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 these existing 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
definitions, although it does mention some basic policy areas
required for the proper implementation and operation of the protocol
mechanisms.
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
current 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].
byte An 8-bit quantity; also known as "octet" in
standardese.
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
asymmetric 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
distributable. As used in this document, this is
distinguished from an asymmetric public/private
key-pair. An example is a user password.
Security Association (SA)
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
information such as sensitivity labelling.
Security Parameters Index (SPI)
A number that indicates a particular set of uni-
directional attributes used under a Security
Association, such as transform(s) and session-
key(s). The number is relative to the IP
Destination, which is the SPI Owner, and is unique
per IP (Next Header) Protocol. That is, the same
value MAY be used by multiple protocols to
concurrently indicate different Security Association
parameters.
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
frequently.
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 (defined elsewhere). For
example, AH-MD5 [RFC-1828] transforms an IP datagram
into a cryptographic hash, and ESP-DES-CBC [RFC-1829]
transforms plaintext to ciphertext and back
again.
Many of these terms are hierarchically related:
Security Association (bi-directional)
- one or more lists of Security Parameters (uni-directional)
-- one or more Attributes
--- may have a key
--- may indicate a transform
Implementors will find details of cryptographic hashing (such as
MD5), encryption algorithms and modes (such as DES), digital
signatures (such as DSS), and other algorithms in [Schneier95].
1.2. Protocol Overview
The Photuris protocol consists of several simple phases:
1. A "Cookie" Exchange guards against simple flooding attacks sent
with bogus IP Sources or UDP Ports. Each party passes a "cookie"
to the other.
In return, a list of supported Exchange-Schemes are offered by the
Responder 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 calculate 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 Parameters.
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
separate 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 is masked for party privacy protection using a
message privacy-key based on the shared-secret. This protects the
identities of the parties, hides the Security Parameter attribute
values, and improves security for the exchange protocol and
security transforms.
4. Additional messages may be exchanged to periodically change the
session-keys, and to establish new or revised Security Parameters.
These exchanges are also masked for party privacy protection in
the same fashion as above.
The sequence of message types and their purposes are summarized in
the diagram below. The first three phases (cookie, exchange, and
identification) must be carried out in their entirety before any
Security Association can be used.
Initiator Responder
========= =========
Cookie_Request ->
<- Cookie_Response
offer schemes
Value_Request ->
pick scheme
offer value
offer attributes
<- Value_Response
offer value
offer attributes
[generate shared-secret from exchanged values]
Identity_Request ->
make SPI
pick SPI attribute(s)
identify self
authenticate
make privacy key(s)
mask/encrypt message
<- Identity_Response
make SPI
pick SPI attribute(s)
identify self
authenticate
make privacy key(s)
mask/encrypt message
[make SPI session-keys in each direction]
SPI User SPI Owner
======== =========
SPI_Needed ->
list SPI attribute(s)
make validity key
authenticate
make privacy key(s)
mask/encrypt message
<- SPI_Update
make SPI
pick SPI attribute(s)
make SPI session-key(s)
make validity key
authenticate
make privacy key(s)
mask/encrypt message
Either party may initiate an exchange at any time. For example, the
Initiator need not be a "caller" in a telephony link.
The Initiator is responsible for recovering from all message losses
by retransmission.
A Photuris exchange between two parties results in a pair of SPI
values (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
Destination, IP (Next Header) Protocol, 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
Cookies (and UDP Ports) keep the exchanges separate. This results in
more than one initial SPI for each Destination.
To create multiple SPIs 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
transmission.
Implementation Notes:
The method used for SPI assignment is implementation dependent.
The only requirement is that the SPI be unique for the IP
Destination and IP (Next Header) Protocol.
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
particular order or range.
1.4. LifeTimes
The Photuris exchange results in two kinds of state, each with
separate 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 individual SPIs that are established.
This state may be viewed as between users and nodes.
The SPI LifeTimes may be shorter or longer than the Exchange
LifeTime. 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 (ELT), and is subject to periodic expiration. This
depends on the physical and logistical security of the machine, and
is typically in the range of 10 minutes to one day (default 30
minutes).
In addition, during a Photuris exchange, an Exchange TimeOut (ETO)
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 30 seconds).
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 ETO.
To prevent synchronization between Photuris exchanges, the
implementation SHOULD randomly vary each Exchange LifeTime within
twice the range of seconds that are required to calculate a new
Exchange-Value. For example, when the Responder uses a base ELT of
30 minutes, and takes 10 seconds to calculate the new Exchange-Value,
the equation might be (in milliseconds):
1790000 + urandom(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
calculated 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 (SPILT) is usually related to the
speed of the link (typically 2 to 30 minutes), but it MUST NOT be
less than thrice the ETO.
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. For example,
when the Responder uses a base SPILT of 5 minutes, and 30 seconds for
the ETO, the equation might be (in milliseconds):
285000 + urandom(30000)
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
traffic 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
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
implementation MAY accept the SPI. The length of time allowed is
highly dependent on clock drift and variable packet round trip time,
and is therefore implementation dependent.
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].
