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syslog Working Group                                           J. Kelsey
Internet-Draft
Expires: November 23, 2005                                     J. Callas
                                                          PGP Corporation
                                                             May 23, 2005


                          Signed syslog Messages
                      draft-ietf-syslog-sign-16.txt

Status of this Memo

    This document is an Internet-Draft and is subject to all provisions
    of Section 3 of RFC 3667.  By submitting this Internet-Draft, each
    author represents that any applicable patent or other IPR claims of
    which he or she is aware have been or will be disclosed, and any of
    which he or she becomes aware will be disclosed, in accordance with
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    Internet-Drafts are working documents of the Internet Engineering
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    This Internet-Draft will expire on November 23, 2005.

Copyright Notice

    Copyright (C) The Internet Society (2005).

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
    11.   References . . . . . . . . . . . . . . . . . . . . . . . . . 29
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 30
        Intellectual Property and Copyright Statements . . . . . . . . 31










































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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.

11.  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.




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    [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.

    [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

    Email: kelsey.j@ix.netcom.com


    Jon Callas
    PGP Corporation

    Email: jon@callas.org





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Intellectual Property Statement

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Copyright Statement

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Acknowledgment

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Kelsey & Callas         Expires November 23, 2005              [Page 31]


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