Network Working Group A. Kukec
Internet-Draft University of Zagreb
Intended status: Standards Track S. Krishnan
Expires: August 16, 2010 Ericsson
S. Jiang
Huawei Technologies Co., Ltd
February 12, 2010
SEND Hash Threat Analysis
draft-ietf-csi-hash-threat-06
Abstract
This document analysis the use of hashes in SEND, possible threats
and the impact of recent attacks on hash functions used by SEND.
Current SEND specification [rfc3971] uses the SHA-1 [sha-1] hash
algorithm and X.509 certificates [rfc5280] and does not provide
support for the hash algorithm agility. The purpose of the document
is to provide analysis of possible hash threats and to decide how to
encode the hash agility support in SEND.
Status of this Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Impact of collision attacks on SEND . . . . . . . . . . . . . 6
3.1. Attacks against CGAs in stateless autoconfiguration . . . 6
3.2. Attacks against X.509 certificates in ADD process . . . . 7
3.3. Attacks against the Digital Signature in the RSA
Signature option . . . . . . . . . . . . . . . . . . . . . 8
3.4. Attacks against the Key Hash field in the RSA
Signature option . . . . . . . . . . . . . . . . . . . . . 8
4. Support for the hash agility in SEND . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.1. Normative References . . . . . . . . . . . . . . . . . . . 11
6.2. Informative References . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
SEND [rfc3971] uses the hash algorithm in different ways. It uses
the SHA-1 hash algorithm to generate Cryptographically Generated
Addresses (CGA) [rfc3972], the contents of the Key Hash field and the
Digital Signature field of the RSA Signature option. It also uses a
hash algorithm (SHA-1, MD5, etc.) within the digital signature in the
X.509 certificates [rfc5280] for the router authorization in the
Authorizaton Delegation Discovery (ADD) process.
There is a great variaty of hash function, but only MD5 and SHA-1 are
in the wide use. They both derive from MD4, which has been well
known for its weaknesses. First hash attacks affected the
compression function of MD5, while the latest hash attacks against
SHA-1 delivered colliding hash in significantlly smaller number of
rounds compared to the brute force attack number of rounds
[sha1-coll]. Attacks against X.509 certificates improved as well,
and researchers demonstrated colliding certificate with MD5 hash,
both for the same and different identities [x509-coll].
Depending on the way of use of hash algorithm, Internet protocol can
be affected by the weakness of the underlaying hash function. This
document analyzes uses of hash algorithms in SEND, possible weakness
that hash attacks could introduce to SEND, and possible ways to make
SEND resistant to such attacks.
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2. Terminology
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 RFC 2119 [rfc2119].
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3. Impact of collision attacks on SEND
Due to the hash attacks demonstrated on the aforesaid hash algorithms
a study was performed to assess the threat of these attacks on the
cryptographic hash usage in internet protocols [RFC4270]. This
document analyzes the hash usage in SEND following the recommended
approach [rfc4270] [new-hashes].
Basic cryptographic properties of a hash function are that it is both
one-way and collision free. There are two attacks against the one-
way property, the first-preimage attack and the second-preimage
attack. In the first-preimage attack, given a knowledge of a
particular hash value h, an attacker finds an input message m such
that hash(m) = h. The second-preimage attack deals with fixed
messages. Given a knowledge of a fixed value m used as the input
message to the hash function, an attacker finds a different value m'
that yields hash(m)=hash(m'). Supposing that the hash function
produces an n-bit long output, since each output is equally likely,
an attack takes an order of 2^n operations to be successful. Due to
the birthday attack, if the hash function is supplied with a random
input, it returns one of the k equally-likely values, and the number
of operations can be reduced to the number of 1.2*2^(n/2) operations.
Attack against the collision-free property deals with two fixed
messages, both produced by an attacker. Attacker produces two
different messages, m and m', such that hash(m)=hash(m'). Up to
date, all demonstrated attacks are attacks against a collision-free
property. Attacks against the one-way property are not yet feasible
[rfc4270].
The strength of Internet protocol does not have to be necessarily
affected by the weakness of the underlaying hash function. The
appropriate way of use of the hash algorithm will keep the protocol
immune, no matter of the hash algorithm weaknesses. Out of many
possible hash algorithm uses, such as non-repudiable digital
signatures, certificate digital signatures, message authentication
with shared secrets, fingerprints, only the first two can introduce
weaknesses to the Internet protocol [rfc4270]. The rest of the
section analyzes the impact of hash attacks, mainly collision
attacks, on SEND by the cases of use. Through our analysis, we also
discuss whether we should support the hash agility in SEND.
3.1. Attacks against CGAs in stateless autoconfiguration
Hash functions are used in the stateless autoconfiguration process
that is based on CGAs. Impacts of collision attacks on current uses
of CGAs and the CGA hash agility are analyzed in the update of the
CGA specification [rfc4982]. CGAs provide the proof-of-ownership of
the private key corresponding to the public key used to generate the
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CGA. The collision attack against the CGA assume that attacker
generates two different sets of CGA Parameters that result in the
same hash value. Since CGAs do not deal with the non-repudiation
feature, and both sets are chosen by the attacker, this attack is not
useful in any real-world scenario. CGA-based protocols, including
SEND, are not affected by the recent collision attacks. If pre-image
attacks were feasible, an attacker would find colliding CGA
Parameters for the victim's CGA, and produce the Key Hash field and
the Digital Signature field afterwards using the new public key.
Since the strength of all hashes in SEND depends on the strength of
the CGA, the pre-image attack is potentially dangerous, but it is not
yet feasible.
3.2. Attacks against X.509 certificates in ADD process
Another use of hash functions is for the router authorization in the
ADD process. Router sends to a host a certification path, which is a
path between a router and the host's trust anchor, consisting of
X.509 certificates. Researchers demonstrated attacks against X.509
certificates with MD5 signature in 2005 [new-hashes] and in 2007
[x509-coll]. In 2005 researchers constructed colliding certificates
with the same distinguished name, different public keys, and
identical signatures, based on different public keys. The problem
for the attacker is that two certificates with the same identity are
not actually useful in real-world scenarios, because the
Certification Authority can take care of not allowing to provide such
two certificates. In addition, attacks against the human-readable
fields demand much more effort than the attacks against non human-
readable fields, such as a public key field. In case of the identity
field, an attacker is faced with the problem of the prediction and
the generation of the false but meaningful identity data, which at
the end might be revealed by the Certification Authority. Thus, in
practice, there is a very low probability that collision attacks
affect non human-readable parts of the certificate. In 2007
researchers demonstrated colliding certificates which differ in the
identity data and in the public key, but still result in the same
signature value. Even in this case, the real-world scenarios prevent
the hash algorithm weaknesses to introduce vulnerabilities of the
certificate or the protocol. Even if an attacker produced such two
colliding certificates in order to claim that he was someone else, he
still needs to predict the content of all fields appearing before the
public key, e.g. the serial number and validity periods. Although a
relying party cannot verify the content of these fields (each
certificate by itself is unsuspicious), the Certification Authority
keeps track of those fields and it can reveal the false certificate
during the fraud analysis. Even though the real-world scenarios make
SEND immune to recent hash attacks introduced through X.509
certificates, theoretically they are possible, but only in case of
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the human-readable fields. Regarding X.509 certificates in SEND,
biggest concer are attacks against the RFC3779 IP address extension
which would enable the bogus router to advertize the changed IP
prefix range, if the IP prefix range used, although, not broader than
the prefix range of the parent certificate in the ADD chain. Adding
some form of randomness to human-readble data would prevent such
attacks, which can be considered once when the collision attack
improve.
3.3. Attacks against the Digital Signature in the RSA Signature option
The computation of the Digital Signature field is described
[rfc3971]. It is produced as the SHA-1 hash over the IPv6 addresses,
the ICMPv6 header, the ND message and other fields, e.g. the Message
Type Tag and ND options, and signed with the sender's private key.
Private key corresponds to the public key in the CGA parameters
structure. It is usually authorized through CGAs. The Digital
Signature field is vulnerable to recent collision attacks. Possible
attack on such explicit digital signature is a typical non-
repudiation attack. An attacker produces two different messages, m
and m', where hash(m) = hash(m'). He underlays one of the messages
to be signed with the key authorized through CGAs, but uses another
message afterwards. However, the structure of at least one of two
messages in a collision attack is strictly predefined. The previous
requirement makes this collision attack to be much more then the
simple collision attack, and requires the attacker to know or predict
the communication context. Theoretically this attack could harm
SEND, but in real-world situation is extremely hard to achieve it.
3.4. Attacks against the Key Hash field in the RSA Signature option
The Key Hash field in the RSA Signature option is a SHA-1 hash of the
public key from the CGA Parameters structure in the CGA option. It
is a fingerprint that provides the integrity protection.
Fingerprints are generally not affected by the collision attacks
because they involve random data as one of the inputs, which prevents
recent collision attacks. In addition, context of the SEND message
and the protocol makes this attack unable to introduce new
vulnerabilities to SEND. An attacker has to produce both keys, k and
k', such that hash(k) = hash(k'). Since the key is authorized
through CGA, and possibily through the certification in the ADD
process, this attack is of no use for the attacker. The pre-image
attack against the Key Hash field, if it was possible, would affect
SEND since the Key Hash field contains a non human-readable data.
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4. Support for the hash agility in SEND
Previous section analyzed that CGAs and fingerprints affected by the
collision attacks (Key Hash field of the Send message) do not
introduce new vulnerabilities to SEND. Digital signatures in the
Digital Signature field of the SEND message and X.509 certificate
theoretically are affected by the recent collision attacks, but the
SEND context prevents those attacks of almost any use in the real-
world scenarios. However, recent attacks indicate the possibility
for the future improved real-world attacks. Researchers advise to
migrate away from currently used hash algorithms. In order to
increase the future security of SEND, we suggest the support for the
hash and algorithm agility in SEND.
o The most effective and secure would be to bind the hash function
option with something that can not be changed at all, like
[rfc4982] does for CGA. It encodes the hash function information
into addresses. We could decide to use by default the same hash
function in SEND as in CGA. The security of all hashes in SEND
depends on CGA, i.e. if an attacker breaks CGA, all other hashes
are automatically broken. The use of the hash algorithm embedded
in CGA protects from the bidding down attacks. From the security
point of view, at the moment, this solution is more reasonable
then defining different hash algorithm for each hash. The
disadvantage of this solution is that it introduces the limitation
for SEND to be used exclusively with CGAs.
o Another solution is to incorporate the Hash algorithm option into
the SEND message. This solution is vulnerable to the bidding down
attack.
o The third possible solution is to encode the algorithm in the CGA.
This would reduce the strength of the CGA and make it vulnerable
to brute force attacks.
o Possible solution is also the hybrid solution which would require
the hash algorithm to be the same as CGA, if CGA option is
present, and to use the Hash agility option if the CGA option is
not present. In such way, SEND is not bound exclusively to CGA.
o None of the previous solutions supports the negotiation of the
hash function. One of possible solutions is the negotiation
approach for the SEND hash agility based on the Supported
Signature Algorithm option described in [sig-agility]. Based on
the processing rules described in [sig-agility] nodes find the
intersection between the sender's and the receiver's supported
signature algorithms set.
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5. Security Considerations
This document analyzes the impact of hash attacks in SEND and offeres
a higher security level for SEND by providing solution for the hash
agility support.
The negotiation approach for the hash agility in SEND based on the
Supported Signature Algorithms option is vulnerable to bidding-down
attacks, which is usual in the case of any negotiation approach.
This issue can be mitigated with the appropriate local policies.
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6. References
6.1. Normative References
[new-hashes]
Bellovin, S. and E. Rescorla, "Deploying a New Hash
Algorithm", November 2005.
[pk-agility]
Cheneau, T., Maknavicius, M., Sean, S., and M. Vanderveen,
"Support for Multiple Signature Algorithms in
Cryptographically generated Addresses (CGAs)",
draft-cheneau-cga-pk-agility-00 (work in progress),
February 2009.
[rfc3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
Neighbor Discovery (SEND)", RFC 3971, March 2005.
[rfc4270] Hoffman, P. and B. Schneier, "Attacks on Cryptographic
Hashed in Internet Protocols", RFC 4270, November 2005.
[rfc4982] Bagnulo, M. and J. Arrko, "Support for Multiple Hash
Algorithms in Cryptographically Generated Addresses
(CGAs)", RFC 4982, July 2007.
[sig-agility]
Cheneau, T. and M. Maknavicius, "Signature Algorithm
Agility in the Secure Neighbor Discovery (SEND) Protocol",
draft-cheneau-send-sig-agility-01 (work in progress),
May 2010.
6.2. Informative References
[rfc2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[rfc5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC rfc5280, May 2008.
[sha-1] NIST, FIBS PUB 180-1, "Secure Hash Standard", April 1995.
[sha1-coll]
Wang, X., Yin, L., and H. Yu, "Finding Collisions in the
Full SHA-1. CRYPTO 2005: 17-36", 2005.
[x509-coll]
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Stevens, M., Lenstra, A., and B. Weger, "Chosen-Prefix
Collisions for MD5 and Colliding X.509 Certificates for
Different Identitites. EUROCRYPT 2007: 1-22", 2005.
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Authors' Addresses
Ana Kukec
University of Zagreb
Unska 3
Zagreb
Croatia
Email: ana.kukec@fer.hr
Suresh Krishnan
Ericsson
8400 Decarie Blvd.
Town of Mount Royal, QC
Canada
Email: suresh.krishnan@ericsson.com
Sheng Jiang
Huawei Technologies Co., Ltd
KuiKe Building, No.9 Xinxi Rd.,
Shang-Di Information Industry Base, Hai-Dian District, Beijing
P.R. China
Email: shengjiang@huawei.com
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