Network Working Group P Karn
Internet Draft Qualcomm
W A Simpson
DayDreamer
expires in six months April 1996
The Photuris Session Key Management Protocol -
draft-simpson-photuris-10.txt |
Status of this Memo
Distribution of this memo is unlimited. -
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Abstract
Photuris is an experimental session-key management protocol intended
for use with the IP Security Protocols (AH and ESP).
Applicability
Photuris is intended for Internet nodes that frequently access or are
accessed by a large and unpredicatable number of other nodes. It
features defense against resource clogging, perfect forward secrecy, |
and (optional) privacy protection of the exchange parties.
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Photuris is primarily used for creating virtual private networks, +
establishing sessions for mobile users and networks operating over
bandwidth-limited links, and short-lived sessions between numerous -
clients and servers. +
Photuris is extensible. A wide variety of authentication, compres- +
sion, encryption, identification, and other operational types are +
supported. +
Photuris is independent of any particular party identification method +
or certificate format. Support for symmetric key party identifica- +
tion is required to be implemented, and asymmetric key party identi- +
fication is optionally supported by extensions.
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1. Introduction
The ultimate goal of Internet Security is to facilitate direct IP
connectivity between sensitive hosts and users across the Internet.
Users will rely on Internet Security to protect the confidentiality
of the traffic they send across the Internet and depend on it to
block unauthorized external access to their internal hosts and net-
works.
Users must have confidence in every Internet Security component,
including key management. Without this confidence, users may erect
barriers that impede legitimate use of the Internet, or forego the
Internet entirely.
Internet Security does not place any significance on easily forged IP
Source addresses. It relies instead on proof of possession of secret
knowledge: that is, a cryptographic key.
However, secure manual distribution and maintainance of these keys is
often cumbersome and problematic. User distribution often leads to
long-lived keys, with concommitant opportunity for compromise of the
keys.
A fundamental role of this key management protocol is to verify the
values exchanged, while ensuring that the resulting key is not known
by another party. It has been shown [DOW92] that key exchange must
be coupled to authentication. Each party requires assurance that an
exchanged key is not shared with an imposter.
Protecting sensitive data on the Internet against compromise --
either by interception or by unauthorized access -- is necessary, but
not sufficient. The computing resources themselves must also be pro-
tected against malicious attack or sabotage.
With these criteria in mind, Photuris [Firefly] is designed:
A. for frequent exchange of limited lifetime individual session-keys,
with a minimum of configuration and effort.
B. for associating security parameters with these session-keys. |
C. to support the use of a variety of authentication methods, and
facilitate the exchange of many identification types.
D. to thwart certain types of denial of service attacks on host
resources.
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E. to provide these services with minimal network activity, balanced |
with computational efficiency. |
This document is primarily intended for implementing the Photuris |
protocol. It is not intended to detail service and application |
interface definitions, although it does mention some basic policy |
areas as required for the proper implementation and operation of the |
protocol mechanisms.
1.1. Terminology
exchange-value The publically distributable value used to calculate
a shared-secret. As used in this document, refers
to a Diffie-Hellman exchange, not the public part of |
a public/private key-pair.
private-key A value that is kept secret, and is part of an asym- |
metric public/private key-pair.
public-key A publically distributable value that is part of an |
asymmetric public/private key-pair.
secret-key A symmetric key that is not publically dis- |
tributable. As used in this document, this is dis- |
tinguished from an asymmetric public/private key- |
pair. An example is a user password.
Security Association
A collection of parameters describing the security
relationship between two nodes. These parameters
include the identities of the parties, the transform
(including algorithm and algorithm mode), the key(s)
(such as a session-key, secret-key, or appropriate
public/private key-pair), and possibly other infor-
mation such as sensitivity labelling. For further
details, see [RFC-1825].
Security Parameters Index (SPI)
A number that indicates the Security Association.
The number is relative to the IP Destination, which
is the SPI Owner.
session-key A key that is independently derived from a shared-
secret by the parties, and used for keying one
direction of traffic. This key is changed fre-
quently.
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shared-secret As used in this document, the calculated result of
the Photuris exchange.
SPI Owner The party that corresponds to the IP Destination; +
the receiver of the datagram. +
SPI User The party that corresponds to the IP Source; the +
sender of the datagram.
transform A cryptographic manipulation of a particular set of
data. As used in this document, refers to certain
well-specified methods (which are defined else-
where). For example, AH-MD5 [RFC-1828] transforms |
an IP datagram into a cryptographic hash, and ESP-
DES-CBC [RFC-1829] transforms plaintext to cipher-
text and back again.
Implementors will find details of cryptographic hashing (such as
MD5), encryption algorithms and modes (such as DES), digital signa-
tures (such as DSS), and other algorithms in [Schneier95].
1.2. Protocol Overview
The Photuris protocol consists of several simple phases:
1. A "Cookie" Exchange guards against simple flooding attacks sent |
with bogus IP Sources or UDP Ports. Each party passes a "cookie" |
to the other.
In addition, supported exchange-schemes are offered by the Respon- |
der for calculating a shared-secret.
2. A Value Exchange establishes a shared-secret between the parties.
Each party passes an Exchange-Value to the other. These values +
are used to establish a shared-secret between the parties. The
Responder remains stateless until a shared-secret has been cre-
ated.
In addition, supported attributes are offered by each party for |
use in establishing new Security Associations.
3. An Identification Exchange identifies the parties to each other,
and verifies the integrity of values sent in phases 1 and 2.
In addition, the shared-secret provides a basis to generate sepa- |
rate Security Association session-keys in each direction, which
are in turn used for conventional authentication or encryption.
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Additional security attributes are also exchanged as needed.
This exchange may also be encrypted for party privacy protection |
using an exchange session-key based on the shared-secret. This |
protects the identities of the parties, hides the security parame- |
ter values, and improves security for the exchange protocol and |
security transforms.
4. Additional messages may be exchanged to periodically change the
session-keys, and to establish new or revised security parameters.
These exchanges may also be encrypted for party privacy protection |
in the same fashion as above.
The sequence of message types and their purposes are summarized in
the diagram below. The first three phases (cookie, exchange, and
identification) must be carried out in their entirety before any
security association can be used.
Initiator Responder
========= =========
Cookie_Request ->
<- Cookie_Response
offer schemes
Value_Request ->
pick scheme
offer value
offer attributes
<- Value_Response
offer value
offer attributes
[generate shared-secret from exchanged values]
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Identity_Request -> |
make SPI
pick SPI attribute(s)
identify self
authenticate
(make protection key) |
(encrypt message) |
<- Identity_Response |
make SPI
pick SPI attribute(s)
identify self
authenticate
(make protection key) |
(encrypt message) |
[make SPI session-keys in each direction]
SPI User SPI Owner |
======== ========= |
SPI_Needed -> |
list SPI attribute(s) |
make integrity key |
authenticate |
(encrypt message) |
<- SPI_Update |
make SPI |
pick SPI attribute(s) |
make SPI session-key(s) |
make integrity key |
authenticate |
(encrypt message) |
Either party may initiate an exchange at any time. For example, the
Initiator need not be a "caller" in a telephony link.
The Initiator is responsible for recovering from all message losses
by retransmission.
A Photuris exchange between two parties results in a pair of SPI val- |
ues (one in each direction). Each SPI is used in creating separate
session-key(s) in each direction.
When both parties initiate Photuris exchanges concurrently, or one
party initiates more than one Photuris exchange, the Initiator Cook-
ies (and UDP Ports) keep the exchanges separate. This results in
more than one initial SPI for each Destination.
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To create multiple Security Associations with different parameters, |
the parties may also send SPI_Updates.
There is no requirement that all such outstanding SPIs be used. The
SPI User (sender) selects an appropriate SPI for each datagram trans-
mission.
1.3. Clogging Defense
To grant access to authorized users regardless of location, it must
be possible to cheaply detect and discard bogus datagrams. Other-
wise, an attacker intent on sabotage might rapidly send datagrams to
exhaust the host's CPU or memory resources.
Using Internet Security authentication facilities, when a datagram
does not pass an authentication check, it can be discarded without
further processing. This is easily done with manual (null) key man-
agement between trusted hosts at relatively little cost, given the
speed of cryptographic hashing functions compared to public-key algo-
rithms.
Unfortunately, such a trusted host will have only a fixed number of
keys available. The keys will tend to have long lifetimes. This
carries significant security risks.
Automatic key management is necessary to generate keys between par-
ties without prior arrangement. But, there is a potential Achilles
heel in the key management protocol.
Because of their use of CPU-intensive operations such as modular
exponentiation, key management schemes based on public-key cryptogra-
phy are vulnerable to resource clogging attacks. Although a complete
defense against such attacks is impossible, Photuris features make
them much more difficult.
1.3.1. Cookie Exchange
Photuris exchanges a pair of "cookies" based on the IP node addresses |
before initiating any extensive computational operations. This |
cookie exchange provides a weak form of message origin authentication |
and verifies the presence of network communications between the par- |
ties, thwarting the saboteur from using random IP Source addresses. |
The simple validation of these cookies uses the same level of |
resources as other Internet Security authentication mechanisms.
This forces the attacker to:
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1) use its own valid IP address, or
2) gain access to a physical transmission link and appropriate those
addresses, or
3) subvert Internet routing for the same purpose.
The first option allows the target to detect and filter out such
attacks, and significantly increases the likelihood of identifying
the attacker. The latter two options are much more difficult than
merely sending large numbers of datagrams with randomly chosen IP
Source addresses from an arbitrary point on the Internet.
The cookie exchange does not protect against an observer that can |
copy a valid cookie, or an interceptor that can modify or substitute
another cookie. These attacks are mitigated somewhat with time- |
variant cookies.
1.3.2. State Limitation
There is a small amount of state associated with the Photuris
exchange itself. This includes the Cookies, Exchange-Values, and the
computed shared-secret.
During the initial Cookie Exchange, the Responder does not maintain
any state for the exchange. This prevents memory resource exhaustion
from a simple flooding attack.
Later exchange phases require saving of state to perform the key
establishment calculations and identity verification. An attacker
that is willing to expose itself to a larger window of detection can |
waste substantial resources by repeating the steps of the Photuris |
process without using the results. |
The Responder combines time-variant cookies with a counter to limit |
the number of multiple concurrent Photuris exchanges with the same |
Internet nodes. Initiators will not be issued additional cookies by |
the Responder until their previous exchanges have concluded or |
expired. This combination also prevents an attack by hoarding valid |
cookies, and then flooding the Responder with a large number of con- |
current exchanges.
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1.3.3. State Precomputation
Prior to accepting Cookie_Requests, the Responder can precompute its |
Exchange-Value. Successive requests from multiple Initiators will
not require additional computation until the Identification Exchange. |
Once Photuris exchange state has been established between nodes, |
repetitive exchanges can use many of the same previously computed
values. This prevents an attacker with more CPU power from easily
exhausting the target.
1.3.4. State Expiration
During a Photuris exchange, the Responder Exchange TimeOut limits the |
wait for the exchange to complete. This includes the packet round |
trips, and the time for completing the Identification Exchange calcu- |
lations. The time is bounded by both the maximum amount of calcula- |
tion delay expected for the processing power of an unknown peer, and |
the minimum user expectation for results (default 60 seconds). |
In addition, all retained exchange state of both parties has an asso- |
ciated Exchange LifeTime, and is subject to periodic expiration. |
This depends on the physical and logistical security of the machine, |
and is typically in the range of 10 minutes to one day (default 30 |
minutes).
When an Exchange-Value expires (or is replaced by a newer value), all |
related exchange state is purged. The periodic expiration and purge
of exchange state reduces the risk of compromise of keys and secrets,
and is an important consideration in attaining Perfect Forward |
Secrecy.
If an attacker has succeeded in overwhelming a target, the target |
will eventually recover its resources as the expired state is purged. |
Implementation Notes:
These Exchange LifeTimes and TimeOuts are implementation dependent |
and are not disclosed in any Photuris message. The paranoid oper- +
ator will have a fairly short Exchange LifeTime, but it MUST NOT +
be less than twice the Exchange TimeOut.
To prevent synchronization between Photuris exchanges, the imple-
mentation SHOULD randomly vary each Exchange LifeTime within twice
the range of seconds that are required to calculate a new
Exchange-Value. For example, if the Responder uses a base
Exchange LifeTime of 30 minutes, and takes 10 seconds to calculate
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the new Exchange-Value, the equation might be (in milliseconds): |
1800000 + random(20000) |
The exchange-scheme, Exchange-Values, and resulting shared-secret |
MAY be cached in short-term storage for the Exchange LifeTime. |
When repetitive Photuris exchanges occur between the same parties, |
and the Exchange-Values are discovered to be unchanged, the pre- |
computed shared-secret can be used to rapidly generate new ses- |
sion-keys.
1.4. Perfect Forward Secrecy
Many security breaches in cryptographic systems have been facilitated
by designs that generate traffic-encrypting keys (or their equiva-
lents) well before they are needed, and then keep them around longer
than necessary. This creates many opportunities for compromise,
especially by insiders. A carefully designed public-key system can
avoid this problem.
The rule is to avoid using any long-lived keys (such as a RSA public-
private key-pair) to encrypt session-keys or actual traffic. Such
keys should be used solely for identification (entity authentication) |
purposes.
All keys for traffic encryption should be randomly generated immedi-
ately before use, and then destroyed immediately after use, so that
they cannot be recovered. The keys should not be based on the values
of any previous keys, or any other long-lived stored information.
The Photuris exchange messages can provide perfect forward secrecy,
as defined by Diffie [Diffie90]. When the calculated shared-secret
is eventually destroyed, it is unrecoverable.
Theft of the private/secret key used to sign the exchanges would
allow the thief to impersonate the party in future conversations, but
it would not decode any past traffic that might have been recorded.
1.5. Security Parameters -
Photuris key management is used to determine a number of parameters
for each Security Association between the communicating parties.
This includes the particular authentication and/or encryption trans-
forms, and the key(s) used to authenticate, encrypt or decrypt the
payload.
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The key management implementation usually maintains a table or list
containing the several parameters for each concurrent Security Asso-
ciation. The implementation needs to access that security parameter
table to determine how to process each datagram. To indicate a par-
ticular table entry, a Security Parameters Index (SPI) is used.
The SPI is assigned by the entity controlling the IP Destination: the
SPI Owner (the receiver). The parties use the combination of SPI and
IP Destination to distinguish the correct association.
Each SPI has an associated LifeTime, specified by the SPI owner
(receiver). This SPI LifeTime is usually related to the speed of the
link (typically 30 seconds to 30 minutes).
The SPI can also be deleted by the SPI Owner using the SPI_Update. |
Once the SPI has expired or been deleted, the parties cease using the
SPI.
Implementation Notes:
The method used for SPI assignment is implementation dependent.
However, selection of a cryptographically random value can help
prevent attacks that depend on a predicatable sequence of values.
To prevent synchronization between Photuris exchanges, the imple-
mentation SHOULD randomly vary each SPI LifeTime by a few seconds.
To prevent resurrection of deleted or expired SPIs, implementa-
tions SHOULD remember those SPIs, but mark them as unusable until
the Photuris exchange shared-secret used to create them also
expires and purges the associated state.
When more than one unexpired SPI is available for the same func-
tion, a common implementation technique is to select the SPI with
the greatest remaining LifeTime. However, selecting randomly
among a large number of SPIs may provide some defense against
traffic analysis. +
When an implementation detects an incoming SPI that has recently +
expired, but the associated state has not yet been purged, the +
implementation MAY accept the SPI. The length of time allowed is +
highly dependent on clock drift and variable packet round trip +
time, and is therefore implementation dependent.
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1.6. LifeTimes
The Photuris exchange results in two kinds of state, each with sepa-
rate LifeTimes.
1) The Exchange LifeTime of the small amount of state associated with |
the Photuris exchange itself. This state may be viewed as between
Internet nodes.
2) The SPI LifeTimes of the multiple Security Associations that are |
established. This state may be viewed as between users and nodes.
The SPI LifeTimes may be shorter or longer than the Exchange Life- |
Time. These LifeTimes are not required to be related to each other.
When an Exchange-Value expires (or is replaced by a newer value), any |
unexpired derived SPIs are not affected. This is important to allow
traffic to continue without interruption during new Photuris
exchanges.
1.7. Identification
Every party requires its own Identification. When the Photuris |
exchange is node to node, such as single user personal computers or
unattended firewalls used in virtual private networks, the nodes
themselves may be viewed as the users.
When required for secure multi-user environments, the Iden- |
tity_Messages can be used to provide separate limited authentication
from each user of one node when communicating with another common
node. To provide bi-directional user-oriented keying, the parties |
can initiate multiple concurrent Photuris exchanges. These may pro-
vide separate user Identification from the Initiator to the Responder
in each direction.
Each secure multi-user operating system MUST be capable of separately
maintaining multiple Identification Exchange SPI values for each
Value Exchange calculated shared-secret. It is the responsibility of
the Source to internally segregate the shared-secret and different
session-keys provided per Destination, and select an appropriate SPI
for each datagram transmission.
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Design Notes:
Successful use of user-oriented keying requires a significant
level of operating system support. Use of multi-user segregated
exchanges likely requires added functionality in the transport API
of the implementation operating system. Such mechanisms are out-
side the scope of this document.
It has been suggested that the Photuris exchange could also be
established between particular application or transport processes
associated with a user of a node. Such mechanisms are emphati-
cally outside the scope of this document.
1.8. Multicast Support
Key management is more difficult in a multicast environment.
Senders to a multicast group may share common a Security Parameters
Index, if all communications are using the same security configura-
tion parameters. In this case, the receiver only knows that the mes-
sage came from a node knowing the SPI for the group, and cannot
authenticate which member of the group sent the datagram.
Multicast groups may also use a separate SPI value for each Source.
If each sender is keyed separately and asymmetric algorithms are
used, data origin authentication is also provided.
A given Destination is not necessarily in control of the selection
process. In the case of multicast groups, a single node or coop-
erating subset of the multicast group may work on behalf of the
entire group to set up a Security Association.
It is anticipated that Photuris would be used first to establish a
distribution SPI and session-key, and that another orthogonal key
distribution mechanism will use that SPI to send the group keys.
This is a matter for future research. Such mechanisms are outside |
the scope of this document.
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2. Protocol Details
The Initiator begins a Photuris exchange under several circumstances: |
- The Initiator has a datagram that it wishes to send with privacy,
and has no current Photuris exchange state with the IP Destina-
tion. This datagram is discarded, and a Cookie_Request is sent
instead.
- The Initiator has received the ICMP message [RFC-1812] Destination |
Unreachable: Communication Administratively Prohibited (Type 3, |
Code 13), and has no current Photuris exchange state with the ICMP |
Source. |
- The Initiator has received the ICMP message [RFC-xxxx] Security
Failures: Bad SPI (Type 40, Code 0), that matches current Photuris |
exchange state with the ICMP Source. |
- The Initiator has received the ICMP message [RFC-xxxx] Security
Failures: Need Authentication (Type 40, Code 4), and has no cur- |
rent Photuris exchange state with the ICMP Source. |
- The Initiator has received the ICMP message [RFC-xxxx] Security
Failures: Need Authorization (Type 40, Code 5), that matches cur- |
rent Photuris exchange state with the ICMP Source.
When the event is an ICMP message, special care MUST be taken that |
the ICMP message actually includes information that matches a previ- |
ously sent IP datagram. Otherwise, this could provide an opportunity |
for a clogging attack, by stimulating a new Photuris Exchange.
2.1. UDP
All Photuris messages use the User Datagram Protocol header
[RFC-768]. The Initiator sends to UDP Destination Port 468.
When replying to the Initiator, the Responder swaps the IP Source and
Destination, and the UDP Source and Destination Ports.
The UDP checksum MUST be correctly calculated when sent. When a mes-
sage is received with an incorrect UDP checksum, it is silently dis-
carded.
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Implementation Note:
It is expected that installation of Photuris will ensure that UDP |
checksum calculations are enabled for the computer operating sys- |
tem and later disabling by operators is prevented.
2.2. Header Format
All of the messages have a format similar to the following, as trans-
mitted left to right in network order (most significant to least sig-
nificant):
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Initiator-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Responder-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type |
+-+-+-+-+-+-+-+-+
Initiator-Cookie 16 octets.
Responder-Cookie 16 octets.
Type one octet. Each message type has a unique value.
Initial values are assigned as follows:
0 Cookie_Request
1 Cookie_Response
2 Value_Request
3 Value_Response
4 Identity_Request
5 Secret_Response
6 Secret_Request
7 Identity_Response
8 SPI_Needed
9 SPI_Update
10 Bad_Cookie
11 Resource_Limit
12 Verification_Failure
13 (reserved)
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Further details and differences are elaborated in the individual mes-
sages.
Design Note:
The fixed size of the cookies was chosen for convenience, based on
the output of commonly available cryptographic hashing functions.
It is anticipated that this size is likely to be more than suffi-
cient to protect against very high bit-rate flooding attacks.
2.3. Variable Precision Numbers
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Size | Value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Size two, four, or eight octets. The number of signifi-
cant bits used in the Value field. Always transmit-
ted most significant octet first.
When the Size is zero, no Value field is present;
there are no significant bits. This means "missing"
or "null". It should not be confused with the value
zero, which includes an indication of the number of
significant bits.
When the most significant octet is in the range 0
through 254 (0xfe), the field is two octets. Both
octets are used to indicate the size of the Value
field, which ranges from 1 to 65,279 significant
bits (in 1 to 8,160 octets).
When the most significant octet is 255 (0xff), the
field is four octets. The remaining three octets
are added to 65,280 to indicate the size of the
Value field, which is limited to 16,776,959 signifi-
cant bits (in 2,097,120 octets).
When the most significant two octets are 65,535
(0xffff), the field is eight octets. The remaining
six octets are added to 16,776,960 to indicate the
size of the Value field. This is vastly too long
for these UDP messages, but is included for com-
pleteness.
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Value Zero or more octets. Always transmitted most sig-
nificant octet first.
The bits used are right justified within octet
boundaries; that is, any unused bits are in the most
significant octet. Unused bits are zero filled.
Shortened forms SHOULD NOT be used when the Value includes a number
of leading zero significant bits. The Size SHOULD indicate the cor-
rect number of significant bits.
Design Notes:
Some of the message fields require a value that may vary in the |
number of bits. These bits may not make up an integral number of
octets.
The numbers are assumed to be unsigned.
The emphasis on significant bits was based on concerns that cryp-
tographic lengths and strengths be readily determined. This is in
contrast to the usual concern that each number have only one
unique (shortest) representation. +
When processing datagrams containing variable size values, the +
length must be checked against the overall datagram length. An +
invalid size (too long or short) that causes a poorly coded +
receiver to abort could be used as a denial of service attack.
2.4. Exchange Schemes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Scheme | Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Scheme two octets. A unique value indicating the exchange-
scheme. See the "Exchange Scheme List". +
Size two octets, ranging from 0 to 65,279. See variable
precision number.
Value Zero or more octets. See variable precision number.
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Selection among several different exchange-schemes is needed to
enable experimental and proprietary extensions without affecting the
basic protocol. The target of the exchange (Responder) specifies a
list of the schemes supported, and the Initiator chooses one that it
also supports.
The scheme list includes alternative algorithms and distinguishing
parameters. These are mixed in the same list for simplicity. The
implementation can easily distinguish between the separate uses of
each supported scheme. These uses are indicated in the "Exchange
Scheme List".
Design Notes:
Although exchange-schemes offer great flexibility, only a few
well-chosen algorithms and parameters are specified. This pro-
vides opportunity for intensive review by the cryptographic commu-
nity, reduces implementation complexity, and improves potential
for interoperability.
Only one exchange-scheme (#2) is required to be supported, and |
SHOULD be present in every Offered-Schemes list.
2.5. Attributes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Value(s) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type one octet. A unique value indicating the kind of
attribute. See the "Attribute List" for details. +
When the Type is zero (padding), no Length field is |
present (always zero).
Length one octet. The size of the Value(s) field in
octets.
When the Length is zero, no Value(s) field is pre-
sent.
Value(s) Zero or more octets. See the "Attribute List" for |
details.
Selection among several different security parameter attributes is
needed to enable future implementation changes without affecting the
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basic protocol. Each party (the sender) offers a list of the
attributes supported and its peer (the receiver) selects from that
list when making its incoming Security Associations.
The attribute list includes authentication, compression, encryption,
identification, and other operational types available for exchange
between the parties. These are mixed in the same number space for
simplicity. The implementation can easily distinguish between the
separate uses of each supported attribute. See the "Attribute List" |
for details. |
The Length MUST NOT be assumed to be constant for a particular Type. |
The same Type MAY be present in a list of attributes with varying |
Lengths.
Design Notes:
Although attributes offer great flexibility, only a few well-
chosen algorithms are specified. This provides opportunity for
intensive review by the cryptographic community, reduces implemen-
tation complexity, and improves potential for interoperability.
The authentication, compression, encryption and identification
mechanisms chosen, as well as the encapsulation modes (if any),
need not be the same in both directions. +
When processing datagrams containing variable length values, the +
length must be checked against the overall datagram length. An +
invalid length (too long or short) that causes a poorly coded +
receiver to abort could be used as a denial of service attack.
2.5.1. Authentication
Authentication decisions are in the SPI Owner (receiver) direction.
Only the receiver can determine that arriving traffic is authentic.
Its need for authentication is indicated by choosing authentication
attributes, and/or authenticated encryption attributes, when creating
each SPI. It enforces authentication through the simple expedient of |
dropping all datagrams with missing or invalid authentication, and |
sending an appropriate ICMP Security Failures message [RFC-xxxx],
such as Need Authentication (Type 40, Code 4) or Need Authorization
(Type 40, Code 5).
Support is required for the "MD5-KDP" and "Simple MD5-DP Verifica- |
tion" Attributes, and they SHOULD be present in every Offered-
Attributes list.
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If the potential SPI Owner (receiver) has not created any authentica-
tion SPIs although Photuris exchange state has been established, but
it sends ICMP Security Failures messages, the prospective SPI User |
(sender) is unable to provide authentication for its datagrams. When
this situation occurs, the prospective SPI User SHOULD log the occu- |
rance, and notify an operator as appropriate.
Design Notes:
This feature is particularly important for deployment and scaling.
It cannot be expected that the prospective SPI User will be omni- |
scient about the upgrade status and policy of potential receivers.
Instead, the datagram receiver indicates its authentication needs. |
The coupling of the ICMP message with the Cookie Exchange provides
additional defense against clogging, at the expense of another
round trip.
2.5.2. Encapsulation
Encapsulation decisions are in the SPI User (sender) direction. Only
the sender can determine whether each datagram needs privacy protec-
tion. It uses an encryption SPI created by the SPI Owner (receiver),
in addition to an authentication SPI (as needed).
Since SPI creation is by the receiver, but privacy (and potentially |
other) decisions are made in the sending direction, a message is
needed to stimulate the SPI creation. When the prospective SPI User
(sender) needs privacy protection for a datagram and Photuris
exchange state has been established, but has no current privacy |
encapsulation SPI from the potential SPI Owner (receiver), an |
SPI_Needed message is sent by the prospective SPI User, listing pri- |
vacy attributes that both parties have previously offered. The orig-
inal datagram is discarded.
Support is required for the "DES-CBC" Attribute, and it SHOULD be
present in every Offered-Attributes list. Where encryption is pro-
hibited in a particular environment, the "DES-CBC" Attribute MAY be
omitted.
If either party has not offered any encryption attributes, the
prospective SPI User (sender) is unable to provide privacy for its |
datagrams. When this situation occurs, the prospective SPI User |
SHOULD log the occurance, and notify an operator as appropriate.
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Implementation Notes: |
Typically, an encryption method is chosen for the primary
attribute of the initial SPI in each direction.
If integrity is needed, and there is no existing separate SPI that
offers authentication, it is recommended that an authentication
method be included as a secondary attribute in the initial SPI.
When both authentication and encryption attributes are used for
the same SPI, care must be exercised that there is no interaction
between the algorithms that might reveal some portion of the ses-
sion-key(s). There is no known interaction between MD5 and DES-
CBC.
3. Cookie Exchange
Initiator Responder +
========= ========= +
Cookie_Request -> +
<- Cookie_Response +
offer schemes +
3.0.1. Send Cookie_Request
The Initiator initializes local state, and generates a "cookie". The |
Initiator-Cookie MUST be different in each new Cookie_Request between |
the same parties. See "Cookie Generation" for details. |
By default, the Responder-Cookie and Counter are set to zero. |
If the new Cookie_Request is in response to a message from a previous |
exchange in which this party was the Responder, the Responder-Cookie |
is set to the previous Initiator-Cookie, and the Counter is set to |
zero. |
Otherwise, the IP Destination for the Responder is examined. If any |
previous exchange between the peer IP nodes has not expired, the |
Responder-Cookie is set to the most recent Responder-Cookie, and the |
request Counter is set to the corresponding Counter.
The Initiator also starts a retransmission timer. If no valid
Cookie_Response arrives within the time limit, the same |
Cookie_Request is retransmitted for the remaining number of Retrans- |
missions. The Initiator-Cookie value MUST be the same in each such |
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retransmission to the same IP Destination and UDP Port. |
When Retransmissions have been exceeded, if a Bad_Cookie message has |
been received during the exchange, the Initiator SHOULD begin the |
Photuris exchange again by sending a new Cookie_Request.
3.0.2. Receive Cookie_Request
On receipt of a Cookie_Request, the Responder determines whether
there are sufficient resources to begin another Photuris exchange.
- When too many SPI values are already in use for this particular
peer, or too many concurrent exchanges are in progress, or some +
other resource limit is reached, a Resource_Limit message is sent. |
- When any previous exchange initiated by this particular peer has |
not exceeded the Exchange TimeOut, and the Responder-Cookie does |
not specify one of these previous exchanges, a Resource_Limit mes- |
sage is sent. |
Otherwise, the Responder returns a Cookie_Response. |
Note that the Responder creates no additional state at this time.
3.0.3. Send Cookie_Response
The IP Source for the Initiator is examined. If any previous +
exchange between the peer IP nodes has not expired, the response +
Counter is set to the most recent exchange Counter plus one (allowing +
for out of order retransmissions). Otherwise, the response Counter +
is set to the request Counter plus one. If the new value is zero +
(modulo 256), the value is set to one. +
The Responder generates a cookie. The Responder-Cookie value in each |
successive response SHOULD be different. See "Cookie Generation" for |
details.
The exchange-schemes available between the peers are listed in the |
Offered-Schemes.
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3.0.4. Receive Cookie_Response
The Initiator validates the Initiator-Cookie, and the Offered- |
Schemes. |
- Whenever an invalid/expired Initiator-Cookie is detected, the mes- |
sage is silently discarded. |
- Whenever the variable length Offered-Schemes do not match the UDP |
Length, or all Offered-Schemes are obviously defective and/or |
insufficient for the purposes intended, the message is silently |
discarded; the implementation SHOULD log the occurance, and notify |
an operator as appropriate. |
- Once a valid message has been received, later Cookie_Responses |
with matching Initiator-Cookies are also silently discarded, until
a new Cookie_Request is sent.
When the message is valid, an exchange-scheme is chosen from the list
of Offered-Schemes.
This Scheme-Choice may affect the next Photuris message sent. By
default, the next Photuris message is a Value_Request.
Design Notes:
Having the scheme chosen by the Initiator allows the greatest pro-
tocol flexibility, and follows the requirement that no state be
kept by the Responder until the shared-secret is calculated.
Unfortunately, this allows the weakest scheme to be chosen by an
attacker.
This is no worse than the alternative: to have the Responder
choose from weak schemes offered by the attacker. |
Various proposals for extensions utilize the Scheme-Choice to
indicate a different message sequence. Such mechanisms are out- |
side the scope of this document.
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3.1. Cookie_Request
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Initiator-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Responder-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Counter | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
Initiator-Cookie 16 octets. A randomized value that identifies the
exchange. The value MUST NOT be zero. See "Cookie |
Generation" for details.
Responder-Cookie 16 octets. Identifies a specific previous exchange.
Copied from a previous Cookie_Response. +
When zero, no previous exchange is specified.
When non-zero, and the Counter is zero, contains the +
Initiator-Cookie of a previous exchange. The speci- |
fied party is requested to be the Responder in this |
exchange, to retain previous party pairings. |
When non-zero, and the Counter is also non-zero, |
contains the Responder-Cookie of a previous |
exchange. The specified party is requested to be |
the Responder in this exchange, to retain previous |
party pairings. |
Also, can be used for bidirectional User, Transport, |
and Process oriented keying. Such mechanisms are |
outside the scope of this document.
Type 0
Counter one octet. Indicates the number of the current +
exchange. Copied from a previous Cookie_Response. +
When zero, no previous Responder is specified.
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3.2. Cookie_Response
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Initiator-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Responder-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Counter | Reserved | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Offered-Schemes ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Initiator-Cookie 16 octets. Copied from the Cookie_Request.
Responder-Cookie 16 octets. A randomized value that identifies the
exchange. The value MUST NOT be zero. See "Cookie |
Generation" for details.
Type 1
Counter one octet. Indicates the number of the current +
exchange. Must be greater than zero.
Reserved two octets. For future use; MUST be set to zero |
when transmitted, and MUST be ignored when received.
Offered-Schemes A list of one or more exchange-schemes supported by
the Responder, beginning with most preferred.
Each scheme is four or more octets (see "Exchange
Scheme List"). Only one of each kind of scheme may
be offered. The end of the list is indicated by the
UDP Length.
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3.3. Cookie Generation
The exact technique by which a Photuris party generates a cookie is
implementation dependent. The method chosen must satisfy some basic
requirements:
1. The cookie MUST depend on the specific parties. This prevents an
attacker from obtaining a cookie using a real IP address and UDP
port, and then using it to swamp the victim with requests from
randomly chosen IP addresses or ports.
2. It MUST NOT be possible for anyone other than the issuing entity
to generate cookies that will be accepted by that entity. This
implies that the issuing entity will use local secret information
in the generation and subsequent verification of a cookie. It
must not be possible to deduce this secret information from any
particular cookie.
3. The cookie generation and verification methods MUST be fast to
thwart attacks intended to sabotage CPU resources.
A recommended technique is to use a cryptographic hashing function |
(such as MD5). |
An incoming cookie can be verified at any time by regenerating it
locally from values contained in the incoming datagram and the local |
secret random value.
3.3.1. Initiator Cookie
The Initiator secret value that affects its cookie SHOULD change for |
each new Photuris exchange, and is thereafter internally cached on a
per Responder basis. This provides improved synchronization and pro-
tection against replay attacks.
An alternative is to cache the cookie instead of the secret value.
Incoming cookies can be compared directly without the computational
cost of regeneration. +
It is recommended that the cookie be calculated over the secret +
value, the IP Source and Destination addresses, and the UDP Source +
and Destination ports.
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3.3.2. Responder Cookie
The Responder secret value that affects its cookies MAY remain the |
same for many different Initiators. However, this secret SHOULD be |
changed periodically to limit the time for use of its cookies (typi- |
cally each 60 seconds), and MUST be changed whenever any precalcu- |
lated Responder Exchange-Value is changed. |
The Responder-Cookie SHOULD include the Counter from the |
Cookie_Response. This provides improved synchronization and protec- |
tion against replay attacks.
It is recommended that the cookie be calculated over the secret |
value, the IP Source and Destination addresses, its own UDP Destina- |
tion port, the Counter, and the Initiator-Cookie. |
On receipt of a Value_Request, the Responder regenerates its cookie
for validation. The cookie is not cached per Initiator to avoid sav-
ing state during the initial Cookie Exchange.
Once the Value_Response is sent, both Initiator and Responder cookies |
are cached to identify the exchange.
4. Value Exchange
Initiator Responder +
========= ========= +
Value_Request -> +
pick scheme +
offer value +
offer attributes +
<- Value_Response +
offer value +
offer attributes +
[generate shared-secret from exchanged values] +
4.0.1. Send Value_Request
The Initiator generates an appropriate Exchange-Value for the Scheme- |
Choice. This Exchange-Value may be precalculated and used for multi- |
ple Responders.
The IP Destination for the Responder is examined, and the attributes |
available between the parties are listed in the Offered-Attributes.
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The Initiator also starts a retransmission timer. If no valid |
Value_Response arrives within the time limit, the same Value_Request |
is retransmitted for the remaining number of Retransmissions. |
When Retransmissions have been exceeded, if a Bad_Cookie message has |
been received during the exchange, the Initiator SHOULD begin the |
Photuris exchange again by sending a new Cookie_Request.
4.0.2. Receive Value_Request
The Responder validates the Responder-Cookie, the Counter, the |
Scheme-Choice, the Exchange-Value, and the Offered-Attributes. |
- Whenever an invalid/expired Responder-Cookie is detected, a |
Bad_Cookie message is sent. |
- Whenever an invalid Scheme-Choice is detected, or the Exchange- |
Value is obviously defective, or the variable length Offered- |
Attributes do not match the UDP Length, the message is silently |
discarded; the implementation SHOULD log the occurance, and notify |
an operator as appropriate. |
When the message is valid, the Responder sets its Exchange timer to |
the Exchange TimeOut, and returns a Value_Response.
The Responder keeps a copy of the incoming Value_Request cookie pair, |
and its Value_Response. If a duplicate Value_Request is received, it
merely resends its previous Value_Response, and takes no further
action.
4.0.3. Send Value_Response
The Responder generates an appropriate Exchange-Value for the Scheme- |
Choice. This Exchange-Value may be precalculated and used for multi- |
ple Initiators.
The IP Source for the Initiator is examined, and the attributes |
available between the parties are listed in the Offered-Attributes.
Implementation Notes:
At this time, the Responder begins calculation of the shared-
secret. Calculation of the shared-secret is executed in parallel
to minimize delay.
This may take a substantial amount of time. The implementor
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should ensure that retransmission is not blocked by this calcula-
tion. This is not usually a problem, as retransmission timeouts
typically exceed calculation time.
4.0.4. Receive Value_Response
The Initiator validates the pair of Cookies, the Exchange-Value, and |
the Offered-Attributes. |
- Whenever an invalid/expired cookie is detected, the message is |
silently discarded. |
- Whenever the Exchange-Value is obviously defective, or the vari- |
able length Offered-Attributes do not match the UDP Length, the |
message is silently discarded; the implementation SHOULD log the |
occurance, and notify an operator as appropriate. |
- Once a valid message has been received, later Value_Responses with |
both matching cookies are also silently discarded, until a new
Cookie_Request is sent.
When the message is valid, the Initiator begins its parallel computa-
tion of the shared-secret. +
When the Initiator completes computation, it sends an Iden- |
tity_Request to the Responder.
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4.1. Value_Request
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Initiator-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Responder-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Counter | Scheme-Choice | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Initiator-Exchange-Value ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initiator-Offered-Attributes ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Initiator-Cookie 16 octets. Copied from the Cookie_Response.
Responder-Cookie 16 octets. Copied from the Cookie_Response.
Type 2
Counter one octet. Copied from the Cookie_Response. |
Scheme-Choice two octets. A value selected by the Initiator from
the list of Offered-Schemes in the Cookie_Response.
Only the Scheme is specified; the size and value(s)
are implicit.
Initiator-Exchange-Value
variable precision number. Provided by the Initia-
tor for calculating a shared-secret between the par-
ties. The Value format is indicated by the Scheme- |
Choice.
The field may be any integral number of octets in
length, as indicated by its Size field. It does not
require any particular alignment. The 32-bit align-
ment shown is for convenience in the illustration.
Initiator-Offered-Attributes
A list of Security Parameter attributes supported by
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the Initiator.
The contents and usage of this list are further
described in "Offered Attributes List". The end of
the list is indicated by the UDP Length.
4.2. Value_Response
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Initiator-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Responder-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Responder-Exchange-Value ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Responder-Offered-Attributes ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Initiator-Cookie 16 octets. Copied from the Value_Request.
Responder-Cookie 16 octets. Copied from the Value_Request.
Type 3
Reserved Three octets. For future use; MUST be set to zero
when transmitted, and MUST be ignored when received.
Responder-Exchange-Value
variable precision number. Provided by the Respon-
der for calculating a shared-secret between the par-
ties. The Value format is indicated by the current |
Scheme-Choice as indicated by the Value_Request.
The field may be any integral number of octets in
length, as indicated by its Size field. It does not
require any particular alignment. The 32-bit align-
ment shown is for convenience in the illustration.
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Responder-Offered-Attributes
A list of Security Parameter attributes supported by
the Responder.
The contents and usage of this list are further
described in "Offered Attributes List". The end of
the list is indicated by the UDP Length.
4.3. Offered Attribute List
This list includes those attributes supported by the party that are
available to the other party. The attribute formats are specified in
the "Attribute List", where mandatory attributes are also specified.
The list is composed of three sections: Identification-Attributes, |
Authentication-Attributes, and Encapsulation-Attributes. Within each
section, the attributes are listed from most to least preferable.
The first section of the list includes methods of identification. An |
Identity-Choice is selected from this list.
The second section of the list begins with "AH-Attributes" (#1). It
includes methods of authentication, and other operational types.
The third section of the list begins with "ESP-Attributes" (#2). It
includes methods of compression, encryption, and other operational
types.
Attribute-Choices are selected from the latter two sections of the
list.
Implementation Notes:
Since the offer is made by the prospective SPI User (sender),
order of preference likely reflects the capabilities and engineer-
ing tradeoffs of a particular implementation.
However, the critical processing bottlenecks are frequently in the
receiver. The SPI Owner (receiver) may express its needs by
choosing a less preferable attribute.
The order may also be affected by operational policy and requested
services for an application. Such considerations are outside the
scope of this document.
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5. Identification Exchange
Initiator Responder |
========= ========= |
Identity_Request -> |
make SPI |
pick SPI attribute(s) |
identify self |
authenticate |
(make protection key) |
(encrypt message) |
<- Identity_Response |
make SPI |
pick SPI attribute(s) |
identify self |
authenticate |
(make protection key) |
(encrypt message) |
[make SPI session-keys in each direction] |
The exchange of messages is ordered, although the formats and mean- |
ings of the messages are identical in each direction. The messages |
are easily distinguished by the parties themselves, by examining the |
Type and Identification fields.
Implementation Notes: |
The amount of time for the calculation may be dependent on the
value of particular bits in secret values used in generating the
shared-secret or identity verification. To prevent analysis of
these secret bits by recording the time for calculation, sending |
of the Identity_Messages SHOULD be delayed until the time expected
for the longest calculation. This will be different for different |
processor speeds, different algorithms, and different length vari- |
ables. Therefore, the method for estimating time is implementa- |
tion dependent.
Any authenticated and/or encrypted user datagrams received before -
the completion of identity verification can be placed on a queue
pending completion of this step. If verification succeeds, the
queue is processed as though the datagrams had arrived subsequent
to the verification. If verification fails, the queue is dis-
carded.
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5.0.1. Send Identity_Request
The Initiator chooses an appropriate Identification, an SPI and SPI +
LifeTime, a set of Attributes for the SPI, calculates the Verifica- +
tion, and optionally encrypts the message for party privacy protec- +
tion (when a Privacy-Method is indicated by the Scheme-Choice).
The Initiator also starts a retransmission timer. If no valid Iden- |
tity_Response arrives within the time limit, its previous Iden- |
tity_Request is retransmitted for the remaining number of Retransmis- |
sions.
When Retransmissions have been exceeded, if a Bad_Cookie message has |
been received during the exchange, the Initiator SHOULD begin the |
Photuris exchange again by sending a new Cookie_Request.
5.0.2. Receive Identity_Request
The Responder validates the pair of Cookies, the Identification, the +
Verification, and the Attribute-Choices. +
- Whenever an invalid/expired cookie is detected, a Bad_Cookie mes- +
sage is sent. +
- Whenever an invalid Identification is detected, or the message +
verification fails, a Verification_Failure message is sent. +
- Whenever the variable length Attribute-Choices do not match the +
UDP Length, or the attributes are not a subset of those in the +
Offered-Attributes, the message is silently discarded. +
- Whenever such a problem is detected, the Security Association is +
not established; the implementation SHOULD log the occurance, and +
notify an operator as appropriate. +
When the message is valid, the Responder sets its Exchange timer to +
the Exchange LifeTime (if this has not already been done for a previ- +
ous exchange). When its parallel computation of the shared-secret is +
complete, the Responder returns an Identity_Response. +
The Responder keeps a copy of the incoming Identity_Request values, +
and its Identity_Response. If a duplicate Identity_Request is +
received, it merely resends its previous Identity_Response, and takes +
no further action.
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5.0.3. Send Identity_Response
The Responder chooses an appropriate Identification, an SPI and SPI |
LifeTime, a set of Attributes for the SPI, calculates the Verifica- |
tion, and optionally encrypts the message for party privacy protec- |
tion (when a Privacy-Method is indicated by the Scheme-Choice). |
The Responder calculates the SPI session-keys in both directions. |
The Responder sets its Update timer to half the value of its SPI |
LifeTime. If no new Photuris exchange occurs within the time limit, |
and the Exchange timer has not expired, an SPI_Update is sent to cre- |
ate another SPI. |
At this time, the Responder begins the authentication and/or encryp- |
tion of user datagrams. |
5.0.4. Receive Identity_Response |
The Initiator validates the pair of Cookies, the Identification, the |
Verification, and the Attribute-Choices. |
- Whenever an invalid/expired cookie is detected, the message is |
silently discarded. |
- Whenever an invalid Identification is detected, or the message |
verification fails, a Verification_Failure message is sent. |
- Whenever the variable length Attribute-Choices do not match the |
UDP Length, or the attributes are not a subset of those in the |
Offered-Attributes, the message is silently discarded. |
- Whenever such a problem is detected, the Security Association is |
not established; the implementation SHOULD log the occurance, and |
notify an operator as appropriate. |
- Once a valid message has been received, later Identity_Responses |
with both matching cookies are also silently discarded, until a |
new Cookie_Request is sent. |
When the message is valid, the Initiator sets its Exchange timer to |
the Exchange LifeTime (if this has not already been done for a previ- |
ous exchange). |
The Initiator calculates the SPI session-keys in both directions. |
The Initiator sets its Update timer to half the value of its SPI |
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LifeTime. If no new Photuris exchange occurs within the time limit, |
and the Exchange timer has not expired, an SPI_Update is sent to cre- |
ate another SPI. |
At this time, the Initiator begins the authentication and/or encryp- |
tion of user datagrams. |
5.1. Identity_Messages |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Initiator-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Responder-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | LifeTime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Security-Parameter-Index |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| Identity-Choice | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
~ Identification ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Verification ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attribute-Choices ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... Padding | Pad Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Initiator-Cookie 16 octets. Copied from the Value_Request.
Responder-Cookie 16 octets. Copied from the Value_Request.
Type 4 (Request) or 7 (Response) |
LifeTime three octets. The number of seconds remaining
before the indicated SPI expires. Must be greater
than zero.
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Security-Parameter-Index
four octets. The SPI to be used for incoming commu-
nications.
When zero, indicates that no SPI is created in this
direction.
Identity-Choice An identity attribute is selected from the list of
Offered-Attributes sent by the peer, and is used to
calculate the Verification.
The field may be any integral number of octets in
length, as indicated by its Length field. It does
not require any particular alignment. The 16-bit
alignment shown is for convenience in the illustra-
tion.
Identification variable precision number, or alternative format |
indicated by the Identity-Choice. See the |
"Attribute List" for details.
The field may be any integral number of octets in
length. It does not require any particular align-
ment. The 32-bit alignment shown is for convenience
in the illustration.
Verification variable precision number, or alternative format |
indicated by the Identity-Choice. The calculation |
of the value is described in "Identity Verifica- |
tion".
The field may be any integral number of octets in |
length. It does not require any particular align-
ment. The 32-bit alignment shown is for convenience
in the illustration.
Attribute-Choices
Zero or more octets. A list of attributes for this
(non-zero) SPI, selected from the list of Offered-
Attributes supported by the peer.
The contents and usage of this list are further
described in "Attribute Choices List". The end of
the list is indicated by the UDP Length after remov- |
ing the Pad Length and Padding fields (UDP Length - |
Pad Length - 1).
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Padding Zero or more octets. Prior to (optional) encryp- |
tion, it is filled to align the Pad Length field at |
a boundary appropriate to the Privacy-Method indi- |
cated by the current Scheme-Choice. The padding |
values begin with the value 0, and count up to the |
number of padding octets. For example, if the Pad |
Length is 5, the padding values are 0, 1, 2, 3, 4.
After (optional) decryption, if the padding octets |
are not the correct values for the Pad Length, then |
verification fails.
Pad Length one octet. The size of the Padding field in octets |
(not including the Pad Length field). The value
typically ranges from 0 to 7, but may be up to 255
to permit hiding of the actual data length.
This field is always present, even when no Padding |
is required. |
The portion of the message after the SPI MAY be encrypted for party |
privacy protection. Such mechanisms are outside the scope of this |
document.
The fields following the SPI are opaque. That is, the values are set |
prior to (optional) encryption, and examined only after (optional) |
decryption.
5.2. Attribute Choices List
This list specifies the attributes of a Security Association. The |
attribute formats are specified in the "Attribute List".
The list is composed of one or two sections: Authentication- |
Attributes, and/or Encapsulation-Attributes.
When sending from the SPI User to the SPI Owner, the attributes are
processed in the order listed. For example,
"ESP-Attributes", |
"DES-CBC", |
"AH-Attributes", |
"MD5-KDP", |
would result in ESP with encryption, and then AH authentication of |
the ESP payload.
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The SPI Owner will naturally process the datagram in the reverse
order.
This ordering also affects the order of key generation. Both SPI +
Owner and SPI User generate the keys in the order listed. +
Implementation Notes:
When choices are made from the list of Offered-Attributes, it is
not required that any Security Association include every kind of
offered attribute in any single SPI, or that a separate SPI be
created for every offered attribute.
Some analysts have recommended that the AH should always be out-
side the ESP. This is a matter for future research.
Some kinds of attributes may be included more than once in a sin- +
gle SPI. The set of allowable combinations of attributes are
dependent on implementation and operational policy. Such consid-
erations are outside the scope of this document.
5.3. Shared-Secret
The shared-secret is used in a number of calculations. Regardless of
the internal representation of the shared-secret, when used in calcu-
lations it is in the same form as the Value part of a Variable Preci-
sion Number:
- most significant octet first. |
- bits used are right justified within octet boundaries. |
- any unused bits are in the most significant octet. |
- unused bits are zero filled. |
5.4. Identity Verification
This message is authenticated using the Identity-Choice. The Verifi- |
cation value is calculated prior to (optional) encryption, and veri- |
fied after (optional) decryption.
The Identity-Choice authentication function is supplied with two |
input values:
- the computed shared-secret. |
- the data to be verified (as a concatenated sequence of octets). |
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The resulting output value is stored in the Verification field. |
The Identity-Choice authentication function is calculated over the |
following concatenated data values:
+ the Initiator Cookie, |
+ the Responder Cookie, |
+ the Responder Offered-Schemes, |
+ the SPI Owner Exchange-Value, |
+ the SPI Owner Offered-Attributes, |
+ the SPI Owner Identification, |
+ the SPI Owner secret-key, |
+ the SPI User Exchange-Value, |
+ the SPI User Offered-Attributes, |
+ the SPI User Identification (when known), |
+ the SPI User secret-key (when known), |
+ the message Type, LifeTime and SPI fields, |
+ the Attribute-Choices following the Verification field,
+ the Padding (if any), |
+ the Pad Length. |
Note that the order of the Exchange-Value and Offered-Attribute
fields is different in each direction. The Identification and SPI |
fields are also likely to be different in each direction. Note also +
that the SPI User Identification and secret-key will be omitted in +
the Identity_Request. +
If the verification fails, the users are notified, and a Verifica- +
tion_Failure message is sent, without adding any Security Associa- +
tions. On success, normal operation begins with the authentication +
and/or encryption of user datagrams.
Implementation Notes:
This is separate from any authentication method specified for |
Security Associations. |
The exact details of the Identification and secret-keys that are |
included in the Verification calculation are dependent on the |
Identity-Choice, as described in the "Attribute List".
Each party may wish to keep their own trusted databases, such as
the Pretty Good Privacy (PGP) web of trust, and accept only those
identities found there. Failure to find the Identification in
either an internal or external database results in the same Veri- |
fication_Failure message as failure of the verification computa-
tion.
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The hash of the Exchange-Value includes both the Size and Value -
fields. The hash of the Offered-Attributes and Attribute-Choices
includes the Type, Length and Value fields.
5.5. Session-Key Computation
Each Security Association SPI has one or more session-keys. These
keys are generated based on the attributes of the Security Associa-
tion. See the "Attribute List" for details.
The Attribute-Choice specified key generation cryptographic hash is |
used to create an SPI session-key for that particular attribute.
This hash is calculated over the following concatenated values:
+ the Initiator Cookie, -
+ the Responder Cookie,
+ the SPI Owner secret-key, |
+ the SPI User secret-key, |
+ the message Verification field, |
+ the computed shared-secret. |
Since the message Verification field is likely to be different in |
each direction, and the order of the secret-keys is different in each
direction, the resulting session-key will usually be different in -
each direction.
When a larger number of keying-bits are needed than are available +
from the specified cryptographic hash, these keying-bits are gener- +
ated by duplicating the trailing shared-secret, and recalculating the +
hash. That is, the first hash will have one trailing copy of the +
shared-secret, the second hash will have two trailing copies of the +
shared-secret, and so forth. +
Implementation Notes:
Inclusion of the Verification field (dependent on the SPI), |
together with the party secret-keys, allows reuse of the same
Exchange-Values and resulting shared-secret among several parties
and multiple users of the same node without generating the same
session-keys.
The exact details of the Verification field and secret-keys that +
are included in the session-key calculation are dependent on the +
Identity-Choices, as described in the "Attribute List". +
To avoid keeping the secret-keys in memory after the initial veri- +
fication, it is often possible to precompute the hash of the +
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initial octets of the concatenated data values for each direction. +
When both authentication and encryption attributes are used for
the same SPI, there may be multiple session-keys associated with -
the same SPI. These session-keys are generated in the order of +
the Attribute-Choices list. +
6. SPI Messages +
SPI User SPI Owner +
======== ========= +
SPI_Needed -> +
list SPI attribute(s) +
make integrity key +
authenticate +
(encrypt message) +
<- SPI_Update +
make SPI +
pick SPI attribute(s) +
make SPI session-key(s) +
make integrity key +
authenticate +
(encrypt message) +
The exchange of messages is not related to the Initiator and Respon- +
der. Instead, either party may send one of these messages at any +
time. The messages are easily distinguished by the parties. +
6.0.1. Send SPI_Needed +
At any time after completion of the Identification Exchange, either +
party can send an SPI_Needed. This message is sent when a prospec- +
tive SPI User needs particular attributes for a datagram (such as +
privacy protection), and no current SPI has those attributes.
The prospective SPI User selects from the intersection of attributes |
that both parties have previously offered, calculates the Verifica- |
tion, and optionally encrypts the message for party privacy protec- |
tion (when a Privacy-Method is indicated by the Scheme-Choice). |
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6.0.2. Receive SPI_Needed |
The potential SPI Owner validates the pair of Cookies, the Verifica- |
tion, and the Attributes-Needed. |
- Whenever an invalid/expired cookie is detected, a Bad_Cookie mes- |
sage is sent. |
- Whenever the message verification fails, a Verification_Failure |
message is sent. |
- Whenever the variable length Attributes-Needed do not match the |
UDP Length, or the attributes are not a subset of those in the |
Offered-Attributes, the message is silently discarded. |
- Whenever such a problem is detected, the Security Association is |
not established; the implementation SHOULD log the occurance, and |
notify an operator as appropriate. |
When the message is valid, the party SHOULD send an SPI_Update that |
includes the necessary attributes.
6.0.3. Send SPI_Update
At any time after completion of the Identification Exchange, either +
party can send an SPI_Update. This message has effect in only one +
direction, from the SPI Owner to the SPI User. +
The SPI Owner chooses an SPI and SPI LifeTime, a set of Attributes +
for the SPI, calculates the Verification, and optionally encrypts the +
message for party privacy protection (when a Privacy-Method is indi- +
cated by the Scheme-Choice).
6.0.4. Receive SPI_Update
The prospective SPI User validates the pair of Cookies, the Verifica- |
tion, and the Attributes-Needed. |
- Whenever an invalid/expired cookie is detected, a Bad_Cookie mes- |
sage is sent. |
- Whenever the message verification fails, a Verification_Failure |
message is sent. |
- Whenever the variable length Attribute-Choices do not match the |
UDP Length, or the attributes are not a subset of those in the |
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Offered-Attributes, the message is silently discarded. |
- Whenever such a problem is detected, the Security Association is |
not established; the implementation SHOULD log the occurance, and |
notify an operator as appropriate. |
When the message is valid, further actions are dependent on the value |
of the SPI LifeTime field, as described later.
6.1. SPI_Needed
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | |
~ Initiator-Cookie ~ |
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | |
~ Responder-Cookie ~ |
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Type | Reserved | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Reserved | |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ |
| | |
~ Verification ~ |
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Attributes-Needed ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
... Padding | Pad Length | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
Initiator-Cookie 16 octets. Copied from the Value_Request. |
Responder-Cookie 16 octets. Copied from the Value_Request. |
Type 8 |
Reserved seven octets. For future use; MUST be set to zero |
when transmitted, and MUST be ignored when received. |
Verification variable precision number, or other format indicated |
by the Scheme-Choice. The calculation of the value |
is described in "Validity Verification". |
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The field may be any integral number of octets in |
length. It does not require any particular align- |
ment. The 32-bit alignment shown is for convenience |
in the illustration. |
Attributes-Needed |
Four or more octets. A list of two or more |
attributes, selected from the list of Offered- |
Attributes supported by the peer. |
The contents and usage of this list are as previ- |
ously described in "Attribute Choices List". The |
end of the list is indicated by the UDP Length after |
removing the Pad Length and Padding fields (UDP |
Length - Pad Length - 1). |
Padding Zero or more octets. Prior to (optional) encryp- |
tion, it is filled to align the Pad Length field at |
a boundary appropriate to the Privacy-Method indi- |
cated by the current Scheme-Choice. The padding |
values begin with the value 0, and count up to the |
number of padding octets. For example, if the Pad |
Length is 5, the padding values are 0, 1, 2, 3, 4. |
After (optional) decryption, if the padding octets |
are not the correct values for the Pad Length, then |
verification fails. |
Pad Length one octet. The size of the Padding field in octets |
(not including the Pad Length field). The value |
typically ranges from 0 to 7, but may be up to 255 |
to permit hiding of the actual data length. |
This field is always present, even when no Padding |
is required. |
The portion of the message after the SPI MAY be encrypted for party |
privacy protection, in the same fashion specified for Iden- |
tity_Messages. |
The fields following the SPI are opaque. That is, the values are set |
prior to (optional) encryption, and examined only after (optional) |
decryption. |
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6.2. SPI_Update |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Initiator-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Responder-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | LifeTime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Security-Parameter-Index |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| |
~ Verification ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attribute-Choices ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... Padding | Pad Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Initiator-Cookie 16 octets. Copied from the Value_Request.
Responder-Cookie 16 octets. Copied from the Value_Request.
Type 9 |
LifeTime three octets. The number of seconds remaining
before the indicated SPI expires. The value zero
indicates deletion of the indicated SPI.
Security-Parameter-Index
four octets. The SPI to be used for incoming commu-
nications. +
This may be a new SPI value (for creation), or an
existing SPI value (for deletion). The value zero
indicates all old SPIs for this IP Destination (used |
for deletion).
Verification variable precision number, or other format indicated |
by the Scheme-Choice. The calculation of the value |
is described in "Validity Verification".
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The field may be any integral number of octets in |
length. It does not require any particular align-
ment. The 32-bit alignment shown is for convenience
in the illustration.
Attribute-Choices
Four or more octets. A list of two or more |
attributes for this SPI, selected from the list of
Offered-Attributes supported by the peer.
The contents and usage of this list are as previ- |
ously described in "Attribute Choices List". The
end of the list is indicated by the UDP Length after |
removing the Pad Length and Padding fields (UDP |
Length - Pad Length - 1).
Padding Zero or more octets. Prior to (optional) encryp- |
tion, it is filled to align the Pad Length field at |
a boundary appropriate to the Privacy-Method indi- |
cated by the current Scheme-Choice. The padding |
values begin with the value 0, and count up to the |
number of padding octets. For example, if the Pad |
Length is 5, the padding values are 0, 1, 2, 3, 4.
After (optional) decryption, if the padding octets |
are not the correct values for the Pad Length, then |
verification fails.
Pad Length one octet. The size of the Padding field in octets |
(not including the Pad Length field). The value
typically ranges from 0 to 7, but may be up to 255
to permit hiding of the actual data length.
This field is always present, even when no Padding |
is required.
The portion of the message after the SPI MAY be encrypted for party |
privacy protection, in the same fashion specified for Iden- |
tity_Messages.
The fields following the SPI are opaque. That is, the values are set |
prior to (optional) encryption, and examined only after (optional) |
decryption.
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6.2.1. Creation
When the SPI LifeTime is greater than zero, the SPI_Update can be |
used to create a new Security Association. Frequently, this message |
is used to create replacement SPIs as the LifeTime of an earlier SPI |
approaches expiration.
In addition, this message allows more rapid SPI creation for high
bandwidth applications. The messages flow in the opposite direction
from the primary traffic flow.
The new session-keys are calculated in the same fashion as the Iden- |
tity_Messages. Since the SPI value is always different than any pre-
vious SPI during the Exchange LifeTime of the shared-secret, the
resulting session-keys will necessarily be different from all others
used in the same direction.
When the peer finds that too many SPI values are already in use for
this party, or some other resource limit is reached, a Resource_Limit |
message is sent.
No retransmission timer is necessary. Success is indicated by the
peer use of the new SPI.
Should all creation attempts fail, eventually the peer will find that
all existing SPIs have expired, and will begin the Photuris exchange |
again by sending a new Cookie_Request. When appropriate, this |
Cookie_Request MAY include a Responder-Cookie to retain previous |
party pairings.
6.2.2. Deletion
When the SPI LifeTime is zero, the SPI_Update can be used to delete |
existing Security Associations. This is especially useful when the |
application that needed them terminates, to prevent another applica- |
tion from replaying the datagrams.
No retransmission timer is necessary. This message is advisory, to
reduce the number of ICMP Security Failures messages.
Should any deletion attempts fail, the peer will learn that the
deleted SPIs are invalid through the normal ICMP Security Failures
messages, and will initiate a Photuris exchange by sending a new |
Cookie_Request.
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6.2.3. Modification
The SPI_Update cannot be used to modify existing Security Associa- |
tions, such as lengthen an existing SPI LifeTime, resurrect an
expired SPI, or add/remove an Attribute-Choice. |
On receipt, such an otherwise valid message is silently discarded.
6.2.4. Validity Verification
This message is authenticated using the Validity-Method indicated by |
the current Scheme-Choice (see "Exchange Scheme List"). The Verifi- +
cation value is calculated prior to (optional) encryption, and veri- +
fied after (optional) decryption. +
The Validity-Method authentication function is supplied with two +
input values: +
- the computed shared-secret, +
- the data to be verified (as a concatenated sequence of octets). +
The resulting output value is stored in the Verification field.
The Validity-Method authentication function is calculated over the |
following concatenated data values:
+ the Initiator Cookie, -
+ the Responder Cookie,
+ the SPI Owner Identity Verification, |
+ the SPI User Identity Verification, |
+ the message Type, LifeTime and SPI fields, |
+ the Attribute-Choices following the Verification field,
+ the Padding (if any), |
+ the Pad Length. |
Note that the order of the Identity Verification fields (from the +
Identity_Messages) is different in each direction.
If the verification fails, the users are notified, and a Verifica- |
tion_Failure message is sent, without adding or deleting any Security
Associations. On success, normal operation begins with the authenti-
cation and/or encryption of user datagrams.
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Implementation Notes:
This is separate from any authentication method specified for |
Security Associations.
The hash of the Identity Verification includes both the Size and |
Value fields. The hash of the Attribute-Choices includes the
Type, Length and Value fields.
7. Error Messages
Issued in response to Photuris state loss or other problems. The
message has effect in only one direction. No retransmission timer is
necessary.
These messages are not encrypted for party privacy protection. |
The receiver checks the Cookies for validity. Special care MUST be +
taken that the Cookie pair in the Error Message actually match a pair +
currently in use, and that the protocol is currently in a state where +
such an Error Message might be expected. Otherwise, these messages +
could provide an opportunity for a denial of service attack. Invalid
messages are silently discarded.
7.1. Bad_Cookie
For the format of the message, see "Header Format". There are no |
additional fields. |
Initiator-Cookie 16 octets. Copied from the offending message. |
Responder-Cookie 16 octets. Copied from the offending message. |
Type 10
This error message is sent when a Value_Request, Identity_Request, |
SPI_Needed, or SPI_Update is received, and the receiver's Cookie is
invalid or the associated Exchange-Value has expired.
During the Photuris exchange, when this error message is received, it |
has no immediate effect on the operation of the protocol phases. |
When Retransmissions have been exceeded, if this error message has |
been received, the Initiator SHOULD begin the Photuris exchange again |
by sending a new Cookie_Request. |
After the Photuris exchange has completed, when this error message is |
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received in response to an SPI_Needed or SPI_Update, the party SHOULD |
initiate a Photuris exchange by sending a new Cookie_Request.
However, existing SPIs are not deleted. They expire normally, and
are purged sometime later. +
Design Notes: +
This message will occur normally at any time after the +
Cookie_Response, whenever the Responder dynamically changes its +
local secret for cookie generation and the secret for generating +
its Exchange-Value, or either party expires its exchange state. +
On the other hand, an observer could attempt to use this message +
for denial of service by copying the valid cookies and sending it +
faster than the round-trip of the valid exchange peer. +
Therefore, the protocol gracefully recovers during the Value and +
Identification Exchanges by using the Retransmission TimeOut to +
give sufficient time for a valid exchange reply to arrive. It +
recovers during the SPI Messages by using cached prior exchange +
values to eliminate the intensive calculations of a new Photuris +
exchange. +
7.2. Resource_Limit
For the format of the message, see "Header Format". There are no |
additional fields. |
Initiator-Cookie 16 octets. Copied from the offending message. |
Responder-Cookie 16 octets. Copied from the offending message. |
Type 11 |
This error message is sent when a Cookie_Request or SPI_Update is |
received, and too many SPI values are already in use for that peer,
or some other Photuris resource is unavailable.
During the Photuris exchange, when this error message is received in |
response to a Cookie_Request, the implementation SHOULD double the |
retransmission timeout for sending another Cookie_Request. |
After the Photuris exchange has completed, when this error message is |
received in response to an SPI_Update, the implementation SHOULD NOT |
send another SPI_Update until it has deleted an existing SPI, or
waited for a cached SPI entry to expire. |
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Design Notes: |
This message will occur normally instead of a Cookie_Response, |
during such events as server recovery after a power failure. It |
is also regulates overly aggressive SPI creation. |
Again, an observer could attempt to use this message for denial of |
service by copying the valid cookies and sending it faster than |
the round-trip of the valid exchange peer. |
Therefore, the protocol gracefully recovers during the Cookie |
Exchange by using the Retransmission TimeOut to give sufficient |
time for a valid exchange reply to arrive. It recovers during the |
SPI Messages by the normal SPI expiration process. |
7.3. Verification_Failure
For the format of the message, see "Header Format". There are no |
additional fields. |
Initiator-Cookie 16 octets. Copied from the offending message. |
Responder-Cookie 16 octets. Copied from the offending message. |
Type 12 |
This error message is sent when an Identity_Message, SPI_Needed or |
SPI_Update is received, and verification fails. |
When this error message is received, the implementation SHOULD log |
the occurance, and notify an operator as appropriate. However, |
receipt has no effect on the operation of the protocol. |
Design Notes:
This message will not occur normally. The principle purpose is to |
notify an operator when an attack has occurred or that the identi- |
fication used is not valid. |
Again, an observer could attempt to use this message for denial of |
service by copying the valid cookies and sending it faster than |
the round-trip of the valid exchange peer.
Therefore, the protocol gracefully recovers during the Identifica- |
tion Exchange by using the Retransmission TimeOut to give suffi- |
cient time for a valid exchange reply to arrive. It recovers dur- |
ing the SPI Messages by using cached prior exchange values to |
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eliminate the intensive calculations of a new Photuris exchange.
8. Public Value Exchanges
Photuris is based in principle on public-key cryptography, specif- |
ically Diffie-Hellman key exchange. Exchange of D-H Exchange-
Values based on private/secret values results in a mutual shared-
secret between the parties. This shared-secret can be used on its
own, or to generate a series of session-keys for authentication
and encryption of subsequent traffic.
Widespread deployment and use of an Internet Security protocol is
possible without public-key cryptography. For example, Kerberos
[RFC-1510] can generate host-pair keys for use in Internet Secu-
rity, much as it now generates session-keys for use by encrypted
telnet and other "kerberized" applications.
The Kerberos model has some widely recognized drawbacks. Foremost
is the requirement for a highly available on-line Key Distribution
Center (KDC), with a database containing every principal's secret-
key. This carries significant security risks.
Public-key cryptography enables decentralization. Entities gener-
ate session-keys without real-time communication with any other
party.
This draft assumes familiarity with the Diffie-Hellman public-key
algorithm. A good description can be found in [Schneier95].
8.1. Modular Exponentiation Groups
The original Diffie-Hellman technique [DH76] specified modular
exponentiation. An Exchange-Value is generated using a generator
(g), raised to a private/secret exponent (x), modulo a prime (p).
(g**x) mod p
When these public-values are exchanged between parties, the par-
ties can calculate a shared-secret value between themselves.
(g**xy) mod p
The security depends on the relative difficulty of calculating
discrete logarithms, compared to the ease of exponentiation in the
same finite field. The prime modulus MUST be sufficiently large
to prevent calculation of its discrete logs within the lifetime of
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the protected data.
When a strong prime modulus and generator pair are well chosen,
the difficulty of a discrete log attack is maximized. By choosing
the pairs in advance, analysis of the pair characteristics is pos-
sible. This analysis can promote confidence in the security of
the implementations. +
The generator (g) and modulus (p) are established by the Scheme- +
Choice (see "Exchange Scheme List" for details). They are offered +
in the Cookie_Response, and one pair is chosen in the +
Value_Request. +
The exponent (x) or (y) is kept secret by the parties. Only the +
public-value result of the modular exponentiation with (x) or (y) +
is sent as the Exchange-Value.
8.2. Moduli Selection
Each implementation proposes one or more moduli in its Offered-
Schemes. Every implementation MUST support up to 4096-bit moduli.
For any particular Photuris node, these moduli need not change for
significant periods of time; likely days or weeks. A background
process can periodically generate new moduli.
8.2.1. Strong Primes
Ideally, each prime modulus (p) should have the property that both
p and (p-1)/2 are prime. This provides the strongest defense
against factoring.
Discovery of strong primes is extremely computationally intensive,
and practically impossible for commercially available processors
to find in a reasonable interactive time. Complete verification
can take hours or days.
8.2.2. Prime-Order Subgroups
An alternative is the use of a large subgroup where q is a prime
factor of (p-1). This technique is described in [OW96], and based |
on [Schnorr91].
Discovery of prime-order subgroups is less computationally inten- |
sive than verification of strong primes. The computational cost |
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of finding such a prime (p) with a prime divisor (q) is only a
little more than finding any random prime.
8.2.3. Unstructured Primes
A random unstructured prime (p), where (p-1) may have small prime |
factors, is subject to a Pohlig-Hellman attack. Strong primes and |
prime-order subgroups prevent this attack.
Discovery of random primes is the bulk of the computational pro-
cessing of the previously described primes. Therefore, they
SHOULD be used instead of unstructured primes.
8.2.4. Non-Primes
Technically, the modulus is not required to be prime. Any suffi-
ciently large modulus would be useful, and provide a minimal
amount of security.
To improve security, a potential modulus should be sieved to
reject those with small prime factors (less than 1,000,000).
However, the security of non-prime moduli is considered insuffi-
cient for data of any long-term value. These SHOULD NOT be used,
except in the most ephemeral cases -- such as cellular telephones,
and other low computational power devices.
8.2.5. Bootstrap Moduli
Each implementation is likely to use a fixed modulus during its
bootstrap, until it can generate another modulus in the back-
ground. As the bootstrap modulus will be widely distributed, and
reused whenever the machine reinitializes, it SHOULD be a strong
prime to provide the greatest long-term protection.
8.2.6. Learning Moduli
As Photuris exchanges are initiated, new moduli will be learned
from the Responder Offered-Schemes. The Initiator MAY cache these
moduli for its own use.
Before offering any learned modulus, the implementation MUST per-
form at least one iteration of probable primality verification.
In this fashion, many processors will perform verification in
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parallel as moduli are passed around.
When primality verification failures are found, the failed moduli |
SHOULD be retained for some (implementation dependent) period of
time, to avoid relearning and retesting after subsequent
exchanges.
8.3. Generator Selection -
The generator (g) should be chosen such that the secret exponents
will generate all possible public-values, evenly distributed
throughout the range of the modulus (p), without cycling through a
smaller subset. Such a generator is called a "primitive root"
(which is trivial to find when p is strong).
Only one generator (2) is required to be supported.
Implementation Notes:
One useful technique is to select the generator, and then limit
the modulus selection sieve to primes with that generator.
2 when p (mod 24) = 11.
3 when p (mod 12) = 5.
5 when p (mod 10) = 3 or 7.
The required generator (2) improves efficiency in multiplica-
tion performance. It is usable even when it is not a primitive
root, as it still covers half of the space of possible
residues.
8.4. Exponent Selection
Each implementation generates a separate random secret exponent
for each different modulus. Then, a D-H Exchange-Value is calcu-
lated for the given modulus, generator, and exponent.
The exponent 0 will generate the public value 1, and exponent 1
will generate the public value g mod p. These exponents do not
qualify as secret.
Although the same exponent and Exchange-Value may be used with
several parties whenever the same modulus and generator are used,
the exponent SHOULD be changed at random intervals. A background
process can periodically destroy the old values, generate a new
random secret exponent, and recalculate the Exchange-Value. This
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limits the exposure of both the secret exponent and shared-secret,
protecting earlier communications, as past secrets are not kept |
for possible discovery by a future intrusion. Also, the secret
exponent currently in use is less likely to be anticipated, as the
element of random timing is introduced.
Since these operations involve several time-consuming modular
exponentiations, moving them to the "background" substantially
improves the apparent execution speed of the Photuris protocol.
It also reduces CPU loading sufficiently to allow a single pub-
lic/private key-pair to be used in several closely spaced Photuris
executions, when creating Security Associations with several dif-
ferent nodes over a short period of time.
Consideration should also be given to the speed versus security
tradeoffs of modular exponentiation. While an exponent may be
used that is shorter than the modulus, the cryptologic literature
is indeterminate as to the minimum proportionate size. This spec-
ification recommends that the exponent length be at least twice
the desired cryptographic strength of the longest session-key
needed by the strongest offered-attribute.
Implementation Notes:
The size of the exponent is entirely implementation dependent,
is unknown to the other party, and can be easily changed.
A single modular exponentiation on a 486-66DX2 processor using
RSAREF and Borland C under MS-DOS took 20 seconds with a
1024-bit prime modulus and a 1024-bit random exponent. This
dropped to about 1 to 1.5 seconds when the random exponent was
shortened to 128 bits, with the same 1024-bit modulus.
Other precomputation suggestions are described in [BGMW93] and
[Rooij94].
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9. Exchange Scheme List
Initial values are assigned as follows:
(0) Reserved.
(1) Reserved. |
(2) Implementation Required. Any modulus (p) with a recommended |
generator (g) of 2. The modulus is contained in the |
Exchange Scheme Value field in the list of Offered-Schemes. |
The "Identification Exchange" and "SPI Messages" Privacy- |
Method is "not protected".
The "SPI Messages" Validity-Method is "MD5-DP". |
(3) Exchange-Schemes 3 to 255 are intended for future well-known |
published schemes.
(256) Exchange-Schemes 256 to 32767 are intended for vendor-
specific unpublished schemes. Implementors wishing a number |
MUST request the number from the authors.
(32768)
Exchange-Schemes 32768 to 65535 are available for cooperat-
ing parties to indicate private schemes, regardless of ven-
dor implementation. These numbers are not reserved, and are |
subject to duplication. Other criteria, such as the IP |
Source and Destination of the Cookie_Request, are used to
differentiate the particular Exchange-Schemes available.
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10. Validity Methods
10.1. MD5-DP
As described in "Validity Verification", the MD5 [RFC-1321] hash |
is calculated over the concatenation of |
MD5( key, data, datafill, key, md5fill ) |
The leading key is not padded to any particular alignment. |
The datafill uses the same pad-with-length technique defined for |
md5fill. The length includes the leading key and data. |
The resulting Verification field is a 128-bit variable precision |
number (18 octets including Size). |
11. Attribute List
Implementors wishing a number MUST request the number from the
authors. Initial values are assigned as follows:
Use Type
- 0* padding
- 1* AH-Attributes
- 2* ESP-Attributes
I 3* Simple MD5-DP Verification |
A 5* MD5-KDP |
E 8* DES-CBC
X 255 Organizational |
A AH Attribute-Choice
E ESP Attribute-Choice
I Identity-Choice
X dependent on list location +
* feature must be supported (mandatory)
Other attributes are specified in companion documents.
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11.1. Padding
+-+-+-+-+-+-+-+-+
| Type |
+-+-+-+-+-+-+-+-+
Type 0
Each attribute may have value fields that are multiple octets. To
facilitate processing efficiency, these fields are aligned on
integral modulo 8 octet (64-bit) boundaries.
Padding is accomplished by insertion of 1 to 7 Type 0 padding
octets before the attribute that needs alignment.
No padding is used after the final attribute in a list.
11.2. AH-Attributes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 1
Length 0
When a list of Attributes is specified, this Attribute begins the
section of the list which applies to the Authentication Header
(AH).
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11.3. ESP-Attributes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 2
Length 0
When a list of Attributes is specified, this Attribute begins the
section of the list which applies to the Encapsulating Security
Payload (ESP).
11.4. Simple MD5-DP Verification
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 3
Length 0
When selected as an Identity-Choice, the immediately following |
Identification field contains an unstructured variable precision |
number. Valid Identifications and symmetric secret-keys are pre- |
configured by the parties.
There is no required format or content for the Identification
value. The value may be a number or string of any kind.
Typically, the Identification is a user name, a Fully Qualified
Domain Name, or an email address which contains a user name and a
domain name. Examples include:
user
node.site.
user@node.site.
rcmd@node.site.
Mundane Name <user@node.site> +
There is no requirement that the domain name match any of the par-
ticular IP addresses in use by the parties.
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The authentication symmetric secret-key (as specified) is selected |
based on the contents of the Identification field. All implemen- +
tations must support at least 62 octets. The selected symmetric +
secret-key SHOULD provide at least 64-bits of cryptographic +
strength. +
As described in "Identity Verification", the MD5 [RFC-1321] hash +
is calculated over the concatenation of: +
MD5( key, data, datafill, key, md5fill ) +
The leading key is not padded to any particular alignment.
The datafill uses the same pad-with-length technique defined for |
md5fill. The length includes the leading key and data. |
The resulting Verification field is a 128-bit variable precision |
number (18 octets including Size). +
For identity verification and session-key calculation, the authen- +
tication symmetric secret-key is also used as the calculation +
secret-key.
11.5. MD5-KDP
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 5
Length 0
May be selected as an AH Attribute-Choice, pursuant to [RFC-1828] +
et sequitur. The selected Exchange Scheme SHOULD provide at least
64-bits of cryptographic strength.
MD5 [RFC-1321] is used as the key generation cryptographic hash |
for generating the SPI session-key, as described in "Session-Key
Computation". The most significant 496-bits (62 octets) of the |
generated hashes are used for the key.
The remaining least significant 16-bits (2 octets) of the last |
hash are discarded. When combined with other uses of key genera- |
tion for the same SPI, the next such attribute will begin with a |
new hash.
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Profile: |
When negotiated with Photuris, the transform differs slightly |
from [RFC-1828].
The form of the authenticated message is: |
MD5( key, keyfill, datagram, datafill, key, md5fill ) |
The additional datafill protects against the attack described |
in [PO96]. This is also filled to the next 512-bit boundary, |
using the same pad-with-length technique defined for MD5. The |
length includes the leading key and data.
11.6. DES-CBC
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Type | Length | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
Type 8
Length 0 |
May be selected as an ESP Attribute-Choice, pursuant to [RFC-1829] +
et sequitur. The selected Exchange Scheme SHOULD provide at least
56-bits of cryptographic strength.
MD5 [RFC-1321] is used as the key generation cryptographic hash |
for generating the SPI session-key, as described in "Session-Key
Computation". The most significant 64-bits of the generated hash |
are used for the key. The least significant bit of each key octet |
is ignored (or set to parity when the implementation requires). |
If the key matches any of the weak, semi-weak or possibly weak |
keys [Schneier95, pages 280-282], that key is discarded; the next |
64-bits of the generated hash are used instead, recursively. |
The remaining octets of the last hash are discarded. When com- |
bined with other uses of key generation for the same SPI, the next |
such attribute will begin with a new hash.
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Profile: |
When negotiated with Photuris, the transform differs slightly |
from [RFC-1829]. |
The IV is always 32-bits.
The 64-bit IV is generated from the 32-bit SPI field followed |
by (concatenated with) the 32-bit IV field. The bit-wise com- |
plement of the 32-bit IV value is XOR'd with the first 32-bits |
(SPI). |
The padding values begin with the value 0, and count up to the |
number of padding octets. For example, if the plaintext length |
is 41, the padding values are 0, 1, 2, 3, 4, and the following |
Pad Length is 5. |
After decryption, if the padding octets are not the correct |
values for the Pad Length, then the payload is discarded, and a |
"Decryption Failed" error is indicated, as described in [RFC- |
xxxx].
11.7. Organizational
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | OUI
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... | Kind | Value(s) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 255 |
Length >= 4
When the Length is four, no Value(s) field is
present.
OUI three octets. The vendor's Organizationally
Unique Identifier, assigned by IEEE 802 (see
[RFC-1700] for contact details). The bits within
the octet are in canonical order, and the most
significant octet is transmitted first.
Kind one octet. Indicates a sub-type for the OUI.
There is no standardization for this field. Each
OUI implements its own values.
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Value(s) Zero or more octets. The details are implementa-
tion specific.
Some implementors might not need or want to publish their propri-
etary algorithms and attributes. This OUI mechanism is available
to specify these without encumbering the authors with proprietary |
number requests.
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A. Automaton
An example automaton is provided to illustrate the operation of
the protocol. It is not yet updated to match the text!!! Where +
conflicts appear between this example and the text, the text takes
precedence.
The finite-state automaton is defined by events, actions and state
transitions. Events include reception of external commands such
as expiration of a timer, and reception of datagrams from a peer.
Actions include the starting of timers and transmission of data-
grams to the peer.
Events
DU13 = Communication Administratively Prohibited
SF0 = Bad SPI
SF4 = Need Authentication
SF5 = Need Authorization
WP = Want Privacy
RCQ+ = Receive Cookie_Request (Good)
RCQ- = Receive Cookie_Request (Bad)
RCR+ = Receive Cookie_Response (Good)
RCR- = Receive Cookie_Response (Bad)
RVQ+ = Receive Value_Request (Good)
RVQ- = Receive Value_Request (Bad)
RVR+ = Receive Value_Response (Good)
RVR- = Receive Value_Response (Bad)
RIQ+ = Receive Identity_Request (Good) |
RIQ- = Receive Identity_Request (Bad) |
RIR+ = Receive Identity_Response (Good) |
RIR- = Receive Identity_Response (Bad) |
RUN+ = Receive SPI_Needed (Good) |
RUN- = Receive SPI_Needed (Bad) |
RUM+ = Receive SPI_Update (Good) |
RUM- = Receive SPI_Update (Bad) |
RBC = Receive Bad Cookie |
RRL = Receive Resource Limit |
RVF = Receive Verification Failure |
TO+ = Timeout with counter > 0
TO- = Timeout with counter expired
UTO = Update TimeOut
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XTO = Exchange TimeOut
Actions
scq = Send Cookie_Request
scr = Send Cookie_Response
svq = Send Value_Request
svr = Send Value_Response
siq = Send Identity_Request |
sir = Send Identity_Response |
sum = Send SPI_Update |
se* = Send error message (see text) |
sbc = Send Bad Cookie |
srl = Send Resource Limit |
svf = Send Verification Failure |
bito = Backoff Initiate TimeOut
buto = Backoff Update TimeOut
ito = Set Initiate TimeOut |
uto = Set Update TimeOut |
xto = Set Exchange TimeOut |
log = log operator message |
A.1. State Transition Table
States are indicated horizontally, and events are read vertically.
State transitions and actions are represented in the form
action/new-state. Multiple actions are separated by commas, and
may continue on succeeding lines as space requires; multiple
actions may be implemented in any convenient order. The state may
be followed by a letter, which indicates an explanatory footnote.
The dash ('-') indicates an illegal transition. -
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Initiator +
| 0 1 2 3 4
| Initial Cookie CookieBad Value ValueBad |
------+--------------------------------------------------
DU13 |ito,scq/1 ito,scq/1 ito,scq/1 3 4 |
SF0 |ito,scq/1 1 2 3 4 |
SF4 |ito,scq/1 1 2 3 4 |
SF5 |ito,scq/1 1 2 3 4 |
WP |ito,scq/1 1 2 3 4 |
|
RCR+ | - ito,svq/3 ito,svq/3 3 4 |
RCR- | 0 1 2 3 4 |
RVR+ | - - - ito,siq/5 ito,siq/5 |
RVR- | 0 1 2 3 4 |
RIR+ | - - - - - |
RIR- | 0 1 2 3 4 |
|
RUN+ | - - - - - |
RUN- | sbc/0 sbc/1 sbc/2 sbc/3 sbc/4 |
RUM+ | - - - - - |
RUM- | sbc/0 sbc/1 sbc/2 sbc/3 sbc/4 |
|
RBC | - 2 2 4 4 |
RRL | - bito/1 bito/2 3 4 |
RVF | - - - - - |
| |
TO+ | - scq/1 scq/2 svq/3 svq/4 |
TO- | - 0 scq/1 0 scq/1 |
UTO | - - - - - |
XTO | - 0 0 0 0
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Initiator |
| 5 6 8 |
|Identity IdentityBad Update |
------+----------------------------- |
DU13 | 5 6 8 |
SF0 | 5 6 ito,scq/1 |
SF4 | 5 6 ito,scq/1 |
SF5 | 5 6 ito,scq/1 |
WP | 5 6 sun/8 |
| |
RCR+ | 5 6 8 |
RCR- | 5 6 8 |
RVR+ | 5 6 8 |
RVR- | 5 6 8 |
RIR+ | uto/8 uto/8 8 |
RIR- | svf/5 svf/6 8 |
| |
RUN+ | - - sum/8 |
RUN- | sbc/5 sbc/6 se*/8 |
RUM+ | - - 8 |
RUM- | sbc/5 sbc/6 se*/8 |
| |
RBC | 6 6 ito,scq/1 |
RRL | 5 6 buto/8 |
RVF | log/5 log/6 log/8 |
| |
TO+ | sim/5 sim/6 - |
TO- | 0 scq/1 - |
UTO | - - sum/8 |
XTO | 0 0 0 |
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Responder |
| 0 7 8 |
| Initial Ready Update |
------+----------------------------- |
WP | - 7 sun/8 |
|
RCQ+ | scr/0 scr/7 scr/8 |
RCQ- | srl/0 srl/7 srl/8 |
RVQ+ |xto,svr/7 svr/7 svr/8 |
RVQ- | sbc/0 sbc/7 sbc/8 |
RIQ+ | - uto,sir/8 sir/8 |
RIQ- | sbc/0 se*/7 se*/8 |
|
RUN+ | - - sum/8 |
RUN- | sbc/0 sbc/7 se*/8 |
RUM+ | - - 8 |
RUM- | sbc/0 sbc/7 se*/8 |
|
RBC | - 7 ito,scq/1 |
RRL | - - buto/8 |
RVF | - - log/8 |
| |
UTO | - - sum/8 |
XTO | - 0 0 |
A.2. States
Following is a more detailed description of each automaton state.
The "Bad" version of a state is to indicate that the Bad_Cookie +
message has been received.
A.2.1. Initial
The Initial state is fictional, in that there is no state between
the parties.
A.2.2. Cookie
In the Cookie state, the Initiator has sent a Cookie_Request, and
is waiting for a Cookie_Response. Both the Restart and Exchange
timers are running.
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Note that the Responder has no Cookie state.
A.2.3. Value
In the Value state, the Initiator has sent its Exchange-Value, and |
is waiting for an Identity_Message. Both the Restart and Exchange |
timers are running.
A.2.4. Identity
In the Identity state, the Initiator has sent an Identity_Request, |
and is waiting for an Identity_Response in reply. Both the
Restart and Exchange timers are running.
A.2.5. Ready
In the Ready state, the Responder has sent its Exchange-Value, and |
is waiting for an Identity_Message. The Exchange timer is run- |
ning.
A.2.6. Update
In the Update state, each party has concluded the Photuris
exchange, and is unilaterally updating expiring SPIs until the
Exchange LifeTime expires. Both the Update and Exchange timers
are running.
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B. Example Bootstrap Moduli -
During the initial bootstrap of the implementation, there may not
be sufficient time to generate a new modulus before a security
association is needed. These moduli are verified examples that
may be used during this bootstrap period.
(512-2)
A 512-bit strong prime (p), expressed in hex: |
da58 3c16 d985 2289 d0e4 af75 6f4c ca92 |
dd4b e533 b804 fb0f ed94 ef9c 8a44 03ed |
5746 50d3 6999 db29 d776 276b a2d3 d412 |
e218 f4dd 1e08 4cf6 d800 3e7c 4774 e833 |
The recommended generator (g) for this prime is 2. |
This prime modulus was randomly generated by a freely available |
program written by Phil Karn, verified using the |
mpz_probab_prime() function Miller-Rabin test in the Gnu Math |
Package (GMP) version 1.3.2; as well as independently developed |
test libraries by Rich Schroeppel (complete Elliptic Curve |
test). |
Currently estimated to provide 64 (pessimistic) bit-equivalents |
of cryptographic strength. Exponent lengths of 128 bits (or |
more) are recommended. |
Using current technology, calculation of the discrete loga- |
rithms is anticipated to take no more than a year. This is |
insufficient for long-term use. |
A modulus of this size is only used with transforms (such as |
DES) that already provide less protection than the estimated |
strength, and where rapid computation is of primary importance. |
(1024-2) |
A 1024-bit strong prime (p), expressed in hex:
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97f6 4261 cab5 05dd 2828 e13f 1d68 b6d3
dbd0 f313 047f 40e8 56da 58cb 13b8 a1bf
2b78 3a4c 6d59 d5f9 2afc 6cff 3d69 3f78
b23d 4f31 60a9 502e 3efa f7ab 5e1a d5a6
5e55 4313 828d a83b 9ff2 d941 dee9 5689
fada ea09 36ad df19 71fe 635b 20af 4703
6460 3c2d e059 f54b 650a d8fa 0cf7 0121
c747 99d7 5871 32be 9b99 9bb9 b787 e8ab
The recommended generator (g) for this prime is 2.
This prime modulus was randomly generated by a freely available
program written by Phil Karn, verified using the
mpz_probab_prime() function Miller-Rabin test in the Gnu Math
Package (GMP) version 1.3.2; and also verified with GMP on
other platforms by Wei Dai and Frank A Stevenson, as well as
independently developed test libraries by Eric Young (Miller-
Rabin test), and Rich Schroeppel (complete Elliptic Curve
test). -
Currently estimated to provide 80 (pessimistic) through 98 |
(optimistic) bit-equivalents of cryptographic strength. Expo- |
nent lengths of 160 to 256 bits (or more) are recommended.
Implementors are encouraged to generate their own bootstrap mod-
uli, and to change bootstrap moduli in successive implementation
releases.
Operational Considerations
The specification provides only a few configurable parameters,
with defaults that should satisfy most situations.
Retransmissions
Default: 3.
Initial Retransmission TimeOut (IRTO)
Default: 10 seconds.
Exchange TimeOut (ETO)
Default: 60 seconds. Minimum: Retransmissions * IRTO. |
Exchange LifeTime (ELT) |
Default: 30 minutes. Minimum: 2 * ETO. |
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SPI LifeTime (SPILT) |
Default: 5 minutes. Minimum: 2 * ELT. +
In addition, each party configures local policy that determines |
what access (if any) is granted to the holder of a particular
identity. For example, the party might allow anonymous FTP, but
prohibit Telnet. Such considerations are outside the scope of
this document.
Security Considerations
Photuris was based on currently available tools, by experienced
network protocol designers with an interest in cryptography,
rather than by cryptographers with an interest in network proto-
cols. This specification is intended to be readily implementable |
without requiring an extensive background in cryptology.
Therefore, only minimal background cryptologic discussion and |
rationale is included in this document. Although some review has |
been provided by the general cryptologic community, it is antici-
pated that design decisions and tradeoffs will be thoroughly anal-
ysed in subsequent dissertations and debated for many years to
come. +
Cryptologic details are reserved for separate documents that may +
be more readily and timely updated with new analysis.
Acknowledgements
Thou shalt make no law restricting the size of integers that
may be multiplied together, nor the number of times that an
integer may be multiplied by itself, nor the modulus by which
an integer may be reduced. [Prime Commandment]
Phil Karn was principally responsible for the design of the proto- |
col phases, particularly the clogging defense, and provided much |
of the design rationale text.
William Simpson designed the packet formats and attributes, and |
additional message types, editing and formatting. All such mis- |
takes are his responsibility.
This protocol was later discovered to have many elements in common
with the Station-To-Station authentication protocol [DOW92].
Angelos Keromytis suggested the cookie exchange rate limitation |
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counter, and developed the first complete independent implementa- |
tion. |
Paul C van Oorschot suggested signing both the public exponents |
and the shared-secret, to provide an authentication-only version
of identity verification. Also, he provided text regarding mod-
uli, generator, and exponent selection.
Bart Preneel and Paul C van Oorschot in [PO96] suggested adding |
padding between the data and trailing key when hashing for authen- |
tication. |
Hilarie Orman suggested adding secret "nonces" to session-key gen- |
eration, and provided extensive review of the protocol details.
Bill Sommerfeld suggested using the Cookie values on successive |
exchanges to provide bi-directional user-oriented keying.
Oliver Spatscheck developed a second independent implementation. |
International interoperability testing provided the impetus for
many of the implementation notes herein.
Randall Atkinson, Steven Bellovin, James Hughes, Brian LaMacchia,
Cheryl Madson, Perry Metzger, Ron Rivest, and Rich Schroeppel pro-
vided useful critiques of earlier versions of this document.
References
[BGMW93] E. Brickell, D. Gordon, K. McCurley, and D. Wilson, "Fast
Exponentiation with Precomputation (Extended Abstract)",
Advances in Cryptology -- EUROCRYPT '92, Lecture Notes in
Computer Science, 658 (1993), Springer-Verlag, 200-207.
Also U.S. Patent #5,299,262, E.F. Brickell, D.M. Gordon,
K.S. McCurley, "Method for exponentiating in crypto-
graphic systems", 29 Mar 1994.
[Diffie90]
Whitfield Diffie, "Authenticated Key Exchange and Secure
Interactive Communication", Northern Telecom, Securicom
'90, Paris France, 16 March 1990.
[DH76] Diffie, W., and Hellman, H.E., "New Directions in Cryp-
tography", IEEE Transactions on Information Theory, v
IT-22 n 6 pp 644-654, November 1976.
Karn & Simpson expires in six months [Page 74]
DRAFT Photuris April 1996
[DOW92] Whitfield Diffie, Paul C van Oorshot, Michael J Wiener,
"Authentication and Authenticated Key Exchanges",
Designs, Codes and Cryptography, v 2 pp 107-125, Kluwer
Academic Publishers, 1992.
[Firefly]
"Photuris" is the latin name for the firefly. "Firefly"
is in turn the name for the USA National Security Admin-
istration's (classified) key exchange protocol for the
STU-III secure telephone. Informed speculation has it
that Firefly is based on very similar design principles.
[OW96] Paul C van Oorshot, Michael J Weiner, "On Diffie-Hellman
Key Agreement with Short Exponents", work in progress. -
[Prime Commandment]
A derivation of an apocryphal quote from the usenet list |
sci.crypt. |
[PO96] Bart Preneel, Paul C van Oorshot, "...Two MACs", work in |
progress.
[RFC-768]
Postel, J., "User Datagram Protocol", STD 6, August 1980.
[RFC-1321]
Rivest, R., "The MD5 Message-Digest Algorithm", RFC-1321, +
MIT Laboratory for Computer Science, April 1992.
[RFC-1510]
Kohl, J., Neuman, B., "The Kerberos Network Authentica-
tion Service (V5)", September 1993.
[RFC-1700]
Reynolds, J., and Postel, J., "Assigned Numbers", STD 2,
USC/Information Sciences Institute, October 1994.
[RFC-1812]
Baker, F., Editor, "Requirements for IP Version 4 +
Routers", Cisco Systems, June 1995.
[RFC-1825]
Atkinson, R., "Security Architecture for the Internet
Protocol", Naval Research Laboratory, July 1995.
[RFC-1828]
Metzger, P., Simpson, W., "IP Authentication using Keyed
MD5", July 1995.
Karn & Simpson expires in six months [Page 75]
DRAFT Photuris April 1996
[RFC-1829]
Karn, P., Metzger, P., Simpson, W., "The ESP DES-CBC
Transform", July 1995.
[RFC-xxxx]
Karn, P., and Simpson, W., "ICMP Security Failures Mes- +
sages", draft-ietf-ipsec-icmp-fail-01.txt, work in +
progress.
[Rooij94]
P. de Rooij, "Efficient exponentiation using precomputa-
tion and vector addition chains", EUROCRYPT '94, pp
403-415.
[Schneier95]
Schneier, B., "Applied Cryptography Second Edition", John
Wiley & Sons, New York, NY, 1995. ISBN 0-471-12845-7.
[Schnorr91]
Schnorr, C.P., "Efficient signature generation by smart |
cards", Cryptology, v 4 pp 161-174, 1991. -
Karn & Simpson expires in six months [Page 76]
DRAFT Photuris April 1996
Contacts |
Comments should be submitted to the photuris@majordomo.soscorp.com +
mailing list. +
Questions about this memo can also be directed to: -
Phil Karn
Qualcomm, Inc.
6455 Lusk Blvd.
San Diego, California 92121-2779
karn@qualcomm.com +
karn@unix.ka9q.ampr.org (preferred) +
William Allen Simpson
Daydreamer
Computer Systems Consulting Services
1384 Fontaine
Madison Heights, Michigan 48071
wsimpson@UMich.edu |
wsimpson@GreenDragon.com (preferred) |
bsimpson@MorningStar.com |
Karn & Simpson expires in six months [Page 77]