[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]
Versions: 02 03 04 05 06 07 08
Network Working Group P Karn
Internet Draft Qualcomm
W A Simpson
DayDreamer
expires in six months November 1995
The Photuris Session Key Management Protocol
draft-ietf-ipsec-photuris-08.txt |
Status of this Memo
This document is a submission to the IP Security Working Group of the
Internet Engineering Task Force (IETF). Comments should be submitted
to the ipsec@ans.net mailing list.
Distribution of this memo is unlimited.
This document is an Internet-Draft. Internet Drafts are working
documents of the Internet Engineering Task Force (IETF), its Areas,
and its Working Groups. Note that other groups may also distribute
working documents as Internet Drafts.
Internet Drafts are draft documents valid for a maximum of six
months, and may be updated, replaced, or obsoleted by other documents
at any time. It is not appropriate to use Internet Drafts as
reference material, or to cite them other than as a ``working draft''
or ``work in progress.''
To learn the current status of any Internet-Draft, please check the
``1id-abstracts.txt'' listing contained in the internet-drafts Shadow
Directories on:
ftp.is.co.za (Africa)
nic.nordu.net (Europe)
ds.internic.net (US East Coast)
ftp.isi.edu (US West Coast)
munnari.oz.au (Pacific Rim)
Abstract
Photuris is an experimental session-key management protocol intended
for use with the IP Security Protocols (AH and ESP).
Karn & Simpson expires in six months [Page i]
DRAFT Photuris November 1995
1. Introduction
The ultimate goal of Internet Security is to facilitate direct IP
connectivity between sensitive hosts and users across the Internet.
Users will rely on Internet Security to protect the confidentiality
of the traffic they send across the Internet and depend on it to
block unauthorized external access to their internal hosts and
networks.
Users must have confidence in every Internet Security component,
including key management. Without this confidence, users may erect
barriers that impede legitimate use of the Internet, or forego the
Internet entirely.
Internet Security does not place any significance on easily forged IP
Source addresses. It relies instead on proof of possession of secret
knowledge: that is, a cryptographic key.
However, secure manual distribution and maintainance of these keys is
often cumbersome and problematic. User distribution often leads to
long-lived keys, with concommitant opportunity for compromise of the
keys.
A fundamental role of this key management protocol is to verify the
values exchanged, while ensuring that the resulting key is not known
by another party. It has been shown [DOW92] that key exchange must
be coupled to authentication. Each party requires assurance that an
exchanged key is not shared with an imposter.
Protecting sensitive data on the Internet against compromise --
either by interception or by unauthorized access -- is necessary, but
not sufficient. The computing resources themselves must also be
protected against malicious attack or sabotage.
With these criteria in mind, Photuris [Firefly] is designed:
1. for frequent exchange of short-lived individual session-keys, with
a minimum of configuration and effort.
2. to support the use of a variety of authentication methods, and
facilitate the exchange of many identification types.
3. to thwart certain types of denial of service attacks on host
resources.
Karn & Simpson expires in six months [Page 1]
DRAFT Photuris November 1995
Design Notes:
Photuris was based on currently available tools, by experienced
network protocol designers with an interest in cryptography,
rather than by cryptographers with an interest in network
protocols. This specification is intended to be readily
implementable without requiring an extensive background in
cryptography.
Therefore, only minimal background cryptographic discussion and
rationale is included in this document. Although some review has
been provided by the general cryptographic community, it is
anticipated that design decisions and tradeoffs will be thoroughly
analysed in subsequent dissertations and debated for many years to
come.
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 [Schneier94].
1.1. Terminology
exchange-value The publically distributable value used to calculate
a shared-secret. As used in this document, refers
to a Diffie-Hellman exchange, as opposed to a
public-key.
private-key A key that is kept secret, and is part of a
public/private key-pair. As used in this document,
refers to RSA key pairs.
public-key A publically distributable value that is part of a
public/private key-pair. As used in this document,
refers to RSA key pairs.
secret-key A key that is not publically distributable, and not
part of a public/private key-pair. An example is a
user password.
Security Association
A collection of parameters describing the security
relationship between two nodes. These parameters
include the identities of the parties, the transform
(including algorithm and algorithm mode), the key(s)
(such as a session-key, secret-key, or appropriate
public/private key-pair), and possibly other
Karn & Simpson expires in six months [Page 2]
DRAFT Photuris November 1995
information 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
frequently.
shared-secret As used in this document, the calculated result of
the Photuris exchange.
transform A cryptographic manipulation of a particular set of
data. As used in this document, refers to certain
well-specified methods (which are defined
elsewhere). For example, AH-MD5 [RFC-1828]
transforms an IP datagram into a one-way hash, and
ESP-DES-CBC [RFC-1829] transforms plaintext to
ciphertext and back again.
1.2. Protocol Description
The Photuris protocol consists of several simple phases:
1. A "Cookie" Exchange guards against simple flooding attacks sent
with bogus IP Sources.
In addition, supported exchange-schemes are offered for |
calculating the shared-secret.
2. A Value Exchange establishes a shared-secret between the parties.
The Responder remains stateless until a shared-secret has been
created.
In addition, supported attributes are offered for use in the
Security Associations.
3. An Identification Exchange identifies the parties to each other,
and verifies the integrity of values sent in phases 1 and 2.
The shared-secret provides a basis to generate separate Security
Association session-keys in each direction, which are in turn used
Karn & Simpson expires in six months [Page 3]
DRAFT Photuris November 1995
for conventional authentication or encryption. Additional
security attributes are also exchanged as needed.
This exchange is also encrypted for privacy using another |
permutation of the shared-secret. This protects the identities of
the parties and hides the security parameter values.
4. Additional messages may be exchanged to periodically change the
session-keys, and to establish new or revised security parameters.
These exchanges are also encrypted for privacy in the same fashion |
as above.
Karn & Simpson expires in six months [Page 4]
DRAFT Photuris November 1995
Initiator Responder
========= =========
Cookie_Request ->
<- Cookie_Response
offer schemes
Exchange_Request ->
pick scheme
offer value
offer attributes
<- Exchange_Response
offer value *
offer attributes
(generate shared-secret from exchanged values)
Identification_Message ->
make SPI
pick SPI attribute(s) |
identify self
authenticate
(make privacy key) |
(encrypt) |
<- Identification_Message
make SPI
pick SPI attribute(s) |
identify self
authenticate
make SPI session-key(s) |
(make privacy key) |
(encrypt) |
make SPI session-key(s) |
(optional)
Change_Message(s) ->
make SPI |
pick SPI attribute(s) |
make SPI session-key(s) |
make integrity key |
authenticate |
(encrypt) |
<- Change_Message(s)
make SPI |
pick SPI attribute(s) |
make SPI session-key(s) |
make integrity key |
authenticate |
Karn & Simpson expires in six months [Page 5]
DRAFT Photuris November 1995
(encrypt) |
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 two SPI values
(one in each direction). Each SPI is used in creating a separate
session-key in each direction.
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 Security Associations with different parameters,
the parties may also send Change_Messages.
There is no requirement that all such outstanding SPIs be used. The
sender selects an appropriate SPI for each datagram transmission.
1.3. Clogging Defense
To grant access to authorized users regardless of location, it must
be possible to cheaply detect and discard bogus datagrams.
Otherwise, an attacker intent on sabotage might rapidly send
datagrams to exhaust the host's CPU or memory resources.
Using Internet Security authentication facilities, when a datagram
does not pass an authentication check, it can be discarded without
further processing. This is easily done with manual (null) key
management between trusted hosts at relatively little cost, given the
speed of cryptographic hashing functions compared to public-key
algorithms.
Unfortunately, such a trusted host will have only a fixed number of
keys available. The keys will tend to have long lifetimes. This
carries significant security risks.
Automatic key management is necessary to generate keys between
parties without prior arrangement. But, there is a potential
Achilles heel in the key management protocol.
Because of their use of CPU-intensive operations such as modular
exponentiation, key management schemes based on public-key
Karn & Simpson expires in six months [Page 6]
DRAFT Photuris November 1995
cryptography are vulnerable to resource clogging attacks. Although a
complete defense against such attacks is impossible, Photuris
features make them much more difficult.
Cookie Exchange
Photuris exchanges a "cookie" before initiating any public-key
operations, thwarting the saboteur from using random IP Source
addresses. The simple validation of this cookie uses the same
level of resources as other Internet Security authentication
mechanisms.
This forces the attacker to:
1) use its own valid IP address, or
2) gain access to a physical transmission link and appropriate
those addresses, or
3) subvert Internet routing for the same purpose.
The first option allows the target to detect and filter out such
attacks, and significantly increases the likelihood of identifying
the attacker. The latter two options are much more difficult than
merely sending large numbers of datagrams with randomly chosen IP
Source addresses from an arbitrary point on the Internet.
The cookie does not protect against an observer that can copy a
valid cookie, or an interceptor that can modify or substitute
another cookie. However, these attacks are mitigated somewhat
with time-variant cookies.
Minimize State
There is a small amount of state associated with the Photuris
exchange itself. This includes the Cookies, Exchange-Values, and
the computed shared-secret.
During the initial Cookie Exchange, the Responder does not
maintain any state for the exchange. This prevents memory
resource exhaustion from a simple flooding attack.
Later exchange phases require saving of state to perform the key
establishment calculations and identity verification. An attacker
that is willing to expose itself to a larger window of detection
can waste substantial resources repeating all the steps of the
Photuris process.
Karn & Simpson expires in six months [Page 7]
DRAFT Photuris November 1995
Precomputation
Once exchange state has been established between nodes, repetitive
exchanges can use many of the same previously computed values.
This prevents an attacker with more CPU power from easily
exhausting the target.
Expiration
All retained exchange state is subject to periodic expiration
(typically 10 minutes). These Exchange LifeTimes are |
implementation dependent and are not disclosed in any Photuris |
message.
When an Exchange-Value expires (or is replaced by a newer value),
all related exchange state is purged. The periodic expiration and
purge of exchange state reduces the risk of compromise of keys and
secrets, and is an important consideration in attaining perfect
forward secrecy.
If an attacker has succeeded in overwhelming a target, the target
will eventually recover as the expired state is purged.
1.4. Traffic Anonymity
Although each datagram carries a cleartext IP Destination, the
ultimate destination can be hidden by "laundering" it through an
encrypted tunnel. The IP Source could be hidden in the same manner.
If the Source has been dynamically allocated, it provides no useful
information to an eavesdropper.
This leaves the identifying information that the parties send during
the Identification Exchange. One would often like to deny this
information to an eavesdropper, especially when this would reveal the
location of a user.
The identification can be easily protected by encrypting the
Identification Exchange with the shared-secret just established.
This keeps a passive eavesdropper from learning the identities of the
parties, either directly from the certificates or by checking
signatures against a known database of public keys.
The scheme is not foolproof. By posing as the Responder, an active
attacker could trick the Initiator into revealing its identity.
However, this active attack is considerably more difficult than
passive vacuum-cleaner monitoring. Unless the attacker can steal the
private/secret key belonging to the Responder, the Initiator will
Karn & Simpson expires in six months [Page 8]
DRAFT Photuris November 1995
discover the deception when verifying the Identification Exchange.
1.5. Security Parameters
Photuris key management is used to determine a number of parameters
for each Security Association between the communicating parties.
This includes the particular authentication and/or encryption
transforms, and the key(s) used to authenticate, encrypt or decrypt
the payload.
The key management implementation usually maintains a table
containing the several parameters for each concurrent Security
Association. The implementation needs to access that security
parameter table to determine how to process each datagram.
The Security Parameters Index (SPI) is assigned by the entity
controlling the IP Destination: the SPI Owner (the receiver). The
parties use the combination of SPI and IP Destination to distinguish
the correct association.
Each SPI has an associated LifeTime, specified by the SPI owner
(receiver). This SPI LifeTime is usually related to the speed of the |
link (typically 30 to 300 seconds). The SPI can also be deleted by
the SPI Owner using the Change_Message. Once the SPI has expired or
been deleted, the parties cease using the SPI, and purge the
associated state.
The SPI LifeTime 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), the |
derived SPIs are not affected. This is important to allow traffic to
continue without interruption during new Photuris exchanges.
Implementation Notes:
The method used for SPI assignment is implementation dependent.
However, selection of a cryptographically random value can help
prevent attacks that depend on a predicatable sequence of values.
To prevent resurrection of old SPIs, implementations SHOULD
remember those deleted or expired SPIs, but mark them as unusable
until the shared-secret used to create them also expires.
Karn & Simpson expires in six months [Page 9]
DRAFT Photuris November 1995
1.6. User Support
The Photuris exchange results in two kinds of state, each with
separate LifeTimes.
1) The small amount of state associated with the Photuris exchange
itself. This state may be viewed as between Internet nodes.
2) The multiple Security Associations that are established. This
state may be viewed as from nodes to users.
Every node requires its own Identification. When the Photuris
exchange is node to node, such as single user personal computers or
unattended firewalls used in virtual private networks, the nodes
themselves may be viewed as the users.
Internet Security protects against threats that come from the
external network, not from mutually hostile users of the nodes
themselves. |
A) A secure multi-user operating system MUST be able to protect its
resources from hostile users, and protect one hostile user from
damaging the resources controlled by another hostile user. |
B) A secure multi-user operating system MUST incorporate strong |
support for user-oriented discretionary access controls. |
C) If the operating system has any security vulnerability, such that |
internal information may be revealed or the information of one
user may be inadvertantly disclosed to another user, then there is |
no basis for separate user-oriented keying. |
When required for secure multi-user environments, the Photuris
Identification can be used to provide separate limited authentication
from each user of one node when communicating with another common
node. To provide user-oriented keying, the nodes can initiate
multiple concurrent Photuris exchanges. These may provide separate
user Identification from the Initiator to the Responder in each
direction.
Each secure multi-user operating system MUST be capable of separately
maintaining multiple Identification Exchange SPI values for each
Value Exchange calculated shared-secret. It is the responsibility of
the Source to internally segregate the shared-secret and different
session-keys provided per Destination, and select an appropriate SPI
for each datagram transmission.
Karn & Simpson expires in six months [Page 10]
DRAFT Photuris November 1995
Implementation Notes:
Once exchange state has been established between nodes, repetitive
exchanges can use many of the same previously computed values.
Successful use of user-oriented keying requires a significant
level of operating system support. Use of multi-user segregated *
exchanges likely requires added functionality in the transport API
of the implementation operating system. Such a mechanism is
outside the scope of this document.
It has been suggested that the Photuris exchange could also be
established between particular application or transport processes
associated with a user of a node. Such a mechanism is
emphatically outside the scope of this document.
1.7. Multicast Support
Key management is more difficult in a multicast environment.
Senders to a multicast group may share common a Security Parameters
Index, if all communications are using the same security
configuration parameters. In this case, the receiver only knows that
the message came from a node knowing the SPI for the group, and
cannot authenticate which member of the group sent the datagram.
Multicast groups may also use a separate SPI value for each Source.
If each sender is keyed separately and asymmetric algorithms are
used, data origin authentication is also provided.
A given Destination is not necessarily in control of the selection
process. In the case of multicast groups, a single node or
cooperating subset of the multicast group may work on behalf of
the entire group to set up a Security Association.
It is anticipated that Photuris would be used first to establish a
distribution SPI and session-key, and that another orthogonal key
distribution mechanism will use that SPI to send the group keys.
This is a matter for future research. Such a mechanism is outside
the scope of this document.
Karn & Simpson expires in six months [Page 11]
DRAFT Photuris November 1995
2. Protocol Details
The Initiator begins a Photuris exchange when it has:
1) a datagram that it wishes to send with privacy, and has no current
Photuris exchange state with the IP Destination.
2) received the ICMP message Destination Unreachable: Communication
Administratively Prohibited (Type 3, Code 13).
3) received the ICMP message Security Failures: Bad SPI (Type 40, |
Code 0), that indicates an expired/invalid SPI.
4) received the ICMP message Security Failures: Need Authentication
(Type 40, Code 4), that indicates a requirement for
authentication.
5) received the ICMP message Security Failures: Need Authorization
(Type 40, Code 5), that indicates a requirement for a different
level of authorization.
6) received an Error_Message indicating that a new Cookie_Request
should be sent.
Other needs to initiate a Photuris exchange are likely to be a matter
for considerable future debate.
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
message is received with an incorrect UDP checksum, it is silently
discarded.
Implementation Note:
It is expected that installation of Photuris will ensure that UDP
checksums are enabled for the computer operating system and later
disabling by operators is prevented.
Karn & Simpson expires in six months [Page 12]
DRAFT Photuris November 1995
2.2. Header Format
All of the messages have a format similar to the following, as
transmitted left to right in network order (most significant to least
significant):
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Initiator-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Responder-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type |
+-+-+-+-+-+-+-+-+
Initiator-Cookie 16 octets.
Responder-Cookie 16 octets.
Type one octet. Each message type has a unique value.
Further details and differences are elaborated in the individual
messages.
Design Note:
The fixed size of the cookies was chosen for convenience, based on
the output of commonly available cryptographic hashing functions.
It is anticipated that this size is likely to be more than
sufficient to protect against very high bit-rate flooding attacks.
2.3. Exchange Schemes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Scheme |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Scheme two octets. A unique value indicating the |
exchange-scheme. |
Selection among several different exchange-schemes is needed to
enable experimental and proprietary extensions without affecting the
Karn & Simpson expires in six months [Page 13]
DRAFT Photuris November 1995
basic protocol. The target of the exchange (Responder) specifies a
list of the schemes supported, and the Initiator chooses one that it
also supports.
The scheme list includes alternative algorithms and distinguishing
parameters. These are mixed in the same list for simplicity. The
implementation can easily distinguish between the separate uses of |
each supported scheme. These uses are indicated in the "Exchange |
Scheme List" Appendix.
Design Notes:
Although exchange-schemes offer great flexibility, only a few |
well-chosen algorithms and parameters are specified. This
provides opportunity for intensive review by the cryptographic
community, reduces implementation complexity, and improves
potential for interoperability.
Only one exchange-scheme (#2) is required to be supported, and |
MUST be present in every Offered-Schemes list.
2.4. Attributes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Value(s) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type one octet. A unique value indicating the kind of |
attribute. |
When the Type is zero (padding), no Length field is |
present. |
Length one octet. The size of the Value(s) field in |
octets. |
When the Length is zero, no Value(s) field is |
present. |
Value(s) Zero or more octets. Further details are elaborated |
in the "Attribute List" Appendix. |
Selection among several different security parameter attributes is
needed to enable future implementation changes without affecting the
basic protocol. Each party (the sender) offers a list of the
attributes supported and its peer (the receiver) selects from that
Karn & Simpson expires in six months [Page 14]
DRAFT Photuris November 1995
list when making its incoming Security Associations.
The attribute list includes authentication, compression, encryption,
identification, and other operational types available for exchange |
between the parties. These are mixed in the same list for
simplicity. The implementation can easily distinguish between the
separate uses of each supported attribute. These uses are indicated |
in the "Attribute List" Appendix.
Encryption decisions are in the SPI User (sender) direction. Only
the sender can determine whether each datagram needs privacy
protection. It uses an encryption SPI created by the receiver, in
addition to an authentication SPI (as needed). When the sender needs
privacy protection for a datagram and Photuris exchange state has
been established, but the potential receiver has no current
encryption SPI, an Error_Message listing encryption attributes is
sent and the original datagram is discarded.
Authentication decisions are in the SPI Owner (receiver) direction.
Only the receiver can determine that arriving traffic is authentic.
Its need for authentication is indicated by choosing authentication
attributes, and/or authenticated encryption attributes, when creating
each SPI. It enforces the authentication through the simple
expedient of dropping all datagrams with missing or invalid
authentication, and sending an appropriate ICMP Security Failures |
message.
Design Notes: |
Although attributes offer great flexibility, only a few well-
chosen algorithms are specified. This provides opportunity for
intensive review by the cryptographic community, reduces
implementation complexity, and improves potential for
interoperability.
Support for some attributes is required (MD5 and DES-CBC), and
SHOULD be present in every Offered-Attributes list. Where
encryption is prohibited in a particular environment, the DES-CBC
attributes MAY be omitted.
Since SPI creation is in the receiver direction, but privacy (and
potentially other) decisions are in the sending direction, a
message is used from the sender to the receiver to stimulate the
SPI creation.
Typically, an encryption method is chosen for the primary
attribute of the initial SPI in each direction. If integrity is
needed, it is recommended that an authentication method be added
Karn & Simpson expires in six months [Page 15]
DRAFT Photuris November 1995
as an additional separate SPI.
When both authentication and encryption attributes are used for |
the same SPI, care must be exercised that there is no interaction
between the algorithms that might reveal some portion of the
session-key. There is no known interaction between MD5 and DES-
CBC.
When choices are made from the set 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. These combinations are
implementation dependent.
The authentication, compression, encryption and identification
mechanisms, as well as the encapsulation mode (if any), need not
be the same in both directions.
2.5. Variable Precision Numbers
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Size | Value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Size two, four, or eight octets. The number of |
significant bits used in the Value field. Always
transmitted 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
significant bits (in 2,097,120 octets).
Karn & Simpson expires in six months [Page 16]
DRAFT Photuris November 1995
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
completeness.
Value Zero or more octets. Always transmitted most |
significant 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
correct number of significant bits.
Design Notes:
Some of the message fields require a value which may vary in the
number of bits. These bits may not make up an integral number of
octets.
The numbers are assumed to be unsigned.
The emphasis on significant bits was based on concerns that
cryptographic lengths and strengths be readily determined. This
is in contrast to the usual concern that each number have only one
unique (shortest) representation.
3. Cookie Exchange
The Initiator initializes local state, and sends a Cookie_Request to
the Responder.
The Initiator also starts a retransmission timer. If no
Cookie_Response is obtained within the time limit, the Cookie_Request
is retransmitted. The Initiator-Cookie value in each such
retransmission to the same IP Destination and UDP Port SHOULD be the
same.
On receipt of a Cookie_Request, the Responder determines if there are
sufficient resources to begin another Photuris exchange. When too
many SPI values are already in use for this particular peer, or some
other resource limit is reached, an Error_Message is sent.
Karn & Simpson expires in six months [Page 17]
DRAFT Photuris November 1995
Otherwise, the Responder generates a cookie, and returns it in a
Cookie_Response. The Responder-Cookie value in each successive
response MAY be different.
Note that the Responder creates no additional state at this time.
On receipt of a Cookie_Response, the Initiator validates the
Initiator-Cookie. Invalid messages are silently discarded.
Karn & Simpson expires in six months [Page 18]
DRAFT Photuris November 1995
3.1. Cookie_Request
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Initiator-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Responder-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type |
+-+-+-+-+-+-+-+-+
Initiator-Cookie 16 octets. A randomized value that identifies the
exchange. The Initiator will use this value to
reject invalid responses.
Responder-Cookie 16 octets. Unused, MUST be set to zero when
transmitted, and MUST be ignored when received.
Type 0
3.2. Cookie_Response
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Initiator-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Responder-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Reserved | Offered-Schemes ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Initiator-Cookie 16 octets. Copied from the Cookie_Request.
Responder-Cookie 16 octets. A randomized value that identifies the
exchange. The Responder will use this value to
reject invalid requests.
Type 1
Karn & Simpson expires in six months [Page 19]
DRAFT Photuris November 1995
Reserved one octet. Unused, MUST be set to zero when
transmitted, and MUST be ignored when received.
Offered-Schemes Two or more octets. A list of one or more |
exchange-schemes supported by the Responder, in
increasing order of preference.
Each value is two octets (see "Exchange Scheme List"
Appendix). The end of the list is indicated by the
UDP Length.
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 must 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 calculate a cryptographic hashing
function (such as MD5) over the IP Source and Destination addresses,
the UDP Source and Destination ports, and a locally generated secret
random value. An incoming cookie can be verified at any time by
regenerating it locally from values contained in the incoming IP
datagram and the local secret random value.
Initiator
The Initiator secret random value that affects the cookie SHOULD
change for each new Photuris exchange, and is thereafter
internally cached on a per Responder basis. This provides
improved synchronization and protection against replay attacks.
Karn & Simpson expires in six months [Page 20]
DRAFT Photuris November 1995
An alternative is to cache the cookie instead of the secret value.
Incoming cookies can be compared directly without the
computational cost of regeneration.
Responder
The Responder secret random value MAY remain the same for many
different Initiators. Instead, this secret SHOULD be changed
whenever the Responder Exchange-Value is changed.
During the initial Cookie Exchange, the Responder regenerates its
cookie for validation. The cookie is not cached per Initiator to
avoid saving state during the initial Cookie Exchange. Once the
Exchange_Request is received, both Initiator and Responder cookies
are cached to identify the exchange.
4. Value Exchange
On receipt of a valid Cookie_Response, the Initiator chooses an |
appropriate exchange-scheme and Exchange-Value, and sends an
Exchange_Request. Later Cookie_Responses from the same Responder are
silently discarded, until a new Cookie_Request is sent.
The Initiator also starts a retransmission timer. If no valid
Exchange_Response is obtained within the time limit, the same
Exchange_Request is retransmitted.
On receipt of an Exchange_Request, the Responder validates the
Responder-Cookie and the Scheme-Choice. Whenever an invalid/expired
cookie or scheme is detected by the Responder, an Error_Message is
sent, and the message is discarded.
When a valid Exchange_Request has been received, the Responder |
chooses an appropriate Exchange-Value for the indicated scheme, and
sends an Exchange_Response.
The Responder keeps a copy of the incoming Exchange_Request values,
and its Exchange_Response. If a duplicate Exchange_Request is
received, it merely resends its previous Exchange_Response, and takes
no further action.
Implementation Notes:
At this time, the Responder begins calculation of the shared-
secret. This may take a substantial amount of time. The
implementor should ensure that retransmission is not blocked by
this calculation. This is not usually a problem, as
Karn & Simpson expires in six months [Page 21]
DRAFT Photuris November 1995
retransmission timeouts typically exceed calculation time.
Karn & Simpson expires in six months [Page 22]
DRAFT Photuris November 1995
4.1. Exchange_Request
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Initiator-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Responder-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Reserved | 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
Reserved one octet. For future use; MUST be set to zero when
transmitted, and MUST be ignored when received.
Scheme-Choice two octets. A value selected by the Initiator from
the list of Offered-Schemes in the Cookie_Response.
Initiator-Exchange-Value
variable precision number. Provided by the
Initiator for calculating a shared-secret between
the parties. The 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
alignment shown is for convenience in the
illustration.
Initiator-Offered-Attributes
Six or more octets. A list of three (mandatory) or |
more Security Parameter attributes supported by the
Karn & Simpson expires in six months [Page 23]
DRAFT Photuris November 1995
Initiator, in increasing order of preference.
This list includes all attributes supported by the
Initiator. Each Responder "-Choice" selects from
this list.
The formats are specified in the "Attribute List"
Appendix, where mandatory attributes are also
specified. The end of the list is indicated by the
UDP Length.
Design Notes:
Having the scheme chosen by the Initiator allows the greatest
protocol flexibility, and follows the requirement that no state be
kept by the Responder until the shared-secret is calculated.
Unfortunately, this allows the weakest scheme to be chosen by an
attacker.
This is no worse than the alternative: to have the Responder
choose from weak schemes offered by the Initiator.
4.2. Exchange_Response
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Initiator-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Responder-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Reserved | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Responder-Exchange-Value ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Responder-Offered-Attributes ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Initiator-Cookie 16 octets. Copied from the Exchange_Request.
Responder-Cookie 16 octets. Copied from the Exchange_Request.
Karn & Simpson expires in six months [Page 24]
DRAFT Photuris November 1995
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
Responder for calculating a shared-secret between
the parties. The 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
alignment shown is for convenience in the
illustration.
Responder-Offered-Attributes
Six or more octets. A list of three (mandatory) or |
more Security Parameter attributes supported by the
Responder, in increasing order of preference.
This list includes all attributes supported by the
Responder. Each Initiator "-Choice" selects from
this list.
The formats are specified in the "Attribute List"
Appendix, where mandatory attributes are also
specified. The end of the list is indicated by the
UDP Length.
5. Identification Exchange *
On receipt of a valid Exchange_Response, the Initiator begins its
parallel computation of the shared-secret. When the Initiator
completes computation, it sends an Identification_Message to the
Responder.
The Initiator also starts a retransmission timer. If no
Identification_Message response is obtained within the time limit,
the same Identification_Message request is retransmitted.
When the Responder completes its parallel computation of the shared-
secret, and upon receipt of a valid Identification_Message, it sends
an Identification_Message to the Initiator.
Karn & Simpson expires in six months [Page 25]
DRAFT Photuris November 1995
The Responder keeps a copy of the incoming Identification_Message
values, and its Identification_Message. If a duplicate
Identification_Message is received, it merely resends its previous
Identification_Message, and takes no further action.
Whenever an invalid/expired cookie or attribute is detected by the
receiver, an Error_Message is sent, and the message is discarded.
Implementation Notes:
Calculation of the shared-secret by the Initiator and Responder is
executed in parallel to minimize delay.
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 cached
shared-secret can be used to rapidly generate new session-keys. |
The paranoid operator will have a fairly short Exchange LifeTime, |
but it SHOULD NOT be zero, to protect against resource clogging |
(described earlier).
Karn & Simpson expires in six months [Page 26]
DRAFT Photuris November 1995
5.1. Identification_Message
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Initiator-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Responder-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | LifeTime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Security-Parameter-Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identity-Choice | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
~ Identification ~ |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Verification ~ |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attribute-Choices ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... Padding | Pad Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Initiator-Cookie 16 octets. Copied from the Exchange_Request.
Responder-Cookie 16 octets. Copied from the Exchange_Request.
Type 4
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
communications.
Identity-Choice Two or more octets. An identity attribute is |
selected from the list of Offered-Attributes sent by
the peer, and is used to calculate the Verification.
Karn & Simpson expires in six months [Page 27]
DRAFT Photuris November 1995
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
illustration. |
Identification Zero or more octets. The content is specified by |
the Identity-Choice attribute.
The field may be any integral number of octets in
length. It does not require any particular
alignment. The 32-bit alignment shown is for
convenience in the illustration.
Verification variable precision number. The content is specified
by the Identity-Choice attribute.
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
alignment shown is for convenience in the
illustration. *
Attribute-Choices
Two or more octets. A list of one or more |
attributes for this SPI, selected from the list of
Attributes supported by the peer.
The formats are specified in the "Attribute List"
Appendix. The end of the list is indicated by the
UDP Length minus the Pad Length and Padding.
Padding Zero or more octets. Prior to encryption, it is |
filled with unspecified implementation dependent
(preferably random) values, to align the Pad Length
field at a boundary appropriate to the Privacy- |
Method.
After decryption, it MUST be ignored.
Pad Length one octet. The size of the Padding field in octets.
It does not include the Pad Length fields. The
value typically ranges from 0 to 7, but may be up to
255 to permit hiding of the actual data length.
This field is always present, even though no |
Privacy-Method is specified or no Padding is
required.
Karn & Simpson expires in six months [Page 28]
DRAFT Photuris November 1995
This field is opaque. That is, the value is set
prior to encryption, and is examined only after
decryption.
5.2. Shared-Secret |
The shared-secret is used in a number of calculations. Regardless of |
the internal representation of the shared-secret, when used in |
calculations it is in the same form as the Value part of a Variable |
Precision Number: |
- most significant octet first. |
- bits used are right justified within octet boundaries. |
- any unused bits are in the most significant octet. |
- unused bits are zero filled. |
5.3. Privacy *
This message is encrypted using the Privacy-Method indicated by the |
current Scheme-Choice. It is separate from any encryption specified |
for Security Associations (see "Exchange Scheme List" Appendix).
The fields protected, the length of the Padding (if any), and other |
details are described for each Privacy-Method. See the "Attribute
List" Appendix for details.
The Privacy-Method specified key generation cryptographic hash is |
used to create a special privacy session-key. This hash is |
calculated over the following concatenated values:
+ the computed shared-secret,
+ the Initiator Cookie, |
+ the Responder Cookie, |
+ the SPI Owner (receiver) Exchange-Value,
+ the SPI User (sender) Exchange-Value,
+ the computed shared-secret again. *
Since the order of the Exchange-Value fields is different in each |
direction, the resulting privacy session-key will usually be
different in each direction. |
Karn & Simpson expires in six months [Page 29]
DRAFT Photuris November 1995
Implementation Notes: |
This Privacy-Method is used to protect both |
Identification_Messages and Change_Messages. |
The chosen algorithm need not provide integrity, as sufficient |
integrity is provided by the Identity Verification.
5.4. Identity Verification |
The two parties now verify the identities received. The indicated |
Identity-Choice method is calculated over the following concatenated |
values:
+ the computed shared-secret,
+ the Offered-Schemes,
+ the SPI Owner (receiver) Exchange-Value,
+ the SPI User (sender) Exchange-Value,
+ the SPI Owner (receiver) Offered-Attributes,
+ the SPI User (sender) Offered-Attributes,
+ the Type, LifeTime and SPI,
+ the Identification (see notes below),
+ the Attribute-Choices following the Verification field, |
+ the authentication secret-key (if any),
+ the computed shared-secret again.
Note that the order of the Exchange-Value and Offered-Attribute |
fields is different in each direction. The SPI and Identification |
fields are also likely to be different in each direction.
If identity verification fails, the users are notified, an
Error_Message is sent, and the Security Association is destroyed.
On success, normal operation begins with the authentication and/or
encryption of user datagrams.
Implementation Notes:
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
discarded.
The exact details of the Identification that are included in the
verification calculation are dependent on the Identity-Choice.
Karn & Simpson expires in six months [Page 30]
DRAFT Photuris November 1995
See the "Attribute List" Appendix for details.
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
Error_Message as failure of the verification computation.
As previously noted, the purpose of verification is to protect
against an insertion or modification attack. To provide anonymity
for mobile nodes, the identity is not coupled with or restricted
to any particular IP address.
Each party implements local policy that determines what access, if
any, is granted to the holder of a particular identity. For
example, the party might allow anonymous FTP, but prohibit Telnet.
Such policy considerations are outside the scope of this document.
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 |
Association. See the "Attribute List" Appendix for details. |
The Attribute-Choice specified key generation cryptographic hash is |
used to create a SPI session-key for that particular attribute. This |
hash is calculated over the following concatenated values:
+ the computed shared-secret,
+ the Initiator Cookie, |
+ the Responder Cookie, |
+ the SPI Owner (receiver) Identity Verification,
+ the SPI User (sender) Identity Verification,
+ the SPI, *
+ the computed shared-secret again.
Since the order of the Identity Verification fields is different in |
each direction, and the SPI is likely to be different in each |
direction, the resulting session-key will usually be different in
each direction.
Implementation Notes: |
Inclusion of the Cookie and Verification fields together with the |
SPI allows reuse of the same Exchange-Values and resulting |
shared-secret among several parties and multiple users of the same
node without generating the same session-keys. |
Karn & Simpson expires in six months [Page 31]
DRAFT Photuris November 1995
When both authentication and encryption attributes are used for |
the same SPI, and such attributes use different key generation |
hashing algorithms, there may be multiple session-keys associated |
with the same SPI.
Karn & Simpson expires in six months [Page 32]
DRAFT Photuris November 1995
6. Other Message Types
The need for these messages has been indicated in previous processing
descriptions. Details of use follow each message.
6.1. Change_Message
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Initiator-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Responder-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | LifeTime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Security-Parameter-Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | *
~ Verification ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attribute-Choices ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... Padding | Pad Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Initiator-Cookie 16 octets. Copied from the Exchange_Request.
Responder-Cookie 16 octets. Copied from the Exchange_Request.
Type 5
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
communications. 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 (typically used for deletion). *
Karn & Simpson expires in six months [Page 33]
DRAFT Photuris November 1995
Verification variable precision number. The calculation of the *
value is described in the Verification section.
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
alignment shown is for convenience in the
illustration. *
Attribute-Choices
Two or more octets. A list of one or more |
attributes for this SPI, selected from the list of
Attributes supported by the peer.
The formats are specified in the "Attribute List"
Appendix. The end of the list is indicated by the
UDP Length minus the Pad Length and Padding.
Padding Zero or more octets. Prior to encryption, it is |
filled with unspecified implementation dependent
(preferably random) values, to align the Pad Length
field at a boundary appropriate to the Privacy- |
Method.
After decryption, it MUST be ignored.
Pad Length one octet. The size of the Padding field in octets.
It does not include the Pad Length fields. The
value typically ranges from 0 to 7, but may be up to
255 to permit hiding of the actual data length.
This field is always present, even though no |
Privacy-Method is specified or no Padding is
required.
This field is opaque. That is, the value is set
prior to encryption, and is examined only after
decryption.
At any time after completion of the Identification Exchange, either
party can send a Change_Message. The message has effect in only one
direction, from the SPI Owner to the SPI User.
This message is required to be encrypted for privacy in the same |
fashion specified for the Identification_Messages.
Whenever an invalid/expired cookie or attribute is detected by the
receiver, an Error_Message is sent, and the message is discarded.
Karn & Simpson expires in six months [Page 34]
DRAFT Photuris November 1995
6.1.1. Creation
This message can be used to create a new Security Association.
Frequently, this message is used to create a separate authentication
SPI when the initial SPI was used for encryption.
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 |
Identification_Message. Since the SPI value is always different than
any previous SPI during the lifetime of the shared-secret, the |
resulting session-keys will necessarily be different from all others
used in the same direction.
When the peer's cookie or public-value has expired, it will send an
Error_Message response.
When the peer finds that too many SPI values are already in use for
this party, or some other resource limit is reached, it will send an
Error_Message response.
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 from the Cookie_Request.
6.1.2. Deletion
This message can be used to delete existing Security Associations.
This is especially useful when all associations need deletion, such
as when the application that needed them terminates.
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 begin the Photuris exchange again from the
Cookie_Request.
Karn & Simpson expires in six months [Page 35]
DRAFT Photuris November 1995
6.1.3. Modification
This message cannot be used to modify existing Security Associations,
such as lengthen an existing SPI LifeTime, resurrect an expired SPI, *
or add or remove an Attribute-Choice.
On receipt, such an otherwise valid message is silently discarded.
6.1.4. Privacy |
This message is encrypted in the same fashion specified for the |
Identification_Messages. |
6.1.5. Integrity Verification
This message is authenticated using the Validity-Method indicated by |
the current Scheme-Choice. It is separate from any authentication |
specified for Security Associations (see "Exchange Scheme List" |
Appendix).
The Validity-Method specified cryptographic hash is calculated over |
the following concatenated values:
+ the computed shared-secret,
+ the Initiator Cookie, |
+ the Responder Cookie, |
+ the SPI Owner (receiver) Identity Verification,
+ the SPI User (sender) Identity Verification,
+ the Type, LifeTime and SPI, |
+ the Attribute-Choices following the Verification field, |
+ the computed shared-secret again.
Note that the order of the Identity Verification fields is different |
in each direction.
If the verification fails, the users are notified, and an
Error_Message is sent, without adding or deleting any Security
Associations. On success, normal operation begins with the
authentication and/or encryption of user datagrams.
Implementation Notes:
Unlike the computationally intensive Identification Exchange, this |
message requires independent verification of integrity when not |
inherently provided by the Privacy-Method, to prevent malicious |
forgery of Security Attributes. |
Karn & Simpson expires in six months [Page 36]
DRAFT Photuris November 1995
The Verification value is calculated prior to encryption for
privacy, and verified after decryption.
The Verification value is different from the SPI session-key that *
is created. However, the initial octets of the concatenated
values are the same, and the implementor may save some calculation
effort when the generating methods are the same.
As usual, the Cookies are checked first before performing |
decryption and checking the Verification. This Verification re- |
checks the Cookies again. Separate external and internal
verification of the expected Cookies prevents clogging by replay
of earlier Change_Messages.
Karn & Simpson expires in six months [Page 37]
DRAFT Photuris November 1995
6.2. Error_Message
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Initiator-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Responder-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Attributes-Needed ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Initiator-Cookie 16 octets. Copied from the offending message.
Responder-Cookie 16 octets. Copied from the offending message.
Type 7
Code Indicates the problem:
0 implementation error
1 invalid/expired
2 resource limit
3 verification failure
4 need attributes
Attributes-Needed
variable. A list of zero or more attributes (for
Code 4).
The formats are specified in the "Attribute List"
Appendix. The end of the list is indicated by the
UDP Length.
Issued in response to Photuris state loss or other problems. The
message has effect in only one direction. No retransmission timer is
necessary.
This message is not encrypted for privacy in the fashion described
for the Identification_Message.
The receiver checks the Cookies for validity. Invalid messages are
silently discarded.
Karn & Simpson expires in six months [Page 38]
DRAFT Photuris November 1995
6.2.1. Implementation Error
This Error_Message Code is sent when a scheme or an attribute is
received which was not offered, or is not allowed for the feature
specified.
When this Code is received, the implementation SHOULD log the
occurance, and notify an operator as appropriate.
6.2.2. Invalid/Expired
This Error_Message Code is sent when certain Photuris messages are
received (as indicated in their accompanying description), and the
receiver's cookie is invalid or the associated public-value has
expired.
When this Code is received, which was not itself discarded for
invalid/expired Cookies, the shared-secret and other state between
the parties is purged, and a new Cookie_Request is sent instead.
However, existing SPIs are not deleted. They expire normally, and
are purged sometime later.
6.2.3. Resource Limit
This Error_Message Code is sent when a Cookie_Request or
Change_Message is received, and too many SPI values are already in
use for that peer, or some other Photuris resource is unavailable.
When this Code is received, the party SHOULD NOT instantiate another
SPI until it has deleted an existing SPI, or waited for a cached SPI
entry to expire.
6.2.4. Verification Failure
This Error_Message Code is sent when an Identification_Message or
Change_Message is received, and verification fails.
When this Code is received, the implementation SHOULD log the
occurance, and notify an operator as appropriate.
Karn & Simpson expires in six months [Page 39]
DRAFT Photuris November 1995
6.2.5. Need Attributes
This Error_Message Code is sent when a party needs particular
attributes for a datagram (such as encryption), and the peer has not
yet created a Security Association (SPI) that has those attributes.
The missing attributes are included in the Error_Message.
When this Code is received, the party SHOULD send a Change_Message
that includes the necessary attributes.
Karn & Simpson expires in six months [Page 40]
DRAFT Photuris November 1995
7. Public Value Exchanges
Photuris is based on public-key cryptography, specifically Diffie-
Hellman key exchange. Exchange of D-H Exchange-Values based on |
private/secret values results in a mutual shared-secret between the
parties. This shared-secret can be used on its own, or to generate a
series of session-keys for authentication and encryption of
subsequent traffic.
Widespread deployment and use of an Internet Security protocol is
possible without public-key cryptography. For example, Kerberos
[RFC-1510] can generate host-pair keys for use in Internet Security,
much as it now generates session-keys for use by encrypted telnet and
other "kerberized" applications.
The Kerberos model has some widely recognized drawbacks. Foremost is
the requirement for a highly available on-line Key Distribution
Center (KDC), with a database containing every principal's secret-
key. This carries significant security risks.
Public-key cryptography enables decentralization. Entities generate
session-keys without real-time communication with any other party.
This draft assumes familiarity with the Diffie-Hellman public-key
algorithm. A good description can be found in [Schneier94].
7.1. Perfect Forward Secrecy
Many security breaches in cryptographic systems have been facilitated
by designs that generate traffic-encrypting keys (or their
equivalents) well before they are needed, and then keep them around
longer than necessary. This creates many opportunities for
compromise, especially by insiders. A carefully designed public-key
system can avoid this problem.
The rule is to avoid using any long-lived keys (such as an RSA key-
pair) to encrypt session-keys or actual traffic. Such keys should be
used solely for authentication purposes.
All keys for traffic encryption should be randomly generated
immediately before use, and then destroyed immediately after use, so
that they cannot be recovered. The keys should not be based on the
values of any previous keys, or any other long-lived stored
information.
The Photuris exchange messages can provide perfect forward secrecy,
as defined by Diffie [Diffie90], using a combination of Diffie-
Karn & Simpson expires in six months [Page 41]
DRAFT Photuris November 1995
Hellman (for key exchange) and authentication, as follows:
1. Agree on a shared-secret using the Diffie-Hellman algorithm.
2. Authenticate the Diffie-Hellman exchanges. This authenticates the
parties to each other, thwarting the "man in the middle" active
attack against Diffie-Hellman.
When the shared-secret generated in step 1 is eventually destroyed,
it is unrecoverable.
Theft of the private/secret key used to sign the exchanges in step 2
would allow the thief to impersonate the party in future
conversations, but it would not decode any past traffic that might
have been recorded.
7.2. Modular Exponentiation Groups
The original Diffie-Hellman technique specified modular
exponentiation. An Exchange-Value is generated using a generator |
(g), raised to a private/secret exponent (x), modulo a prime (p).
(g**x) mod p
When two of these values are exchanged between parties, the parties
can calculate a shared-secret value between themselves.
Each modular exponentiation prime (p) must have the property that
both p and (p-1)/2 are prime. A small set of such recommended strong
primes for use as Photuris moduli are specified.
The prime moduli used will be well-known, and embedded in the
implementations. Use of a very limited number of moduli has one
minor and two very significant advantages:
Overhead
Trivially avoids sending the full modulus.
Prime and Generator Pair Selection
Discovery of strong primes is extremely computationally intensive,
and practically impossible for commercially available processors
to find in a reasonable interactive time. Verification can take
hours or days.
The generator (g) should be chosen such that the secret exponents
Karn & Simpson expires in six months [Page 42]
DRAFT Photuris November 1995
will generate all possible public exponential values, evenly
distributed throughout the range, without cycling through a
smaller subset. Such a generator is called a "primitive root"
(which is trivial to find when p is strong).
When the strong prime and generator pair are well chosen, the
difficulty of a discrete log attack is maximized. By choosing the
pairs in advance, analysis of the pair characteristics is
possible. This analysis can promote confidence in the security of
the implementations.
Precomputation
Each party can precompute the D-H Exchange-Value. |
A background process can periodically destroy the old values,
generate a new random secret exponent, and recalculate the |
Exchange-Value. This limits the exposure of both the secret
exponent and shared-secret, as past secrets are not kept for
possible discovery by a future intrusion, protecting earlier
communications. Also, the secret exponent currently in use is
less likely to be anticipated, as the element of random timing is
introduced.
Since these operations involve several time-consuming modular
exponentiations, moving them to the "background" substantially
speeds up the apparent execution speed of the Photuris protocol.
It also reduces CPU loading sufficiently to allow a single
public/private key-pair to be used in several closely spaced
Photuris executions, when creating Security Associations with
several different hosts over a short period of time.
Other precomputation suggestions are described in [BGMW93].
7.2.1. Moduli Strengths
The cryptographic strength (resistance to discovery of the shared-
secret) of modular exponentiation directly depends on the length of
the prime moduli. Unfortunately, considerable additional length is
needed for a few more bits of strength.
The difficulty of finding the calculated shared-secret has long been
presumed to be the same complexity as factoring an integer of the
same size which is the product of two large primes:
e ** ((ln p)**1/2 * (ln (ln p))**1/2)
Karn & Simpson expires in six months [Page 43]
DRAFT Photuris November 1995
More recent work (number field sieve) estimates that for large
numbers the strength may approach an equivalent of:
e ** (1.93 * (ln p)**1/3 * (ln (ln p))**2/3)
This is significantly smaller, and raises questions about how much
longer modular exponentiation will be a viable key exchange
technique.
There is another long-term problem with use of well-known moduli.
The discrete logarithms for small(-ish) primes in the modulus field
need only be calculated once, and the results can be combined to
easily determine all past recorded shared-secret exchanges and all
subsequent shared-secrets.
Design Notes:
The pessimistic estimates given for modular exponentiation in the |
"Exchange Scheme List" Appendix are based on the number field |
sieve. |
The number field sieve formula is still theoretical. Experimental
results can be used to estimate a time constant that should be
applied to the formula in order to fit the formula to actual
execution times. |
The relevant data point for this document is that a 400-bit number
can be factored in 250 MIPS years; this result is recent (circa
1995). This yields an estimate of the time constant at roughly
1.5 * 10**-15 MIPS years.
These calculations yield an estimate of 30,000 MIPS years for |
512-bit moduli. Current processing power could produce the |
discrete logarithm tables for any well-known 512-bit modulus in |
about a year. This is unacceptable. |
The estimate rises to 2.5*10**11 MIPS years for 1024-bit moduli. |
This is considered sufficient to be infeasable to compute the
discrete logarithm tables for any single well-known moduli within
a decade.
Ideally, cooperating parties will also use their own moduli. |
Additional well-known moduli can be added at any time. Should any
particular moduli be compromised, a vast number of alternatives
may be easily substituted.
Karn & Simpson expires in six months [Page 44]
DRAFT Photuris November 1995
7.2.2. Exponent Selection
The cryptographic strength of the shared-secret is also dependent on |
the work factor of solving for the randomly chosen secret exponents.
Revealing the exponent(s) of either party will unravel the shared-
secret. Each party depends upon the other to provide sufficiently
strong exponents.
There is surprisingly little guidance in the literature about the
length of the secret exponents. The size of these exponents
dramatically affects the speed of modular exponentiation. It is
desirable to use the smallest random exponent that is consistent with
good security.
The exponent 0 will generate the public value 1, and exponent 1 will
generate the public value g mod p. These exponents do not qualify as
secret.
The most conservative advice received to date [Hellman95] is to make
the random exponents twice as long as the strength required of the
intended session-key. This ensures that any space/time "meet in the
middle" attack on the discrete logarithm problem will be at least as
complex as a brute-force search on the resulting session-key space.
Implementation Notes:
A single modular exponentiation on a 486-66DX2 processor using
RSAREF and Borland C under MS-DOS took 20 seconds with a 1024-bit
prime modulus and a 1024-bit random exponent. This dropped to
about 1 to 1.5 seconds when the random exponent was shortened to
128 bits, with the same 1024-bit modulus.
The size of the exponent is entirely implementation dependent, is
unknown to the other party, and can be easily changed.
7.3. Elliptic Curve Groups
The points on an elliptic curve form a group under addition. This
group can be used as the basis for the efficient implementation of
the Diffie-Hellman operations. To uniquely specify the computation,
the implementor must know the finite field for representation of the
point coordinates, the elliptic curve, and the generator.
The elliptic curve "addition" formulas are more complicated than
straight-forward component-wise addition, and the use of finite
fields further complicates the description of the algorithms. A good
reference for a succinct description of elliptic curves with finite
Karn & Simpson expires in six months [Page 45]
DRAFT Photuris November 1995
fields is [P1363]; a more general treatment can be found in
[Menezes]. A treatise on efficient software implementation is in
[Schroeppel].
Note that while the literature uses the term "addition" for the
group operation, it is directly analogous to "multiplication" in
modular exponentiation groups. Thus, the analogous term for
"g**r" is "r*g" (that is, the scalar multiple r of g).
The generator is specified as the (x,y) coordinates of an elliptic
curve point, and the representation of x and y is given with respect
to a finite field. Multiples of the generator are (x,y) pairs.
Thus, the Initiator and Responder Exchange-Values are to be
interpreted as two concatenated bit values in the order (x,y). The
lengths of the coordinates are implicit in the specification of the
field.
The field representation is uniquely determined by the irreducible
polynomial specified in the group description. See the "Exchange
Scheme List" Appendix for details.
The principle advantage enjoyed by use of these elliptic curves is
that calculation times are significantly reduced compared to modular
exponentiation of the same cryptographic strength. The advantage of
elliptic curves becomes more pronounced as the length of the shared-
secret increases.
Use of a very limited number of fields has similar advantages to
those cited for modular exponentiation: reduced overhead, field and
generator selection, and precomputation of the Exchange-Values. It |
also allows great latitude in code optimization.
Overhead
Avoids sending the several polynomial parameters.
Field Selection
Discovery of elliptic curves with a large prime divisor is
extremely computationally intensive, and practically impossible
for commercially available processors to find in a reasonable
interactive time. Verification can take days or weeks.
Karn & Simpson expires in six months [Page 46]
DRAFT Photuris November 1995
7.3.1. Field Strengths
Although analysis is still in early stages, it appears that elliptic
curves do not suffer some limitations of modular exponentiation. The
Diffie-Hellman algorithm, when implemented over elliptic curve
groups, is not subject to the index calculus attack that leads to the
equivalence of the difficulty of factoring with the difficulty of
solving modular exponentiation.
This means that elliptic curve groups have an overall strength that
is equal to one half of the log of the group size. Increasing the
length of the parameters appears to have a proportional improvement
in cryptographic strength.
7.3.2. Exponent Selection
As in modular exponentiation, the cryptographic strength of the
shared-secret is also dependent on the work factor of solving for the |
randomly chosen secret exponents. Unlike modular exponentiation, the
size of the exponents does not dominate the speed of calculating the
elliptic curve groups.
The lengths of the randomly chosen secret exponents are usually the |
same as the number of bits in the generated keying material. This is
twice as long as the resulting overall strength.
Karn & Simpson expires in six months [Page 47]
DRAFT Photuris November 1995
A. Exchange Scheme List
Up-to-date values for the Exchange Schemes are specified in the most |
recent "Assigned Numbers" [RFC-1700]. Implementors wishing a number |
indicated as "unassigned" MUST request the number from IANA. Initial |
values are assigned as follows:
(0) Reserved.
(1) Implementation Optional. Elliptic curve:
curve: y^2 + xy = x^3 + 0x7338F
generator: (0x7B, 0x1C8)
irreducible polynomial: u^155 + u^62 + 1
Provides 155 bits of keying material (in 160 bits). The
cryptographic strength is currently estimated to be equivalent
to 155/2 bits. The x and y coordinates are 155 significant
bits each (310 bits total). Exponent lengths are always 155 |
bits each.
The Identification_Message and Change_Message Privacy-Method is |
DES-CBC-64. |
The Change_Message Validity-Method is MD5. |
Supplied by Hilarie Orman <ho@cs.arizona.edu>.
(2) Implementation Required. Modular Exponentiation using a 1024-
bit strong prime (p), expressed in hex:
97f6 4261 cab5 05dd 2828 e13f 1d68 b6d3
dbd0 f313 047f 40e8 56da 58cb 13b8 a1bf
2b78 3a4c 6d59 d5f9 2afc 6cff 3d69 3f78
b23d 4f31 60a9 502e 3efa f7ab 5e1a d5a6
5e55 4313 828d a83b 9ff2 d941 dee9 5689
fada ea09 36ad df19 71fe 635b 20af 4703
6460 3c2d e059 f54b 650a d8fa 0cf7 0121
c747 99d7 5871 32be 9b99 9bb9 b787 e8ab
The recommended generator (g) for this prime is 2.
Provides 1024 bits of keying material. The cryptographic
strength is currently estimated to be equivalent to 86 bits
(pessimistic) through 98 bits (optimistic). Exponent lengths
of 196 to 256 bits are recommended. *
Karn & Simpson expires in six months [Page 48]
DRAFT Photuris November 1995
The Identification_Message and Change_Message Privacy-Method is |
DES-CBC-64.
The Change_Message Validity-Method is MD5. |
This prime modulus was randomly generated by a freely available |
program written by Phil Karn, verified using the |
mpz_probab_prime() function Miller-Rabin test in the Gnu Math |
Package (GMP) version 1.3.2; and also verified with GMP on |
other platforms by Wei Dai and Frank A Stevenson, as well as |
independently developed test libraries by Eric Young (Miller- |
Rabin test), and Rich Schroeppel (complete Elliptic Curve |
test). |
(3) Moduli-indices of 3 to 8 are currently reserved, and are |
specified in a companion document. |
(9) Moduli-indices of 9 to 255 are currently unassigned.
(256) Moduli-indices of 256 to 65535 are available for cooperating |
parties to indicate private schemes.
Karn & Simpson expires in six months [Page 49]
DRAFT Photuris November 1995
B. Attribute List
Up-to-date values for the Attribute Type are specified in the most |
recent "Assigned Numbers" [RFC-1700]. Implementors wishing a number
indicated as "unassigned" MUST request the number from IANA. Initial
values are assigned as follows:
A I Type |
0 padding |
1 reserved |
+ + 2 MD2 |
3 reserved |
+ + 4 MD4 |
* * 5 MD5 |
+ + 6 SHA |
7-11 unassigned (hashing) |
+ 12 RC2 |
13 reserved |
+ 14 RC4 |
+ 15 RC5 |
+ 16 DES-CBC, 0-bit IV |
* 17 DES-CBC, 32-bit IV |
* 18 DES-CBC, 64-bit IV |
19 reserved |
+ 20 Triple DES-CBC, 0-bit IV |
+ 21 Triple DES-CBC, 32-bit IV |
+ 22 Triple DES-CBC, 64-bit IV |
23 reserved |
+ 24 IDEA |
+ 25 DSS |
+ 26 PKCS |
+ 27 DNS-SIG certificate |
+ 28 PGP certificate |
+ 29 X.509 certificate chain |
30-31 unassigned (certificates) |
+ 32 Sensitivity Label |
+ 33 VJ Header Compression |
+ 34 LZ77 |
+ 35 Stac LZS |
+ 36 AH-Sequence |
37-254 unassigned |
x x 255 Organizational |
A Initiator/Responder Attribute-Choice |
I Identity-Choice |
* mandatory algorithm must be supported |
+ feature must be supported |
when algorithm optionally supported |
Karn & Simpson expires in six months [Page 50]
DRAFT Photuris November 1995
x feature may be supported |
when algorithm optionally supported |
Attributes that are required to be supported are included in this
document. Other attributes are specified in companion documents. |
B.1. Padding |
+-+-+-+-+-+-+-+-+
| Type |
+-+-+-+-+-+-+-+-+
Type 0 |
Each attribute may have value fields that are multiple octets. To
facilitate processing efficiency, these fields are aligned on |
integral modulo 8 octet (64-bit) boundaries.
Padding is accomplished by insertion of 1 to 7 Type 0 padding |
octets before the attribute that needs alignment.
No padding is used after the final attribute in a list.
B.2. MD5 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 5 |
Length 0
The selected Exchange Scheme SHOULD provide at least 64-bits of |
cryptographic strength.
Attribute-Choice *
When selected as an Initiator or Responder Attribute-Choice,
pursuant to [RFC-1828], MD5 is also used as the key generation |
cryptographic hash for generating the SPI session-key. All |
128-bits of the generated hash are used for the key.
Karn & Simpson expires in six months [Page 51]
DRAFT Photuris November 1995
Identity-Choice |
When selected as an Identity-Choice, the resulting Verification field
is 128-bits (18 octets including Size).
The MD5 hash is calculated as described in "Identity Verification".
The authentication secret-key (as specified) is selected based on the
contents of the Identification field.
The Identification field contains a variable precision number. Valid
Identifications and secret-keys are preconfigured by the parties.
There is no required format or content for the Identification value.
The value may be a number or string of any kind.
Typically, the Identification is a user name, a Fully Qualified
Domain Name, or an email address which contains a user name and a
domain name. Examples include:
user
node.site.
user@node.site.
rcmd@node.site.
There is no requirement that the domain name match any of the
particular IP addresses in use by the parties.
Validity-Method |
When selected as a Validity-Method, the resulting Verification field |
is 128-bits (18 octets including Size).
The hash is calculated as described in "Change Verification". The
leading shared-secret is not padded to any particular alignment.
B.3. DES-CBC
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 16, 17 or 18 |
Length 0
The selected Exchange Scheme SHOULD provide at least 56-bits of |
Karn & Simpson expires in six months [Page 52]
DRAFT Photuris November 1995
cryptographic strength. |
Attribute-Choice
When selected as an Initiator or Responder Attribute-Choice,
pursuant to [RFC-1829], MD5 is used as the key generation |
cryptographic hash for generating the SPI session-key. The most |
significant 64-bits of the generated hash are used for the key. |
The least significant bit of each octet is ignored (or set to |
parity).
Privacy-Method |
When selected as a Privacy-Method, MD5 is used as the key generation |
cryptographic hash for generating the privacy session-key. The most |
significant 64-bits of the generated hash are used for the key. The |
least significant bit of each octet is ignored (or set to parity).
The 64-bit Initialization Vector (IV) is set to the Type, LifeTime, *
and SPI fields. Encryption begins with the next field, and continues
to the end of the data indicated by the UDP Length.
Karn & Simpson expires in six months [Page 53]
DRAFT Photuris November 1995
B.4. Organizational
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | OUI
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... | Kind | Value(s) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 255
Length >= 4 |
When the Length is four, no Value(s) field is |
present.
OUI three octets. The vendor's Organizationally Unique
Identifier, assigned by IEEE 802 (see [RFC-1700] for
contact details). The bits within the octet are in
canonical order, and the most significant octet is
transmitted first.
Kind one octet. Indicates a sub-type for the OUI. There
is no standardization for this field. Each OUI
implements its own values.
Value(s) Zero or more octets. The details are implementation |
specific.
Some implementors might not need or want to publish their proprietary
algorithms and attributes. This OUI mechanism is available to
specify these without encumbering the IANA with proprietary number
requests.
Karn & Simpson expires in six months [Page 54]
DRAFT Photuris November 1995
C. Scenarios
Herewith are a few simplified examples of exchanges. These are not
intended to be overly inclusive. There are opportunities for
optimization, many of which are indicated in Implementation Notes in
the main text.
C.1. Virtual Private Network
An administrator has two networks, and wishes all traffic between
them to be encrypted. The boundary routers are 1.0.0.1 and 2.0.0.2.
The commands to tunnel and encrypt are added. In the former router,
route addp 2.0.0.0/8 tunnel 2.0.0.2
secure 2.0.0.2
and in the latter router,
route addp 1.0.0.0/8 tunnel 1.0.0.1
secure 1.0.0.1
In each router, the lazy administrator uses the same username and
password,
user "root" "abracadabra"
When the first datagram arrives at router 1.0.0.1 destined for node
2.2.2.2, the routing table indicates that it should be encapsulated
for 2.0.0.2. However, the more specific routing table entry for
2.0.0.2 indicates that security is required. None is currently
available, so the encapsulated datagram is tossed in the bit-bucket,
and a Photuris exchange is initiated instead.
Router 1.0.0.1 selects a random number (0x1111) and a UDP port
(4097). It hashes these together with its IP Source and the IP
Destination, and saves the hash value as its unique cookie for this
exchange. A Cookie_Request is sent to 2.0.0.2 at UDP Port 468
containing the Initiator_Cookie.
Router 2.0.0.2 also generates a random number (0x2222) and saves it
in a global variable.
When Router 2.0.0.2 receives the Cookie_Request, it hashes the UDP
Ports (4097 and 468) together with its IP Source, the IP Destination,
and its global random number, and sends a Cookie_Response. The reply
also indicates that only Exchange Scheme 2 is offered.
Karn & Simpson expires in six months [Page 55]
DRAFT Photuris November 1995
When Router 1.0.0.1 receives (and validates) the Cookie_Response, it
generates a second random number with enough bits of strength for the
strongest attributes supported (default 128-bits: 0x1111...1111), and |
calculates an Exchange-Value for Exchange Scheme 2. An
Exchange_Request is sent to 2.0.0.2 containing the scheme and value,
and offering three attributes (MD5, DES-CBC-32, DES-CBC-64).
When Router 2.0.0.2 receives (and validates) the Exchange_Request, it
also generates a second random number with enough bits of strength
for the strongest attributes supported (default 128-bits:
0x2222...2222), calculates an Exchange-Value for Exchange Scheme 2, |
and sends an Exchange_Response. The reply also indicates that the
next steps will be encrypted with DES-CBC-64, as well as offering
three attributes (MD5, DES-CBC-32, DES-CBC-64).
Router 2.0.0.2 begins calculating the shared-secret.
When Router 1.0.0.1 receives (and validates) the Exchange_Response,
it also calculates the shared-secret.
When Router 1.0.0.1 finishes calculating the shared-secret, it
generates a third random number for its SPI (0x12345678), and
indicates an appropriate LifeTime (300 seconds).
It chooses a set of Attributes (MD5 and DES-CBC-32). Based on its |
Identity-Choice (MD5), Identification (root), and the secret
(abracadabra), it calculates a hash over the many values exchanged,
and stores this in the Verification field.
Then, the Type through SPI fields are used as an Initialization
Vector, and DES-CBC-64 is used to encrypt the remainder of the
message after the SPI. The resulting Identification_Message is sent
to 2.0.0.2.
When Router 2.0.0.2 receives (and validates) the
Identification_Message, and finishes calculating the shared-secret,
it also generates a third random number for its SPI (0xdeadbeef),
indicates an appropriate LifeTime (600 seconds), and performs the
same functions described above.
In this example, the Identification and secret-key are the same for
both parties. 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, the calculation order of the fields
is different and the verification result will have a very high
probability of difference.
Karn & Simpson expires in six months [Page 56]
DRAFT Photuris November 1995
C.2. 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 3.0.0.3.
Each user adds commands to tunnel and authenticate.
route addp 3.0.0.0/8 tunnel 3.0.0.3
secure 3.0.0.3 authenticate-only
The administrator gives each user a different username and password,
together with a separate username and password for the router. (A
lazy administrator can simply give one username and password to all
users for both the user and the router, as described previously.)
user "wanderer" "FalDaRee"
user "router" "FalDaRah"
The mobile host is assigned a temporary local IP address (4.0.0.4).
When the first datagram is generated destined for node 3.3.3.3, the
routing table indicates that it should be encapsulated for 3.0.0.3.
However, the more specific routing table entry for 3.0.0.3 indicates
that authentication is required. None is currently available, so the
encapsulated datagram is tossed in the bit-bucket, and a Photuris
exchange is initiated instead.
The Photuris exchange proceeds as previously described, except that
two Identifications are involved. Router 3.0.0.3 may be configured
with all the usernames permitted, or more likely will access an
external database of usernames and passwords using a mechanism such
as RADIUS.
Even though router 3.0.0.3 includes DES-CBC-32 in its Attribute-
Choices, the mobile node configuration does not require that every
datagram be encrypted. That is, the specific policy of this mobile
node is that an AH be added whenever traffic is sent to 3.0.0.3, but
that no ESP is used.
In this example, the boundary router has no configured policy with
respect to the mobile node. This would be difficult, as the actual
IP address assignment is unpredictable.
However, a serendipitous SPI has been created by the mobile node.
When the router prepares to forward a datagram from inside net 3, it
will discover that the routing table entry for 4.0.0.4 includes a
security association. The router implementor could legitimately use
Karn & Simpson expires in six months [Page 57]
DRAFT Photuris November 1995
that SPI for AH and/or ESP in the absence of contrary policy
configuration.
Therefore, the mobile node must be capable of receiving encapsulated
authenticated and/or encrypted traffic using its SPI. It must also
be capable of receiving unauthenticated and unencrypted traffic.
C.3. Automated Firewall Bypass
Although the previous examples may be adequate in early stages of
deployment, where many nodes have not yet been upgraded to include
Internet Security, the "ultimate goal" is direct IP connectivity.
This is particularly required when both nodes are mobile, and there
are no fixed well-known routers between them. This will also reduce
configuration, and facilitate network renumbering.
Instead of relying on an administrator, the users are empowered to
select their own usernames and passwords. They may change them at
any time with the standard tools provided by their operating systems.
(A lazy user can simply have one username and password on all
systems, as described previously.)
user "myself@LapTop.WhereEver" "o,WabM,"
user "myself@DeskTop.WhereEver" "o,WabD!"
The user may find it convenient (or be required by the operating
system) to use a more formal naming syntax (above), simply to keep
the many accessible systems separate.
When the LapTop is attempting to access the DeskTop, it may be
obstructed by an intervening router acting as a firewall. This is
indicated by the ICMP message Destination Unreachable: Communication
Administratively Prohibited (Type 3, Code 13).
The Photuris exchange proceeds as previously described, except that
rather than sending to the firewall, the exchange is attempted to the
actual target system first. This will work when the firewall is
capable of passing Photuris datagrams, as well as AH and ESP
protected datagrams.
If the implementation continues to receive ICMP messages in response
to the Cookie_Request, it should abandon the exchange and attempt a
new Photuris exchange with the intervening router instead. This may
be problematic when the router does not have a public-key form of
signature available, as by definition the user has not configured the
presence of this router. Such a mechanism is outside the scope of
this document.
Karn & Simpson expires in six months [Page 58]
DRAFT Photuris November 1995
C.4. Multi-User Access Control
In certain extremely security conscious environments, all valid users
sharing a node might not have the same level of authorization. Each
user or process requires its own identification and access contol.
These "Multi-Level Secure" environments require two or more Photuris
exchanges.
These exchanges involve 4 or more entities:
user "myself@SomeWhere" "mairsydotes"
user "system@SomeWhere" "doesydotes"
user "myself@ElseWhere" "liddellamsydivy"
user "system@ElseWhere" "kiddellydivy"
The first Photuris exchange might be triggered when sending a
datagram from SomeWhere to ElseWhere. The Initiator would naturally
use the Identification which is associated with the triggering
transport-level process (myself@SomeWhere).
Because Photuris is a network-level session key management protocol,
the Responder Identification will be associated with the operating
system which is processing the network-level exchange
(system@ElseWhere). Thus, the resulting SPI could be described as
myself-system-SPI:ElseWhere.
Note that myself-system-SPI:ElseWhere was authenticated by the
Initiator. Since authentication policy is in the receiver direction,
when datagrams arrive at ElseWhere with the given SPI, the access
policy will be dependent on the Initiator Identification.
Likewise, when datagrams arrive at SomeWhere with system-myself-
SPI:SomeWhere, the access policy will be dependent on the Responder
Identification. This may not match the authorization level required
for access. This is indicated by the ICMP message Security Failures:
Need Authorization (Type 40, Code 5).
The Responder in turn becomes the Initiator in a second exchange, and
delivers the Identification which is associated with the triggering
transport-level process (myself@ElseWhere), creating myself-system-
SPI:SomeWhere. The peer might use its system-wide Identification
(system@SomeWhere), creating system-myself-SPI:ElseWhere.
When repetitive Photuris exchanges occur between the same parties, |
and the Exchange-Values are discovered to be unchanged, the cached
shared-secret can be used to rapidly generate new session-keys.
Only myself-system-SPI:SomeWhere and myself-system-SPI:ElseWhere
Karn & Simpson expires in six months [Page 59]
DRAFT Photuris November 1995
provide user-level keying in each direction. Depending on policy,
these might be the only SPIs used.
C.5. Long LifeTime SPIs |
In certain minimalist security environments, it may be desirable to |
reduce the number of Photuris Exchanges. |
For example, repeated crashes of servers will normally cause the |
clients to re-initiate a Photuris Exchange as the system reboots and |
comes on-line. When thousands of nodes are involved, the Photuris |
Exchanges may consume a large amount of computation, effectively |
lengthening the outage. |
In addition, when perfect forward secrecy is not needed (such as |
authenticated firewall traversal without encryption), there is no |
requirement that the SPI session-key LifeTime be kept short, or |
limited to a certain amount of data. |
Instead, the SPI session-keys may be cached in long-term storage. |
The size of the Photuris SPI LifeTime is sufficient for weeks or |
months of long-term storage (2**24-1 seconds is about half a year). |
This depends upon a design feature of Photuris. The SPI LifeTime is |
not related to the Photuris Exchange LifeTime. That is, any |
particular SPI session-key can be stored for long periods of time |
without compromising other uses of the same calculated shared-secret. |
Implementation Notes: |
The careful reader will undoubtedly notice that this scenario is |
somewhat tongue-in-cheek. Repeated crashes of servers might be |
fixed by changing to a different vendor.... |
The shared-secrets MUST NOT be cached to long-term storage. |
Encryption session-keys MUST NOT be cached to long-term storage. |
Karn & Simpson expires in six months [Page 60]
DRAFT Photuris November 1995
Security Considerations
Security issues are the primary topic of this memo.
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].
Each Photuris exchange generates a calculated shared-secret. The
strength of the shared-secret is essential to the strength of the
Security Associations. Discovery of the underlying shared-secret
would compromise the Security Associations relying upon it.
Photuris generation of session-keys involves a cryptographic hash
over the shared-secret. The shared-secret is itself only indirectly
used for creating those keys that actually protect session traffic.
This protects the shared-secret from discovery, and allows repeated
use of the shared-secret for generating multiple session-keys.
Discovery of one such key should not reveal related session-keys.
This use of the calculated shared-secret, for message integrity, for
privacy, and for creating multiple session-keys by hashing with a new
SPI, substantially depends on the quality of the chosen cryptographic
hashing function(s) that generate the keys. This is mitigated by |
carefully organized differences in calculation of the integrity,
privacy, and SPI session-keys in each direction, and the optional *
concealment of the algorithms chosen for each SPI.
When an interceptor can modify or substitute another SPI, alteration
of the SPI will interrupt communication, but the attacker will gain
no additional information.
The Verification method must not allow "message recovery", to prevent
determination of the shared-secret or any long-term distributed
secret-key (where applicable). More specifically, it should not be
feasible to compute any of the bits of an authenticated message from
the verification value.
In general, where a secret (such as the shared-secret or session-
Karn & Simpson expires in six months [Page 61]
DRAFT Photuris November 1995
keys) is involved in any calculation, the algorithms selected should
not reveal information about the secret, either directly or
indirectly.
The modular exponentiation, elliptic curve, and key generation
algorithms provide a differing number of bits of keying material. It
is important to distinguish the characteristics of these bits.
A. The length of the shared-secret depends on the modulus or field
size.
B. The strength of the shared-secret depends on the modulus or field
selection, and the minimum exponent length used by either party.
C. The length of the generated keying material depends on the details
of the key generation algorithm.
D. The strength of the generated keying material depends on the
strength of the shared-secret, and is limited by the length of the
generated keying material itself.
E. The length of the session-key used by a transform depends on the
details of the transform.
F. The strength of the session-key used by a transform depends on the
strength of the keying material, and is limited by the length of
the session-key used by the transform itself.
The length of the shared-secret affects the difficulty of finding
hash collisions that might reveal the shared-secret. The lengths of
the shared-secret and hashing function combinations are considered
sufficient to provide greater robustness than the cryptographic
strength of the public-key exchange itself.
That is, it is likely to be easier to find the underlying shared-
secret through analysis of the Exchange-Values than to determine |
collisions for a particular generated key.
Use of exponents or a key generation algorithm that produces less
strength than required for a selected transform results in less
robust security than would otherwise be expected.
Different attributes require different levels of session-key
strength. Each party should offer exchange-schemes and use exponents |
that provide the length and strength required for the strongest
offered attribute.
Once an exchange-scheme has been selected, the implementation should
Karn & Simpson expires in six months [Page 62]
DRAFT Photuris November 1995
limit attribute choices to those which need no more than the shared-
secret strength provided.
It is the responsibility of the implementor to choose a useful set of
attributes for each Security Association, that provide the best
tradeoff of security and performance for a given application. In
general, when more than one attribute providing the same function is
offered, the strongest algorithm should be selected.
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 is principally responsible for the design of the protocol
phases, particularly the clogging defense, and initial internet
security protocol implementation experience spanning more than 4
years.
William Simpson was responsible for adding attributes, other message
types, editing and formatting. All such mistakes are his
responsibity.
This protocol was later discovered to have many elements in common
with the Station-To-Station authentication protocol, described in
[DOW92].
Paul C van Oorschot suggested signing both the public exponents and *
the session-key, to provide an authentication-only version of
identity verification.
Hilarie Orman provided text regarding elliptic curves, and extensive
review of the protocol details.
Randall Atkinson, James Hughes, Angelos Keromytis, Brian LaMacchia,
Cheryl Madson, Perry Metzger, Ron Rivest, Rich Schroeppel, and Bill |
Sommerfeld 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
Karn & Simpson expires in six months [Page 63]
DRAFT Photuris November 1995
Computer Science, 658 (1993), Springer-Verlag, 200-207.
[Diffie90]
Whitfield Diffie, "Authenticated Key Exchange and Secure
Interactive Communication", Northern Telecom, Securicom '90,
Paris France, 16 March 1990.
[DOW92] Whitfield Diffie, Paul C van Oorshot, Michael J Wiener,
"Authentication and Authenticated Key Exchanges", Designs,
Codes and Cryptography, v 2 pp 107-125, Kluwer Academic
Publishers, 1992.
[Firefly]
"Photuris" is the latin name for the firefly. "Firefly" is
in turn the name for the USA National Security
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.
[Hellman95]
Martin Hellman, personal communication.
[Menezes]
Alfred J. Menezes, "Elliptic Curve Public Key
Cryptosystems", Kluwer Academic Publishers, 1993.
[P1363] Alfred J. Menezes, Minghua Qu, and Scott A. Vanstone,
"Standard for RSA, Diffie-Hellman and Related Public Key
Cryptography", Working Draft of IEEE P1363 Standard, Oct.
30, 1994. http://www.rsa.com/pub/p1363/draft/
[Prime Commandment]
A derivation of an apocryphal quote from the usenet
sci.crypt.
[RFC-768]
Postel, J., "User Datagram Protocol", STD 6, August 1980.
[RFC-1510]
Kohl, J., Neuman, B., "The Kerberos Network Authentication
Service (V5)", September 1993.
[RFC-1700]
Reynolds, J., and Postel, J., "Assigned Numbers", STD 2,
USC/Information Sciences Institute, October 1994.
[RFC-1750]
Eastlake, Crocker & Schiller, "Randomness Recommendations
Karn & Simpson expires in six months [Page 64]
DRAFT Photuris November 1995
for Security", December 1994.
[RFC-1825]
Atkinson, R., "Security Architecture for the Internet
Protocol", Naval Research Laboratory, July 1995.
[RFC-1828]
Metzger, P., Simpson, W., "IP Authentication using Keyed
MD5", July 1995.
[RFC-1829]
Karn, P., Metzger, P., Simpson, W., "The ESP DES-CBC
Transform", July 1995.
[Schneier94]
Schneier, B., "Applied Cryptography", John Wiley & Sons, New
York, NY, 1994. ISBN 0-471-59756-2.
[Schroeppel95]
R. Schroeppel, H. Orman, S. O'Malley, and O. Spatscheck,
"Fast Key Exchange with Elliptic Curve Systems", Advances in
Cryptology -- Crypto '95, Santa Barbara, California, August
1995. ftp://ftp.cs.arizona.edu/reports/1995/TR95-03.ps
Karn & Simpson expires in six months [Page 65]
DRAFT Photuris November 1995
Author's Address(es)
Questions about this memo can also be directed to:
Phil Karn
Qualcomm, Inc.
6455 Lusk Blvd.
San Diego, California 92121-2779
karn@qualcomm.com
karn@unix.ka9q.ampr.org (prefered)
William Allen Simpson
Daydreamer
Computer Systems Consulting Services
1384 Fontaine
Madison Heights, Michigan 48071
Bill.Simpson@um.cc.umich.edu
bsimpson@MorningStar.com (prefered)
Karn & Simpson expires in six months [Page 66]
DRAFT Photuris November 1995
Table of Contents
1. Introduction .......................................... 1
1.1 Terminology ..................................... 2
1.2 Protocol Description ............................ 3
1.3 Clogging Defense ................................ 6
1.4 Traffic Anonymity ............................... 8
1.5 Security Parameters ............................. 9
1.6 User Support .................................... 10
1.7 Multicast Support ............................... 11
2. Protocol Details ...................................... 12
2.1 UDP ............................................. 12
2.2 Header Format ................................... 13
2.3 Exchange Schemes ................................ 13
2.4 Attributes ...................................... 14
2.5 Variable Precision Numbers ...................... 16
3. Cookie Exchange ....................................... 17
3.1 Cookie_Request .................................. 19
3.2 Cookie_Response ................................. 19
3.3 Cookie Generation ............................... 20
4. Value Exchange ........................................ 21
4.1 Exchange_Request ................................ 23
4.2 Exchange_Response ............................... 24
5. Identification Exchange ............................... 25
5.1 Identification_Message .......................... 27
5.2 Shared-Secret ...................................29
5.3 Privacy ......................................... 29
5.4 Identity Verification ........................... 30
5.5 Session-Key Computation ......................... 31
6. Other Message Types ................................... 33
6.1 Change_Message .................................. 33
6.1.1 Creation ........................................ 35
6.1.2 Deletion ........................................ 35
6.1.3 Modification .................................... 36
6.1.4 Privacy .........................................36
6.1.5 Integrity Verification .......................... 36
6.2 Error_Message ................................... 38
6.2.1 Implementation Error ............................ 39
6.2.2 Invalid/Expired ................................. 39
6.2.3 Resource Limit .................................. 39
6.2.4 Verification Failure ............................ 39
6.2.5 Need Attributes ................................. 40
Karn & Simpson expires in six months [Page ii]
DRAFT Photuris November 1995
7. Public Value Exchanges ................................ 41
7.1 Perfect Forward Secrecy ......................... 41
7.2 Modular Exponentiation Groups ................... 42
7.2.1 Moduli Strengths ................................ 43
7.2.2 Exponent Selection .............................. 45
7.3 Elliptic Curve Groups ........................... 45
7.3.1 Field Strengths ................................. 47
7.3.2 Exponent Selection .............................. 47
APPENDICES ................................................... 48
A. Exchange Scheme List .................................. 48
B. Attribute List ........................................ 50
B.1 Padding ......................................51
B.2 MD5 .......................................... 51
B.3 DES-CBC ......................................... 52
B.4 Organizational .................................. 54
C. Scenarios ............................................. 55
C.1 Virtual Private Network ......................... 55
C.2 Authenticated Firewall Traversal ................ 57
C.3 Automated Firewall Bypass ....................... 58
C.4 Multi-User Access Control ....................... 59
C.5 Long LifeTime SPIs ..............................60
SECURITY CONSIDERATIONS ...................................... 61
ACKNOWLEDGEMENTS ............................................. 63
REFERENCES ................................................... 63
AUTHOR'S ADDRESS .............................................66 |
Html markup produced by rfcmarkup 1.129d, available from
https://tools.ietf.org/tools/rfcmarkup/