IPSEC Working Group Bill Sommerfeld
INTERNET-DRAFT Hewlett Packard
draft-ietf-ipsec-inline-isakmp-01.txt March 1997
Inline Keying within the ISAKMP Framework.
<draft-ietf-ipsec-inline-isakmp-01.txt>
STATUS OF THIS MEMO
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
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To learn the current status of any Internet-Draft, please check the
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ftp.isi.edu (US West Coast).
Distribution of this document is unlimited.
This particular Internet Draft is being published in order to focus
discussion on protocols for lightweight inline keying in the context
of ISAKMP/Oakley. It is intended that this version will raise more
questions than it answers; there many ways to do inline keying and
the author does not pretend to have all the answers. A future
version of this draft might become suitable for submission to the
IESG as a standards track protocol.
ABSTRACT
The current proposal for IP-layer key management [ISAKMP, OAKLEY,
ISAOAK] has fairly high overhead. Before a security association can
be established, at least one pair of messages need to be exchanged
between the communicating peers. For efficiency, this suggests that
ISAKMP setup should be infrequent. However, general principles of
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key management suggest that individual keys should be used as little
as practical and changed as frequently as possible. Steve Bellovin
has suggested that, ideally, different security associations should
be used for each different transport-level connection[BADESP].
This document discusses a way of structuring a protocol to permit
this to happen with minimal overhead, both in round-trip delay at
connection setup, and in bandwidth once the connection is
established.
The general concept of inline or in-band keying here was inspired by
SKIP[SKIP]. However, SKIP's approach is burdened by the addition of
an extra intermediate header of perhaps 20 to 28 bytes to every
protected packet, which doubles the bandwidth overhead of protected
traffic compared with ESP with fixed keying. In order to minimise
the per-packet overhead, an inline keying header should be used only
until the desired security association is established, at which point
the peers will fall back to pure ESP/AH.
IN-BAND SECURITY ASSOCIATION SETUP
AH and ESP security associations are identified by a 32-bit
identifier known as an SPI, which is assigned by the receiving end of
the association. This makes it difficult to create a new security
association at the request of a traffic originator without at least
one round trip.
However, most packets are sent either in response to another packet,
or in the expectation that a response will be received.
We can exploit this, by having the sender allocate an SPI for return
traffic, and include a message informing the recipient of it in the
first message. The reponse to this message can also contain a return
SPI, so further packets in the conversation can be directed to the
appropriate security association for the conversation.
There are at least two applications for this protocol. Hosts may
wish to use separate SPIs for separate transport-layer connections.
Similarly, routers doing tunnel-mode ESP may wish to establish
separate SPIs for each active host-pair they tunnel.
In the former case, for a typical TCP exchange, the in-band keying
headers would piggyback on the SYN and SYN/ACK packets, and would
then be absent from subsequent traffic.
No explicit acknowledgement of the creation of the new SPI is needed,
as use of the reply-SPI by the peer would (implicitly) acknowledge
the key change.
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No explicit retransmission of in-band-keying requests is needed; if
the upper-level protocol retransmits before the full SPI pair is
created, the inline keying header is included in the retransmission.
PACKET PROCESSING
Some additional logic needs to be added to an implementation's
outbound and inbound policy engines. As this protocol has not yet
been implemented, the following is necessarily vague; suggestions on
how to improve it are welcomed.
When sending a packet, if there isn't a valid outbound SPI we want to
use to reach the peer, and we have reason to believe the peer accepts
inline keying headers, we look up the inbound SPI we want our peer to
use (or create it if it's nonexistant) and insert an in-band keying
header in the outbound packet.
On the other hand, if there is a valid outbound SPI, we use it. If
there isn't a valid inbound SPI for a response, we create the inbound
SPI we want to receive replies on and insert an in-band-keying header
in the outbound packet, then send it using the outbound SPI.
If an inbound packet contains a valid in-band-keying header with a
reply SPI in it, we create an outbound SPI with the appropriate TTL,
and then process the rest of the packet.
PARENT SPI PARAMETERS
The parent SPI must have the following parameters specified or
negotiated:
- a shared secret
- the pseudo-random function (prf) to be used for key derivation.
- the algorithm to be used for encryption, and any algorithm-specific
parameters (other than the key).
- the algorithm to be used for message integrity protection, and any
algorithm-specific parameters (other than the key).
ESTABLISHING THE PARENT SPI.
This scheme can only "spawn" new SPIs from old SPIs; therefore, there
needs to be a way to establish the "parent" SPI.
There are several possible alternatives:
One of the ISAKMP exchanges could be used to establish the parent
SPI.
Another alternative, very similar to SKIP, would be for each system
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to publish a certificate containing a Diffie-Hellman public key, a
"well known" SPI, and the additional parent-SPI parameters listed
above. The shared secret actually used in the inline-keying protocol
would depend on the source address of the packet. It is not clear
whether this variation is of interest and is included only for
completeness.
ENCODING OPTIONS
There are a number of potential ways of encoding this protocol, with
different advantages and disadvantages; at this point, it is not
clear which way is best. The primary areas of variability include:
- how the SPI-creation message is framed
- what is contained in the SPI-creation message
- how the user payload "along for the ride" is protected.
In order to minimize complexity, in the end, there should only be one
answer for each of these questions. Several requirements on the
encoding exist:
1) the SPI-creation message must be optional
2) there should be some "cryptographic distance" between the three
kinds of keys involved:
- the key(s) used to authenticate and/or encrypt the user
payload which is along for the ride
- the key(s) used to authenticate the SPI-creation message
- the key(s) used by the newly-created reply SPI.
3) there should be as much commonality in encoding between this
protocol and the various sub-protocols of ESP and ISAKMP with similar
functionality.
4) the inline keying header should be small.
FRAMING
Three alternatives suggest themselves:
- special-case encoding within an ESP transform, perhaps using a bit
out of the pad length by mutual agreement between the communicating
peers.
- a new IP-layer protocol
- use of the "IPv6 destination options" IP-layer protocol.
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Using a new IP-layer protocol seems cleanest, though there has been
strong opposition to it from some implementors. The use of the IPv6
destination options protocol would limits the size of a single option
to no more than 255 octets, which may be a significant constraint.
REPLY-SPI-CREATION CONTENTS
There are essentially two possibilities for the reply-SPI-creation
message: either an encapsulated quick mode message, or a custom
message simpler than, but similar to quick mode.
Encapsulating ISAKMP/Oakley
A simpler, but less efficient, alternative would be to encapsulate
various ISAKMP/Oakley quick-mode messages as the reply-SPI-creation
message.
Custom Format
Notation here is derived from the notation used in the Oakley draft.
We assume the existance of a shared secret, sSPI between the
communicating parties, and a parent SPI corresponding to the shared
secret. This can be established in a number of ways, which will be
discussed later.
Fields:
reply-SPI
reply-SPI-scope
reply-SPI-lifetime
reply-SPI-nonce
reply-SPI-parameters
peer-SPI-nonce
spi-creation-fields is a set of fields which tell the receiver how to
create an outbound SPI for reponses to the payload of this datagram.
This set includes:
Reply-SPI is the number of an SPI created by the sender for responses
to this packet.
Reply-SPI-scope indicates the scope of the reply SPI; exact encoding
is TBS and should probably be similar to the way the scope is handled
in [DOI]. The scope should be able to select among (at least) four
possible scopes: host-range, host, protocol, protocol+port.
Reply-SPI-lifetime is the lifetime (exact encoding TBS), of the
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newly-created SPI. A similar encoding to the one used by [DOI]
should be used here.
reply-SPI-parameters are other parameters of the newly created SPI,
including the algorithm, and any algorithm-specific parameters. An
encoding similar to the one used by [DOI] should be used.
Peer-spi-nonce is either all zeros, or the peer's reply-SPI-nonce.
The key(s) associated with the newly-created SPI are derived from:
prf (sSPI, reply-TAG | peer-spi-nonce | reply-spi-nonce |
reply-SPI)
reply-TAG is a well-known constant, TBD.
sSPI is the shared secret associated with the parent SPI.
prf(a, b) is a pseudo-random function as in Oakley; the exact
function to use is a parameter of the parent SPI.
It may be desirable to include other fields from the header in both
of the key-derivation hashes.
KEYING OF THE USER PAYLOAD
If the "parent" algorithm is judged to be sufficently strong, the
initial user payload could simply be encrypted using the ESP in the
parent SPI. However, the use of inline keying seems to suggest a
desire to segregate different communications under different keys.
A extension to ESP suggests itself. We can negotiate the addition of
a "key generation nonce" to the cleartext part of the ESP header, and
then generate the key for encrypting the ciphertext portion of the
packet using a keyed hash of the shared secret and the nonce:
prf (sSPI, current-packet-nonce | SPI | other)
sSPI is the SPI's shared secret;
current-packet-nonce is a 128-bit random value generated by the
sender (ideally different for each packet).
"SPI" is the numeric SPI value.
"other" are any additional fields which might make sense to include
in the key-generation hash.
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MISCELLANEOUS ISSUES
There are a number of miscellaneous problems which arise as a result
of constructing a protocol of this form. None of them seem to be
particularly insurmountable. These considerations are based on the
author's experience with vaguely similar security protocols.
Interaction With MTU Discovery
Because insertion of the inline keying header will change the size of
the packet, there are likely to be interactions between this protocol
and MTU discovery when the first packet triggering the creation of a
new SA pair is near maximum size.
Several potential solutions come to mind:
1) With some cooperation between layers, it may be possible to reduce
the amount of data in the outbound packet (this may be possible for
TCP; it's almost certainly not possible for other protocols).
2) MTU discovery could be deferred, by sending the packet with DF
cleared.
Neither of these seem really satisfactory; suggestions on how to
handle this case are welcomed.
SPI expiration and clock skew.
This section perhaps belongs as an appendix to the a future version
of the Internet DOI specification [DOI].
Inline SPI's expire after a defined time. Since relative timestamps
are used in the protocol, clocks synchronized in phase are not
assumed; however, some minimal accuracy in frequency is assumed.
It is assumed here that SPI timestamps are maintained in absolute
form internally and converted to and from relative form only for use
over the network.
The explicit expiration time of the SPI indicates the time after
which the sender should no longer send to that SPI. The recipient
should still honor inbound packets to that SPI for an additional
grace period of:
K1 * MSL + K2 * nominal-lifetime
where "MSL" is the maximum lifetime of a packet in the network as in
TCP, "K1" is a fudge factor allowing some margin around MSL, and "K2"
is a fudge factor allowing for a difference in clock frequency
between systems.
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Proposed default values:
MSL: 120 (same as TCP)
K1: 2 (same as TCP)
K2: 1/32 (allow for ~3% frequency difference)
It may be useful for debugging purposes to allow these parameters to
be adjusted.
With the above constants, a nominal 10-minute SPI would be valid for
about 14 minutes; and a nominal 2-hour SPI would be valid for
approximately 2 hours and 8 minutes.
EXAMPLE
The following is a simplified example of the messages involved in
setting up a new SPI pair using inline keying. A lot of detail has
been omitted to clarify the presentation.
Assume per-connection inline keying is in use between two hosts A and
B, with a security association X existing from A to B.
A wishes to establish a TCP connection to B. Because per-connection
keying is in use, A would first create a new inbound SPI "Y", and
send:
ESP [A-to-B, inline[N1, seq 0, Y], TCP [port 4352 to 21, SYN]]
Upon receipt, B creates a new outbound SPI "Y", and a new inbound SPI
"Z", and hands off the TCP packet to its TCP. It then responds with:
ESP [Y, inline[N2, seq 0, Z], TCP [port 21 to 4352, SYN+ACK]]
Upon receipt, A creates a new outbound SPI "Z".
Receipt of a packet on spi Y acknowleges receipt of the inline-keying
message, so subsequent messages for this connection do not contain an
inline keying header, so A responds with:
ESP [Z, TCP [port 4352 to 21, ACK, data1]]
Similarly, B responds with:
ESP [Y, TCP [port 21 to 4352, ACK, data2]]
and the conversation would continue like this until the TCP
connection closed.
SECURITY CONSIDERATIONS
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This entire document concerns key management for the IP security
protocols.
This protocol does not provide for Perfect Forward Secrecy by itself,
but if used in conjunction with ISAKMP, may provide a reasonable
engineering tradeoff between security and performance.
As with Oakley's Quick Mode, this protocol can consume the entropy of
the shared secret[ISAOAK]. Implementors should take note of this fact
and be able to limit the number of inline-keying messages allowed per
shared secret. This draft does not proscribe such a limit.
REFERENCES
[BADESP] Bellovin, S., "Problem Areas for the IP Security
Protocols", Proceedings of the Sixth Usenix UNIX security
symposium, July 1996 (available from
ftp://ftp.research.att.com/dist/smb/badesp.ps)
[AH] Atkinson, R., "IP Encapsulating Security Payload (ESP)",
RFC1826, August 1995.
[DOI] Piper, D., "The Internet IP Security Domain Of
Interpretation for ISAKMP", version 1, draft-ietf-ipsec-
ipsec-doi-00.txt.
[ESP] Atkinson, R., "IP Encapsulating Security Payload (ESP)",
RFC1827, August 1995.
[IPSEC] Randall Atkinson, "Security Architecture for the Internet
Protocol", Internet Draft draft-ietf-ipsec-arch-sec-01.txt,
10 November 1996
[ISAKMP] Maughhan, D., Schertler, M., Schneider, M., and Turner, J.,
"Internet Security Association and Key Management Protocol
(ISAKMP)", version 6, draft-ietf-ipsec-isakmp-06.{ps,txt}.
[ISAOAK] Harkins, D., Carrel, D., "The resolution of ISAKMP with
Oakley", draft-ietf-ipsec-isakmp-oakley-02.txt.
[OAKLEY] Orman, H., "The Oakley Key Determination Protocol", version
1, draft-ietf-ipsec-oakley-01.txt.
[SKIP] Aziz, A., Markson, T., Prafullchandra, H., "Simple Key-
Management For Internet Protocols", draft-ietf-ipsec-
skip-07.txt.
AUTHOR'S ADDRESS:
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Bill Sommerfeld <sommerfeld@apollo.hp.com>
Hewlett Packard
300 Apollo Drive
Chelmsford MA 01824
Telephone: +1 508 436 4352
Sommerfeld Expires 1 October 1997 [Page 10]
