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syslog Working Group                                           J. Kelsey
Internet-Draft                                                      NIST
Expires: September 30, 2004                                    J. Callas
                                                         PGP Corporation
                                                              April 2004



             The syslog Protocol and Signed syslog Messages
                     draft-ietf-syslog-sign-14.txt


Status of this Memo


   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.


   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
   groups may also distribute working documents as Internet-Drafts.


   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."


   The list of current Internet-Drafts can be accessed at http://
   www.ietf.org/ietf/1id-abstracts.txt.


   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.


   This Internet-Draft will expire on September 30, 2004.


Copyright Notice


   Copyright (C) The Internet Society (2004). All Rights Reserved.


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.












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Table of Contents


   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.    syslog Message Format  . . . . . . . . . . . . . . . . . . .  6
   3.    Signature Block Format and Fields  . . . . . . . . . . . . .  7
   3.1   syslog Packets Containing a Signature Block  . . . . . . . .  7
   3.2   Cookie . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   3.3   Version  . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   3.4   Reboot Session ID  . . . . . . . . . . . . . . . . . . . . .  8
   3.5   Signature Group and Signature Priority . . . . . . . . . . .  8
   3.6   Global Block Counter . . . . . . . . . . . . . . . . . . . . 10
   3.7   First Message Number . . . . . . . . . . . . . . . . . . . . 11
   3.8   Count  . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
   3.9   Hash Block . . . . . . . . . . . . . . . . . . . . . . . . . 11
   3.10  Signature  . . . . . . . . . . . . . . . . . . . . . . . . . 11
   4.    Payload and Certificate Blocks . . . . . . . . . . . . . . . 12
   4.1   Preliminaries: Key Management and Distribution Issues  . . . 12
   4.2   Building the Payload Block . . . . . . . . . . . . . . . . . 12
   4.3   Building the Certificate Block . . . . . . . . . . . . . . . 13
   4.3.1 Cookie . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
   4.3.2 Version  . . . . . . . . . . . . . . . . . . . . . . . . . . 14
   4.3.3 Reboot Session ID  . . . . . . . . . . . . . . . . . . . . . 14
   4.3.4 Signature Group and Signature Priority . . . . . . . . . . . 15
   4.3.5 Total Payload Block Length . . . . . . . . . . . . . . . . . 15
   4.3.6 Index into Payload Block . . . . . . . . . . . . . . . . . . 15
   4.3.7 Fragment Length  . . . . . . . . . . . . . . . . . . . . . . 15
   4.3.8 Signature  . . . . . . . . . . . . . . . . . . . . . . . . . 15
   5.    Redundancy and Flexibility . . . . . . . . . . . . . . . . . 16
   5.1   Redundancy . . . . . . . . . . . . . . . . . . . . . . . . . 16
   5.1.1 Certificate Blocks . . . . . . . . . . . . . . . . . . . . . 16
   5.1.2 Signature Blocks . . . . . . . . . . . . . . . . . . . . . . 16
   5.2   Flexibility  . . . . . . . . . . . . . . . . . . . . . . . . 17
   6.    Efficient Verification of Logs . . . . . . . . . . . . . . . 18
   6.1   Offline Review of Logs . . . . . . . . . . . . . . . . . . . 18
   6.2   Online Review of Logs  . . . . . . . . . . . . . . . . . . . 19
   7.    Security Considerations  . . . . . . . . . . . . . . . . . . 21
   7.1   Cryptography Constraints . . . . . . . . . . . . . . . . . . 21
   7.2   Packet Parameters  . . . . . . . . . . . . . . . . . . . . . 21
   7.3   Message Authenticity . . . . . . . . . . . . . . . . . . . . 22
   7.4   Sequenced Delivery . . . . . . . . . . . . . . . . . . . . . 22
   7.5   Replaying  . . . . . . . . . . . . . . . . . . . . . . . . . 22
   7.6   Reliable Delivery  . . . . . . . . . . . . . . . . . . . . . 22
   7.7   Sequenced Delivery . . . . . . . . . . . . . . . . . . . . . 22
   7.8   Message Integrity  . . . . . . . . . . . . . . . . . . . . . 23
   7.9   Message Observation  . . . . . . . . . . . . . . . . . . . . 23
   7.10  Man In The Middle  . . . . . . . . . . . . . . . . . . . . . 23
   7.11  Denial of Service  . . . . . . . . . . . . . . . . . . . . . 23
   7.12  Covert Channels  . . . . . . . . . . . . . . . . . . . . . . 23




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   8.    IANA Considerations  . . . . . . . . . . . . . . . . . . . . 25
   8.1   Version Field  . . . . . . . . . . . . . . . . . . . . . . . 25
   8.2   SIG Field  . . . . . . . . . . . . . . . . . . . . . . . . . 27
   8.3   Key Blob Type  . . . . . . . . . . . . . . . . . . . . . . . 27
   9.    Authors and Working Group Chair  . . . . . . . . . . . . . . 28
   10.   Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 29
         References . . . . . . . . . . . . . . . . . . . . . . . . . 30
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 31
         Intellectual Property and Copyright Statements . . . . . . . 32











































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1. Introduction


   This document describes a mechanism that adds origin authentication,
   message integrity, replay resistance, message sequencing, and
   detection of missing messages to syslog.  Essentially, this is
   accomplished by sending a cryptographically signed syslog message
   containing the signatures of previously sent syslog messages.  The
   contents of this message is called a Signature Block. Each Signature
   Block contains, in effect, a detached signature on some number of
   previously sent messages. While most implementations of syslog
   involve only a single device as the generator of each message and a
   single receiver as the collector of each message, provisions need to
   be made to cover messages being sent to multiple receivers. This is
   generally performed based upon 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 device. A Signature Block always
   belongs to exactly one signature group and it always signs messages
   belonging only to that signature group.


   Additionally, a device will send a Certificate Block to provide key
   management information between the sender and the receiver.  This
   Certificate Block has a field to denote the type of key material
   which may be such things as a PKIX certificate, an OpenPGP
   certificate, or even an indication that a key had been
   predistributed.  In all cases, these messages still use the syslog
   packet format described in this document.  In the cases of
   certificates being sent, the certificates may have to be split across
   multiple packets.


   The receiver of the previous messages may verify that the digital
   signature of each received message matches the signature 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.


   This specification is independent of the actual transport protocol
   selected.  It may be used with syslog packets over traditional UDP
   [5] as described in RFC 3164 [20].  It may be used with other event
   notification protocols, and it may be used with the Reliable Delivery
   of syslog as described in RFC 3195 [21]. Other efforts to define
   event notification messages should consider this specification in




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   their design.



















































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2. syslog Message Format


   This specification does not rely upon any specific syslog message
   format. It MAY be transported over a traditional syslog message
   format such as that defined in the informational RFC 3164 [20], or it
   MAY be used over the Reliable Delivery of syslog Messages as defined
   in RFC 3195 [21].  Care must be taken when choosing a transport for
   this mechanism, however.  Since the device generating the Signature
   Block message signs each message in its entirety, it is imperative
   that the messages MUST NOT be changed in transit.  It is equally
   imperative that the syslog-sign messages MUST NOT be changed in
   transit.  Specifically, a relay, as described in RFC 3164 MAY make
   changes to a syslog packet if specific fields are not found.  If this
   occurs, the entire mechanism is rendered useless.


   For convenience, this document will use the syslog message format in
   the terms described in RFC 3164.  That document describes the 3 parts
   of a syslog message; the PRI, HEADER, and MSG parts.  The MSG part is
   composed of TAG and CONTENT parts.  Space characters separate each of
   the fields.
































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3. Signature Block Format and Fields


   This section describes the Signature Block format and the fields used
   within the Signature Block.


3.1 syslog Packets Containing a Signature Block


   Signature Block messages MUST be encompassed within completely formed
   syslog messages. It SHOULD also contain a valid TAG field. It is
   RECOMMENDED that the TAG field have the value of "syslog " (without
   the double quotes) to signify that this message was generated by the
   syslog process. The CONTENT field of the syslog Signature Block
   messages MUST have the following fields. Each of these fields are
   separated by a single space character.


   The Signature Block is composed of the following fields. Each field
   must be printable ASCII, and any binary values are base-64 encoded.


       Field                       Designation        Size in bytes
       -----                       -----------        ---- -- -----


       Cookie                         Cookie                8


       Version                          Ver                 4


       Reboot Session ID               RSID                1-10


       Signature Group                  SIG                 1


       Signature Priority              SPRI                1-3


       Global Block Counter             GBC                1-10


       First Message Number             FMN                1-10


       Count                           Count               1-2


       Hash Block                     Hash Block   variable, size of hash
                                                  (base-64 encoded binary)


       Signature                      Signature         variable
                                                  (base-64 encoded binary)


   These fields are described below.


3.2 Cookie


   The cookie is a eight-byte sequence to signal that this is a




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   Signature Block. This sequence is "@#sigSIG" (without the double
   quotes).  As noted, a space character follows this, and all other
   fields.


3.3 Version


   The signature group version field is 4 characters in length and is
   terminated with a space character. 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 bytes), the hash algorithm (1 byte) and the signature
   scheme (1 byte).


      Protocol Version - 2 bytes with the first version as described in
      this document being value of 01 to denote Version 1.


      Hash Algorithm - 1 byte with the definition that 1 denotes SHA1 as
      defined in FIPS-180-1.1995 [2].


      Signature Scheme - 1 byte with the definition that 1 denotes
      OpenPGP DSA - RFC 2440 [18], FIPS.186-1.1998 [1].


   As such, the version, hash algorithm and signature scheme defined in
   this document may be represented as "0111" (without the quote marks).


3.4 Reboot Session ID


   The reboot session ID is a value between 1 and 10 bytes, which is
   required to never repeat or decrease.  The acceptable values for this
   are between 0 and 9999999999.  If the value latches at 9999999999,
   then manual intervention may be required to reset it to 0.
   Implementors MAY wish to consider using the snmpEngineBoots value as
   a source for this counter as defined in RFC 2574 [19].


3.5 Signature Group and Signature Priority


   The SIG identifier as described above may take on any value from 0-3
   inclusive.  The SPRI may take any value from 0-191.  Each of these
   fields are followed by a space character.  These fields taken
   together allows network administrators to associate groupings of
   syslog messages with appropriate Signature Blocks and Certificate
   Blocks.  For example, in some cases, network administrators may send
   syslog messages of Facilities 0 through 15 to one destination while
   sending messages with Facilities 16 through 23 to another.
   Associated Signature Blocks should be sent to these different syslog
   servers as well.





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   In some cases, an administrator may wish the Signature Blocks to go
   to the same destination as the syslog messages themselves.  This may
   be to different syslog servers if the destinations of syslog messages
   is being controlled by the Facilities or the Severities of the
   messages.  In other cases, administrators may wish to send the
   Signature Blocks to an altogether different destination.


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


   a.  '0' -- There is only one signature group.  All Signature Block
       messages use a single PRI value which is the same SPRI value.  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.  As
       one example, if SIG=0, then PRI and SPRI may be 46 to indicate
       that they are informational messages from the syslog daemon.  If
       the device is configured to send all messages with the local5
       Facility (21), then the PRI and SPRI may be 174 to indicate that
       they are also from the local5 Facility with a Severity of 6.


   b.  '1' -- Each PRI value has its own signature group. Signature
       Blocks for a given signature group have SPRI = PRI for that
       signature group.  In this case, the administrator of a device may
       not know where any of the syslog messages will ultimately go.
       This use ensures that a Signature Block follows each of the
       syslog messages to each destination.  This may be seen to be
       inefficient if groups of syslog messages are actually going to
       the same syslog server.  Examine an example of a device being
       configured to have a SIG value of 1, which generates 16 syslog
       messages with


              4 from PRI=132  (Facility 16, Severity 4),
              4 from PRI=148  (Facility 18, Severity 4),
              4 from PRI=164, (Facility 20, Severity 4), and
              4 from PRI=180  (Facility 22, Severity 4).


        In actuality, the messages from Facilities local0 and local2 go
       to one syslog server and messages from Facilities local4 and
       local6 go to a different one.  Then, the first syslog server
       receives 2 Signature Blocks, the first with PRI=134 and the
       second from PRI=150 - the PRI values matching the SPRI values.
       The second syslog server would also receive two Signature Block
       messages, the first from PRI=164 and the second from PRI=180.  In
       each of those Signature Blocks, the SPRI values matches their
       respective PRI values.  In each of these cases, the Signature




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       Blocks going to each respective syslog server could have been
       combined.  One way to do this more efficiently is explained using
       SIG=2.


   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 in that signature group.  For flexibility, the
       PRI does not have to be that upper-boundary SPRI value.
       Continuing the above example, the administrator of the device may
       configure SIG=2 with upper-bound SPRIs of 151 and 191.  The lower
       group contains all PRIs between 0 and 151, and the second group
       contains all PRIs between 152 and 191.  The administrator may
       then wish to configure the lower group to send all of the lower
       group Signature Blocks using PRI=150 (Facility 18, Severity 6),
       and the upper group using PRI=182 (Facility 22, Severity 6).  The
       receiving syslog servers then each receive a single Signature
       Block describing the 8 syslog messages sent to it.


   d.  '3' -- Signature groups are not assigned with any simple
       relationship to PRI values. This has to be some predefined
       arrangement between the sender and the intended receivers.  In
       this case, the administrators of the devices and syslog servers
       may, as an example, use SIG=3 with a SPRI of 1 to denote that all
       Warning and above syslog messages from all Facilities are sent
       using a PRI of 46 (Facility 5, Severity 6).


   One reasonable way to configure some installations is to have only
   one signature group with SIG=0.  The devices send messages to many
   collectors and also send a copy of each Signature Block to each
   collector. This won't allow any collector to detect gaps in the
   messages, but it allows all messages that arrive at each collector to
   be put into the right order, and to be verified.  It also allows each
   collector to detect duplicates and any messages that are not
   associated with a Signature Block.


3.6 Global Block Counter


   The global block counter is a value representing the number of
   Signature Blocks sent out by syslog-sign before this one, in this
   reboot session. This takes at least 1 byte and at most 10 bytes
   displayed as a decimal counter and the acceptable values for this are
   between 0 and 9999999999.  If the value latches at 9999999999, then
   the reboot session counter must be incremented by 1 and the global
   block counter resumes at 0.  Note that this counter crosses signature
   groups; it allows us to roughly synchronize when two messages were
   sent, even though they went to different collectors.





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3.7 First Message Number


   This is a value between 1 and 10 bytes.  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 will be numbered "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.


3.8 Count


   The count is a 1 or 2 byte field displaying the number of message
   hashes to follow.  The valid values for this field are between 1 and
   99.


3.9 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. The hashing algorithm used
   effectively specified by the Version field determines the size of
   each hash, but the size MUST NOT be shorter than 160 bits. It is
   base-64 encoded as per RFC 2045.


3.10 Signature


   This is a digital signature, encoded in base-64, as per RFC 2045. The
   signature is calculated over all fields but excludes the space
   characters between them.  The Version field effectively specifies the
   original encoding of the signature. The signature is a signature over
   the entire data, including all of the PRI, HEADER, and hashes in the
   hash block.  To reiterate, the signature is calculated over the
   completely formatted syslog-message, excluding spaces between fields,
   and also excluding this signature field.
















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4. Payload and Certificate Blocks


   Certificate Blocks and Payload Blocks provide key management in
   syslog-sign.


4.1 Preliminaries: Key Management and Distribution Issues


   The purpose of Certificate Blocks is to support key management using
   public key cryptosystems. All devices send at least one Certificate
   Block at the beginning of a new reboot session, carrying useful
   information about the reboot session.


   There are three key points to understand about Certificate Blocks:


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


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


   c.  The kind of Payload Block that is expected is determined by what
       kind of key material is on the collector that receives it. The
       device and collector (or offline log viewer) has both some key
       material (such as a root public key, or predistributed public
       key), and an acceptable value for the Key Blob Type in the
       Payload Block, below. The collector or offline log viewer MUST
       NOT accept a Payload Block of the wrong type.



4.2 Building the Payload Block


   The Payload Block is built when a new reboot session is started.
   There is a one-to-one correspondence of reboot sessions to Payload




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   Blocks. That is, each reboot session has only one Payload Block,
   regardless of how many signature groups it may support.  Like syslog
   packets containing the Signature Block, Payload Block messages MUST
   be completely formed syslog messages. It is RECOMMENDED that the TAG
   field have the value of "syslog " (without the double quotes) to
   signify that this message was generated by the syslog process. The
   CONTENT field of the syslog Payload Block messages MUST have the
   following fields. Each of these fields are separated by a single
   space character.


   a.  Unique identifier of sender; by default, the sender's IP address
       in dotted-decimal (IPv4) or colon-separated (IPv6) notation.


   b.  Full local time stamp for the device at the time the reboot
       session started.  This must be in TIMESTAMP-3339 format.


   c.  Key Blob Type, a one-byte field which holds one of five values:


       1.  'C' -- a PKIX certificate.


       2.  'P' -- an OpenPGP certificate.


       3.  'K' -- the public key whose corresponding private key is
           being used to sign these messages.


       4.  'N' -- no key information sent; key is predistributed.


       5.  'U' -- installation-specific key exchange information


   d.  The key blob, consisting of the raw key data, if any, base-64
       encoded.



4.3 Building the Certificate Block


   The Certificate Block must get the Payload Block to the collector.
   Since certificates can legitimately be much longer than 1024 bytes,
   each Certificate Block carries a piece of the Payload Block. Note
   that the device MAY make the Certificate Blocks of any legal length
   (that is, any length less than 1024 bytes) which holds all the
   required fields. Software that processes Certificate Blocks MUST deal
   correctly with blocks of any legal length.


   The Certificate Block is composed of the following fields. Each field
   must be printable ASCII, and any binary values are base-64 encoded.







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       Field                       Designation        Size in bytes
       -----                       -----------        ---- -- -----


       Cookie                         Cookie                8


       Version                          Ver                 4


       Reboot Session ID               RSID                1-10


       Signature Group                  SIG                 1


       Signature Priority              SPRI                1-3


       Total Payload Block Length      TPBL                1-8


       Index into Payload Block        Index               1-8


       Fragment Length                FragLen              1-3


       Payload Block Fragment         Fragment          variable
                                                  (base-64 encoded binary)


       Signature                      Signature         variable
                                                  (base-64 encoded binary)



4.3.1 Cookie


   The cookie is a eight-byte sequence to signal that this is a
   Signature Block. This sequence is "@#sigCER" (without the double
   quotes).  As noted, a space character follows this, and all other
   fields.


4.3.2 Version


   The signature group version field is 4 characters in length and is
   terminated with a space character. This field is identical in nature
   to the Version field described in Section 3.3. As such, the version,
   hash algorithm and signature scheme defined in this document may be
   represented as "0111" (without the quote marks).


4.3.3 Reboot Session ID


   The Reboot Session ID is identical in characteristics to the RSID
   field described in Section 3.4.







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4.3.4 Signature Group and Signature Priority


   The SIG field is identical in characteristics to the SIG field
   described in Section 3.10.  Also, the SPRI field is identical to the
   SPRI field described there.


4.3.5 Total Payload Block Length


   The Total Payload Block Length is a value representing the total
   length of the Payload Block in bytes in decimal.  This will be one to
   eight bytes.


4.3.6 Index into Payload Block


   This is a value between 1 and 8 bytes.  It contains the number of
   bytes into the Payload Block where this fragment starts.  The first
   byte of the first fragment is numbered "1".


4.3.7 Fragment Length


   The total length of this fragment expressed as a decimal integer.
   This will be one to three bytes.


4.3.8 Signature


   This is a digital signature, encoded in base-64, as per RFC 2045. The
   signature is calculated over all fields but excludes the space
   characters between them.  The Version field effectively specifies the
   original encoding of the signature. The signature is a signature over
   the entire data, including all of the PRI, HEADER, and hashes in the
   hash block.  This is consistent with the method of calculating the
   signature as specified in Section 3.10.  To reiterate, the signature
   is calculated over the completely formatted syslog-message, excluding
   spaces between fields, and also excluding this signature field.


















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5. Redundancy and Flexibility


   There is a general rule that determines how redundancy works and what
   level of flexibility the device and collector have in message
   formats: in general, the device is allowed to send Signature and
   Certificate Blocks multiple times, to send Signature and Certificate
   Blocks of any legal length, to include fewer hashes in hash blocks,
   etc.


5.1 Redundancy


   Syslog messages are sent over unreliable transport, which means that
   they can be lost in transit. However, the collector must receive
   Signature and Certificate Blocks or many messages may not be able to
   be verified. Sending Signature and Certificate Blocks multiple times
   provides redundancy; since the collector MUST ignore Signature/
   Certificate Blocks it has already received and authenticated, the
   device can in principle change its redundancy level for any reason,
   without communicating this fact to the collector.


   Although the device isn't 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, here
   we define some redundancy parameters below which may be useful in
   controlling redundant transmission from the device to the collector.


5.1.1 Certificate Blocks


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


   certResendDelay  = maximum time delay in seconds to delay before next
   redundant sending.


   certResendCount  = maximum number of sent messages to delay before
   next redundant sending.


5.1.2 Signature Blocks


   sigNumberResends = number of times a Signature Block is resent.


   sigResendDelay   = maximum time delay in seconds from original
   sending to next redundant sending.


   sigResendCount   = maximum number of sent messages to delay before
   next redundant sending.






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5.2 Flexibility


   The device may change many things about the makeup of Signature and
   Certificate Blocks in a given reboot session. The things it cannot
   change are:


      * The version


      * The number or arrangements of signature groups


   It is legitimate for a device to send out short Signature Blocks, in
   order to keep the collector able to verify messages quickly. In
   general, unless something verified by the Payload Block or
   Certificate Blocks is changed within the reboot session ID, any
   change is allowed to the Signature or Certificate Blocks during the
   session.




































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6. Efficient Verification of Logs


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


6.1 Offline Review of Logs


   When the collector stores logs and reviewed later, they can be
   authenticated offline just before they are reviewed. Reviewing these
   logs offline is simple and relatively cheap in terms of resources
   used, so long as there is enough space available on the reviewing
   machine. Here, we consider that the stored log files have already
   been separated by sender, reboot session ID, and signature group.
   This can be done very easily with a script file. We then do the
   following:


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


   b.  We sort the Certificate Block file by index value, and check to
       see if 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've 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 firstMessageNumber. We now
       create an authenticated log file, which consists of some header
       information, and then a sequence of message number, message text
       pairs. We next go through the Signature Block file. For each
       Signature Block in the file, we do the following:


       1.  Verify the signature on the Block.


       2.  For each hashed message in the Block:


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


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






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       3.  Skip all other Signature Blocks with the same
           firstMessageNumber.


   d.  The resulting authenticated log file contains all messages that
       have been authenticated, and implicitly indicates (by missing
       message numbers) all gaps in the authenticated messages.


   It's pretty easy to 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
   haven't discussed error-recovery, which may be necessary for the
   Certificate Blocks. In practice, a very simple error-recovery
   strategy is probably good enough -- if the Payload Block doesn't come
   out as valid, then we can just try an alternate instance of each
   Certificate Block, if such are available, until we get the Payload
   Block right.


   It's easy for an attacker to flood us with plausible-looking
   messages, Signature Blocks, and Certificate Blocks.


6.2 Online Review of Logs


   Some processes on the collector machine may need to monitor log
   messages in something very close to real-time. This can be done with
   syslog-sign, though it is somewhat more complex than the offline
   analysis. This is done as follows:


   a.  We have an output queue, into which we write (message number,
       message text) pairs which have been authenticated. Again, we'll
       assume we're handling only one signature group, and only one
       reboot session ID, at any given time.


   b.  We have three data structures: A queue into which (message
       number, hash of message) pairs is kept in sorted order, a queue
       into which (arrival sequence, hash of message) is kept in sorted
       order, and a hash table which stores (message text, count)
       indexed by hash value. In this file, 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're sent first.)




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       Once that's done, any Certificate Block that arrives is
       discarded.


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


   e.  Whenever a Signature Block arrives, we first check to see if the
       firstMessageNumber value is too old, or if another Signature
       Block with that firstMessageNumber has already been received. If
       so, we discard the Signature Block unread. Otherwise, we check
       its signature, and discard it if the signature isn't valid. A
       Signature Block contains a sequence of (message number, message
       hash) pairs. For each pair, we first check to see if the message
       hash is in the hash table. If so, we write out the (message
       number, message text) in the authenticated message queue.
       Otherwise, we write the (message number, message hash) to the
       message number queue. This generally involves rolling the oldest
       entry out of this queue: before this is done, that entry's hash
       value is again searched for in the hash table. If a matching
       entry is found, the (message number, message text) pair is
       written out to the authenticated message queue. In either case,
       the oldest entry is then discarded.


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


   It's not too hard to 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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7. Security Considerations


   Normal syslog event messages are unsigned and have most of the
   security attributes described in Section 6 of RFC 3164.  This
   document also describes Certificate Blocks and Signature Blocks which
   are signed syslog messages.  The Signature Blocks contains signature
   information of previously sent syslog event messages.  All of this
   information may be used to authenticate syslog messages and to
   minimize or obviate many of the security concerns described in RFC
   3164.


7.1 Cryptography Constraints


   As with any technology involving cryptography, you should check the
   current literature to determine if any algorithms used here have been
   found to be vulnerable to attack.


   This specification uses Public Key Cryptography technologies. The
   proper party or parties must control the private key portion of a
   public-private key pair. Any party that controls a private key may
   sign anything they please.


   Certain operations in this specification involve the use of random
   numbers. An appropriate entropy source should be used to generate
   these numbers. See RFC 1750 [8].


7.2 Packet Parameters


   The message length must not exceed 1024 bytes.  Various problems may
   result if a device sends out messages with a length greater than 1024
   bytes.  As seen in RFC 3164, relays MAY truncate messages with
   lengths greater than 1024 bytes which would result in a problem for
   receivers trying to validate a hash of the packet.  In this case, as
   with all others, it is best to be conservative with what you send but
   liberal in what you receive, and accept more than 1024 bytes.


   Similarly, senders must rigidly enforce the correctness of the
   message body. This document specifies an enhancement to the syslog
   protocol but does not stipulate any specific syslog message format.
   Nonetheless, problems may arise if the receiver does not fully accept
   the syslog packets sent from a device, or if it has problems with the
   format of the Certificate Block or Signature Block messages.


   Finally, receivers must not malfunction if they receive syslog
   messages containing characters other than those specified in this
   document.






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7.3 Message Authenticity


   Event messages being sent through syslog do not strongly associate
   the message with the message sender.  That fact is established by the
   receiver upon verification of the Signature Block as described above.
   Before a Signature Block is used to ascertain the authenticity of an
   event message, it may be received, stored and reviewed by a person or
   automated parser.  Both of these should maintain doubt about the
   authenticity of the message until after it has been validated by
   checking the contents of the Signature Block.


   With the Signature Block checking, an attacker may only forge
   messages if they can compromise the private key of the true sender.


7.4 Sequenced Delivery


   Event messages may be recorded and replayed by an attacker.  However
   the information contained in the Signature Blocks allows a reviewer
   to determine if the received messages are the ones originally sent by
   a device.  This process also alerts the reviewer to replayed
   messages.


7.5 Replaying


   Event messages may be recorded and replayed by an attacker.  However
   the information contained in the Signature Blocks will allow a
   reviewer to determine if the received messages are the ones
   originally sent by a device.  This process will also alert the
   reviewer to replayed messages.


7.6 Reliable Delivery


   RFC 3195 may be used for the reliable delivery of all syslog
   messages.  This document acknowledges that event messages sent over
   UDP may be lost in transit.  A proper review of the Signature Block
   information may pinpoint any messages sent by the sender but not
   received by the receiver.  The overlap of information in subsequent
   Signature Block information allows a reviewer to determine if any
   Signature Block messages were also lost in transit.


7.7 Sequenced Delivery


   Related to the above, syslog messages delivered over UDP not only may
   be lost, but they may arrive out of sequence.  The information
   contained in the Signature Block allows a receiver to correctly order
   the event messages. Beyond that, the timestamp information contained
   in the packet may help the reviewer to visually order received
   messages even if they are received out of order.




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7.8 Message Integrity


   syslog messages may be damaged in transit.  A review of the
   information in the Signature Block determines if the received message
   was the intended message sent by the sender. A damaged Signature
   Block or Certificate Block will be evident since the receiver will
   not be able to validate that it was signed by the sender.


7.9 Message Observation


   Event messages, Certificate Blocks and Signature Blocks are all sent
   in plaintext. Generally this has had the benefit of allowing 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.


7.10 Man In The Middle


   It is conceivable that an attacker may intercept Certificate Blocks
   and insert their own Certificate information.  In that case, the
   attacker would be able to receive event messages from the actual
   sender and then relay modified messages, insert new messages, or
   deleted messages.  They would then be able to construct a Signature
   Block and sign it with their own private key.  The network
   administrators should verify that the key contained in the
   Certificate Block is indeed the key being used on the actual device.
   If that is indeed the case, then this MITM attack will not succeed.


7.11 Denial of Service


   An attacker may be able to overwhelm a receiver by sending it invalid
   Signature Block messages.  If the receiver is attempting to process
   these messages online, it may consume all available resources.  For
   this reason, it may be appropriate to just receive the Signature
   Block messages and process them as time permits.


   As with any system, an attacker may also just overwhelm a receiver by
   sending more messages to it than can be handled by the infrastructure
   or the device itself. Implementors should attempt to provide features
   that minimize this threat. Such as only receiving syslog messages
   from known IP addresses.


7.12 Covert Channels


   Nothing in this protocol attempts to eliminate covert channels.
   Indeed, the unformatted message syntax in the packets could be very
   amenable to sending embedded secret messages.  In fact, just about
   every aspect of syslog messages lends itself to the conveyance of




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   covert signals.  For example, a collusionist could send odd and even
   PRI values to indicate Morse Code dashes and dots.


















































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8. IANA Considerations


   Two syslog packet types are specified in this document; the Signature
   Block and the Certificate Block.  Each of these has several fields
   specified that should be controlled by the IANA.  Essentially these
   packet types may be differentiated based upon the value in the Cookie
   field.  The Signature Block packet may be identified by a value of
   "@#sigSIG" in the Cookie field.  The Certificate Block packet may be
   identified by a value of "@#sigCER" in the Cookie field.  Each of
   these packet types share fields that should be consistent;
   specifically, the Certificate Block packet types may be considered to
   be an announcement of capabilities and the Signature Block packets
   SHOULD have the same values in the fields described in this section.
   This document allows that there may be some really fine reason for
   the values to be different between the two packet types but the
   authors and contributors can't see any valid reason for that at this
   time.


   This document also upholds the Facilities and Severities listed in
   RFC 3164 [20].  Those values range from 0 to 191.  This document also
   instructs the IANA to reserve all other possible values of the
   Severities and Facilities above the value of 191 and to distribute
   them via the consensus process as defined in RFC 2434 [17].


   The following fields are to be controlled by the IANA in both the
   Signature Block packets and the Certificate Block packets.


8.1 Version Field


   The Version field (Ver) is a 4 byte field.  The first two bytes of
   this field define the version of the Signature Block packets and the
   Certificate Block Packets.  This allows for future efforts to
   redefine the subsequent fields in the Signature Block packets and
   Certificate Block packets.  A value of "00" is reserved and not used.
   This document describes the fields for the version value of "01".  It
   is expected that this value be incremented monotonically with decimal
   values up through "50" for IANA assigned values. Values "02" through
   "50" will be assigned by the IANA using the "IETF Consensus" policy
   defined in RFC 2434 [17].  It is not anticipated that these values
   will be reused. Values of "51" through "99" will be vendor-specific,
   and values in this range are not to be assigned by the IANA.


   In the case of vendor-specific assigned Version numbers, all
   subsequent values defined in the packet will then have
   vendor-specific meaning.  They may, or may not, align with the values
   assigned by the IANA for these fields.  For example, a vendor may
   choose to define their own Version of "51" still containing values of
   "1" for the Hash Algorithm and Signature Scheme which aligns with the




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   IANA assigned values as defined in this document.  However, they may
   then choose to define a value of "5" for the Signature Group for
   their own reasons.


   The third byte of the Ver field defines the Hash Algorithm.  It is
   envisioned that this will also be a monotonically increasing value
   with a maximum value of "9".  The value of "1" is defined in this
   document as the first assigned value and is SHA1 FIPS-180-1.1995 [2].
   Subsequent values will be assigned by the IANA using the "IETF
   Consensus" policy defined in RFC 2434 [17].


   The forth and final byte of the Ver field defines the Signature
   Scheme.  It is envisioned that this too will be a monotonically
   increasing value with a maximum value of "9".  The value of "1" is
   defined in this document as OpenPGP DSA - RFC 2440 [18],
   FIPS.186-1.1998 [1]. Subsequent values will be assigned by the IANA
   using the "IETF Consensus" policy defined in RFC 2434 [17].  The
   fields, values assigned in this document and ranges are illustrated
   in the following table.


   Field    Value Defined     IANA Assigned   Vendor Specific
           in this Document       Range            Range
   -----   ----------------   -------------   ---------------
   Ver
    ver           01              01-50            50-99
    hash           1               0-9             -none-
    sig            1               0-9             -none-


   If either the Hash Algorithm field or the Signature Scheme field is
   needed to go beyond "9" within the current version (first two bytes),
   the IANA should increment the first two bytes of this 4 byte field to
   be the next value with the definition that all of the subsequent
   values of fields described in this section are reset to "0" while
   retaining the latest definitions given by the IANA.  For example,
   consider the case that the first two characters are "23" and the
   latest Signature Algorithm is 4.  Let's say that the latest Hash
   Algorithm value is "9" but a better Hash Algorithm is defined.  In
   that case, the IANA will increment the first two bytes to become
   "24", retain the current Hash Algorithm to be "0", define the new
   Hash Algorithm to be "1" in this scheme, and define the current
   Signature Scheme to also be "0".  This example is illustrated in the
   following table.


     Current        New - Equivalent       New with Later
                      to "Current"          Algorithms
     -------        --------------        ---------------
     ver = 23          ver = 24              ver = 24
     hash = 9          hash = 0              hash = 1




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     sig = 4           sig = 0               sig = 0



8.2 SIG Field


   The SIG field values are numbers as defined in Section 3.5. Values
   "0" through "3" are assigned in this document.  The IANA shall assign
   values "4" through "7" using the "IETF Consensus" policy defined in
   RFC 2434 [17]. Values "8" and "9" shall be left as vendor specific
   and shall not be assigned by the IANA.


8.3 Key Blob Type


   Section Section 4.2 defines five, one character identifiers for the
   key blob type.  These are the uppercase letters, "C", "P", "K", "N",
   and "U".  All other uppercase letters shall be assigned by the IANA
   using the "IETF Consensus" policy defined in RFC 2434 [17].
   Lowercase letters are left as vendor specific and shall not be
   assigned by the IANA.

































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9. Authors and Working Group Chair


   The working group can be contacted via the mailing list:


         syslog-sec@employees.org


   The current Chair of the Working Group may be contacted at:


         Chris Lonvick
         Cisco Systems
         Email: clonvick@cisco.com


   The authors of this draft are:


         John Kelsey
         Email: kelsey.j@ix.netcom.com


         Jon Callas
         Email: jon@callas.org

































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10. Acknowledgements


   The authors wish to thank Alex Brown, Chris Calabrese, Carson Gaspar,
   Drew Gross, Chris Lonvick, Darrin New, Marshall Rose, Holt Sorenson,
   Rodney Thayer, Andrew Ross, Rainer Gerhards, Albert Mietus, and the
   many Counterpane Internet Security engineering and operations people
   who commented on various versions of this proposal.













































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References


   [1]   National Institute of Standards and Technology, "Digital
         Signature Standard", FIPS PUB 186-1, December 1998, <http://
          csrc.nist.gov/fips/fips1861.pdf>.


   [2]   National Institute of Standards and Technology, "Secure Hash
         Standard", FIPS PUB 180-1, April 1995, <http://
          www.itl.nist.gov/fipspubs/fip180-1.htm>.


   [3]   American National Standards Institute, "USA Code for
         Information Interchange", ANSI X3.4, 1968.


   [4]   Menezes, A., van Oorschot, P. and S. Vanstone, ""Handbook of
         Applied Cryptography", CRC Press", 1996.


   [5]   Postel, J., "User Datagram Protocol", STD 6, RFC 768, August
         1980.


   [6]   Mockapetris, P., "Domain names - concepts and facilities", STD
         13, RFC 1034, November 1987.


   [7]   Mockapetris, P., "Domain names - implementation and
         specification", STD 13, RFC 1035, November 1987.


   [8]   Eastlake, D., Crocker, S. and J. Schiller, "Randomness
         Recommendations for Security", RFC 1750, December 1994.


   [9]   Malkin, G., "Internet Users' Glossary", RFC 1983, August 1996.


   [10]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
         Extensions (MIME) Part One: Format of Internet Message Bodies",
         RFC 2045, November 1996.


   [11]  Oehler, M. and R. Glenn, "HMAC-MD5 IP Authentication with
         Replay Prevention", RFC 2085, February 1997.


   [12]  Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing
         for Message Authentication", RFC 2104, February 1997.


   [13]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", BCP 14, RFC 2119, March 1997.


   [14]  Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
         2279, January 1998.


   [15]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
         Specifications: ABNF", RFC 2234, November 1997.




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   [16]  Hinden, R. and S. Deering, "IP Version 6 Addressing
         Architecture", RFC 2373, July 1998.


   [17]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
         Considerations Section in RFCs", BCP 26, RFC 2434, October
         1998.


   [18]  Callas, J., Donnerhacke, L., Finney, H. and R. Thayer, "OpenPGP
         Message Format", RFC 2440, November 1998.


   [19]  Blumenthal, U. and B. Wijnen, "User-based Security Model (USM)
         for version 3 of the Simple Network Management Protocol
         (SNMPv3)", RFC 2574, April 1999.


   [20]  Lonvick, C., "The BSD Syslog Protocol", RFC 3164, August 2001.


   [21]  New, D. and M. Rose, "Reliable Delivery for syslog", RFC 3195,
         November 2001.


   [22]  Klyne, G. and C. Newman, "Date and Time on the Internet:
         Timestamps", RFC 3339, July 2002.


   [23]  Schneier, B., "Applied Cryptography Second Edition: protocols,
         algorithms, and source code in C", 1996.



Authors' Addresses


   John Kelsey
   NIST


   EMail: kelsey.j@ix.netcom.com



   Jon Callas
   PGP Corporation


   EMail: jon@callas.org














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Intellectual Property Statement


   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
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   proprietary rights by implementors or users of this specification can
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   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
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   this standard. Please address the information to the IETF Executive
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Full Copyright Statement


   Copyright (C) The Internet Society (2004). All Rights Reserved.


   This document and translations of it may be copied and furnished to
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Internet-Draft    The syslog Protocol and Signed syslog Messages
April 2004



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Acknowledgment


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