Signed syslog MessagesNISTjohn.kelsey@nist.govPGP Corporationjon@callas.orgCisco Systemsalex@cisco.com
Security
syslog Working Groupsyslogsyslog-sign
This document describes a mechanism to add origin authentication, message integrity,
replay resistance, message sequencing, and detection of missing messages to the transmitted
syslog messages. This specification is intended to be used in conjunction with the work
defined in RFC 5424, "The syslog Protocol".
This document describes a mechanism, called syslog-sign in this document,
that adds origin
authentication, message integrity, replay resistance, message
sequencing, and detection of missing messages to syslog. Essentially,
this is accomplished by sending a special syslog message.
The contents of this syslog message is called a Signature Block.
Each Signature Block contains, in effect, a detached signature on
some number of previously sent messages. It is cryptographically signed and contains
the hashes of previously sent syslog messages.
The originator of syslog-sign messages is also simply referred to as "signer".
The signer can be the same originator as the originator whose messages it signs,
or it can be a separate originator.
While most implementations
of syslog involve only a single originator
and a single collector of each message,
provisions need to be made to cover situations in which messages are
sent to multiple collectors.
This concerns, in particular, situations in which different messages
from the same originator are sent to different collectors,
which means that some messages are sent
to some collectors but not to others.
The required differentiation of messages is generally performed
based on the Priority value of the individual messages.
For example, messages from any Facility
with a Severity value of 3, 2, 1, or 0 may be sent to one collector
while all messages of Facilities 4, 10, 13, and 14 may be sent to
another collector. Appropriate syslog-sign messages must be kept
with their proper syslog messages. To address this, syslog-sign
uses a Signature Group. A Signature Group identifies a group of
messages that are all kept together for signing purposes by the
signer. A Signature Block always belongs to exactly one Signature
Group and always signs messages belonging only to that Signature
Group.
Additionally, a signer sends Certificate Blocks to provide key
management information between the signer and the collector. A
Certificate Block has a field to denote the type of key material
which may be such things as a PKIX certificate, an OpenPGP certificate,
or even an indication that a key had been pre-distributed.
In the cases of certificates being sent, the
certificates may have to be split across multiple Certificate Blocks
carried in separate messages.
It is possible that the same host contains multiple signers that
each use their own keys to sign syslog messages. In this case,
each signer sends its own Certificate Block and Signature
Blocks. Furthermore, each signer defines its own
Signature Groups. Each signer on a given host needs to use a distinct
combination of APP-NAME and PROCID for its
Signature Block and Certificate Block message.
(This implies that the combination of HOSTNAME, APP-NAME and PROCID
uniquely distinguishes originators of syslog-sign messages across hosts,
provided that the signers use a unique HOSTNAME.)
The collector may verify that the hash of
each received message matches the signed hash contained
in the corresponding Signature Block.
A collector may process these Signature
Blocks as they arrive, building an authenticated log file.
Alternatively, it may store all the log messages in the order they
were received. This allows a network operator to authenticate the
log file at the time the logs are reviewed.
Because the mechanism that is described in this specification uses the
concept of STRUCTURED-DATA elements defined in ,
compliant implementations of this specification MUST also implement
. It is conceivable that the concepts
underlying this specification could
also be used in conjunction with other message delivery mechanisms.
Designers of other efforts to define event notification mechanisms are
therefore encouraged to consider this specification in their designs.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
"SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in .
This specification is intended to be used in conjunction with the syslog
protocol as defined in
. The syslog protocol therefore
MUST be supported by implementations of this specification.
Because the originator generating the
Signature Block message, also simply referred to as "signer",
signs each message in its entirety,
the messages MUST NOT be changed in transit. By the same token,
the syslog-sign messages MUST NOT be changed in transit.
One of the effects of
such behavior, including message alteration by relays,
would be to render any signing invalid
and hence make the mechanism useless.
Likewise, any truncation of messages
that occurs between sending and receiving renders the mechanism useless.
For this reason, syslog signer and collector implementations implementing this
specification MUST support messages of up to and including 2048 octets in length,
in order to minimize the chance of truncation.
While syslog signer and collector implementations MAY support messages with a
length larger than 2048 octets, implementers need to be aware that any message
truncations that occur render the mechanism useless.
In such cases, it is up to the operator to ensure that the syslog messages
can be received properly and can be validated.
recommends using the TLS transport and deliberately
constrains the use of UDP. UDP is NOT RECOMMENDED for use with signed syslog,
because its recommended payload size of 480 octets is too restrictive for
the purposes of syslog-sign. A 480 octet Signature Block could sign only
9 normal messages, meaning that at a significant proportion of messages would
be Signature Block messages. The 480 octet limitation is primarily geared towards
small embeded systems with significant resource constraints which, because of those
constraints, would not implement syslog-sign in the first place. In addition,
the use of UDP is geared towards syslog messages that are primarily
intended for troubleshooting,
a very different purpose from the application targeted by syslog-sign.
Where syslog UDP transport is used, it is the responsibility of operators to ensure
that network paths are configured in a way that messages of sufficient length
(up to and including 2048 octets) can be properly delivered.
This specification uses the syslog message format
described in .
Along with other fields, that document describes the concept of Structured Data (SD).
Structured Data is defined in terms of SD ELEMENTS (SDEs).
An SDE consists of a name and a set of parameter name - value pairs.
The SDE name is referred to as SD-ID.
The name-value pairs are referred to as SD-PARAM, or SD Parameters,
with the name constituting the SD-PARAM-NAME, and the value constituting the SD-PARAM-VALUE.
The syslog messages defined in this document carry the data that is associated with
Signature Blocks and Certificate Blocks
as Structured Data.
The special syslog messages defined in this document
include for this purpose definitions
of SDEs to convey parameters that relate to the signing of syslog messages.
The MSG part of the syslog messages defined in this document SHOULD
simply be empty --
the content of the messages is not intended for interpretation by humans but by applications
that use those messages to build an authenticated log.
Because the syslog messages defined in this document adhere to the format
described in , they identify the machine that
originates the syslog message in the HOSTNAME field. Therefore, the Signature Block and Certificate
Block data do not need to include any additional parameter to identify the machine that orginates the
message.
In addition, several signers MAY sign messages on a single host
independently of each other, each using their own Signature Groups.
In that case, each unique signer is distinguished by the combination
of APP-NAME and PROCID. (By the same token, the same message might be
signed by multiple signers.) Each unique signer MUST have a unique
APP-NAME and PROCID on each host. (This implies that the
combination of HOSTNAME, APP-NAM and PROCID uniquely distinguishes the
originator of syslog-sign messages, provided that the signers use a
unique HOSTNAME.) A Signature Block message MUST use the same
combination of HOSTNAME, APP-NAME, and PROC-ID that was used to send
the corresponding Certificate Block messages containing the Payload
Block.
This section describes the format of the Signature Block and the fields used
within the Signature Block, as well as the syslog messages used to carry the
Signature Block.
There is a need to distinguish the Signature Block itself from the syslog message
that is used to carry a Signature Block.
Signature Blocks MUST be encompassed within completely formed
syslog messages. Syslog messages that contain a Signature Block are also referred to as
Signature Block messages.
A Signature Block message
is identified by the presence of an
SD ELEMENT with an SD-ID with the value "ssign".
In addition, a Signature Block message
MUST contain valid APP-NAME, PROCID, and MSGID fields to be compliant with
.
This specification does not mandate particular values for these fields; however,
for consistency, a signer MUST use the
same values for APP-NAME, PROCID, and MSGID fields for
every Signature Block message that is sent, whichever values are chosen.
It MUST also use the same value for its HOSTNAME field.
To allow for the possibility of multiple signers per host,
the combination of APP-NAME and PROCID MUST be unique for each such signer on any
given host.
If a signer daemon is restarted, it MAY use a new PROCID for what is otherwise the
same signer but MUST continue to use the same APP-NAME.
If it uses a new PROCID, it MUST send a new Payload Block using Certificate Block messages
that use the same new PROCID (and the same APP-NAME).
It is RECOMMENDED (but not required) to use 110 as value
for the PRI field, corresponding to facility 13 (log audit) and severity 6 (informational).
The Signature Block is
carried as Structured Data within the Signature Block message, per the definitions
that follow in the next section.
A Signature Block
message MAY carry other Structured Data besides the Structured Data of the
Signature Block itself.
The MSG part of a Signature Block message SHOULD be empty.
The syslog messages defined as part of syslog-sign themselves
(Signature Block messages and Certificate Block messages) MUST NOT be
signed by a Signature Block. Collectors that
implement syslog-sign know to distinguish syslog messages that are associated with syslog-sign
from those that are subjected to signing and
process them differently. The intent of syslog-sign is to sign a stream of syslog
messages, not to alter it.
The content of a Signature Block message is the Signature Block.
The Signature Block MUST
be encoded as an SD ELEMENT, as defined in
.
The SD-ID MUST have the value of "ssign".
The SDE contains the fields of the Signature Block encoded as
SD Parameters, as specified in the following.
The Signature Block is composed of the following fields. The value of each field
MUST be printable ASCII, and any binary values MUST be
base 64 encoded, as defined in .
The fields MUST be provided in the order listed. Each SD parameter MUST occur once
and only once in the Signature Block. New SD parameters MUST NOT be added unless a new Version
of the protocol is defined. (Implementations that wish to add proprietary extensions will need
to define a separate SD ELEMENT.)
A Signature Block is accordingly encoded as follows, where xxx denotes a placeholder for the
particular values:
[ssign VER="xxx" RSID="xxx" SG="xxx" SPRI="xxx" GBC="xxx"
FMN="xxx" CNT="xxx" HB="xxx" SIGN="xxx"]
Values of the fields constitute SD parameter values and are hence enclosed in quotes,
per .
The fields are separated by single spaces and are described in the subsequent subsections.
The Version field is an alphanumeric value
that has a length of 4 octets, which may include leading zeroes.
The first two octets and the last octet contain a
decimal character in the range of "0" to "9", whereas the third octet
contains an alphanumeric character in the range of "0" to "9", "a" to "z",
or "A" to "Z".
The value in this field specifies
the version of the syslog-sign protocol. This is extensible to allow
for different hash algorithms and signature schemes to be used in
the future. The value of this field is the grouping of the protocol
version (2 octets), the hash algorithm (1 octet) and the signature
scheme (1 octet).
Protocol Version - 2 octets, with "01" as the value for
the protocol version that is described in this document.
Hash Algorithm - 1 octet, where, in conjunction
with Protocol Version 01, a value of "1" denotes SHA1 and
a value of "2" denotes SHA256, as defined in
. (This is the octet
that can have a value of not just "0" to "9" but also "a" to "z" and "A" to "Z".)
Signature Scheme - 1 octet, where, in conjunction
with Protocol Version 01, a value of "1" denotes
OpenPGP DSA, defined in and
.
The version, hash algorithm and signature scheme defined in
this document would accordingly be represented as "0111" (if SHA1 is used as Hash Algorithm)
and "0121" (if SHA256 is used as Hash Algorithm), respectively
(without the quotation marks).
The values of the Hash Algorithm and Signature Scheme are
defined relative to the Protocol Version. If the single-octet representation of the values
for Hash Algorithm and Signature Scheme were to ever represent a limitation,
this limitation could be overcome by defining a new Protocol Version with additional
Hash Algorithms and/or Signature Schemes, and having implementations support both
Protocol Versions concurrently.
As long as sender and receiver are both adhering to
, the prerequisites are in place so that
signed messages can be received by the receiver and validated
with a Signature Block. To ensure immediate validation of received
messages all implementations MUST support SHA1, and SHA256 SHOULD be
supported.
The Reboot Session ID is a decimal value that has a length between 1 and 10 octets.
The acceptable values for
this are between 0 and 9999999999. Leading zeroes MUST be omitted.
A Reboot Session ID is expected to strictly monotonically increase
(i.e., to never repeat or decrease) whenever a signer
reboots in order to allow collectors to distinguish messages and
message signatures across reboots. There are several ways in which
this may be accomplished. In one way, the Reboot Session ID may
increase by 1, starting with a value of 1. Note that in this case, a
signer is required to retain the previous Reboot Session ID across
reboots. In another way, a value of the unix time (number of seconds
since 1 January 1970) may be used. Implementers of this
method need to beware of the possibility of multiple reboots
occurring within a single second. Implementers need to also beware
of the year 2038 problem, which will cause the 32-bit representation of
unix time to wrap in the year 2038. In yet another way,
implementations where the SNMP engine and the signer always
reboot at the same time might consider using the snmpEngineBoots
value as a
source for this counter as defined in .
In cases where a signer is not able to guarantee that the Reboot Session ID is always
increased after a reboot, the Reboot Session ID MUST always be set to a value of 0.
If the value can no longer be increased (e.g., because it reaches 9999999999),
it SHOULD be reset to a value of 1. Implementations SHOULD ensure that
such a reset does not go undetected,
for example by requesting operator
acknowledgment when a reset is performed upon reboot.
(Operator acknowledgment may not be possible in
all situations, e.g. in the case of embedded devices.)
If a reboot of a signer takes place, Signature Block messages MAY use a new PROCID.
However, Signature Block messages of the same signer MUST continue to use the
same HOSTNAME, APP-NAME, and MSGID.
The SG parameter may take any value from
0-3 inclusive. The SPRI parameter may take any value from 0-191 inclusive.
These fields
taken together allow network administrators to associate
groupings of syslog messages with appropriate Signature Blocks and
Certificate Blocks.
Groupings of syslog messages that are signed together are also
called Signature Groups. A Signature Block contains only hashes
of those syslog messages that are part of the same Signature Group.
For example, in some cases, network
administrators might have originators send syslog messages of Facilities 0 through 15
to one collector and those with Facilities 16
through 23 to another. In such cases, associated Signature Blocks should
likely be sent to the corresponding collectors as well, signing the syslog
messages that are intended for each collector separately. This way, each
collector receives Signature Blocks for all syslog messages
that it receives, and only for those.
The ability to associate different categories of syslog messages with different
Signature Groups, signed in separate Signature Blocks,
provides administrators with flexibility in this regard.
Syslog-sign provides four options for handling Signature Groups,
linking them with PRI values so they may be routed to the
destination commensurate with the corresponding syslog messages. In
all cases, no more than 192 distinct Signature Groups (0-191) are permitted.
The Signature Group to which a Signature Block pertains is indicated by
the Signature Priority (SPRI) field.
The Signature Group (SG) field indicates how to interpret the Signature
Priority field. (Note that the SG field does not indicate the Signature Group itself,
as its name might suggest.) The SG field can have one of the following values:
"0" -- There is only one Signature Group.
In this case, the administrators want all Signature
Blocks to be sent to a single destination; in all likelihood,
all of the syslog messages will also be going to that same
destination. Signature Blocks contain signatures for
all messages regardless of their PRI value.
This means that, in effect,
the Signature Block's SPRI value can be ignored.
However, it is RECOMMENDED that a single SPRI value be used for all
Signature Blocks.
Furthermore, it is RECOMMENDED to set that value
to the same value as the
PRI field of the Signature Block message. This way, the PRI of the Signature
Block message matches the SPRI of the Signature Block that it contains.
"1" -- Each PRI value is associated with its own Signature Group. Signature
Blocks for a given Signature Group have SPRI = PRI for that
Signature Group. In other words, the SPRI of the Signature Block matches
the PRI value of the syslog messages that are part of the Signature Group
and hence signed by the Signature Block.
An SG value of 1 can, for example, be used when the administrator of a signer
does not know where any of the syslog messages will ultimately
go but anticipates that messages with different PRI values will be collected and
processed separately. Having a Signature Group per PRI value provides
administrators with
a large degree of flexibility with regard to how to divide
up the processing of syslog messages and their signatures after they
are received, at the same time allowing
Signature Blocks to follow the corresponding syslog messages to their
eventual destination.
"2" -- Each Signature Group contains a range of PRI values.
Signature Groups are assigned sequentially. A Signature Block for
a given Signature Group has its own SPRI value denoting the
highest PRI value of syslog messages in that Signature Group.
The lowest PRI value of syslog messages in that Signature Group will
be one larger than the SPRI value of the previous Signature Group or "0"
in case there is no other Signature Group with a lower SPRI value.
The specific Signature Groups and ranges they are associated with
are subject to configuration by a system administrator.
"3" -- Signature Groups are not assigned with any of the above
relationships to PRI values of the syslog messages they
sign. Instead, another scheme is used, which is outside the scope of
this specification. There has to be some predefined
arrangement between the originator and the intended collectors as to which
syslog messages are to be included in which Signature Group, requiring
configuration by a system administrator. This provides administrators also
with the flexibility to group syslog messages into Signature Groups according to
criteria that are not tied to the PRI value.
Note that this option is is not intended for
deployments which lack such an arrangement, as in those cases a collector could
misinterpret the intended meaning of the Signature Group.
A collector that receives Signature Block messages of a Signature Group
of whose scheme it is not aware SHOULD
bring this fact to the attention of the system administrator. The particular
mechanism used for that is implementation-specific and outside
the scope of this specification.
One reasonable way to configure some installations is to have only
one Signature Group, indicated with SG=0, and have the signer send a copy of
each Signature Block to each collector. In that case, collectors that are not
configured to receive every syslog message will still receive signatures for
every message, even ones they are not supposed to receive.
While the collector will not be able to detect gaps in the
messages (because the presence of a signature of a message that is missing
does not tell the collector whether
or not the corresponding message would be of the collector's concern),
it does allow all messages that do arrive at each collector
to be put into the right order and to be verified. It also
allows each collector to detect duplicates.
Likewise, configuring only one Signature Group can be a reasonable way to
configure installations that involve relay chains,
where one or more interim relays may or may not relay all messages to the
same destination.
The Global Block Counter is a decimal value representing the number of
Signature Blocks sent by syslog-sign before the current one, in this
reboot session. This takes at least 1 octet and at most 10 octets
displayed as a decimal counter. The acceptable values for this
are between 0 and 9999999999, starting with 0. Leading zeroes MUST be omitted.
If the value of the Global Block Counter
has reached 9999999999 and the Reboot Session ID has a value other than 0
(indicating the fact that persistence of the Reboot Session ID is supported),
then the Reboot Session ID MUST be incremented by 1 and the
Global Block Counter resumes at 0. When
the Reboot Session ID is 0 (i.e., persistent
Reboot Session IDs are not supported) and the Global Block Counter
reaches its maximum value, then the Global Block Counter is reset to 0
and the Reboot Session ID MUST remain at 0.
Note that the Global Block Counter
crosses Signature Groups; it allows one to roughly synchronize when
two messages were sent, even though they went to different
collectors and are part of different Signature Groups.
Because a reboot results in the start of a new reboot session, the signer MUST
reset the Global Block Counter to 0 after a reboot occurs.
Applications need to take into account the possibility that a
reboot occurred when authenticating
a log, and situations in which reboots occur frequently may result
in losing the ability to verify the proper sequence in which messages were
sent, hence jeopardizing the integrity of the log.
This is a decimal value between 1 and 10 octets, with leading zeroes omitted.
It contains the unique
message number within this Signature Group of the first message
whose hash appears in this block. The very first message of the
reboot session is numbered "1". This implies that when the Reboot Session ID
increases, the message number is reset to 1.
For example, if this Signature Group has processed 1000 messages so
far and message number 1001 is the first message whose hash appears
in this Signature Block, then this field contains 1001. The
message number is relative to the Signature Group to which it belongs;
hence, a message number does not identify a message beyond its Signature Group.
Should the message number reach 9999999999 within the same reboot session and
Signature Group, the message number subsequently restarts at 1.
In such event, the Global Block Counter will be vastly different
between two occurrences of the same message number.
The count is a 1 or 2 octet field that indicates the number of message
hashes to follow. The valid values for this field are 1 through
99. The number of hashes included in the Signature
Block MUST be chosen such that the length of the
resulting syslog message does not exceed the maximum permissible syslog
message length.
The hash block is a block of hashes, each separately encoded in
base 64. Each hash in the hash block is the hash of the entire
syslog message represented by the hash, independent of the underlying
transport. Hashes are ordered from left to right in the order of occurrence
of the syslog messages that they represent. The space character is
used to separate the hashes. Note, the hash block constitutes a single SD-Param;
a Signature Block message MUST include all its hashes in a single hash block and
MUST NOT spread its hashes across several hash blocks.
The "entire syslog message" refers to what is described as the syslog
message excluding
transport parts that are described in
and
,
and excluding other parts that may be defined
in future transports. The hash value
will be the result of the hashing algorithm run across the syslog message,
starting with the "<" of the PRI portion of the header part of the
message. The hash algorithm used
and indicated by the Version field determines the size of
each hash, but the size MUST NOT be shorter than 160 bits without the use of
padding. It is
base 64 encoded as per .
The number of hashes in a hash block SHOULD be chosen such that the resulting
Signature Block message does not exceed a length of 2048 octets in order to
avoid the possibility that truncation occurs. When more
hashes need to be sent than fit inside a Signature Block message, it is
advisable to start a new Signature Block.
This is a digital signature, encoded in base 64
per .
The signature is calculated over the completely formatted Signature
Block message (starting from the first octet of PRI and continuing
to the last octet of MSG, or STRUCTURED-DATA if MSG is not present),
before the SIGN parameter (SD Parameter Name and the space before it
[" SIGN"], "=", and the corresponding value) is added.
/**** EDITOR'S NOTE: PLEASE ENSURE THAT '[ SIGN"],' APPEARS ON THE
SAME LINE AND IN PARTICULAR THAT THE LINE DOES NOT WRAP AFTER THE '[ ';
THE SPACE ' ' HAS SIGNIFICANCE AND IS MISSED IF THE LINE WRAPS.
ALSO, PLEASE DELETE THIS NOTE. ****/
(In other words, the digital signature is calculated over the whole
message, with the "SIGN=value" portion removed.)
For the OpenPGP DSA signature scheme, the value of the signature field
contains the DSA values r and s,
encoded as two multiprecision integers
(see , Sections 5.2.2 and 3.2), concatenated,
and then encoded in base 64 .
An example of a Signature Block message is depicted below, broken into lines to fit
internet-draft publication rules. There is a space at the end of each line, with the
exception of the last line which ends with "]".
The message is of syslog-sign protocol version "01". It uses SHA1 as hash algorithm
and an OpenPGP DSA signature scheme. Its reboot session ID is 1.
Its Signature Group is 0 which means that all syslog messages go to the same destination;
its Signature Priority (which can effectively be ignored because all syslog messages
will be
signed regardless of their PRI value) is 0. Its Global Block Counter is 2. The first
message number is 1; the message contains 7 message hashes.
Certificate Blocks and Payload Blocks provide key management for
syslog-sign. Their purpose is to support key management that uses
public key cryptosystems.
A Payload Block contains public key certificate information that is to be conveyed
to the collector. A Payload Block is sent at the
beginning of a new reboot session, carrying public key
information in effect for the reboot session.
However, a Payload Block is not sent directly, but in (one or more) fragments.
Those fragments are termed Certificate Blocks. Therefore, signers send at
least one Certificate Block at the beginning of a new reboot session.
There are three key points to understand about Certificate Blocks:
They handle a variable-sized payload, fragmenting it if
necessary and transmitting the fragments as legal syslog
messages. This payload is built (as described below) at the
beginning of a reboot session and is transmitted in pieces with
each Certificate Block carrying a piece. There is
exactly one Payload Block per reboot session.
The Certificate Blocks are digitally signed. The signer does not
sign the Payload Block, but the signatures on the Certificate
Blocks ensure its authenticity. Note that it may not even be
possible to verify the signature on the Certificate Blocks
without the information in the Payload Block; in this case the
Payload Block is reconstructed, the key is extracted, and then
the Certificate Blocks are verified. (This is necessary even
when the Payload Block carries a certificate, because some other
fields of the Payload Block are not otherwise verified.) In
practice, most installations keep the same public key over
long periods of time, so that most of the time, it is easy to
verify the signatures on the Certificate Blocks, and use the
Payload Block to provide other useful per-session information.
The kind of Payload Block that is expected is determined by what
kind of key material is on the collector that receives it. The
signer and collector (or offline log viewer) both have some key
material (such as a root public key or pre-distributed public
key) and an acceptable value for the Key Blob Type in the
Payload Block, below. The collector or offline log viewer MUST
NOT accept a Payload Block of the wrong type.
The Payload Block is built when a new reboot session is started.
There is a one-to-one correspondence between reboot sessions and Payload
Blocks.
A signer creates a new Payload Block after each reboot. The Payload
Block is used until the next reboot.
A Payload Block MUST have the following fields:
Full local time stamp for the signer at the time the reboot session started.
This
must be in the time stamp format specified in
(essentially, time stamp format per
with some further restrictions).
Key Blob Type, a one-octet field containing one of five values:
'C' -- a PKIX certificate (per ).
'P' -- an OpenPGP KeyID and and OpenPGP certificate (a Transferable Public Key as defined in
, Section 11.1).
The first 8 octets of the key blob field contain the
OpenPGP KeyID (identifying which key or subkey inside the
OpenPGP certificate is used), followed by the
OpenPGP certificate itself.
'K' -- the public key whose corresponding private key is
being used to sign these messages.
For the OpenPGP DSA signature scheme,
the key blob field contains the DSA prime p, DSA group order q,
DSA group generator g,
and DSA public-key value y, encoded as four multiprecision integers
(see , Sections 5.5.2 and 3.2).
'N' -- no key information sent; key is pre-distributed.
'U' -- installation-specific key exchange information.
The key blob, if any, base 64
encoded per and
consisting of the raw key data.
The fields are separated by single space characters.
Because a Payload Block is not carried in a
syslog message directly, only the corresponding Certificate Blocks,
it does not need to be encoded as an SD ELEMENT.
The Payload Block does not contain a field that identifies the reboot
session; instead, the reboot session can be inferred from the
Reboot Session ID parameter of the Certificate Blocks that are used to
carry the Payload Block.
To ensure that the sender and receiver have at least one common key
blob type, for immediate validation of received messages, all
implementations MUST support key blob type "C" (PKIX certificate).
When a PKIX certificate is used ("C" key blob type), it is the
certificate specified in .
Per , syslog messages
may be transported over the TLS protocol, even where there is no PKI.
If that transport is used, then the
device will already have a PKIX certificate and it MAY use the
private key associated with that certificate to sign messages.
In the case where there is no PKI, the chain of trust of a PKIX
certificate must still be established to meet conventional security
requirements.
The methods for doing this are described in .
When the collector receives a Payload Block, it needs to determine
whether the signatures are to be trusted. The following methods are
in scope of this specification:
X.509 certification path validation: The collector is configured
with one or more trust anchors (typically root CA certificates),
which allow it to verify a binding between the subject name and
the public key. Certification path validation is performed
as specified in .
If the HOSTNAME contains an FQDN or an IP address, it is then
compared against the certificate as described in ,
Section 5.2. Comparing other forms of HOSTNAMEs is beyond the
scope of this specification.
Collectors SHOULD support this method.
Note that due to message size restrictions, syslog-sign sends
only the end-entity certificate in the Payload Block. Depending
on the PKI deployment, the collector may need to obtain
intermediate certificates by other means (for example, from a
directory).
X.509 end-entity certificate matching: The collector is
configured with information necessary to identify the valid
end-entity certificates of its valid peers, and for each peer,
the HOSTNAME(s) it is authorized to use.
To ensure interoperability, collectors
MUST support
fingerprints of X.509 certificates as described below. Other
methods MAY be supported.
Collectors MUST support key blob type 'C', and specifying the
list of valid peers using certificate fingerprints. The
fingerprint is calculated and formatted as specified in
, Section 4.2.2.
For each peer, the collector MUST support specifying a list of
HOSTNAME(s) this peer is allowed to use either as FQDNs or IP
addresses. Other forms of HOSTNAMEs are beyond the scope of this
specification.
If the locally configured FQDN is an internationalized domain
name, conforming implementations MUST convert it to the ASCII
Compatible Encoding (ACE) format for performing comparisons as
specified in Section 7 of .
An exact case-insensitive
string match MUST be supported, but the implementation MAY also
support wildcards of any type ("*", regular expressions, etc.)
in locally configured names.
Signer implementations MUST provide a means to generate a
key pair and self-signed certificate in the case that a key pair
and certificate are not available through another mechanism, and
MUST make the certificate fingerprint available through a
management interface.
OpenPGP V4 fingerprints: Like X.509 fingerprints, except key
blob type 'P' is used, and the fingerprint is calculated as
specified in , Section 12.2.
When the fingerprint value
is display or configured, each byte is represented in
hexadecimal (using two uppercase ASCII characters), and space is
added after every second byte. For example: "0830 2A52 2CD1 D712
6E76 6EEC 32A5 CAE1 03C8 4F6E".
Signers and collectors MAY support this method.
Other methods, such as "web of trust", are beyond the scope of this
document.
This section describes the format of the Certificate Block and the fields used
within the Certificate Block, as well as the syslog messages used to carry
Certificate Blocks.
Certificate Blocks are used to get the Payload Block to the collector.
As with a Signature Block, each Certificate Block is carried in its
own syslog message,
called Certificate Block message.
In case separate collectors are associated with different Signature Groups,
Certificate Block messages need to be sent to each collector.
Because certificates can legitimately be much longer than 2048 octets,
the Payload Block can be split up into several pieces, with
each Certificate Block carrying a piece of the Payload Block.
Note
that the signer MAY make the Certificate Blocks of any legal length
(that is, any length that keeps the entire Certificate Block message
within 2048 octets) that holds all the
required fields. Software that processes Certificate Blocks MUST
deal correctly with blocks of any legal length.
The length of the fragment of the Payload Block that a Certificate Block
carries MUST be at least 1 octet. The length SHOULD be chosen
such that the length of the Certificate
Block message does not exceed 2048 octets.
A Certificate Block message
is identified by the presence of an
SD ELEMENT
with an SD-ID with the value "ssign-cert".
In addition, a Certificate Block message
MUST contain valid APP-NAME, PROCID, and MSGID fields to be compliant with
syslog protocol.
Syslog-sign does not mandate particular values for these fields; however,
for consistency, a signer MUST use the
same value for APP-NAME, PROCID, and MSGID fields for
every Certificate Block message, whichever values are chosen.
It MUST also use the same value for its HOSTNAME field.
To allow for the possibility of multiple signers per host,
the combination of APP-NAME and PROCID MUST be unique for each such originator.
If a signer daemon is restarted, it MAY use a new PROCID for what is otherwise
the same signer. The combination of APP-NAME and PROCID MUST be the same
that is used for Signature Block messages of the same signer; however, a
different MSGID MAY be used for Signature Block and Certificate Block messages.
It is RECOMMENDED to use 110 as value
for the PRI field, corresponding to facility 13 (log audit)
and severity 6 (informational).
The Certificate Block is
carried as Structured Data within the Certificate Block message.
A Certificate Block
message MAY carry other Structured Data besides the Structured Data of the
Certificate Block itself.
The MSG part of a Certificate Block message SHOULD be empty.
The contents of a Certificate Block message is the Certificate Block itself.
Like a Signature Block, the Certificate Block is encoded as an SD ELEMENT.
The SD-ID of the Certificate Block is "ssign-cert".
The Certificate Block is composed of the following fields, each of which is
encoded as an SD Parameter with parameter name as indicated. Each field
must be printable ASCII, and any binary values are base 64 encoded per
.
The fields MUST be provided in the order listed.
New SD parameters MUST NOT be added unless a new Version
of the protocol is defined. (Implementations that wish to add proprietary extensions
will need to define a separate SD ELEMENT.)
A Certificate Block is accordingly encoded as follows, where xxx denotes a
placeholder for the particular values:
[ssign-cert VER="xxx" RSID="xxx" SG="xxx" SPRI="xxx" TPBL="xxx"
INDEX="xxx" FLEN="xxx" FRAG="xxx" SIGN="xxx"]
Values of the fields constitute SD parameter values and are hence enclosed in quotes,
per .
The fields are separated by single spaces and are described below. Each SD parameter MUST
occur once and only once.
The Version field is 4 octets in length.
This field is identical in format and meaning to the
Version field described in .
The Reboot Session ID is identical in format and meaning to the
RSID field described in
.
The SIG field is identical in format and meaning to the SIG field described in
.
The SPRI field is identical in format and meaning to the SPRI field described there.
A signer SHOULD send separate Certificate Block messages for each Signature Group.
This ensures that each collector that is associated with a Signature Group will
receive the necessary key material in the case that messages of different Signature
Groups are sent to different collectors. Note that the signer needs to get the same
Payload Block to each collector, as for any given signer there is a one-to-one
relationship between Payload
Block and Reboot Session across all Signature Groups. Deployments that wish
to associate different key material (and hence different Payload Blocks) with
different Signature Groups can use separate
signers for that purpose, each distinguished by its own combination
of HOSTNAME, APP-NAME, PROCID.
The Total Payload Block Length is a value representing the total length
of the Payload Block in octets, expressed as a decimal with one to eight octets
with leading zeroes omitted.
This is a decimal value between 1 and 8 octets,
with leading zeroes omitted.
It contains the number of octets
into the Payload Block at which this fragment starts. The first octet of
the first fragment is numbered "1". (Note, it is not numbered "0".)
The total length of this fragment expressed as a decimal integer
with one to four octets with leading zeroes omitted.
The fragment length must be at least 1.
The Payload Block Fragment contains a fragment of the payload block.
Its length must match the indicated fragment length.
This is a digital signature, encoded in base 64, as per
. The Version field effectively specifies the
original encoding of the signature.
The signature is calculated over the completely formatted
Certificate Block message, before the SIGN parameter is added (see
).
For the OpenPGP DSA signature scheme, the value of the signature field
contains the DSA values r and s,
encoded as two multiprecision integers
(see , Sections 5.2.2 and 3.2), concatenated,
and then encoded in base 64 .
An example of a Certificate Block message is is depicted below, broken into lines to fit
internet-draft publication rules. There are no spaces at the end of the lines that contain
the key blob and the signature.
The message is of syslog-sign protocol version "01". It uses SHA1 as hash algorithm and an
OpenPGP DSA signature scheme. Its reboot session ID is 1.
Its Signature Group is 0; its Signature Priority is 0. The Total Payload Block Length
is 587. The index into the payload block is 1 (meaning this is the first fragment).
The length of the fragment is 587 (meaning that the Certificate Block message contains the
entire Payload Block). The Payload Block has the time stamp 2009-05-03T14:00:39.519005+02:00.
The Key Blob Type is 'K', meaning that it contains a public key whose corresponding
private key is being used to sign these messages.
Note that the Certificate Block message in this example has a time stamp
that is very close to the time stamp in the Payload Block.
The fact that the time stamps are so close implies that this is the first
Certificate Block message
sent in this reboot session; additional Certificate Block messages can be
sent later with a later time stamp, which will carry the same Payload Block
that will still contain the same time stamp.
As described in Section 8.5 of ,
a transport sender may
discard syslog messages. Likewise, when syslog messages are sent over
unreliable transport, they can be lost in transit.
However, if a collector does not receive
Signature and Certificate Blocks, many messages may not be able to
be verified. The signer is allowed to send Signature and
Certificate Blocks multiple times. Sending Signature and Certificate Blocks
multiple times provides redundancy with the intent to ensure that
the collector or relay does get the Signature Blocks and in particular the
Payload Block at some point in time. In the meantime, any online review of logs
as described in is delayed until the
needed blocks are received.
The collector MUST ignore duplicats of
Signature Blocks and Certificate Blocks that it has already received and
authenticated. The signer can in principle change its redundancy
level for any reason, without communicating this fact to the
collector.
A signer that is also the originator of messages that it signs
does not need to queue up other messages while sending
redundant Certificate Block and Signature Block messages. It MAY send redundant
Certificate Block messages even after Signature Block messages
and regular syslog messages have been sent. By the same token, it MAY
send redundant Signature Block messages even after newer syslog messages
that are signed by a subsequent Signature Block have been sent, or even
after a subsequent Signature Block message.
In addition, the signer has flexibility in how many hashes to include within
a Signature Block. It is legitimate for an originator to send short Signature
Blocks to allow the collector to verify messages with minimal delay.
Although the transport sender is not constrained in how it decides to send
redundant Signature and Certificate Blocks, or even in whether it
decides to send along multiple copies of normal syslog messages,
we define some redundancy parameters below which may be useful
in controlling redundant transmission from the transport sender to the
transport receiver, and which may be useful for administrators to configure.
Certificate Blocks are always sent at the beginning of a new reboot
session. One technique to ensure reliable delivery (see Section 8.5) is to send
multiple copies. This can be controlled by a "certInitialRepeat" parameter:
certInitialRepeat = number of times each Certificate Block should be
sent before the first message is sent.
It is also useful to resend Certificate Blocks every now and then
for long-lived reboot sessions. This can be controlled by the certResendDelay
and certResendCount parameters:
certResendDelay = maximum time delay in seconds until resending the
Certificate Block.
certResendCount = maximum number of other syslog messages to send until resending
the Certificate Block.
In some cases, it may be desirable to allow for configuration of the
transport sender such that Certificate Blocks are not sent at all after
the first normal syslog message has been sent.
This could be expressed by
setting both certResendDelay and certResendCount to "0".
However, it is RECOMMENDED
to configure the transport sender to send redundant Certificate Blocks even after the
first message, in particular when the UDP transport
is used.
In one set of circumstances, the receiver may receive a Certificate
Block, some group of syslog messages and some corresponding Signature
Blocks. If the receiver reboots after that then the conditions of
recovery will vary depending upon the transport.
For UDP , the receiver SHOULD continue to use the cached
Certificate Block, but MUST validate the RSID value to make sure
that it has the most current one. If the receiver cannot validate that it has
the most current Certificate Block, then it MUST wait
for a retransmission of the Certificate Block, which may be controlled
by the certResendDelay and certResendCount parameters. It is up to
the operators to ensure that Certificate Blocks are sent frequently
enough to meet this set of circumstances.
For TLS transport , the sender
MUST send a fresh Certificate
Block when a session is established. This will keep the sender and
receiver synchronized with the most current Certificate Block.
Implementations which support sending syslog messages of different
Signature Groups to different collectors and which wish to offer very granular
controls MAY allow the above
parameters to be configured on a per Signature Group basis.
The choice of reasonable values in a given deployment
depends on several factors,
including the acceptable delay
that that may be incurred from the receipt of a syslog message
until the corresponding Signature Block is received,
whether UDP or TLS transport is used,
and the available management bandwidth.
The following might be a reasonable choice for a deployment
in which reliability of underlying transport and of collector implementation
are of little concern:
certInitialRepeat=1, certResendDelay=1800 seconds, certResendCount=10000
The following might be a reasonable choice for a deployment
in which reliability of transmission over UDP transport could be an issue:
certInitialRepeat=2, certResendDelay=300 seconds, certResendCount=1000
Verification of log messages involves a certain delay of time that is caused by the lag
in time
between the sending of the message itself and the corresponding Signature Block.
The following configuration parameter can be useful to limit the time lag that will
be incurred (note that the maximum message length may also force
generating a Signature Block; see Sections and
):
sigMaxDelay = generate a new Signature Block if this many seconds
have elapsed since the message with the First Message Number of the Signature Block
was sent.
Retransmissions of Signature Blocks are not sent immediately after
the original transmission, but slightly later. The following
parameters control when those retransmissions are done:
sigNumberResends = number of times a Signature Block is resent.
(It is recommended so select a value of greater than "0"
in particular when the UDP transport is used.)
sigResendDelay = send the next retransmission when this many seconds have elapsed
since the previous sending of this Signature Block.
sigResendCount = send the next retransmission when this many other syslog messages have
been sent since the previous sending of this Signature Block.
The choice of reasonable values in a given deployment
depends on several factors,
including the acceptable delay that that may be incurred from the
receipt of a syslog message until the corresponding Signature Block
is received so that the syslog message can be verified,
the reliability of the underlying transport,
and the available management bandwidth.
The following might be a reasonable choice for a
deployment where reliability of transport and collector are of little concern
and where there is a need to have syslog messages generally signed within 5 minutes:
sigMaxDelay=300 seconds, sigNumberResends=2, sigResendDelay=300 seconds, sigResendCount=500
The following would be a reasonable choice for a deployment
that needs to validate syslog messages typically within 60 seconds,
but no more than 3 minutes after receipt:
sigMaxDelay=30 seconds, sigNumberResends=5, sigResendDelay=30 seconds, sigResendCount=100
Notwithstanding the fact that the signer is not constrained in whether it
decides to send redundant Signature Block messages, Signature Blocks SHOULD NOT overlap.
This facilitates their processing by the receiving collector.
This means that an originator of
Signature Block messages, after having sent a first message with some
First Message Number and a Count, SHOULD NOT send a second message with the same
First Message Number but a different Count.
It also means that an originator of Signature Block messages SHOULD NOT
send a second message whose First Message Number is greater than the First Message
Number, but smaller than the First Message Number plus the Count indicated in the
first message.
That said, the possibility of Signature Blocks that overlap does provide additional
flexibility with regards to redundancy; it provides an additional option that may
be desirable in some deployments.
Therefore collectors MUST be designed in a way that they can cope with
overlapping Signature Blocks when confronted with them. The collector MUST
ignore hashes of messages that it has already received and validated.
The logs secured with syslog-sign may be reviewed either online or
offline. Online review is somewhat more complicated and
computationally expensive, but not prohibitively so.
This section outlines a method for online and a method for
offline verification of logs which implementations MAY
choose to implement to verify logs efficiently.
Implementations MAY also choose to implement a different method; it is
ultimately up to each implementation how to process the messages that
it receives.
When the collector stores logs to be reviewed later, they can be
authenticated offline just before they are reviewed. Reviewing these
logs offline is simple and relatively inexpensive in terms of resources
used, so long as there is enough space available on the reviewing
machine.
To do so, we first go through the stored log file. Each message
in the log file is classified
as a normal message, a Signature Block message, or a
Certificate Block message.
Signature Blocks and Certificate Blocks are then separated by
signer (as identified by HOSTNAME, APP-NAME, PROCID), Reboot Session ID,
and Signature Group, and stored in their own
files. Normal messages are stored in a keyed file, indexed on
their hash values. They are not separated by signer, as their
(HOSTNAME, APP-NAME, PROCID) identifies the application that
generated the message. The application that generated the message
does not have to coincide with the signer.
For each signer, Reboot Session ID,
and Signature Group, we then do the following:
We sort the Certificate Block file by INDEX value, and check to
see whether we have a set of Certificate Blocks that can reconstruct
the Payload Block. If so, we reconstruct the Payload Block,
verify any key-identifying information, and then use this to
verify the signatures on the Certificate Blocks we have received.
When this is done, we have verified the reboot session and key
used for the rest of the process.
We sort the Signature Block file by First Message Number. We now
create an authenticated log file, which consists of some
header information and then a sequence of message number,
message text pairs.
We next go through the Signature Block file.
We initialize a cursor for the last message number processed with the number 0.
For each Signature Block in the file, we do the following:
Verify the signature on the Signature Block.
If the value of the First Message Number of the Signature Block is less than or equal
to the last message number processed, skip the first (last message number processed
minus First Message Number plus 1) hashes.
For each remaining hashed message in the Signature Block:
Look up the hash value in the keyed message file.
If the message is found, write (message number, message
text) to the authenticated log file.
Set the last message number processed to the value of the
First Message Number plus the Count of the Signature Block
minus 1.
Skip all other Signature Blocks with the same
First Message Number unless one with a larger Count is encountered.
The resulting authenticated log file contains all messages
that have been authenticated. In addition, it implicitly indicates
all gaps in the authenticated messages (specifically
in the case when all messages of the same Signature Group
are sent to the same collector), because their
message numbers are missing.
One can see that, assuming sufficient space for building
the keyed file, this whole process is linear in the number of
messages (generally two seeks, one to write and the other to read,
per normal message received), and O(N lg N) in the number of
Signature Blocks. This estimate comes with two caveats: first, the
Signature Blocks arrive very nearly in sorted order, and so can
probably be sorted more cheaply on average than O(N lg N) steps.
Second, the signature verification on each Signature Block
almost certainly is more expensive than the sorting step in
practice. We have not discussed error-recovery, which may be
necessary for the Certificate Blocks. In practice, a simple
error-recovery strategy is probably enough: if the Payload
Block is not valid, then we can just try alternate
instances of each Certificate Block, if such are available, until we
get the Payload Block right.
It is easy for an attacker to flood us with plausible-looking
messages, Signature Blocks, and Certificate Blocks.
Some collector implementations may need to monitor log
messages in close to real-time. This can be done with
syslog-sign, though it is somewhat more complex than offline
verification.
This is done as follows:
We have an authenticated message file, into which we write (message number,
message text) pairs which have been authenticated. We will
assume that we are handling only one signer, Signature Group, and
Reboot Session ID at any given time.
(For the concurrent support of multiple signers, Signature Groups, and Reboot Session IDs,
the same procedure is applied analogously to each.
Signature Block mesages and Certificate Block
messages clearly indicate their respective signer, Signature Group, and Reboot Session ID.)
We have two data structures: A "Waiting for Signature" queue
in which (arrival sequence, hash of message) pairs are kept in sorted
order, and a "Waiting for Message" queue in which (message number, hash of message)
pairs are kept in sorted order. In addition, we have a hash table that stores
(message text, count) pairs
indexed by hash value. In the hash table, count may be any number
greater than zero; when count is zero, the entry in the hash
table is cleared.
Note: The "Waiting For Signature" queue gets used in the normal
case, when the signature arrives after the message itself.
It holds messages that have been received but whose signature
has yet to arrive.
The "Waiting for Message" queue gets used in the case that
messages are lost or misordered (either in
the network or in relays). It holds signatures that have been
received but whose corresponding messages have yet to arrive.
Since a single Signature Block can
cover only a limited number of messages (due to size restrictions),
and massive reordering/delaying is rare, it is expected that both
queues would be relatively small.
We must receive all the Certificate Blocks before any other
processing can really be done. (This is why they are sent first.)
Once that is done, any additional Certificate Block message that arrives is
discarded.
Any syslog messages or Signature Block messages that arrive before all
Certificate Blocks have been received need to be buffered. Once all
Certificate Blocks have been received, the messages in the buffer can be
retrieved and processed as if they were just arriving.
Whenever a normal message arrives, we first check if its hash
value is found in the "Waiting For Message" queue. If it is,
we write the message number (from the "Waiting for Message" queue)
and the message into the authenticated message file and remove
the entry from the queue.
Otherwise, we add (arrival sequence, hash of message) to the
"Waiting For Signature" queue. If our hash table already has an entry
for the message's hash value, we increment its count by one;
otherwise, we create a new entry with count = 1.
If the "Waiting For Signature" message queue is full, we remove
the oldest message from the queue. That message could not be validated
close enough to real time.
In order to update
the hash table accordingly, we use that entry's hash to
index the hash table. If that entry has count 1, we delete the
entry from the hash table; otherwise, we decrement its count.
By removing the message from the "Waiting for Signature" message
queue without having actually received the message's signature,
we make it impossible to authenticate the message
should its signature arrive later.
Implementors therefore need to ensure that queues
are dimensioned sufficiently large to not expose the collector against
DoS attacks that attempt to flood the collector with unsigned messages.
Whenever a Signature Block message arrives, we check its
originator, (i.e. the signer) by way of HOSTNAME, APP-NAME,
PROCID, as well as its Signature Group and Reboot Session ID to
ensure it matches our Certificate Blocks. We then check to see
whether the First Message Number value is too old to still be of
interest, or if another Signature Block with that First Message
Number and the same Count or a greater Count has already been
received. If so, we discard the Signature Block.
We then check
the signature. Again, we discard the Signature Block
if the signature is not valid.
Otherwise, we proceed with processing the hashes in the Signature Block.
A Signature Block contains a sequence of hashes, each of which is associated
with a message number, starting with the First Message Number for the first
hash and incrementing by one for each subsequent hash.
For each hash, we first check to see whether the message
hash is in the hash table. If this is the case, it means that
we have received the
signature for a message that was received earlier, and we do the following:
We check if a message with the same message
number is already in the authenticated message file.
If that is the case, the signed hash is a duplicate and we
discard it.
Otherwise (the signed hash is not a duplicate),
we write the (message
number, message text) into the authenticated message file.
We also update
the hash table accordingly, using that entry's hash to
index the hash table. If that entry has count 1, we delete the
entry from the hash table; otherwise, we decrement its count.
Otherwise (the message hash is not in the hash table),
we write the (message number, message hash) to the
"Waiting for Message" queue.
If the "Waiting for Message" queue is full, we remove the oldest
entry. In that case, a message that was signed by the signer
could not be validated by the receiver, either because the message was
lost, or because the signature arrived way ahead of the actual message.
By removing the entry from the "Waiting for Message" queue without
having actually received the message, we make it impossible to authenticate the
a legitimate message should that message still arrive later.
Implementors need to ensure queues
are dimensioned sufficiently large so that the chances of such a
scenario actually occcurring is minimized.
The result of this is a sequence of messages in the
authenticated message file. Each message in the message
file has been
authenticated. The sequence is labeled with numbers showing the
order in which the messages were originally transmitted.
One can see that this whole process is roughly linear
in the number of messages, and also in the number of Signature
Blocks received. The process is susceptible to flooding attacks; an
attacker can send enough normal messages that the messages roll off
their queue before their Signature Blocks can be processed.
Normal syslog event messages are unsigned and have most of the security attributes
described in Section 8
of . This document also describes Certificate Blocks
and Signature Blocks, which are signed syslog messages. The Signature Blocks contain
signature information for previously sent syslog event messages. All of this
information can be used to authenticate syslog messages and to minimize or obviate
many of the security concerns described in .
The model for syslog-sign is a direct trust system where the certificate transferred
is its own trust anchor. If a transport sender sends a stream of syslog messages that is
signed using a certificate, the operator or application will transfer to the transport receiver
the certificate that was used when signing. There is no need for a certificate chain.
As with any technology involving cryptography, it is advisable to check
the current literature to determine whether any algorithms used here
have been found to be vulnerable to attack.
This specification uses Public Key Cryptography
technologies. The proper party or parties have to control
the private key portion of a public-private key pair.
Any party that controls a private key can sign anything
it pleases.
Certain operations in this specification involve the use of
random numbers. An appropriate entropy source SHOULD be used to
generate these numbers. See
and .
As a signer, it is advisable to avoid message lengths exceeding 2048 octets.
Various problems might result
if a signer were to send messages with a length greater than 2048
octets, because relays MAY truncate messages with lengths
greater than 2048 octets which would make it impossible for collectors to
validate a hash of the packet. To increase
the chance of interoperability, it tends to be
best to be conservative with what
you send but liberal in what you are able to receive.
Signers need to rigidly adhere to the RFC5424 format when
sending messages. If a collector receives a message
that is not formatted properly then it might drop it, or it
may modify it while receiving it. (See A.2 of .)
If that were to happen, the hash of the sent message would not match
the hash of the received message.
Collectors are not to malfunction in case they receive malformed syslog messages or
messages containing characters other than those specified in this document. In other
words, they are to ignore such messages and continue working.
Syslog does not strongly associate the message
with the message originator. That association is established by the collector upon
verification of the Signature Block. Before a Signature Block is used to
ascertain the authenticity of an event message, it might be received, stored, and
reviewed by a person or automated parser. It is advisable not to assume a message is
authentic until after a message has been
validated by checking the contents of the Signature Block.
With the Signature Block checking, an attacker may only forge messages if he or she
can compromise the private key of the true originator.
Event messages might be recorded and replayed by an
attacker. Using the information contained in the
Signature Blocks, a reviewer can determine whether the received messages are the ones
originally sent by an originator. The reviewer can also identify messages that have
been replayed. Using a method for verification of logs such as the one
outlined in , a replayed message can be detected by checking
prior to writing a message to the authenticated log file whether the message is
already contained in it.
Event messages sent over UDP might be lost in transit.
can be used for
the reliable delivery of syslog messages;
however, it does not protect against loss of syslog messages at the
application layer, for example if the TCP connection or TLS session has been
closed by the transport receiver for some reason.
A reviewer can identify any messages sent by the originator but not
received by the collector by reviewing the Signature Block information.
In addition, the information in
subsequent Signature Blocks allows a
reviewer to determine whether any Signature Block messages
were lost in transit.
Syslog messages delivered over UDP might not only be lost, but
also arrive out of sequence.
A reviewer can determine the original order of syslog messages and identify
which messages were delivered out of order by examining the information
in the Signature Block
along with any timestamp information in the message.
Syslog messages might be damaged in transit. A review of
the information in the Signature Block determines whether
the received message was the intended message sent by
the originator. A damaged Signature Block or Certificate
Block is evident because the collector will not be
able to validate that it was signed by the signer.
Unless TLS is used as a secure transport ,
event messages, Certificate Blocks, and Signature Blocks are all sent in plaintext.
This allows network administrators to read the
message when sniffing the wire. However, this also allows an attacker to see the
contents of event messages and perhaps to use that information for malicious purposes.
It is conceivable that an attacker might intercept Certificate Block messages and insert
its own Certificate information. In that case, the attacker would be able to receive
event messages from the actual originator and then relay modified messages, insert new
messages, or delete messages. It would then be able to construct a Signature Block
and sign it with its own private key. Network administrators need to verify
that the key contained in the Payload Block is indeed the key being used on the
actual signer. If that is the case, then this MITM attack will not succeed.
Methods for establishing a chain of trust are also described
in .
An attacker might send invalid Signature Block messages to overwhelm the collector's
processing capability and consume all available resources.
For this reason, it can be appropriate to simply
receive the Signature Block messages and process them only as time permits.
An attacker might also just overwhelm a collector by sending more
messages to it than it can handle.
Implementers are advised to consider features that minimize this threat,
such as only accepting syslog messages from known IP addresses.
Nothing in this protocol attempts to eliminate covert
channels. In fact, just about every aspect of
syslog messages lends itself to the conveyance of covert
signals. For example, a collusionist could send odd and
even PRI values to indicate Morse Code dashes and dots.
With regard to ,
IANA is requested to add the following values to the registry entitled "syslog
Structured Data id values":
In addition, several fields need to be controlled by the IANA in both
the Signature Block and the Certificate Block, as outlined in the following
sections.
IANA is requested to create three registries, each associated with a different subfield
of the Version field of Signature Blocks and Certificate Blocks, described in
and , respectively.
The first registry that IANA is requested to create
is entitled "syslog-sign protocol version values".
It is for the values of the Protocol Version subfield. The Protocol Version subfield
constitutes the first 2 octets in the Version field.
New values shall be assigned by the IANA using the "IETF Review" policy
defined in .
Assigned numbers are to be increased by 1, up to a maximum value of "50".
Protocol Version numbers of "51" through "99" are vendor-specific;
values in this range are not to be assigned by the IANA.
IANA is requested to register the Protocol Version values shown below.
The second registry that IANA is requested to create
is entitled "syslog-sign hash algorithm values".
It is for the values of the Hash Algorithm subfield. The Hash Algorithm subfield
constitutes the third octet in the Version field Signature Blocks and Certificate Blocks.
New values shall be assigned by the IANA using the "IETF Consensus" policy
defined in . Assigned values are to
be increased sequentially, first up to a maximum value of "9", then from "a" to "z", then
from "A" to "Z".
The values are registered relative to the Protocol Version. This means that the same
Hash Algorithm value can be reserved for different Protocol Versions, possibly referring
to a different hash algorithm each time. This makes it possible to
deal with future scenarios in which the single octet representation becomes a limitation,
as more Hash Algorithms can be supported by defining additional Protocol Versions that
implementations might support concurrently.
IANA is requested to register the Hash Algorithm values shown below.
The third registry that IANA is requested to create
is entitled "syslog-sign signature scheme values".
It is for the values of the Signature Scheme subfield. The Signature Scheme subfield
constitutes the fourth octet in the Version field of Signature Blocks and Certificate
Blocks. New values shall be assigned by the IANA using the "IETF Consensus" policy
defined in . Assigned values are to
be increased by 1, up to a maximum value of "9". This means that the same
Signature Scheme value can be reserved for different Protocol Versions, possibly in each
case referring to a different Signature Scheme each time. This makes it possible to
deal with future scenarios in which the single octet representation becomes a limitation,
as more Signature Schemes can be supported by defining additional Protocol Versions that
implementations might support concurrently.
IANA is requested to register the Signature Scheme values shown below.
IANA is requested to create a registry entitled "syslog-sign sg field values".
It is for values of the SG Field as defined in .
New values shall be assigned by
the IANA using the "IETF Consensus" policy defined in
. Assigned values are to be incremented by 1,
up to a maximum value of "7".
Values "8" and "9" shall be left as vendor specific and shall not be assigned by the IANA.
IANA is requested to register the SG Field values shown below.
IANA is requested to create a registry entitled "syslog-sign key blob type values".
It is to register one-character identifiers for the key blob type, per
. New values shall be assigned by
the IANA using the "IETF Consensus" policy defined in
. Uppercase letters may be assigned as values.
Lowercase letters are left as vendor specific and shall not be assigned by the IANA.
IANA is requested to register the key blob type values shown below.
The working group can be contacted via the mailing list:
The current Chairs of the Working Group can be contacted at:
The authors wish to thank Alex Brown, Chris Calabrese, Steve Chang,
Pasi Eronen, Carson
Gaspar, Rainer Gerhards, Drew Gross, David Harrington, Chris Lonvick, Albert Mietus, Darrin New,
Marshall Rose,
Andrew Ross, Martin Schuette, Holt Sorenson, Rodney Thayer,
and the many Counterpane Internet Security engineering and
operations people who commented on various versions of this proposal.
Digital Signature StandardNational Institute of Standards and TechnologySecure Hash StandardNational Institute of Standards and TechnologyKey words for use in RFCs to Indicate Requirement LevelsHarvard University1350 Mass. Ave.CambridgeMA 02138- +1 617 495 3864-
General
keyword
In many standards track documents several words are used to signify
the requirements in the specification. These words are often
capitalized. This document defines these words as they should be
interpreted in IETF documents. Authors who follow these guidelines
should incorporate this phrase near the beginning of their document:
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
RFC 2119.
The force of these words is modified by the requirement
level of the document in which they are used.
The Base16, Base32, and Base64 Data EncodingsOpenPGP Message Format
Security
pretty good privacyPGPsecurityGuidelines for Writing an IANA Considerations Section in RFCsIBM Corporation3039 Cornwallis Ave.PO Box 12195 - BRQA/502Research Triangle ParkNC 27709-2195919-254-7798narten@raleigh.ibm.comGooglePirsenteretN-7005 TrondheimNorway+47 73 54 57 97Harald@Alvestrand.no
General
Internet Assigned Numbers AuthorityIANA
Many protocols make use of identifiers consisting of constants and
other well-known values. Even after a protocol has been defined and
deployment has begun, new values may need to be assigned (e.g., for a
new option type in DHCP, or a new encryption or authentication
algorithm for IPSec). To insure that such quantities have consistent
values and interpretations in different implementations, their
assignment must be administered by a central authority. For IETF
protocols, that role is provided by the Internet Assigned Numbers
Authority (IANA).
In order for the IANA to manage a given name space prudently, it
needs guidelines describing the conditions under which new values can
be assigned. If the IANA is expected to play a role in the management
of a name space, the IANA must be given clear and concise
instructions describing that role. This document discusses issues
that should be considered in formulating a policy for assigning
values to a name space and provides guidelines to document authors on
the specific text that must be included in documents that place
demands on the IANA.
Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile
The syslog ProtocolTLS Transport Mapping for syslogTransmission of syslog Messages over UDPNIST Special Publication 800-90: Recommendation for Random Number Generation using Deterministic Random Bit GeneratorsNational Institute of Standards and TechnologyDate and Time on the Internet: TimestampsUser-based Security Model (USM) for version 3 of the Simple Network Management Protocol (SNMPv3)Randomness Recommendations for SecurityDigital Equipment Corporation550 King StreetLKG2-1/BB3LittletonMA01460US+1 508 486 6577dee@lkg.dec.comMassachusetts Institute of Technology77 Massachusetts AvenueCambridgeMA02139US+1 617 253 0161jis@mit.eduCyberCash Inc.2086 Hunters Crest WayViennaVA22181US+1 703 620 1222+1 703 391 2651crocker@cybercash.com
Security systems today are built on increasingly strong cryptographic algorithms
that foil pattern analysis attempts. However, the security of these systems is
dependent on generating secret quantities for passwords, cryptographic keys, and
similar quantities. The use of pseudo-random processes to generate secret
quantities can result in pseudo-security. The sophisticated attacker of these
security systems may find it easier to reproduce the environment that produced the
secret quantities, searching the resulting small set of possibilities, than to
locate the quantities in the whole of the number space.
Choosing random quantities to foil a resourceful and motivated adversary is
surprisingly difficult. This paper points out many pitfalls in using traditional
pseudo-random number generation techniques for choosing such quantities. It
recommends the use of truly random hardware techniques and shows that the existing
hardware on many systems can be used for this purpose. It provides suggestions to
ameliorate the problem when a hardware solution is not available. And it gives
examples of how large such quantities need to be for some particular applications.