Network Working Group                                       John Kelsey
Category: INTERNET-DRAFT                                       Certicom
Expires June 2003                                            Jon Callas
December 2002                                           PGP Corporation

                             Syslog-Sign Protocol

Copyright Notice

   Copyright 2002 by The Internet Society. All Rights Reserved.

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 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

       The list of Internet-Draft Shadow Directories can be accessed at

       This work is a product of the IETF syslog Working Group. More
       information about this effort may be found at

   Comments about this draft should be directed to the syslog working
   group at the mailing list of

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119.


   This document describes syslog-sign, a mechanism adding origin
   authentication, message integrity, replay-resistance, message
   sequencing, and detection of missing messages to syslog. Syslog-sign
   provides these security features in a way that has minimal

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   requirements and minimal impact on existing syslog implementations.
   It is possible to support syslog-sign and gain some of its security
   attributes by only changing the behavior of the devices generating
   syslog messages. Some additional processing of the received syslog
   messages and the syslog-sign messages on the relays and collectors
   may realize additional security benefits.

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Table of Contents

          Copyright Notice                                            1
          Status of this Memo                                         1
          Abstract                                                    1
          Table of Contents                                           3
   1.     Introduction                                                4
   2.     Required syslog Format                                      4
   2.1.   PRI Part                                                    5
   2.2.   HEADER Part                                                 6
   2.3.   MSG Part                                                    7
   2.4.   Examples                                                    7
   3.     Signature Block Format and Fields                           8
   3.1.   syslog Packets Containing a Signature Block                 8
   3.2.   Cookie                                                      9
   3.3.   Version                                                     9
   3.4.   Reboot Session ID                                          10
   3.5.   Signature Group                                            10
   3.6.   Global Block Counter                                       12
   3.7.   First Message Number                                       12
   3.8.   Count                                                      12
   3.9.   Hash Block                                                 12
   3.10.  Signature                                                  13
   4.     Payload and Certificate Blocks                             13
   4.1.   Preliminaries: Key Management and Distribution Issues      13
   4.2.   Building the Payload Block                                 14
   4.3.   Building the Certificate Block                             15
   5.     Redundancy and Flexibility                                 15
   5.1.   Redundancy                                                 16
   5.1.1. Certificate Blocks                                         16
   5.1.2. Signature Blocks                                           16
   5.2.   Flexibility                                                16
   6.     Efficient Verification of Logs                             17
   6.1.   Offline Review of Logs                                     17
   6.2.   Online Review of Logs                                      18
   7.     Security Considerations                                    19
   8.     IANA Considerations                                        20
   9.     Authors and Working Group Chair                            20
   10.    Acknowledgments                                            20
   11.    References                                                 20
   12.    Full Copyright Statement                                   21

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1. Introduction

   Syslog-sign is an enhancement to syslog [RFC3164] that adds origin
   authentication, message integrity, replay resistance, message
   sequencing, and detection of missing messages to syslog. This
   mechanism makes no changes to the syslog packet format but does
   require strict adherence to that format. A syslog-sign message
   contains a signature block within the MSG part of a syslog message.
   This signature block contains a separate digital signature for each
   of a group of previously sent syslog messages. The overall message
   is also signed as the last value in this message.

   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
   utilizes 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

   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, and OpenPGP
   certificate, or even an indication that a key had been
   predistributed.  In all cases, these messages will still utilize the
   syslog packet format.  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.

2. Required syslog Format

   The essential format of syslog messages is defined in RFC 3164. The
   basis of the format is that anything delivered to UDP port 514 MUST
   be accepted as a valid syslog message. However, there is a
   RECOMMENDED format laid out in that work which this work REQUIRES.

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   Packets conforming to this specification will REQUIRE this format.

   While syslog as defined in RFC 3164 requires UDP, this document
   neither requires nor encourages it. Syslog-sign may be used using
   any transport, and encourages reliable transport of messages. Adding
   Syslog-sign to a reliable transport for the messages adds benefits
   beyond either alone.

   The full format of a syslog sign message seen on the wire has three
   discernible parts. The first part is called the PRI, the second part
   is the HEADER, and the third part is the MSG. The total length of
   the packet MUST be 1024 bytes or less. There is no minimum length of
   the syslog message although sending a syslog packet with no contents
   is worthless and SHOULD NOT be transmitted.

   The definitions of the fields are slightly changed in this document
   from RFC 3164. While the format described in RFC 3164 is correct for
   packet formation, the Working Group evaluating this work determined
   that it would be better if the TAG field were to become a part of
   the HEADER part rather than the CONTENT part. While IETF
   documentation does not allow the specification of an API, people
   developing code to adhere to this specification have found it
   helpful to think about the parts in this format.

   syslog-sign messages from devices MUST conform to this format. Other
   syslog messages from devices SHOULD also conform to this format. If
   they do not conform to this format, they may be reformatted by a
   relay as described in Section 4.3 of RFC 3164. That would change the
   format of the original messages and any cryptographic signature of
   the original message would not match the cryptographic signature of
   the changed message.

2.1. PRI Part

   The PRI part MUST have three, four, or five characters and will be
   bound with angle brackets as the first and last characters. The PRI
   part starts with a leading "<" ('less-than' character), followed by
   a number, which is followed by a ">" ('greater-than' character). The
   code set used in this part MUST be seven-bit ASCII in an eight- bit
   field as described in RFC 2234 [RFC2234]. These are the ASCII codes
   as defined in "USA Standard Code for Information Interchange" [3].
   In this, the "<" character is defined as the Augmented Backus-Naur
   Form (ABNF) %d60, and the ">" character has ABNF value %d62. The
   number contained within these angle brackets is known as the
   Priority value and represents both the Facility and Severity as
   described below. The Priority value consists of one, two, or three
   decimal integers (ABNF DIGITS) using values of %d48 (for "0")
   through %d57 (for "9").

   The Facilities and Severities of the messages are defined in RFC
   3164. The Priority value is calculated by first multiplying the
   Facility number by 8 and then adding the numerical value of the

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   Severity. For example, a kernel message (Facility=0) with a Severity
   of Emergency (Severity=0) would have a Priority value of 0. Also, a
   "local use 4" message (Facility=20) with a Severity of Notice
   (Severity=5) would have a Priority value of 165. In the PRI part of
   a syslog message, these values would be placed between the angle
   brackets as <0> and <165> respectively. The only time a value of "0"
   will follow the "<" is for the Priority value of "0". Otherwise,
   leading "0"s MUST NOT be used.

2.2. HEADER Part

   The HEADER part contains a time stamp, an indication of the hostname
   or IP address of the device, and a string indicating the source of
   the message. The HEADER part of the syslog packet MUST contain
   visible (printing) characters. The code set used MUST also been
   seven-bit ASCII in an eight-bit field like that used in the PRI
   part. In this code set, the only allowable characters are the ABNF
   VCHAR values (%d33-126) and spaces (SP value %d32).

   The HEADER contains three fields called the TIMESTAMP, the HOSTNAME,
   and the TAG fields. The TIMESTAMP will immediately follow the
   trailing ">" from the PRI part and single space characters MUST
   follow each of the TIMESTAMP and HOSTNAME fields. HOSTNAME will
   contain the hostname, as it knows itself. If it does not have a
   hostname, then it will contain its own IP address. If a device has
   multiple IP addresses, it has usually been seen to use the IP
   address from which the message is transmitted. An alternative to
   this behavior has also been seen. In that case, a device may be
   configured to send all messages using a single source IP address
   regardless of the interface from which the message is sent. This
   will provide a single consistent HOSTNAME for all messages sent from
   a device.

   The TIMESTAMP field is the local time and is in the format of "Mmm
   dd hh:mm:ss" (without the quote marks) where:

       Mmm is the English language abbreviation for the month of the
       year with the first character in uppercase and the other two
       characters in lowercase. The following are the only acceptable

           Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec

       dd is the day of the month. If the day of the month is less than
       10, then it MUST be represented as a space and then the number.
       For example, the 7th day of August would be represented as "Aug
       7", with two spaces between the "g" and the "7".

       hh:mm:ss is the local time. The hour (hh) is represented in a
       24-hour format. Valid entries are between 00 and 23, inclusive.
       The minute (mm) and second (ss) entries are between 00 and 59

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   A single space character MUST follow the TIMESTAMP field.

   The HOSTNAME field will contain only the hostname, the IPv4 address,
   or the IPv6 address of the originator of the message. The preferred
   value is the hostname. If the hostname is used, the HOSTNAME field
   MUST contain the hostname of the device as specified in STD-13 [4].
   The Domain Name MUST NOT be included in the HOSTNAME field. If the
   IPv4 address is used, it MUST be shown as the dotted decimal
   notation as used in STD-13 [5]. If an IPv6 address is used, any
   valid representation used in RFC-2373 [6] MAY be used. A single
   space character MUST also follow the HOSTNAME field.

   The TAG is a string of ABNF alphanumeric characters and other
   certain special characters, that MUST NOT exceed 32 characters in
   length. There are three special characters that are acceptable to
   use in this field as well.

       [  ABNF %d91
       ]  ABNF %d93
       :  ABNF %d58

   The first occurrence of a colon (":") character will terminate the
   TAG field. Generally, the TAG will contain the name of the process
   that generated the message. It may OPTIONALLY contain additional
   information such as the numerical process ID of that process bound
   within square brackets ("[" and "]"). A colon MUST be the last
   character in this field.

2.3. MSG Part

   The MSG part contains the details of the message. This has
   traditionally been a freeform message that gives some detailed
   information of the event. The MSG part of the syslog packet MUST
   contain visible (printing) characters. The code set used MUST also
   been seven-bit ASCII in an eight-bit field like that used in the PRI
   part. In this code set, the only allowable characters are the ABNF
   VCHAR values (%d33-126) and spaces (SP value %d32). Two message
   types will be defined in this document. Each will have unique fields
   within the MSG part and they will be described below.

   Unless otherwise stated, binary data will be base64 encoded, as
   defined in RFC2045 [RFC2045].

2.4. Examples

   The following examples are given.

       Example 1

           <34>Oct 11 22:14:15 mymachine su: 'su root' failed for
           lonvick on /dev/pts/8

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   In this example, as it was originally described in RFC 3164, the PRI
   part is "<34>". In this work, however, the HEADER part consists of
   the TIMESTAMP, the HOSTNAME, and the TAG fields. The TIMESTAMP is
   "Oct 11 22:14:15 ", the HOSTNAME is "mymachine ", and the TAG value
   is "su:". The CONTENT field is " 'su root' failed for lonvick...".
   The CONTENT field starts with a leading space character in this

       Example 2

           <165>Aug 24 05:34:00 myproc[10]:%% It's time to
           make the do-nuts. %%  Ingredients: Mix=OK, Jelly=OK #
           Devices: Mixer=OK, Jelly_Injector=OK, Frier=OK # Transport:
           Conveyer1=OK, Conveyer2=OK # %%

   In this example, the PRI part is <165> denoting that it came from a
   locally defined facility (local4) with a severity of Notice. The
   HEADER part has a proper TIMESTAMP field in the message. A relay
   will not modify this message before sending it. The HOSTNAME is an
   IPv4 address and the TAG field is "myproc[10]:". The MSG part starts
   with "%% It's time to make the do-nuts. %%  Ingredients: Mix=OK,
   ..." this time without a leading space character.

3. Signature Block Format and Fields

   Since the device generating the signature block message signs the
   entire syslog message, it is imperative that the message MUST NOT be
   changed in transit. In adherence with Section 4 of [RFC3164], a
   fully formed syslog message containing a PRI part and a HEADER part
   containing TIMESTAMP and HOSTNAME fields MUST NOT be changed or
   modified by any relay.

3.1. syslog Packets Containing a Signature Block

   Signature block messages MUST be completely formed syslog messages.
   Signature block messages have PRI, HEADER, and MSG parts as
   described in Sections 4.1.1 and 4.1.3 of [RFC3164]. The PRI part
   MUST have a valid Priority value bounded by angled brackets. The
   HEADER part MUST have a valid TIMESTAMP field as well as a HOSTNAME
   field. 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
   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.

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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

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

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.

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       Signature Scheme - 1 byte with the definition that 1 denotes
       OpenPGP DSA [RFC2440], [DSA94].

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

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].

3.5. Signature Group

   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 will allow 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.

   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.

   SIG values are identified as follows:

    a. '0' -- There is only one signature group.  All signature block
       messages will use a single PRI value which will be 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

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       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 will ensure that a signature block will follow
       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
       will receive 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 will match
       their respective PRI values.  In each of these cases, the
       signature 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 will have 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 will contain all PRIs between 0 and 151,
       and the second group will contain 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 will
       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 will have to be some predefined
       arrangement between the sender and the intended receivers.  In
       this case, the administrators of the devices and syslog servers

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       may, as an example, use SIG=3 with a SPRI of 1 to denote that
       all Warning and above syslog messages from all Facilities will
       be 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 will send messages to
   many collectors and will 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 will allow all messages that arrive at each
   collector to be put into the right order, and to be verified.  It
   will also allow 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 will resume 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

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.

   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

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 RFC2045.

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3.10. Signature

   This is a digital signature, encoded in base-64, as per RFC2045. 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.

4. Payload and Certificate Blocks

   Certificate blocks and payload blocks provide key management in

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 will 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.

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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
   blocks. That is, each reboot session has only one payload block,
   regardless of how many signature groups it may support.

   The payload block consists of the following:

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

    b. Full local time stamp for the device, including year if
       available, at time reboot session started.

    c. Signature Group Descriptor. This consists of a one-character
       field specifying how signature groups are assigned. The
       possibilities are:

       (i) '0' -- Only one signature group supported. For all signature
           blocks and certificate blocks, sig == pri == XXX.

      (ii) '1' -- Each pri value gets its own signature group. For each
           signature/certificate block, sig == pri.

     (iii) '2' -- Signature groups are assigned in some way with no
           simple relationship to pri values; for all
           signature/certificate blocks, pri = XXX.

      (iv) '3' -- Signature groups are assigned to ranges of pri
           values. For each signature/certificate block, pri = largest
           pri contained within that signature group.

    d. Highest SIG Value -- a one, two, or three byte field, must be a
       number between 0 and 191, inclusive.

    e. Key Blob Type, a one-byte field which holds one of four values:

       (i) 'C' -- a PKIX certificate.

      (ii) 'P' -- an OpenPGP certificate.

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

      (iv) 'N' -- no key information sent; key is predistributed.

       (v) 'U' -- installation-specific key exchange information

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

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 will hold all the
   required fields. Software that processes certificate blocks MUST
   deal correctly with blocks of any legal length.

   The certificate block is built as follows:

    a. Cookie                -- an eight byte string, "@#sigCer".

    b. Version               -- two bytes with 01 being the version
                                described in this document.

    c. Reboot Session ID     -- as above, a 10-byte quantity, which is
                                required to never repeat or decrease in
                                the lifetime of the device.

    d. Signature Group       -- 1 to 3 bytes as described above.

    e. Total Payload Length  -- 8 bytes numbering the total length
                                in bytes in decimal.

    f. Index into Payload    -- 1 to 8 bytes numbering the length into
                                the payload

    g. Fragment Length       -- 12 bits base-64 encoded as two bytes.

    h. Payload Fragment      -- a fragment of the payload, as specified
                                by the above fields. This fragment is a
                                piece of the certificate. When all the
                                fragments are combined, the resulting
                                data segment is the valid certificate.

    i. Signature             -- a digital signature on fields a-h.

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,

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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

   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

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.

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

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   It is legitimate for a device to send our 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. The device may send shorter signature and certificate
   blocks for

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 will 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 will consist 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:

       (i) Verify the signature on the block.

      (ii) For each hashed message in the block:

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           (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.

     (iii) Skip all other signature blocks with the same

    d. The resulting authenticated log file will contain all messages
       that have been authenticated, and will indicate (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 will 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 will
   almost certainly be 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.

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    c. We must receive all the certificate blocks before any other
       processing can really be done. (This is why they're sent first.)
        Once that's done, any certificate block that arrives is

    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.

7. Security Considerations

     * 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.

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     * Certain operations in this specification involve the use of
       random numbers. An appropriate entropy source should be used to
       generate these numbers. See [RFC1750].

8. IANA Considerations

   As specified in this document, the Priority field contains options
   for a hash algorithm and signature scheme. Values of zero are
   reserved. A value of 1 is defined to be SHA-1, and OpenPGP-DSA,
   respectively. Values 2 through 63 are to be assigned by IANA using
   the "IETF Consensus" policy defined in RFC2434. Capability Code
   values 64 through 127 are to be assigned by IANA, using the "First
   Come First Served" policy defined in RFC2434. Capability Code values
   128 through 255 are vendor-specific, and values in this range are
   not to be assigned by IANA.

9. Authors and Working Group Chair

   The working group can be contacted via the current chair:

       Chris Lonvick
       Cisco Systems

   The authors of this draft are:

       John Kelsey

       Jon Callas
       PGP Corporation

10. Acknowledgments

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

11. References

   [DSA94]         NIST, FIPS PUB 186, "Digital Signature Standard",
                   May 1994.

   [FIPS-180-1]    "Secure Hash Standard", National Institute of
                   Standards and Technology, U.S. Department Of
                   Commerce, April 1995.

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                   Also known as: 59 Fed Reg 35317 (1994).

   [MENEZES]       Alfred Menezes, Paul van Oorschot, and Scott
                   Vanstone, "Handbook of Applied Cryptography," CRC
                   Press, 1996.

   [RFC1750]       D. Eastlake, S. Crocker, and J. Schiller,
                   "Randomness Recommendations for Security", RFC
                   1750, December 1994.

   [RFC1983]       Malkin, G., "Internet Users' Glossary", FYI 18, RFC
                   1983, August 1996.

   [RFC2045]       N. Freed, N. Borenstein, "Multipurpose Internet Mail
                   Extensions (MIME) Part One: Format of Internet
                   Message Bodies

   [RFC2085]       M. Oehler and R. Glenn, "HMAC-MD5 IP Authentication
                   with Replay Prevention", RFC 2085, February 1997.

   [RFC2104]       H. Krawczyk, M. Bellare, and R. Canetti, "HMAC:
                   Keyed-Hashing for Message Authentication", RFC 2104
                   February 1997.

   [RFC2119]       S. Bradner, "Key words for use in RFCs to Indicate
                   Requirement Level", BCP 14, RFC 2119, March 1997.

   [RFC2234]       D. Crocker, P. Overell, "Augmented BNF for Syntax
                   Specifications: ABNF", RFC 2234, November 1997

   [RFC2434]       T. Narten and H. Alvestrand, "Guidelines for
                   Writing an IANA Considerations Section in RFCs",
                   RFC 2434, October 1998

   [RFC2440]       J. Callas, L. Donnerhacke, H. Finney, and R.
                   Thayer, "OpenPGP Message Format", RFC 2440, November

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

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

   [SYSLOG-REL]    D. New, M. Rose, "Reliable Delivery for syslog",
                   work in progress.

12. Full Copyright Statement

   Copyright 2002 by The Internet Society. All Rights Reserved.

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   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph
   are included on all such copies and derivative works. However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an

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