Internet Engineering Task Force Brian Weis
INTERNET-DRAFT Cisco Systems
Document: draft-ietf-msec-ipsec-signatures-00.txt December, 2003
Expires: June, 2004
The Use of RSA Signatures within ESP and AH
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Abstract
This memo describes the use of the RSA Signature algorithm [RSA] as
an authentication algorithm within the revised IPSEC Encapsulating
Security Payload [ESPbis] and the revised IPSEC Authentication Header
[AHbis]. The use of a digital signature algorithm such as RSA
provides data origin authentication when a secret key method (e.g.,
HMAC) cannot do so. For example, when ESP and AH are used to protect
IP multicast data flows.
Further information on the other components necessary for ESP and AH
implementations is provided by [ROADMAP].
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The Use of RSA Signatures with ESP and AH December, 2003
Table of Contents
1.0 Introduction......................................................2
2.0 Algorithm and Mode................................................3
3.0 Performance.......................................................3
4.0 Interaction with the ESP Cipher Mechanism.........................4
5.0 Key Management Considerations.....................................4
6.0 Security Considerations...........................................5
6.1 Eavesdropping...................................................5
6.2 Replay..........................................................5
6.3 Message Insertion...............................................5
6.4 Deletion........................................................5
6.5 Modification....................................................6
6.6 Man in the middle...............................................6
6.7 Denial of Service...............................................6
7.0 IANA Considerations...............................................6
8.0 Acknowledgements..................................................7
9.0 References........................................................7
9.1 Normative References............................................7
9.2 Informative References..........................................7
Authors Addresses.....................................................8
1.0 Introduction
AH and ESP payloads can be used to protect both unicast traffic and
group traffic, such as IPv4 and IPv6 multicast traffic. When unicast
traffic is protected between a pair of entities, HMAC transforms
(such as [HMAC-SHA] and [HMAC-MD5]) are sufficient to prove data
origin authentication. An HMAC is sufficient protection in that
scenario because only one other entity knows the key, and proof-of-
possession of the key in the HMAC construct authenticates the sender.
However when AH and ESP protect group traffic, this property is no
longer valid -- all group members share the single HMAC key and the
identity of the sender is not uniquely established.
Some group applications require true data origin authentication,
where one group member cannot successfully impersonate another group
member. The use of asymmetric encryption algorithms, such as RSA, can
provide true data origin authentication.
Witj asymmetric encryption 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 the private key is used to encrypt the output of a
cryptographic hash algorithm, the cipher text is called a "digital
signature". A receiver of the digital signature uses the public key
to decrypt the hash, which is then compared to a hash generated by
the reveiver. If the hashes match, then the receiver has confidence
in the claimed origin of the packet.
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This memo describes how RSA digital signatures can be applied as an
authentication mechanism to AH and ESP payloads 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, and is not encumbered by intellectual property
claims.
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 P MUST be the default,
which is the hash of an empty string. The mask generation function
MUST be MGF1 as defined in [RSA, Section B.2.1].
Ephemeral RSA key pairs generated for use during the lifetime of an
SA may be generated. Doing so reduces the amount of time the RSA
private key will be used, which may allow for a use of shorter keys.
Note that relatively short key sizes are acceptable because the
security model for short-live RSA authentication keys is different
from the usual model.
The size of the RSA modulus used can vary, but 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].)
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,
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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 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 61 to 256 bytes) does
not cause packet fragmentation.
Both key pair generation and encryption (or signing) are
substantially more expensive operations, 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 key is needed for the application.
Smaller key 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,
so a key that is normally considered weak may be satisfactory. (If
the RSA public key algorithm were used to encrypt the packet, then
the lifetime would be substantially longer.)
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 AH or ESP 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
A key management mechanism 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, which is especially
important for devices with constrained processors that might not be
able to handle the verification of signatures using larger key sizes.
A receiver must have the RSA public key in order to decrypt 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
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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 with a pair-wise key management system such as
the Internet Key Exchange [IKEv2] is NOT RECOMMENDED. The value of
using this transform for a pair-wise connection is limited,
considering the performance impact.
6.0 Security Considerations
This document provides a method of authentication for AH and ESP
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.
As suggested by [RFC3552], the following sections describe various
attacks that could be deployed against using RSA signatures as a
means of 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 in any way.
6.2 Replay
This document does not address replay attacks. That function, if
desired, is addressed through use of AH and ESP sequence numbers as
defined in [AH] and [ESP].
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.
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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, 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.
RSA modulus sizes must be chosen carefully to ensure that the time it
takes a man in the middle to determine the RSA private key through
cryptanalysis is longer than the amount of time that packets are
signed with that private key.
6.7 Denial of Service
A receiver of an ESP and AH packet starts by looking up the Security
Association in the SADB. If one is found, the ESP or AH sequence
number in the packet is verified. If the sequence number falls
within the window, the authentication step is performed.
An attacker that sends an ESP or AH packet which matches a valid SA
on the system and which also has a valid sequence number will cause
the receiver to perform the AH or ESP authentication step. Because
the process of verifying an RSA digital signature consumes
relatively large amounts of processing, this could lead to a denial
of service attack on the receiver if many such packets were sent.
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 by encapsulating an ESP or AH payload
in an AH payload using an HMAC transform. In this case, the HMAC
transform would be validated first, and would indicate that a group
member rather than an attacker sent the packet.
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.
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8.0 Acknowledgements
Scott Fluhrer and David McGrew provided advice regarding applicable
key sizes.
9.0 References
9.1 Normative References
[AHbis] Kent, S., "IP Authentication Header", draft-ietf-ipsec-
rfc2402bis-05.txt, September 2003.
[ESPbis] Kent, S., "IP Encapsulating Security Payload (ESP)", draft-
ietf-ipsec-esp-v3-06.txt, July 2003.
[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.
[RFC2401bis] Kent, S., "Security Architecture for the Internet
Protocol", draft-ietf-ipsec-rfc2401bis-00.txt, October 2003.
[RFC3552] E. Rescorla, et. al., "Guidelines for Writing RFC Text on
Security Considerations", RFC 3552, July 2003.
[ROADMAP] Thayer, R., Doraswamy, N., and R. Glenn, "IP Security
Document Roadmap", RFC 2411, November 1998.
[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
[GDOI] M. Baugher, B. Weis, T. Hardjono, H. Harney, "The Group Domain
of Interpretation", RFC 3547, December 2002.
[HMAC-MD5] Madson, C., and R. Glenn, "The Use of HMAC-MD5-96 within
ESP and AH"", RFC 2403, November 1998.
[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-ikev2-11.txt, October 9, 2003.
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Authors Address
Brian Weis
Cisco Systems
170 W. Tasman Drive,
San Jose, CA 95134-1706, USA
(408) 526-4796
bew@cisco.com
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