Network Working Group W A Simpson
DayDreamer
D A Wagner
Berkeley
expires in six months April 1996
Internet Security Transform Enhancements
draft-simpson-ipsec-enhancement-00.txt
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Distribution of this memo is unlimited.
Abstract
This document describes several generic security transform enhance-
ments for the IP Security Protocols (AH and ESP).
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1. Introduction
The implementation of automated key management provides the opportu-
nity for enhancement of the basic Internet Protocol Security trans-
forms. Rapid key updates are available for better replay protection.
Larger quantities of key material are available to improve the qual-
ity of the security transforms.
These features are not fundamental to operation of Internet Security,
as they duplicate features already available through other protocol
mechanisms or more powerful transforms. However, automated key man-
agement makes these features cheaply available.
Selection of these enhancements are the domain of particular key man-
agement mechanisms. Operational policy considerations are outside
the scope of this document.
2. Replay Protection
When an adversary resends an earlier intercepted IP datagram, the
target would like to detect and discard that datagram. It is desir-
able that detection occur before computationally intensive opera-
tions, such as decryption.
Replay protection provides cryptographically secure at-most-once
datagram delivery. This is distinguished from the ordinary trans-
port-layer delivery mechanisms that would be exercised later in the
protocol processing.
Each implementation maintains a sequence number to provide replay
protection. This sequence number is unique for each security associ-
ation IP Source, Destination and SPI. Replay protection is accom-
plished by authenticating the sequence number. Secure replay protec-
tion requires that the key used for authentication be changed before
the sequence number repeats. This is made practical through auto-
mated key management.
When sending, the sequence number MUST be initialized to 1 for the
first datagram, and MUST be incremented (as an unsigned integer)
after each datagram is sent.
On receipt, the sequence number is checked against a list of previ-
ously accepted numbers from the same IP Source. There is no require-
ment that datagrams arrive in order. As each datagram arrives, the
sequence number is stored so that it won't be accepted again.
The exact algorithm is implementation dependent, but it MUST reject
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datagrams with duplicate sequence numbers, and should make a best-
effort to accept as many valid (non-duplicate) out-of-order datagrams
as possible.
2.1. AH Sequencing
A 16-bit sequence number is sent in the previously Reserved field of
the Authentication Header (AH) [RFC-1826]. The value zero MUST NOT
be sent.
The receiver validates this number within an implementation dependent
range of expected values. Any AH protected datagram that fails this
test is silently discarded.
Receipt of the value zero indicates that the range has been
exhausted, or that the sender has not correctly implemented replay
protection.
2.2. ESP Sequencing
A 32-bit sequence number is sent immediately following the SPI in the
Encapsulating Security Payload (ESP) [RFC-1827]. When an Initializa-
tion Vector (IV) is required by the transform (as in [RFC-1829]),
this sequence number is the same as the IV.
2.3. Combination
When both AH and ESP Sequencing are present, only the AH sequence
number is examined, and the ESP sequence number is not used for
replay prevention.
2.4. Implementation
A full-size (2**16 or 2**32 bit) array for storing the status of each
sequence number received is probably impractical. The following sim-
ple algorithm is one possible improvement for single IP Source traf-
fic.
A low-water mark L is maintained; arriving sequence numbers less than
(earlier than) the low-water mark are automatically rejected.
A fixed window size W is chosen, depending on storage constraints.
An array A[L..L+W-1] of size W is maintained, where each element
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maintains the state (stale or fresh) of the corresponding sequence
number.
The following steps are applied to each incoming sequence number S:
1. If S < L
discard the datagram.
2. If S < L+W
2a. If A[S] == stale
discard the datagram.
2b. Else
set A[S] = stale,
accept the datagram.
(note: S > L+W-1)
3. If A[L] == stale
Let x = L;
While A[x] == stale
Do set x = x + 1.
Let y = L;
Let L = x.
(shift the array A[] down by y-L elements in memory if necessary,
so now A[] has the new bounds L..L+W-1)
Set A[j] = fresh for y+W-1 < j < L+W.
Goto Step 1.
(note: S > L+W-1 and A[L] == fresh)
4. Let y = L;
Let L = S-W+1.
Shift the array A[] down by y-L elements in memory if necessary,
so now A[] has the new bounds L..L+W-1 (or L..S).
Set A[j] = fresh for y+W-1 < j < L+W.
Set A[S] = stale and accept the datagram.
Two invariants are maintained in this algorithm. First, all sequence
numbers S < L are stale. Second, all sequence numbers S > L+W-1 are
fresh.
Note that step 4 forgets some state information, and may cause out-
of-order datagrams that were sent earlier but received later to be
(incorrectly) judged stale and discarded. Though this algorithm may
inadvertenly reject a fresh datagram as stale, the important point is
that it will never accept a replayed datagram.
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An implementation may wish to go through with step 4 only for packets
that pass the authentication verification.
This is only one possible algorithm; implementators may choose
another, so long as it rejects replayed datagrams with duplicate
sequence numbers.
3. Secret Initialization Vector
When an Initialization Vector (IV) is specified by the transform (as
in [RFC-1829]), this IV may be enhanced by combination with a secret
value. Provision of a secret IV for each security association is
made practical through automated key management.
In most cases, a secret IV is not a requirement, as the [VK83] speci-
fied attacks are impractical for the current Internet Security trans-
forms. The existing transforms already provide that the IV is dif-
ferent for each datagram in the same security association. Choosing
a secret IV during session establishment may ensure that the result-
ing IV is more likely to be different for every security association
when there are a large number of security associations between the
same parties.
3.1. XOR
The most simple technique is to generate a secret IV of the appropri-
ate size, and XOR the secret value with the IV carried in the ESP
header.
3.2. Hash
A more robust technique is to regenerate a new IV using a one-way
hash function of a (variable length) secret key and the IV carried in
each ESP header, extracting an appropriate size IV from the result.
This provides a stronger IV that does not appear related to previous
IVs.
4. Whitening
A simple method for strengthening common block cipher transforms (as
in [RFC-1829]) involves XOR of additional secret values with each
block. Provision of these additional secret values for each security
association is made practical through automated key management.
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Indeed, so-called "whitening" is merely an application of repeated
key XOR, the most common cheap method of hiding data. It is insecure
without combination with a robust cipher.
4.1. Ciphertext
The ciphertext may be XOR'd with a secret value before output to the
datagram. This requires a secret the same size as the block. When
Cipher Block Chaining (CBC) is used, this hides the chaining values
used as an IV for the next block.
4.2. Plaintext
The plaintext may be XOR'd with a secret value before input to the
block cipher. This requires a secret the same size as the block.
The effect is similar to a secret IV applied to all the blocks.
4.3. With Pseudo-random Sequence
Another variant is to generate a pseudo-random sequence of secrets to
XOR with either the ciphertext or plaintext. This requires a (vari-
able length) secret to be used as a seed to the sequence generator.
4.4. With Hash Sequence
A more robust variant is to generate a sequence of secrets using a
one-way hash function of a (variable length) secret key and each
block chaining vector (CV), extracting an appropriate size block from
the result. This effect is similar to a secret hash IV applied to
all the blocks.
4.5. Combinations
These variants can be easily combined. For example, an XOR of a
secret with both the ciphertext and plaintext when used as a sandwich
around DES is called DES-XEX2 (DES key plus one XOR key) or DES-XEX3
(DES key plus two separate XOR keys) [KR96].
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Security Considerations
Analyses of these simple techniques are readily available in the lit-
erature. Further details may be found in [Schneier95].
The IV or CV hashing techniques are particularly useful in conjunc-
tion with replay protection, as they prevent undetectable modifica-
tion of the predictable Sequence numbers.
Acknowledgements
Much of the text and implementation details of Replay Protection were
provided by David Wagner. Additional suggestions were provided by
Steve Kent.
Special thanks to John Ioannidis for inspiration and experimentation
which began this most recent round of IP Mobility and IP Security
development. Some of the text on Replay Protection was derived from
[swIPe].
Robert Baldwin suggested the XOR protection of the IV.
Bart Preneel suggested the hash protection of the IV.
The use of "whitening" is further described in [Schneier95].
References
[KR96] Kaliski, B., and Robshaw, M., "Multiple Encryption: Weighing
Security and Performance", Dr. Dobbs Journal, January 1996.
[RFC-1826]
Atkinson, R., "IP Authentication Header", RFC-1826, Naval
Research Laboratory, July 1995.
[RFC-1827]
Atkinson, R., "IP Encapsulating Security Protocol (ESP)",
RFC-1827, Naval Research Laboratory, July 1995.
[RFC-1829]
Karn, P., Metzger, P., Simpson, W., "The ESP DES-CBC Trans-
form", July 1995.
[Schneier95]
Schneier, B., "Applied Cryptography Second Edition", John
Wiley & Sons, New York, NY, 1995. ISBN 0-471-12845-7.
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[swIPe] Ioannidis, J., and Blaze, M., "The Architecture and Imple-
mentation of Network-Layer Security Under Unix", Fourth
Usenix Security Symposium Proceedings, October 1993.
[VK83] Voydock, V.L., and Kent, S.T., "Security Mechanisms in High-
level Networks", ACM Computing Surveys, Vol. 15, No. 2, June
1983.
Contacts
Comments should be submitted to the ipsec-dev@terisa.com mailing
list.
Questions about this memo can also be directed to:
William Allen Simpson
Daydreamer
Computer Systems Consulting Services
1384 Fontaine
Madison Heights, Michigan 48071
wsimpson@UMich.edu
wsimpson@GreenDragon.com (preferred)
bsimpson@MorningStar.com
David A Wagner
Computer Science Department
University of California
Berkeley, California 94720
daw@cs.berkeley.edu
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