One document matched: draft-ietf-msec-ipsec-signatures-03.txt
Differences from draft-ietf-msec-ipsec-signatures-02.txt
Internet Engineering Task Force Brian Weis
INTERNET-DRAFT Cisco Systems
Document: draft-ietf-msec-ipsec-signatures-03.txt November, 2004
Expires: May, 2005
The Use of RSA Signatures within ESP and AH
Status of this Memo
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and any of which I become aware will be disclosed, in accordance with
RFC 3668.
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Abstract
This memo describes the use of the RSA Digital Signature algorithm as
an authentication algorithm within the revised IP Encapsulating
Security Payload (ESP) as described in RFC XXXX and the revised IP
Authentication Header (AH) as described in RFC YYYY. The use of a
digital signature algorithm, such as RSA, provides data origin
authentication in applications when a secret key method (e.g., HMAC)
does not provide this property. One example is the use of ESP and AH
to authenticate the sender of an IP multicast packet.
-- Note to RFC Editor: Please replace RFC XXXX with the RFC
-- number that is assigned to draft-ietf-ipsec-esp-v3 and
-- replace RFC YYYY with the RFC number assigned to
-- draft-ietf-ipsec-rfc2402bis. Please also modify normative
-- references [ESP] and [AH] that point to these drafts with
-- their respective RFC numbers. Lastly, informative reference
-- [IKEV2] should be changed to its assigned RFC number, assuming
-- it is published before this document.
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Table of Contents
1.0 Introduction......................................................2
2.0 Algorithm and Mode................................................3
2.1 Key size discussion.............................................4
3.0 Performance.......................................................5
4.0 Interaction with the ESP Cipher Mechanism.........................6
5.0 Key Management Considerations.....................................6
6.0 Security Considerations...........................................6
6.1 Eavesdropping...................................................7
6.2 Replay..........................................................7
6.3 Message Insertion...............................................7
6.4 Deletion........................................................7
6.5 Modification....................................................7
6.6 Man in the middle...............................................7
6.7 Denial of Service...............................................8
7.0 IANA Considerations...............................................8
8.0 Acknowledgements..................................................8
9.0 References........................................................8
9.1 Normative References............................................8
9.2 Informative References..........................................9
Authors Address.......................................................9
Full Copyright Statement..............................................9
1.0 Introduction
Encapsulating Security Payload (ESP) [ESP] and Authentication Header
(AH) [AH] headers can be used to protect both unicast traffic and
group (e.g., IPv4 and IPv6 multicast) traffic. When unicast traffic
is protected between a pair of entities, HMAC transforms (such as
[HMAC-SHA]) are sufficient to prove data origin authentication. An
HMAC is sufficient protection in that scenario because only the two
entities involved in the communication have access to the key, and
proof-of-possession of the key in the HMAC construct authenticates
the sender. However when ESP and AH authenticate group traffic, this
property no longer holds because all group members share the single
HMAC key. In the group case the identity of the sender is not
uniquely established, since any of the key holders has the ability to
form the HMAC transform. Although the HMAC transform establishes a
group-level security property, data origin authentication is not
achieved.
Some group applications require true data origin authentication,
where one group member cannot successfully impersonate another group
member. The use of asymmetric digital signature algorithms, such as
RSA, can provide true data origin authentication.
With asymmetric algorithms, the sender generates a pair of keys, one
of which is never shared (called the "private key") and one of which
is distributed to other group members (called the "public key"). When
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the private key is used to sign the output of a cryptographic hash
algorithm, the result is called a "digital signature". A receiver of
the digital signature uses the public key, the signature value, and
the hash to determine whether or not the claimed origin of the packet
is correct.
This memo describes how RSA digital signatures can be applied as an
ESP and AH authentication mechanism to provide data origin
authentication.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT","SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in [RFC2119].
2.0 Algorithm and Mode
The RSA Public Key Algorithm [RSA] is a widely deployed public key
algorithm commonly used for digital signatures. Compared to other
public key algorithms, signature verification is relatively
efficient. This property is useful for groups where receivers may
have limited processing capabilities. The RSA algorithm is commonly
supported in hardware.
Several schemes for the RSA algorithm are described in [RSA]. Two
schemes (RSASSA-PKCS1-v1.5 and RSASSA-PSS) combine the generation of
a hash from a message, and the signing of that hash. However, this
combination of cryptographic operations is not always appropriate for
IPsec, where a variety of hardware and software modules may be used.
In addition, one signature method (RSASSA-PPKCS1-v1.5) encodes the
hash type into the signature data block, and this encoding is not
necessary because the hash algorithm is pre-determined in IPsec.
The RSAES-OAEP raw RSA scheme [RSA, Section 7.1] MUST be used as the
encryption scheme. As recommended in [RSA, Section B.1], SHA-1 MUST
be used as the signature hash algorithm both as the message to be
encrypted by the RSA algorithm, and as the encoding parameter for the
OAEP encoding. The value of parameter string L MUST be the default,
which is a SHA-1 hash of an empty string (SHA-1("")). The mask
generation function MUST be MGF1 as defined in [RSA, Section B.2.1].
The distribution mechanism of the RSA public key and its replacement
interval are a local policy matter. The use of an ephemeral key pair
with a lifetime of the ESP or AH SA is RECOMMENDED. This recommended
policy reduces the exposure of the RSA private key to the lifetime of
the data being signed by the private key. Also, this obviates the
need to revoke or transmit the validity period of the key pair.
The size of the RSA modulus MUST be at least 496 bits. This
restriction is a function of the size of the SHA-1 hash and the
number of bits needed for OAEP encapsulation. (For more information,
see [RSA, Section 7.1].)
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2.1 Key size discussion
The choice of RSA modulus size must be made carefully. If too small
of a modulus size is chosen, an attacker may be able to reconstruct
the private key used to sign packets before the key is no longer
used by the sender to sign packets. This order of events may result
in the data origin authentication property being compromised.
However, choosing a modulus size larger than necessary will result
in an unnecessarily high cost of CPU cycles for the sender and all
receivers of the packet.
Recent guidance on key sizes make estimates as to the amount of
effort an attacker would need to expend in order to reconstruct an
RSA private key. Section 2.1 of RFC 3766 [PK-STRENGTHS] suggests
that an attacker may need 8000 MIPS years (MYs) to factor a 512 bit
modulus, based on a previously factored 512 bit value [RSA155].
(However, factoring a 528 bit modulus was subsequently found to take
only 3000 MYs, so this smaller value is used as a more conservative
starting point for determining the difficulty of factoring a 512 bit
modulus.) It should be assumed that an attacker could harness 1
million PCs with state-of-the-art processors for the sieving phase
of Number Field Sieve (NFS) processing, and that a sufficiently fast
processor with substantial memory is available for the matrix
reduction phase. Assuming half of the processing applies to each
phase, this results in a lifetime of about 5 days. However, since
the formulae used to compute this value are empirical, a more
conservative value of 1 hour is chosen as a recommended maximum
lifetime for a key pair with a 512 bit modulus.
A cost based security analysis of key lengths [COST] suggests an
extrapolation of other RSA private key sizes from a private key with
a 512 bit modulus. For example, a table in [COST] shows that the
real time needed to complete NFS on a 576 bit modulus should be
about 10.9 times as long as a 512 bit modulus. This table was used
to extrapolate the key lifetime values below for values up to 796
bit keys. (Although this extrapolation can be applied to 1024 bit
keys and higher, as a precaution against over-optimism we do not
propose they be used for more than one year.)
Table 1 summarizes the above discussion regarding the maximum length
of time that selected modulus sizes should be used.
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Number of Recommended Maximum
Modulus Bits Lifetime
------------ -------------------
496-512 1 hour
576 10 hours
640 4 days
704 30 days
796 8 months
1024 1 year
Table 1. RSA Key Use Lifetime Recommendations
3.0 Performance
The RSA asymmetric key algorithm is very costly in terms of
processing time compared to the HMAC algorithms. However, processing
cost is decreasing over time. Faster general-purpose processors are
being deployed, faster software implementations are being developed,
and hardware acceleration support for the algorithm is becoming more
prevalent. However, care should always be taken that RSA signatures
are not used for applications that expect to have bandwidth
requirements that would be adversely affected.
The RSA asymmetric key algorithm is best suited to protect network
traffic for which:
o The sender has a substantial amount of processing power, whereas
receivers are not guaranteed to have substantial processing
power, and
o The network traffic is small enough that adding a relatively
large authentication tag (in the range of 62 to 256 bytes) does
not cause packet fragmentation.
RSA key pair generation and signing are substantially more expensive
operations than signature verification, but these are isolated to the
sender.
The size of the RSA modulus can affect the processing required to
create and verify RSA digital signatures. Care should be taken to
determine what the size of modulus is needed for the application.
Smaller modulus sizes may be chosen as long as the network traffic
protected by the private key flows for less time than it is estimated
that an attacker would take to discover the private key. This
lifetime is considerably smaller than most public key applications
that store the signed data for a period of time. But since the
digital signature is used only for sender verification purposes, a
modulus that is considered weak in another context may be
satisfactory.
The size of the RSA public exponent can affect the processing
required to verify RSA digital signatures. Low-exponent RSA
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signatures may result in a lower verification processing cost. At the
time of this writing, no attacks are known against low-exponent RSA
signatures that would allow an attacker to create a valid signature
using the RSAES-OAEP raw RSA scheme.
The addition of a digital signature as an authentication tag adds a
significant number of bytes to the packet. This increases the
likelihood that the packet encapsulated in ESP or AH may be
fragmented.
4.0 Interaction with the ESP Cipher Mechanism
There are no known issues that preclude the use of the RSA
signatures algorithm with any specific cipher algorithm.
5.0 Key Management Considerations
Key management mechanisms negotiating the use of RSA Signatures MUST
include the length of the RSA modulus during policy negotiation. This
gives a device the opportunity to decline use of the algorithm. This
is especially important for devices with constrained processors that
might not be able to verify signatures using larger key sizes.
A receiver must have the RSA public key in order to verify integrity
of the packet. When used with a group key management system (e.g.,
RFC 3547 [GDOI]), the public key SHOULD be sent as part of the key
download policy. If the group has multiple senders, the public key of
each sender SHOULD be sent as part of the key download policy.
Use of this transform to obtain data origin authentication for
pairwise SAs is NOT RECOMMENDED. In the case of pairwise SAs (such as
negotiated by the Internet Key Exchange [IKEv2]), data origin
authentication can be achieved with an HMAC transform. Because the
performance impact of an RSA signature is typically greater than an
HMAC, the value of using this transform for a pairwise connection is
limited.
6.0 Security Considerations
This document provides a method of authentication for ESP and AH
using digital signatures. This feature provides the following
protections:
o Message modification integrity. The digital signature allows the
receiver of the message to verify that it was exactly the same as
when the sender signed it.
o Host authentication. The asymmetric nature of the RSA public key
algorithm allows the sender to be uniquely verified, even when
the message is sent to a group.
Non-repudiation is not claimed as a property of this transform. At
times, the property of non-repudiation may be applied to digital
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signatures on application level objects (e.g., electronic mail).
However, this document describes a means of authenticating network
level objects (i.e., IP packets), which are ephemeral and not
directly correlated to any application. Non-repudiation is not
applicable to network level objects (i.e., IP packets).
A number of attacks are suggested by [RFC3552]. The following
sections describe the risks those attacks present when RSA signatures
are used for ESP and AH packet authentication.
6.1 Eavesdropping
This document does not address confidentiality. That function, if
desired, must be addressed by an ESP cipher that is used with the
RSA Signatures authentication method. The RSA signature itself does
not need to be protected from an eavesdropper.
6.2 Replay
This document does not address replay attacks. That function, if
desired, is addressed through use of ESP and AH sequence numbers as
defined in [ESP] and [AH].
6.3 Message Insertion
This document directly addresses message insertion attacks. Inserted
messages will fail authentication and be dropped by the receiver.
6.4 Deletion
This document does not address deletion attacks. It is only
concerned with validating the legitimacy of messages that are not
deleted.
6.5 Modification
This document directly addresses message modification attacks.
Modified messages will fail authentication and be dropped by the
receiver.
6.6 Man in the middle
As long as a receiver is given the sender RSA public key in a
trusted manner (e.g., by a key management protocol), it will be able
to verify that the digital signature is correct. A man in the middle
will not be able to spoof the actual sender unless it acquires the
RSA private key through some means.
The RSA modulus size must be chosen carefully to ensure that the time
a man in the middle needs to determine the RSA private key through
cryptanalysis is longer than the amount of time that packets are
signed with that private key.
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6.7 Denial of Service
According to IPsec processing rules, a receiver of an ESP and AH
packet begins by looking up the Security Association in the SADB. If
one is found, the ESP or AH sequence number in the packet is
verified. No further processing will be applied to packets with an
invalid sequence number.
An attacker that sends an ESP or AH packet matching a valid SA on
the system and also having a valid sequence number will cause the
receiver to perform the ESP or AH authentication step. Because the
process of verifying an RSA digital signature consumes relatively
large amounts of processing, many such packets could lead to a
denial of service attack on the receiver.
If the message was sent to an IPv4 or IPv6 multicast group all group
members that received the packet would be under attack
simultaneously.
This attack can be mitigated against most attackers by encapsulating
ESP or AH using an RSA Signature for authentication within ESP or AH
using an HMAC transform for authentication. In this case, the HMAC
transform would be validated first, and as long as the attacker does
not possess the HMAC key no digital signatures would be evaluated on
the attacker packets. However, if the attacker does possess the HMAC
key (e.g., they are a legitimate member of the group using the SA)
then the DoS attack cannot be mitigated.
7.0 IANA Considerations
An assigned number is required in the "IPSec Authentication
Algorithm" name space in the ISAKMP registry [ISAKMP-REG]. The
mnemonic should be "SIG-RSA".
A new "IPSEC Security Association Attribute" is required in the
ISAKMP registry to pass the RSA modulus size. The attribute class
should be called "Authentication Key Length", and it should a
Variable type.
8.0 Acknowledgements
Scott Fluhrer and David McGrew provided advice regarding applicable
key sizes. Scott Fluhrer also provided advice regarding key
lifetimes.
9.0 References
9.1 Normative References
[AH] Kent, S., "IP Authentication Header", draft-ietf-ipsec-
rfc2402bis-07.txt, March 2004.
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[ESP] Kent, S., "IP Encapsulating Security Payload (ESP)", draft-
ietf-ipsec-esp-v3-08.txt, March 2004.
[ISAKMP-REG] http://www.iana.org/assignments/isakmp-registry
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Level", BCP 14, RFC 2119, March 1997.
[RFC3552] E. Rescorla, et. al., "Guidelines for Writing RFC Text on
Security Considerations", RFC 3552, July 2003.
[RSA] Jonsson, J., B. Kaliski, "Public-Key Cryptography Standard
(PKCS) #1: RSA Cryptography Specifications Version 2.1", RFC 3447,
February 2003.
9.2 Informative References
[COST] R. Silverman, "A Cost-Based Security Analysis of Symmetric and
Asymmetric Key Lengths", RSA Laboratories Bulletin, Number 13, April
2000 (Revised November 2001).
[GDOI] Baugher, M., Weis, B., Hardjono, T., and H. Harney, "The Group
Domain of Interpretation", RFC 3547, December 2002.
[HMAC-SHA] Madson, C., and R. Glenn, "The Use of HMAC-SHA-1-96 within
ESP and AH", RFC 2404, November 1998.
[IKEV2] C. Kaufman, "Internet Key Exchange (IKEv2) Protocol", draft-
ietf-ipsec-ikev2-15.txt, August 13, 2004.
[PK-STRENGTHS] Orman, H., and P. Hoffman, "Determining Strengths For
Public Keys Used For Exchanging Symmetric Keys", RFC 3766, April
2004.
[RSA155] RSA Laboratories, "RSA-155 is factored!",
http://www.rsasecurity.com/rsalabs/node.asp?id=2098.
Authors Address
Brian Weis
Cisco Systems
170 W. Tasman Drive,
San Jose, CA 95134-1706, USA
(408) 526-4796
bew@cisco.com
Full Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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