT
                        HB
                        SIGN

          ssign-cert
                        VER
                        RSID
                        SG
                        SPRI
                        TPBL
                        INDEX
                        FLEN
                        FRAG
                        SIGN

   In addition, several fields need to be controlled by the IANA in both
   the Signature Block and the Certificate Block, as outlined in the
   following sections.

9.2.  Version Field

   IANA is requested to create three registries, each associated with a
   different subfield of the Version field of Signature Blocks and
   Certificate Blocks, described in Section 4.2.1 and Section 5.3.2.1,
   respectively.

   The first registry that IANA is requested to create is entitled
   "syslog-sign protocol version values".  It is for the values of the
   Protocol Version subfield.  The Protocol Version subfield constitutes
   the first 2 octets in the Version field.  New values shall be
   assigned by the IANA using the "IETF Review" policy defined in
   [RFC5226].  Assigned numbers are to be increased by 1, up to a



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   maximum value of "50".  Protocol Version numbers of "51" through "99"
   are vendor-specific; values in this range are not to be assigned by
   the IANA.

   IANA is requested to register the Protocol Version values shown
   below.

         VALUE                    PROTOCOL VERSION
         -----                    ----------------
         00                       Reserved
         01                       Defined in RFC yyyy

   The second registry that IANA is requested to create is entitled
   "syslog-sign hash algorithm values".  It is for the values of the
   Hash Algorithm subfield.  The Hash Algorithm subfield constitutes the
   third octet in the Version field Signature Blocks and Certificate
   Blocks.  New values shall be assigned by the IANA using the "IETF
   Consensus" policy defined in [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 is requested to register the Hash Algorithm values shown below.

         VALUE     PROTOCOL VERSION     HASH ALGORITHM
         -----     ----------------     --------------
         0         01                   Reserved
         1         01                   SHA1
         2         01                   SHA256

   The third registry that IANA is requested to create is entitled
   "syslog-sign signature scheme values".  It is for the values of the
   Signature Scheme subfield.  The Signature Scheme subfield constitutes
   the fourth octet in the Version field of Signature Blocks and
   Certificate Blocks.  New values shall be assigned by the IANA using
   the "IETF Consensus" policy defined in [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



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   additional Protocol Versions that implementations might support
   concurrently.

   IANA is requested to register the Signature Scheme values shown
   below.

         VALUE     PROTOCOL VERSION    SIGNATURE SCHEME
         -----     ----------------    ----------------
         0         01                  Reserved
         1         01                  OpenPGP DSA

9.3.  SG Field

   IANA is requested to create a registry entitled "syslog-sign sg field
   values".  It is for values of the SG Field as defined in
   Section 4.2.3.  New values shall be assigned by the IANA using the
   "IETF Consensus" policy defined in [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 is requested to register the SG Field values shown below.

         VALUE     MEANING
         -----     -------
         0         per RFC yyyy
         1         per RFC yyyy
         2         per RFC yyyy
         3         per RFC yyyy

9.4.  Key Blob Type

   IANA is requested to create a registry entitled "syslog-sign key blob
   type values".  It is to register one-character identifiers for the
   key blob type, per Section 5.2.  New values shall be assigned by the
   IANA using the "IETF Consensus" policy defined in [RFC5226].
   Uppercase letters may be assigned as values.  Lowercase letters are
   left as vendor specific and shall not be assigned by the IANA.

   IANA is requested to register the key blob type values shown below.

         VALUE     KEY BLOB TYPE
         -----     ------------
         'C'       a PKIX certificate
         'P'       an OpenPGP certificate
         'K'       the public key whose corresponding private key is
                   used to sign the messages
         'N'       no key information sent, key is pre-distributed



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         'U'       installation-specific key exchange information


















































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10.  Working Group

   The working group can be contacted via the mailing list:

         syslog@ietf.org

   The current Chairs of the Working Group can be contacted at:

         Chris Lonvick
         Cisco Systems
         Email: clonvick@cisco.com

         David Harrington
         Huawei Technologies (USA)
         Email: ietfdbh@comcast.net
                dharrington@huawei.com
         Tel: +1-603-436-8634


































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11.  Acknowledgements

   The authors wish to thank Alex Brown, Chris Calabrese, Steve Chang,
   Pasi Eronen, Carson Gaspar, Rainer Gerhards, Drew Gross, David
   Harrington, Chris Lonvick, 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.











































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12.  References

12.1.  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/fips186-2/
              fips186-2-change1.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.

   [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.

12.2.  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/



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              publications/nistpubs/800-90/
              SP800-90_DRBG-June2006-final.pdf>.

   [RFC3164]  Lonvick, C., "The BSD syslog Protocol", RFC 3164,
              August 2001.

   [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.




































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Authors' Addresses

   John Kelsey
   NIST

   Email: john.kelsey@nist.gov


   Jon Callas
   PGP Corporation

   Email: jon@callas.org


   Alexander Clemm
   Cisco Systems

   Email: alex@cisco.com

































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