Recent attacks to currently used hash functions have motivated a
considerable amount of concern in the Internet community. The
recommended approach   to deal with this issue is first to
analyze the impact of these attacks on the different Internet
protocols that use hash functions and second to make sure that the
different Internet protocols that use hash functions are capable of
migrating to an alternative (more secure) hash function without a
major disruption in the Internet operation.
This document performs such analysis for the Cryptographically
Generated Addresses (CGAs) defined in . The first conclusion of
the analysis is that the security of the protocols using CGAs is not
affected by the recently available attacks against hash functions.
The second conclusion of the analysis is that the hash function used
is hard coded in the CGA specification. This document updates the
CGA specification  to enable the support of alternative hash
functions. In order to do so, this document creates a new registry
managed by IANA to register the different hash algorithms used in
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 .
3. Impact of Collision Attacks in CGAs
Recent advances in cryptography have resulted in simplified attacks
against the collision-free property of certain commonly used hash
functions  , including SHA-1 that is the hash function used by
CGAs . The result is that it is possible to obtain two messages,
M1 and M2, that have the same hash value with much less than 2^(L/2)
attempts. We will next analyze the impact of such attacks in the
currently proposed usages of CGAs.
As we understand it, the attacks against the collision-free property
of a hash function mostly challenge the application of such hash
functions, for the provision of non-repudiation capabilities. This
is because an attacker would be capable to create two different
messages that result in the same hash value and it can then present
any of the messages interchangeably (for example after one of them
has been signed by the other party involved in the transaction).
However, it must be noted that both messages must be generated by the
As far as we understand, current usages of CGAs does not include the
provision of non-repudiation capabilities, so attacks against the
collision-free property of the hash function do not enable any useful
attack against CGA-based protocols.
Current usages of the CGAs are basically oriented to prove the
ownership of a CGA and then bind it to alternative addresses that can
be used to reach the original CGA. This type of application of the
o The application of CGAs to protect the shim6 protocol . In
this case, CGAs are used as identifiers for the established
communications. CGA features are used to prove that the owner of
the identifier is the one that is providing the alternative
addresses that can be used to reach the initial identifier. This
is achieved by signing the list of alternative addresses available
in the multihomed host with the private key of the CGA.
o The application of CGAs to secure the IPv6 mobility support
protocol  as proposed in . In this case, the CGAs are used
as Home Addresses and they are used to prove that the owner of the
Home Address is the one creating the binding with the new Care-off
Address. Similarly to the previous case, this is achieved by
signing the Binding Update message carrying the Care-off Address
with the private key of the CGA.
o The application of CGA to Secure Neighbour Discovery . In this
case, the CGA features are used to prove the address ownership, so
that it is possible to verify that the owner of the IP address is
the one that is providing the layer 2 address information. This
is achieved by signing the layer 2 address information with the
private key of the CGA.
Essentially, all the current applications of CGAs rely on CGAs to
protect a communication between two peers from third party attacks
and not to provide protection from the peer itself. Attacks against
the collision-free property of the hash functions suppose that one of
the parties is generating two messages with the same hash value in
order to launch an attack against its communicating peer. Since CGAs
are not currently used to providing this type of protection, it is
then natural that no additional attacks are enabled by a weaker
collision resistance of the hash function.
4. Options for Multiple Hash Algorithm Support in CGAs
CGAs, as currently defined in , are intrinsically bound to the
SHA-1 hash algorithm and no other hash function is supported.
Even though the attacks against the collision-free property of the
hash functions do not result in new vulnerabilities in the current
applications of CGAs, it seems wise to enable multiple hash function
support in CGAs. This is mainly for two reasons: first, potential
future applications of the CGA technology may be susceptible to
attacks against the collision-free property of SHA-1. Supporting
alternative hash functions would allow applications that have
stricter requirements on the collision-free property to use CGAs.
Second, one lesson learned from the recent attacks against hash
functions is that it is possible that one day we need to start using
alternative hash functions because of successful attacks against
other properties of the commonly used hash functions. Therefore, it
seems wise to modify protocols in general and the CGAs in particular
to support this transition to alternative hash functions as easy as
4.1. Where to Encode the Hash Function?
The next question we need to answer is where to encode the hash
function that is being used. There are several options that can be
One option would be to include the hash function used as an input to
the hash function. This basically means to create an extension to
the CGA Parameter Data Structure, as defined in , that codifies
the hash function used. The problem is that this approach is
vulnerable to bidding down attacks or downgrading attacks as defined
in . This means that even if a strong hash function is used, an
attacker could find a CGA Parameter Data Structure that uses a weaker
function but results in an equal hash value. This happens when the
original hash function H1 and CGA Parameters Data Structure
indicating H1 result in value X, and another hash function H2 and CGA
Parameters Data Structure indicating H2 also result in the same value
In other words, the downgrading attack would work as follows: suppose
that Alice generates a CGA CGA_A using the strong hash function
HashStrong and using a CGA Parameter Data Structure CGA_PDS_A. The
selected hash function HashStrong is encoded as an extension field in
the CGA_PDS_A. Suppose that by using a brute force attack, an
attacker X finds an alternative CGA Parameter Data Structure
CGA_PDS_X whose hash value, by using a weaker hash function, is
CGA_A. At this point, the attacker can pretend to be the owner of
CGA_A and the stronger hash function has not provided additional
The conclusion from the previous analysis is that the hash function
used in the CGA generation must be encoded in the address itself.
Since we want to support several hash functions, we will likely need
at least 2 or 3 bits for this.
One option would be to use more bits from the hash bits of the
interface identifier. However, the problem with this approach is
that the resulting CGA is weaker because less hash information is
encoded in the address. In addition, since those bits are currently
used as hash bits, it is impossible to make this approach backward
compatible with existent implementations.
Another option would be to use the "u" and the "g" bits to encode
this information, but this is probably not such a good idea since
those bits have been honoured so far in all interface identifier
generation mechanisms, which allow them to be used for the original
purpose (for instance we can still create a global registry for
unique interface identifiers). Finally, another option is to encode
the hash value used in the Sec bits. The Sec bits are used to
artificially introduce additional difficulty in the CGA generation
process in order to provide additional protection against brute force
attacks. The Sec bits have been designed in a way that the lifetime
of CGAs are extended, when it is feasible to attack 59-bits long hash
values. However, this is not the case today, so in general CGA will
have a Sec value of 000. The proposal is to encode in the Sec bits,
not only information about brute force attack protection but also to
encode the hash function used to generate the hash. So for instance,
the Sec value 000 would mean that the hash function used is SHA-1 and
the 0 bits of hash2 (as defined in RFC 3972) must be 0. Sec value of
001 could be that the hash function used is SHA-1 and the 16 bits of
hash2 (as defined in RFC 3972) must be zero. However, the other
values of Sec could mean that an alternative hash function needs to
be used and that a certain amount of bits of hash2 must be zero. The
proposal is not to define any concrete hash function to be used for
other Sec values, since it is not yet clear that we need to do so nor
is it clear which hash function should be selected.
Note that since there are only 8 Sec values, it may be necessary to
reuse Sec values when we run out of unused Sec values. The scenario
where such an approach makes sense is where there are some Sec values
that are no longer being used because the resulting security has
become weak. In this case, where the usage of the Sec value has long
been abandoned, it would be possible to reassign the Sec values.
However, this must be a last resource option, since it may affect
interoperability. This is because two implementations using
different meanings of a given Sec value would not be able to
interoperate properly (i.e., if an old implementation receives a CGA
generated with the new meaning of the Sec value, it will fail and the
same for a new implementation receiving a CGA generated with the old
meaning of the Sec value). In case the approach of reassigning a Sec
value is followed, a long time is required between the deprecation of
the old value and the reassignment in order to prevent
misinterpretation of the value by old implementations.
An erroneous interpretation of a reused Sec value, both on the CGA
owner's side and the CGA verifier's side, would have the following
result, CGA verification would fail in the worst case and both nodes
would have to revert to unprotected IPv6 addresses. This can happen
only with obsolete CGA parameter sets, which would be considered
insecure anyway. In any case, an implementation must not
simultaneously support two different meanings of a Sec value.
5. CGA Generation Procedure
The SEC registry defined in the IANA considerations section of this
document contains entries for the different Sec values. Each of
these entries points to an RFC that defines the CGA generation
procedure that MUST be used when generating CGAs with the associated
It should be noted that the CGA generation procedure may be changed
by the new procedure not only in terms of the hash function used but
also in other aspects, e.g., longer Modifier values may be required
if the number of 0s required in hash2 exceed the currently defined
bound of 112 bits. The new procedure (which potentially involves a
longer Modifier value) would be described in the RFC pointed to by
the corresponding Sec registry entry.
In addition, the RFC that defines the CGA generation procedure for a
Sec value MUST explicitly define the minimum key length acceptable
for CGAs with that Sec value. This is to provide a coherent
protection both in the hash and the public key techniques.
6. IANA Considerations
This document defines a new registry entitled "CGA SEC" for the Sec
field defined in RFC 3972  that has been created and is maintained
by IANA. The values in this name space are 3-bit unsigned integers.
Initial values for the CGA Extension Type field are given below;
future assignments are to be made through Standards Action .
Assignments consist of a name, the value, and the RFC number where
the CGA generation procedure is defined.
The following initial values are assigned in this document:
Name | Value | RFCs
SHA-1_0hash2bits | 000 | 3972, 4982
SHA-1_16hash2bits | 001 | 3972, 4982
SHA-1_32hash2bits | 010 | 3972, 4982
7. Security Considerations
This document is about security issues and, in particular, about
protection against potential attacks against hash functions.
Russ Housley, James Kempf, Christian Vogt, Pekka Nikander, and Henrik
Levkowetz reviewed and provided comments about this document.
Marcelo Bagnulo worked on this document while visiting Ericsson
Research Laboratory Nomadiclab.
9.1. Normative References
 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
 Aura, T., "Cryptographically Generated Addresses (CGA)",
RFC 3972, March 2005.
 Bagnulo, M. and J. Arkko, "Cryptographically Generated
Addresses (CGA) Extension Field Format", RFC 4581,
 Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
Neighbor Discovery (SEND)", RFC 3971, March 2005.
9.2. Informative References
 Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434,
 Hoffman, P. and B. Schneier, "Attacks on Cryptographic Hashes
in Internet Protocols", RFC 4270, November 2005.
 Nordmark, E. and M. Bagnulo, "Multihoming L3 Shim Approach",
Work in Progress, July 2005.
 Johnson, D., Perkins, C., and J. Arkko, "Mobility Support in
IPv6", RFC 3775, June 2004.
 Arkko, J., "Applying Cryptographically Generated Addresses and
Credit-Based Authorization to Mobile IPv6", Work in Progress,
 Bellovin, S. and E. Rescorla, "Deploying a New Hash Algorithm",
NDSS '06, February 2006.
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