Network Working Group P Karn (Qualcomm)
Internet Draft W A Simpson (DayDreamer)
expires in six months May 1997
Photuris: Session Key Management Protocol
draft-simpson-photuris-12.txt
Status of this Memo
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Distribution of this memo is unlimited.
Abstract
Photuris is a session-key management protocol intended for use with
the IP Security Protocols (AH and ESP). This document defines the
basic protocol mechanisms.
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1. Introduction
Photuris [Firefly] establishes short-lived session-keys between two
parties, without passing the session-keys across the Internet. These
session-keys directly replace the long-lived secret-keys (such as
passwords and passphrases) that have been historically configured for
security purposes.
The basic Photuris protocol utilizes the previously configured
secret-keys for identification of the parties. This is intended to
speed deployment and reduce administrative configuration changes.
This document is primarily intended for implementing the Photuris
protocol. It does not detail service and application interface defi-
nitions, although it does mention some basic policy areas as required
for the proper implementation and operation of the protocol mecha-
nisms.
Since the basic Photuris protocol is extensible, new data types and
protocol behaviour should be expected. The implementor is especially
cautioned not to depend on values that appear in examples to be cur-
rent or complete, since their purpose is primarily pedagogical.
1.1. Terminology
In this document, the key words "MAY", "MUST, "MUST NOT", "optional",
"recommended", "SHOULD", and "SHOULD NOT", are to be interpreted as
described in [RFC-2119].
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.
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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.
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 intended recipient of a protected datagram.
SPI User The party that corresponds to the IP Source; the
sender of a protected 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].
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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. The Responder remains
stateless until a shared-secret has been created.
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 session-keys in each direction, which are in turn used for
conventional authentication or encryption. 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. This (optional) facility is specified in
companion documents.
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.
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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]
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]
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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.
1.3. Security Associations
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.
The SPI is assigned by the entity controlling the IP Destination: the
SPI Owner (receiver). The parties use the combination of IP Destina-
tion and SPI to distinguish the correct Security Association.
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.
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.
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Implementation Notes:
The method used for SPI assignment is implementation dependent.
The only requirement is that the SPI be unique for the IP Destina-
tion.
However, selection of a cryptographically random SPI value can
help prevent attacks that depend on a predicatable sequence of
values. The implementor MUST NOT expect SPI values to have a par-
ticular order or range.
1.4. 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.
1.4.1. Exchange LifeTimes
All retained exchange state of both parties has an associated
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 min-
utes).
In addition, during a Photuris exchange, an Exchange TimeOut limits
the wait for the exchange to complete. This timeout includes the
packet round trips, and the time for completing the Identification
Exchange calculations. The time is bounded by both the maximum
amount of calculation delay expected for the processing power of an
unknown peer, and the minimum user expectation for results (default
60 seconds).
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These Exchange LifeTimes and TimeOuts are implementation dependent
and are not disclosed in any Photuris message. The paranoid operator
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 implemen-
tation 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 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 previously com-
puted shared-secret can be used to rapidly generate new session-keys.
1.4.2. SPI LifeTimes
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.
To prevent synchronization between multiple Photuris exchanges, the
implementation SHOULD randomly vary each SPI LifeTime by a few sec-
onds.
There is no requirement that a long LifeTime be accepted by the SPI
User. The SPI User might never use an established SPI, or cease
using the SPI at any time.
When more than one unexpired SPI is available to the SPI User for the
same function, a common implementation technique is to select the SPI
with the greatest remaining LifeTime. However, selecting randomly
among a large number of SPIs might provide some defense against traf-
fic analysis.
To prevent resurrection of deleted or expired SPIs, SPI Owners SHOULD
remember those SPIs, but mark them as unusable until the Photuris
exchange shared-secret used to create them also expires and purges
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the associated state.
When the SPI Owner detects an incoming SPI that has recently expired,
but the associated exchange state has not yet been purged, the imple-
mentation 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.
1.5. Random Number Generation
The security of Photuris critically depends on the quality of the
secret random numbers generated by each party. A poor random number
generator at either party will compromise the shared-secret produced
by the algorithm.
Generating cryptographic quality random numbers on a general purpose
computer without hardware assistance is a very tricky problem. In
general, this requires using a cryptographic hashing function to
"distill" the entropy from a large number of semi-random external
events, such as the timing of key strokes. An excellent discussion
can be found in [RFC-1750].
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
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current 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.
Implementation Notes:
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.
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.
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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 (optional)
6 Secret_Request (optional)
7 Identity_Response
8 SPI_Needed
9 SPI_Update
10 Bad_Cookie
11 Resource_Limit
12 Verification_Failure
13 Message_Reject
Further details and differences are elaborated in the individual mes-
sages.
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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.
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.
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Implementation Note:
The numbers are assumed to be unsigned.
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 Num-
ber".
The Size MUST NOT be assumed to be constant for a particular Scheme.
However, only one of each kind of Scheme will be present in any list
of schemes.
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
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present.
Value(s) Zero or more octets. See the "Attribute List" for
details.
The Length MUST NOT be assumed to be constant for a particular Type.
Multiple attributes of the same Type with varying Lengths MAY be pre-
sent in any list of attributes.
Support is required for the "MD5-KDpKp Symmetric Verification" (#3)
and "MD5-KpDpKp Integrity" (#5) Attributes, and they SHOULD be pre-
sent in every Offered-Attributes list.
Implementation Note:
The authentication, compression, encryption and identification
mechanisms chosen, as well as the encapsulation modes (if any),
need not be the same in both directions.
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 unique
"cookie". The Initiator-Cookie MUST be different in each new
Cookie_Request between the same parties. See "Cookie Generation" for
details.
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.
For example, a Bad_Cookie message is received from the Initiator.
That message has an Initiator-Cookie of A, and a Responder-Cookie
of B. The Responder-Cookie is replaced with A, and a new Initia-
tor-Cookie C is generated.
If any previous exchange between the peer IP nodes has not expired,
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the Responder-Cookie is set to the most recent Responder-Cookie, and
the request Counter is set to the corresponding Counter.
For example, a new Virtual Private Network (VPN) tunnel is about
to be established to an existing partner. The Counter is the same
value received in the prior Cookie_Response, the Responder-Cookie
remains B, and a new Initiator-Cookie C is generated.
Otherwise, the Responder-Cookie and Counter are both set to zero.
By default, the Initiator operates in the same manner as when all
of its previous exchange state has expired. The Responder will
update the Counter appropriately when not all of its own exchange
state has expired.
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
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.
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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 (through rollover of the Counter) the new Counter value is zero
(modulo 256), the value is set to one.
If this new Counter value matches some previous exchange initiated by
this particular peer that has not yet exceeded the Exchange TimeOut,
the Counter is incremented again, until a unique Counter value is
reached.
Nota Bene:
No more than 254 concurrent exchanges between the same two peers
are supported.
The Responder generates a unique cookie. The Responder-Cookie value
in each successive response SHOULD be different. See "Cookie Genera-
tion" for details.
The exchange-schemes available between the peers are listed in the
Offered-Schemes.
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.
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This Scheme-Choice may affect the next Photuris message sent. By
default, the next Photuris message is a Value_Request.
Implementation Notes:
Only the Initiator-Cookie is used to identify the exchange. The
Counter and Responder-Cookie will both be different from the
Cookie_Request.
Various proposals for extensions utilize the Scheme-Choice to
indicate a different message sequence. Such mechanisms are out-
side the scope of this document.
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.
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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.
3.2. Cookie_Response
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Initiator-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Responder-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Counter | 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.
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 Initiator-Cookie. The
Responder-Cookie MUST include the Counter (that it returned in 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.
The cookie is not cached per Initiator to avoid saving state during
the initial Cookie Exchange. On receipt of a Value_Request, the
Responder regenerates its cookie for validation.
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
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available between the parties are listed in the Offered-Attributes.
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.
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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
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 will match
the Initiator-Exchange-Value, and the 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.
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Initiator-Offered-Attributes
A list of Security Parameter attributes supported by
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
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require any particular alignment. The 32-bit align-
ment shown is for convenience in the illustration.
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 two or three sections: Identification-
Attributes, Authentication-Attributes, and (optional) 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 authentication, compression, encryption, and
other operational types. When no Encapsulation-Attributes are
offered, the "ESP-Attributes" attribute itself is omitted from the
list.
Attribute-Choices are selected from the latter two sections of the
list.
Support is required for the "MD5-KDpKp Symmetric Verification" (#3)
and "MD5-KpDpKp Integrity" (#5) Attributes, and they SHOULD be pre-
sent in every Offered-Attributes list.
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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.
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
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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.
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.
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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.
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.
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- 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
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.
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5.1. Identity_Messages
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Initiator-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Responder-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | LifeTime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Security-Parameter-Index |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| Identity-Choice | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
~ Identification ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Verification ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attribute-Choices ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... Padding | PadLength |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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.
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.
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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 PadLength and Padding fields (UDP Length -
PadLength - 1).
Padding Zero or more octets. Prior to (optional) encryp-
tion, it is filled to align the PadLength field at a
boundary appropriate to the Privacy-Method indicated
by the current Scheme-Choice. The padding values
begin with the value 0, and count up to the number
of padding octets (zero relative). For example, if
the PadLength is 5, the padding values are 0, 1, 2,
3, 4.
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After (optional) decryption, if the padding octets
are not the correct values for the PadLength, then
verification fails.
PadLength one octet. The size of the Padding field in octets
(not including the PadLength field). The value typ-
ically 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-KpDpKp Integrity",
would result in ESP with encryption (inside), and then AH authentica-
tion (outside) of the ESP payload.
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.
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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 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.
The shared-secret does not include a Size field.
5.4. Identity Verification
These messages are authenticated using the Identity-Choice. The Ver-
ification value is calculated prior to (optional) encryption, and
verified 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).
The resulting output value is stored in the Verification field.
The Identity-Choice authentication function is calculated over the
following concatenated data values:
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+ 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 (response only),
+ the SPI User secret-key (response only),
+ the message Type, LifeTime and SPI fields,
+ the Attribute-Choices following the Verification field,
+ the Padding (if any),
+ the PadLength.
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 distinct 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.
The Exchange-Value data includes both the Size and Value fields.
The Offered-Attributes and Attribute-Choices data includes the
Type, Length and Value fields.
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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 function is used to
create the SPI session-key for that particular attribute. This func-
tion 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.
When a larger number of keying-bits are needed than are available
from the specified function, these keying-bits are generated by
duplicating the trailing shared-secret, and recalculating the func-
tion. That is, the first iteration will have one trailing copy of
the shared-secret, the second iteration will have two trailing copies
of the shared-secret, and so forth.
Implementation Notes:
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 function over the
initial octets of the concatenated data values for each direction,
and append the trailing copies of the shared-secret.
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.
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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).
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
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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, 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
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.
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6.1. SPI_Needed
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Initiator-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Responder-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| |
~ Verification ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attributes-Needed ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... Padding | PadLength |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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".
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.
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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 PadLength and Padding fields (UDP
Length - PadLength - 1).
Padding Zero or more octets. Prior to (optional) encryp-
tion, it is filled to align the PadLength field at a
boundary appropriate to the Privacy-Method indicated
by the current Scheme-Choice. The padding values
begin with the value 0, and count up to the number
of padding octets (zero relative). For example, if
the PadLength 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 PadLength, then
verification fails.
PadLength one octet. The size of the Padding field in octets
(not including the PadLength field). The value typ-
ically 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 | PadLength |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 PadLength and Padding fields (UDP
Length - PadLength - 1).
Padding Zero or more octets. Prior to (optional) encryp-
tion, it is filled to align the PadLength field at a
boundary appropriate to the Privacy-Method indicated
by the current Scheme-Choice. The padding values
begin with the value 0, and count up to the number
of padding octets (zero relative). For example, if
the PadLength 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 PadLength, then
verification fails.
PadLength one octet. The size of the Padding field in octets
(not including the PadLength field). The value typ-
ically 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.
6.3. Validity Verification
These messages are authenticated using the Validity-Method indicated
by the current Scheme-Choice (see "Exchange Scheme List"). The Veri-
fication value is calculated prior to (optional) encryption, and ver-
ified 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 PadLength.
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 distinct from any authentication method specified for
Security Associations.
The Identity Verification data includes both the Size and Value
fields. The Attribute-Choices data includes the Type, Length and
Value fields.
7. Error Messages
These messages are issued in response to Photuris state loss or other
problems. A message has effect in only one direction. No retrans-
mission 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.
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.
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,
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receipt has no effect on the operation of the protocol.
7.4. Message_Reject
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 13
This error message is sent when an optional message Type is received
that is not supported, or an optional format of a supported message
is not recognized.
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.
8. Public Value Exchanges
Photuris is based in principle on public-key cryptography, specifi-
cally Diffie-Hellman key exchange. Exchange of public 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.
This document 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 expo-
nentiation. 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 parties
can calculate a shared-secret value between themselves.
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(g**xy) mod p
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 private exponents (x) and (y) are kept secret by the parties.
Only the public-value result of the modular exponentiation with (x)
or (y) is sent as the Initiator and Responder Exchange-Value.
These public-values are represented in single Variable Precision Num-
bers. The Size of these Exchange-Values will match the Size of the
modulus (p).
8.2. Moduli Selection
Each implementation proposes one or more moduli in its Offered-
Schemes. Every implementation MUST support up to 1024-bit moduli.
For any particular Photuris node, these moduli need not change for
significant periods of time; likely days or weeks. A background pro-
cess can periodically generate new moduli.
For 512-bit moduli, current estimates would provide 64 (pessimistic)
bit-equivalents of cryptographic strength.
For 1024-bit moduli, current estimates would range from 80 (pes-
simistic) through 98 (optimistic) bit-equivalents of cryptographic
strength.
These estimates are used when choosing moduli that are appropriate
for the Security Association attributes.
8.2.1. Bootstrap Moduli
Each implementation is likely to use a fixed modulus during its boot-
strap, until it can generate another modulus in the background. 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.
Implementors are encouraged to generate their own bootstrap moduli,
and to change bootstrap moduli in successive implementation releases.
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8.2.2. 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 perform
at least one iteration of probable primality verification. In this
fashion, many processors will perform verification in 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 re-learning and re-testing after subsequent exchanges.
8.3. Generator Selection
The generator (g) should be chosen such that the private-secret expo-
nents 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 multiplication
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 private-secret expo-
nent for each different modulus. Then, a D-H Exchange-Value is cal-
culated 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. Other small exponents such that
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(g**x) < p
will be easily visible. These exponents do not qualify as secret.
This specification recommends that the exponent length be at least
twice the desired cryptographic strength of the longest session-key
needed by the strongest offered-attribute.
Based on the estimates in "Moduli Selection" (above):
For 512-bit moduli, exponent lengths of 128 bits (or more) are
recommended.
For 1024-bit moduli, exponent lengths of 160 to 256 bits (or more)
are recommended.
Although the same exponent and Exchange-Value may be used with sev-
eral 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 pri-
vate-secret exponent, and recalculate the Exchange-Value.
Implementation Notes:
The size of the exponent is entirely implementation dependent, is
unknown to the other party, and can be easily changed.
Avoidance of small exponents can be assured by setting at least
one bit in the most significant half of the exponent.
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.
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 Privacy-Method is "not protected".
The "SPI Messages" Validity-Method is "MD5-KDpKp".
(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 cooperating
parties to indicate private schemes, regardless of vendor
implementation. These numbers are not reserved, and are sub-
ject to duplication. Other criteria, such as the IP Source and
Destination of the Cookie_Request, are used to differentiate
the particular Exchange-Schemes available.
10. Validity Methods
10.1. MD5-KDpKp
As described in "Validity Verification", the MD5 [RFC-1321] hash is
calculated over the concatenation of
MD5( key, data, datafill, key, md5fill )
where the key is the computed shared-secret.
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.
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The resulting Verification field is a 128-bit Variable Precision Num-
ber (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* MD5-KDpKp Symmetric Verification
X 5* MD5-KpDpKp Integrity
E 8 DES-CBC
X 255 Organizational
A AH-only Attribute-Choice
E ESP-only Attribute-Choice
I Identity-Choice
X dependent on list location
* feature must be supported (mandatory)
Other attributes are specified in companion documents.
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 inte-
gral 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.
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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).
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 Pay-
load (ESP).
11.4. MD5-KDpKp Symmetric Verification
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 3
Length 0
When selected as an Identity-Choice, the immediately following Iden-
tification field contains an unstructured Variable Precision Number.
Valid Identifications and symmetric secret-keys are preconfigured by
the parties.
There is no required format or content for the Identification value.
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The value may be a number or string of any kind.
The authentication symmetric secret-key (as specified) is selected
based on the contents of the Identification field. All implementa-
tions 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 )
where the key is the computed shared-secret.
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 Num-
ber (18 octets including Size).
For identity verification and session-key calculation, the authenti-
cation symmetric secret-key is also used as the calculation secret-
key.
11.5. MD5-KpDpKp Integrity
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 SPI session-key generation function, 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 generation for
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the same SPI, the next such attribute will begin with a new hash.
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 )
where the key is the SPI session-key.
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 SPI session-key generation function, as
described in "Session-Key Computation". The most significant 64-bits
(8 octets) 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 combined
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 field is always 32-bits.
The full 64-bit DES-CBC IV is generated from the 32-bit SPI field
followed by (concatenated with) the 32-bit IV field. The bit-wise
complement of the 32-bit IV field is XOR'd with the first 32-bits
(SPI field).
The padding values begin with the value 0, and count up to the
number of padding octets (zero relative). For example, if the
plaintext length is 41, the padding values are 0, 1, 2, 3, 4, and
the following PadLength is 5.
After decryption, if the padding octets are not the correct values
for the PadLength, then the payload is discarded, and a "Decryp-
tion 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 pre-
sent.
OUI three octets. The vendor's Organizationally Unique
Identifier, assigned by IEEE 802 or IANA (see
[RFC-1700] for contact details). The bits within
the octet are in canonical order, and the most sig-
nificant 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 implementation
specific.
Some implementors might not need nor 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 incomplete and non-deterministic; many of the
Good/Bad semantic decisions are policy-based or too difficult to rep-
resent in tabular form. 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 datagrams
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
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UTO = Update TimeOut
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
brto = Backoff Retransmission TimeOut
buto = Backoff Update TimeOut
rto = Set Retransmission 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 imple-
mented in any convenient order. The state may be followed by a let-
ter, which indicates an explanatory footnote. The dash ('-') indi-
cates an illegal transition.
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Initiator
| 0 1 2 3 4
| Initial Cookie CookieBad Value ValueBad
------+--------------------------------------------------
DU13 |rto,scq/1 rto,scq/1 rto,scq/1 3 4
SF0 |rto,scq/1 1 2 3 4
SF4 |rto,scq/1 1 2 3 4
SF5 |rto,scq/1 1 2 3 4
WP |rto,scq/1 1 2 3 4
|
RCR+ | - rto,svq/3 rto,svq/3 3 4
RCR- | 0 1 2 3 4
RVR+ | - - - rto,siq/5 rto,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 | - brto/1 brto/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 rto,scq/1
SF4 | 5 6 rto,scq/1
SF5 | 5 6 rto,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 rto,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 rto,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 mes-
sage 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 running.
A.2.6. Update
In the Update state, each party has concluded the Photuris exchange,
and is unilaterally updating expiring SPIs until the Exchange Life-
Time expires. Both the Update and Exchange timers are running.
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B. Use of Identification and Secrets
Implementation of the base protocol requires support for operator
configuration of participant identities and associated symmetric
secret-keys.
The form of the Identification and Secret fields is not constrained
to be a readable string. In addition to a simpler quoted string con-
figuration, an implementation MUST allow configuration of an arbi-
trary stream of octets.
B.1. Identification
Typically, the Identification is a user name, a site 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 partic-
ular IP addresses in use by the parties.
B.2. Group Identity With Group Secret
A simple configuration approach could use a single Identity and
Secret, distributed to all the participants in the trusted group.
This might be appropriate between routers under a single administra-
tion comprising a Virtual Private Network over the Internet.
Nota Bene:
The passwords used in these examples do not meet the "MD5-KDpKp
Symmetric Verification" recommendation for at least 64-bits of
cryptographic strength.
The administrator configures each router with the same username and
password:
identity local "Tiny VPN 1995 November" "abracadabra"
identity remote "Tiny VPN 1995 November" "abracadabra"
When the Initiator sends its Identity_Request, the SPI Owner Identi-
fication field is "Tiny VPN 1995 November" and the SPI Owner secret-
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key is "abracadabra". The SPI User is considered unknown (despite
the fact that only one possible user has been configured). Thus, the
SPI User Identification and SPI User secret-key are omitted from the
Identity Verification calculation.
When the Responder sends its Identity_Response, the SPI Owner Identi-
fication field is "Tiny VPN 1995 November" and the SPI Owner secret-
key is "abracadabra". The SPI User Identification is "Tiny VPN 1995
November" (taken from the request), and the SPI User secret-key is
"abracadabra".
Note that even in the face of implementations with very poor random
number generation yielding the same random numbers for both parties
at every step, with completely identical configuration, the addition
of the SPI User fields in the response calculation is highly likely
to produce a different Verification value. In turn, the different
Verification values affect the calculation of SPI session-keys that
are highly likely to be different in each direction.
B.3. Multiple Identities With Group Secrets
A more robust configuration approach could use a separate Identity
and Secret for each party, distributed to the participants in the
trusted group. This might be appropriate for Authenticated Firewall
Traversal.
An administrator has one or more networks, and a number of mobile
users. It is desirable to restrict access to authorized external
users. The boundary router is 10.0.0.1.
The administrator gives each user a different username and password,
together with a group username and password for the router.
The administrator configures (in part):
identity local "199511@router.site" "FalDaRah"
identity remote "Happy_Wanderer@router.site" "FalDaRee"
Each mobile user adds commands to tunnel and authenticate.
route addprivate 10.0.0.0/8 tunnel 10.0.0.1
secure 10.0.0.1 authenticate-only
identity local "Happy_Wanderer@router.site" "FalDaRee"
identity remote "199511@router.site" "FalDaRah"
identity remote "199512@router.site" "FalDaHaHaHaHaHaHa"
When the mobile Initiator sends its Identity_Request, the SPI Owner
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Identification field is "Happy_Wanderer@router.site" and the SPI
Owner secret-key is "FalDaRee". The SPI User is considered unknown
(despite the fact that the mobile user has only a single configura-
tion). Thus, the SPI User Identification and SPI User secret-key are
omitted from the Identity Verification calculation.
When the firewall Responder sends its Identity_Response, the SPI
Owner Identification field is "199511@router.site" and the SPI Owner
secret-key is "FalDaRah". The SPI User Identification field is
"Happy_Wanderer@router.site" (taken from the request), and the SPI
User secret-key is "FalDaRee".
In this example, the mobile user is already prepared for a monthly
password changeover, where the router might identify itself as
"199512@router.site".
B.4. Multiple Identities With Multiple Secrets
Greater security might be achieved through configuration of a pair of
secrets between each party. As before, one secret is used for ini-
tial contact to any member of the group, but another secret is used
between specific parties. Compromise of one secret or pair of
secrets does not affect any other member of the group. This might be
appropriate between the routers forming a boundary between cooperat-
ing Virtual Private Networks that establish local policy for each VPN
member access.
One administrator configures:
identity local "Apple" "all for one"
identity local "Apple-Baker" "Apple to Baker" "Baker"
identity remote "Baker" "one for all"
identity remote "Baker-Apple" "Baker to Apple"
Another configures:
identity local "Baker" "one for all"
identity local "Baker-Apple" "Baker to Apple" "Apple"
identity remote "Apple" "all for one"
identity remote "Apple-Baker" "Apple to Baker"
When the Initiator sends its Identity_Request, the SPI Owner Identi-
fication field is "Apple" and the SPI Owner secret-key is "all for
one". The SPI User is unknown (many more destination parties could
be configured). Thus, the SPI User Identification and SPI User
secret-key are omitted from the Identity Verification calculation.
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When the Responder sends its Identity_Response, finding that the spe-
cial pairing exists for "Apple" (in this example, indicated by a
third field), the SPI Owner Identification field is "Baker-Apple" and
the SPI Owner secret-key is "Baker to Apple". The SPI User Identifi-
cation is "Apple" (taken from the request), and the SPI User secret-
key is "all for one".
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.
SPI LifeTime (SPILT)
Default: 5 minutes. Minimum: 2 * ELT.
Each party configures a list of known identities and symmetric
secret-keys.
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 Tel-
net. Such considerations are outside the scope of this document.
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Security Considerations
Photuris was based on currently available tools, by experienced net-
work protocol designers with an interest in cryptography, rather than
by cryptographers with an interest in network protocols. This speci-
fication is intended to be readily implementable without requiring an
extensive background in cryptology.
Therefore, only minimal background cryptologic discussion and ratio-
nale is included in this document. Although some review has been
provided by the general cryptologic community, it is anticipated that
design decisions and tradeoffs will be thoroughly analysed in subse-
quent 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 protocol
phases, particularly the "cookie" anti-clogging defense, developed
the initial testing implementation, and provided much of the design
rationale text (now removed to a separate document).
William Simpson was responsible for the packet formats and
attributes, additional message types, editing and formatting. All
such mistakes 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
counter. Also, he developed the first complete independent implemen-
tation (circa October 1995).
Paul C van Oorschot suggested signing both the public exponents and
the shared-secret, to provide an authentication-only version of iden-
tity verification. Also, he provided text regarding moduli, genera-
tor, and exponent selection (now removed to a separate document).
Hilarie Orman suggested adding secret "nonces" to session-key genera-
tion (now removed to a separate document), and provided extensive
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review of the protocol details.
Bart Preneel and Paul C van Oorschot in [PO96] suggested adding
padding between the data and trailing key when hashing for authenti-
cation.
Niels Provos developed a third independent implementation (circa May
1997), ported to AIX, Linux, OpenBSD, and Solaris.
Bill Sommerfeld suggested using the Cookie values on successive
exchanges to provide bi-directional user-oriented keying, and includ-
ing the authentication secret-key in the session-key generation.
Oliver Spatscheck developed the second independent implementation
(circa December 1995) for the Xkernel.
International interoperability testing provided the impetus for many
of the implementation notes herein.
Randall Atkinson, Steven Bellovin, Wataru Hamada, James Hughes, Brian
LaMacchia, Cheryl Madson, Perry Metzger, Bob Quinn, Ron Rivest, Rich
Schroeppel, and Norman Shulman provided 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 cryptographic
systems", 29 Mar 1994.
[DH76] Diffie, W., and Hellman, H.E., "New Directions in Cryptogra-
phy", IEEE Transactions on Information Theory, v IT-22 n 6
pp 644-654, November 1976.
[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 Pub-
lishers, 1992.
[Firefly]
"Photuris" is the latin name for the firefly. "Firefly" is
in turn the name for the USA National Security
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Administration's (classified) key exchange protocol for the
STU-III secure telephone. Informed speculation has it that
Firefly is based on very similar design principles.
[Prime Commandment]
A derivation of an apocryphal quote from the usenet list
sci.crypt.
[PO96] Bart Preneel, Paul C van Oorshot, "On the security of two
MAC algorithms", Advances in Cryptology -- Eurocrypt '96,
Lecture Notes in Computer Science 1070 (May 1996), Springer-
Verlag, pages 19-32.
[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-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 Proto-
col", Naval Research Laboratory, July 1995.
[RFC-1828]
Metzger, P., Simpson, W., "IP Authentication using Keyed
MD5", July 1995.
[RFC-1829]
Karn, P., Metzger, P., Simpson, W., "The ESP DES-CBC Trans-
form", July 1995.
[RFC-2119]
Bradner, S., "Key words for use in RFCs to Indicate Require-
ment Levels", BCP 14, Harvard University, March 1997.
[RFC-xxxx]
Karn, P., and Simpson, W., "ICMP Security Failures Mes-
sages", draft-ietf-ipsec-icmp-fail-01.txt, work in progress.
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DRAFT Photuris Protocol May 1997
[Rooij94]
P. de Rooij, "Efficient exponentiation using precomputation
and vector addition chains", Advances in Cryptology -- Euro-
crypt '94, Lecture Notes in Computer Science, Springer-
Verlag, pages 403-415.
[Schneier95]
Schneier, B., "Applied Cryptography Second Edition", John
Wiley & Sons, New York, NY, 1995. ISBN 0-471-12845-7.
Contacts
Comments about this document should be discussed on the
photuris@majordomo.soscorp.com mailing list.
Questions about this document 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
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