|Title||Hash-Based Addresses (HBA)
Network Working Group M. Bagnulo
Request for Comments: 5535 UC3M
Category: Standards Track June 2009
Hash-Based Addresses (HBA)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
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This memo describes a mechanism to provide a secure binding between
the multiple addresses with different prefixes available to a host
within a multihomed site. This mechanism employs either
Cryptographically Generated Addresses (CGAs) or a new variant of the
same theme that uses the same format in the addresses. The main idea
in the new variant is that information about the multiple prefixes is
included within the addresses themselves. This is achieved by
generating the interface identifiers of the addresses of a host as
hashes of the available prefixes and a random number. Then, the
multiple addresses are generated by prepending the different prefixes
to the generated interface identifiers. The result is a set of
addresses, called Hash-Based Addresses (HBAs), that are inherently
bound to each other.
Table of Contents
1. Introduction ....................................................3
2. Terminology .....................................................4
3. Overview ........................................................4
3.1. Threat Model ...............................................4
3.2. Overview ...................................................4
3.3. Motivations for the HBA Design .............................5
4. Cryptographic Generated Addresses (CGAs) Compatibility
5. Multi-Prefix Extension for CGA ..................................8
6. HBA-Set Generation ..............................................9
7. HBA Verification ...............................................11
7.1. Verification That a Particular HBA Address
Corresponds to a Given CGA Parameter Data Structure .......11
7.2. Verification That a Particular HBA Address Belongs to the
HBA Set Associated with a Given CGA Parameter Data
8. Example of HBA Application in a Multihoming Scenario ...........13
8.1. Dynamic Address Set Support ...............................16
9. DNS Considerations .............................................17
10. IANA Considerations ...........................................18
11. Security Considerations .......................................18
11.1. Security Considerations When Using HBAs in the
Shim6 Protocol ...........................................20
11.2. Privacy Considerations ...................................22
11.3. SHA-1 Dependency Considerations ..........................22
11.4. DoS Attack Considerations ................................22
12. Contributors ..................................................23
13. Acknowledgments ...............................................23
14. References ....................................................24
14.1. Normative References .....................................24
14.2. Informative References ...................................24
In order to preserve inter-domain routing system scalability, IPv6
sites obtain addresses from their Internet Service Providers (ISPs).
Such an addressing strategy significantly reduces the amount of
routes in the global routing tables, since each ISP only announces
routes to its own address blocks, rather than announcing one route
per customer site. However, this addressing scheme implies that
multihomed sites will obtain multiple prefixes, one per ISP.
Moreover, since each ISP only announces its own address block, a
multihomed site will be reachable through a given ISP if the ISP
prefix is contained in the destination address of the packets. This
means that, if an established communication needs to be routed
through different ISPs during its lifetime, addresses with different
prefixes will have to be used. Changing the address used to carry
packets of an established communication exposes the communication to
numerous attacks, as described in , so security mechanisms are
required to provide the required protection to the involved parties.
This memo describes a tool that can be used to provide protection
against some of the potential attacks, in particular against future/
premeditated attacks (aka time shifting attacks in ).
This memo describes a mechanism to provide a secure binding between
the multiple addresses with different prefixes available to a host
within a multihomed site.
It should be noted that, as opposed to the mobility case where the
addresses that will be used by the mobile node are not known a
priori, the multiple addresses available to a host within the
multihomed site are pre-defined and known in advance in most of the
cases. The mechanism proposed in this memo employs either
Cryptographically Generated Addresses (CGAs)  or a new variant of
the same theme that uses the same format in the addresses. The new
variant, Hash-Based Address (HBA), takes advantage of the address set
stability. In either case, a secure binding between the addresses of
a node in a multihomed site can be provided. CGAs employ public key
cryptography and can deal with changing address sets. HBAs employ
only symmetric key cryptography, and have smaller computational
For the purposes of the Shim6 protocol, the other characteristics of
the CGAs and HBAs are similar. Both can be generated by the host
itself without any reliance on external infrastructure. Both employ
the same format of addresses and same format of data fed to generate
the addresses. It is not required that all interface identifiers of
a node's addresses be equal, preserving some degree of privacy
through changes in the addresses used during the communications.
The main idea in HBAs is that information about the multiple prefixes
is included within the addresses themselves. This is achieved by
generating the interface identifiers of the addresses of a host as
hashes of the available prefixes and a random number. Then, the
multiple addresses are obtained by prepending the different prefixes
to the generated interface identifiers. The result is a set of
addresses that are inherently bound. A cost-efficient mechanism is
available to determine if two addresses belong to the same set, since
given the prefix set and the additional parameters used to generate
the HBA, a single hash operation is enough to verify if an HBA
belongs to a given HBA set.
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 .
3.1. Threat Model
The threat analysis for the multihoming problem is described in .
This analysis basically identifies attacks based on redirection of
packets by a malicious attacker towards addresses that do not belong
to the multihomed node. There are essentially two types of
redirection attacks: communication hijacking and flooding attacks.
Communication hijacking attacks are about an attacker stealing on-
going and/or future communications from a victim. Flooding attacks
are about redirecting the traffic generated by a legitimate source
towards a third party, flooding it. The HBA solution provides full
protection against the communication hijacking attacks. The Shim6
protocol  protects against flooding attacks. Residual threats are
described in the "Security Considerations" section.
The basic goal of the HBA mechanism is to securely bind together
multiple IPv6 addresses that belong to the same multihomed host.
This allows rerouting of traffic without worrying that the
communication is being redirected to an attacker. The technique that
is used is to include a hash of the permitted prefixes in the
low-order bits of the IPv6 address.
So, eliding some details, say the available prefixes are A, B, C, and
D, the host would generate a prefix list P consisting of (A,B,C,D)
and a random number called Modifier M. Then it would generate the
A || H(M || A || P)
B || H(M || B || P)
C || H(M || C || P)
D || H(M || D || P)
Thus, given one valid address out of the group and the prefix list P
and the random Modifier M it is possible to determine whether another
address is part of the group by computing the hash and checking
against the low-order bits.
3.3. Motivations for the HBA Design
The design of the HBA technique was driven by the following
First of all, the goal of HBA is to provide a secure binding between
the IPv6 address used as an identifier by the upper-layer protocols
and the alternative locators available in the multihomed node so that
redirection attacks are prevented.
Second, in order to achieve such protection, the selected approach
was to include security information in the identifier itself, instead
of relying on third trusted parties to secure the binding, such as
the ones based on repositories or Public Key Infrastructure. This
decision was driven by deployment considerations, i.e., the cost of
deploying the trusted third-party infrastructure.
Third, application support considerations described in  resulted
in selecting routable IPv6 addresses to be used as identifiers.
Hence, security information is stuffed within the interface
identifier part of the IPv6 address.
Fourth, performance considerations as described in  motivated the
usage of a hash-based approach as opposed to a public-key-based
approach based on pure Cryptographic Generated Addresses (CGA), in
order to avoid imposing the performance of public key operations for
every communication in multihomed environments. The HBA approach
presented in this document presents a cheaper alternative that is
attractive to many common usage cases. Note that the HBA approach
and the CGA approaches are not mutually exclusive and that it is
possible to generate addresses that are both valid CGA and HBA
addresses providing the benefits of both approaches if needed.
4. Cryptographic Generated Addresses (CGAs) Compatibility
As described in the previous section, the HBA technique uses the
interface identifier part of the IPv6 address to encode information
about the multiple prefixes available to a multihomed host. However,
the interface identifier is also used to carry cryptographic
information when Cryptographic Generated Addresses (CGAs)  are
used. Therefore, conflicting usages of the interface identifier bits
may result if this is not taken into account during the HBA design.
There are at least two valid reasons to provide CGA-HBA
First, the current Secure Neighbor Discovery (SeND) specification 
uses the CGAs defined in  to prove address ownership. If HBAs are
not compatible with CGAs, then nodes using HBAs for multihoming
wouldn't be able to do Secure Neighbor Discovery using the same
addresses (at least the parts of SeND that require CGAs). This would
imply that nodes would have to choose between security (from SeND)
and fault tolerance (from IPv6 multihoming support provided by the
Shim6 protocol ). In addition to SeND, there are other protocols
that are considered to benefit from the advantages offered by the CGA
scheme, such as mobility support protocols . Those protocols
could not be used with HBAs if HBAs are not compatible with CGAs.
Second, CGAs provide additional features that cannot be achieved
using only HBAs. In particular, because of its own nature, the HBA
technique only supports a predetermined prefix set that is known at
the time of the generation of the HBA set. No additions of new
prefixes to this original set are supported after the HBA set
generation. In most of the cases relevant for site multihoming, this
is not a problem because the prefix set available to a multihomed set
is not very dynamic. New prefixes may be added in a multihomed site
when a new ISP is available, but the timing of those events are
rarely in the same time scale as the lifetime of established
communications. It is then enough for many situations that the new
prefix is not available for established communications and that only
new communications benefit from it. However, in the case that such
functionality is required, it is possible to use CGAs to provide it.
This approach clearly requires that HBA and CGA approaches be
compatible. If this is the case, it then would be possible to create
HBA/CGA addresses that support CGA and HBA functionality
simultaneously. The inputs to the HBA/CGA generation process will be
both a prefix set and a public key. In this way, a node that has
established a communication using one address of the CGA/HBA set can
tell its peer to use the HBA verification when one of the addresses
of its HBA/CGA set is used as locator in the communication or to use
CGA (public-/private-key-based) verification when a new address that
does not belong to the HBA/CGA set is used as locator in the
So, because of the aforementioned reasons, it is a goal of the HBA
design to define HBAs in such a way that they are compatible with
CGAs as defined in  and their usages described in 
(consequently, to understand the rest of this note, the reader should
be familiar with the CGA specification defined in ). This means
that it must be possible to generate addresses that are both an HBA
and a CGA, i.e., that the interface identifier contains cryptographic
information of CGA and the prefix-set information of an HBA. The CGA
specification already considers the possibility of including
additional information into the CGA generation process through the
usage of Extension Fields in the CGA Parameter Data Structure. It is
then possible to define a Multi-Prefix extension for CGA so that the
prefix set information is included in the interface identifier
Even though a CGA compatible approach is adopted, it should be noted
that HBAs and CGAs are different concepts. In particular, the CGA is
inherently bound to a public key, while an HBA is inherently bound to
a prefix set. This means that a public key is not required to
generate an HBA-only address. Because of that, we define three
different types of addresses:
- CGA-only addresses: These are addresses generated as specified in
 without including the Multi-Prefix extension. They are bound
to a public key and to a single prefix (contained in the basic CGA
Parameter Data Structure). These addresses can be used for SeND
; if used for multihoming, their application will have to be
based on the public key usage.
- CGA/HBA addresses: These addresses are CGAs that include the
Multi-Prefix extension in the CGA Parameter Data Structure used
for their generation. These addresses are bound to a public key
and a prefix set and they provide both CGA and HBA
functionalities. They can be used for SeND as defined in  and
for any usage defined for HBA (such as a Shim6 protocol).
- HBA-only addresses: These addresses are bound to a prefix set but
they are not bound to a public key. Because HBAs are compatible
with CGA, the CGA Parameter Data Structure will be used for their
generation, but a random nonce will be included in the Public Key
field instead of a public key. These addresses can be used for
HBA-based multihoming protocols, but they cannot be used for SeND.
5. Multi-Prefix Extension for CGA
The Multi-Prefix extension has the following TLV format as defined in
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
| Extension Type | Extension Data Length |
|P| Reserved |
+ Prefix +
+ Prefix +
. . .
. . .
. . .
+ Prefix[n] +
Ext Type: 16-bit type identifier of the Multi-Prefix extension (see
the "IANA Considerations" section).
Ext Len: 16-bit unsigned integer. Length of the Extension in
octets, not including the first 4 octets.
P flag: Set if a public key is included in the Public Key field of
the CGA Parameter Data Structure, reset otherwise.
Reserved: 31-bit reserved field. MUST be initialized to zero, and
ignored upon receipt.
Prefix[1...n]: Vector of 64-bit prefixes, numbered 1 to n.
6. HBA-Set Generation
The HBA generation process is based on the CGA generation process
defined in Section 4 of . The goal is to require the minimum
amount of changes to the CGA generation process. It should be noted
that the following procedure is only valid for Sec values of 0, 1,
and 2. For other Sec values, RFC 4982  has defined a CGA SEC
registry that will contain the specifications used to generate CGAs.
The generation procedures defined in such specifications must be used
for Sec values other than 0, 1, or 2.
The CGA generation process has three inputs: a 64-bit subnet prefix,
a public key (encoded in DER as an ASN.1 structure of the type
SubjectPublicKeyInfo), and the security parameter Sec.
The main difference between the CGA generation and the HBA generation
is that while a CGA can be generated independently, all the HBAs of a
given HBA set have to be generated using the same parameters, which
implies that the generation of the addresses of an HBA set will occur
in a coordinated fashion. In this memo, we will describe a mechanism
to generate all the addresses of a given HBA set. The generation
process of each one of the HBA address of an HBA set will be heavily
based in the CGA generation process defined in . More precisely,
the HBA set generation process will be defined as a sequence of
lightly modified CGA generations.
The changes required in the CGA generation process when generating a
single HBA are the following: First, the Multi-Prefix extension has
to be included in the CGA Parameter Data Structure. Second, in the
case that the address being generated is an HBA-only address, a
random nonce will have to be used as input instead of a valid public
key. For backwards compatibility issues with pure CGAs, the random
nonce MUST be encoded as a public key as defined in . In
particular, the random nonce MUST be formatted as a DER-encoded ASN.1
structure of the type SubjectPublicKeyInfo, defined in the Internet
X.509 certificate profile . The algorithm identifier MUST be
rsaEncryption, which is 1.2.840.1135188.8.131.52, and the random nonce
MUST be formatted by using the RSAPublicKey type as specified in
Section 2.3.1 of RFC 3279 . The random nonce length is 384 bits.
The resulting HBA-set generation process is the following:
The inputs to the HBA generation process are:
o A vector of n 64-bit prefixes,
o A Sec parameter, and
o In the case of the generation of a set of HBA/CGA addresses, a
public key is also provided as input (not required when generating
The output of the HBA generation process are:
o An HBA-set
o their respective CGA Parameter Data Structures
The steps of the HBA-set generation process are:
1. Multi-Prefix extension generation. Generate the Multi-Prefix
extension with the format defined in Section 5. Include the
vector of n 64-bit prefixes in the Prefix[1...n] fields. The Ext
Len field value is (n*8 + 4). If a public key is provided, then
the P flag is set to one. Otherwise, the P flag is set to zero.
2. Modifier generation. Generate a Modifier as a random or
pseudorandom 128-bit value. If a public key has not been provided
as an input, generate the Extended Modifier as a 384-bit random or
pseudorandom value. Encode the Extended Modifier value as an RSA
key in a DER-encoded ASN.1 structure of the type
SubjectPublicKeyInfo defined in the Internet X.509 certificate
3. Concatenate from left to right the Modifier, 9 zero octets, the
encoded public key or the encoded Extended Modifier (if no public
key was provided), and the Multi-Prefix extension. Execute the
SHA-1 algorithm on the concatenation. Take the 112 leftmost bits
of the SHA-1 hash value. The result is Hash2.
4. Compare the 16*Sec leftmost bits of Hash2 with zero. If they are
all zero (or if Sec=0), continue with step (5). Otherwise,
increment the Modifier by one and go back to step (3).
5. Set the 8-bit collision count to zero.
6. For i=1 to n (number of prefixes) do:
6.1. Concatenate from left to right the final Modifier value,
Prefix[i], the collision count, the encoded public key or the
encoded Extended Modifier (if no public key was provided), and
the Multi-Prefix extension. Execute the SHA-1 algorithm on the
concatenation. Take the 64 leftmost bits of the SHA-1 hash
value. The result is Hash1[i].
6.2. Form an interface identifier from Hash1[i] by writing the
value of Sec into the three leftmost bits and by setting bits 6
and 7 (i.e., the "u" and "g" bits) both to zero.
6.3. Generate address HBA[i] by concatenating Prefix[i] and the
64-bit interface identifier to form a 128-bit IPv6 address with
the subnet prefix to the left and interface identifier to the
right as in a standard IPv6 address .
6.4. Perform duplicate address detection if required. If an
address collision is detected, increment the collision count by
one and go back to step (6). However, after three collisions,
stop and report the error.
6.5. Form the CGA Parameter Data Structure that corresponds to
HBA[i] by concatenating from left to right the final Modifier
value, Prefix[i], the final collision count value, the encoded
public key or the encoded Extended Modifier, and the Multi-
Note: most of the steps of the process are taken from .
7. HBA Verification
The following procedure is only valid for Sec values of 0, 1, and 2.
For other Sec values, RFC 4982  has defined a CGA SEC registry
that will contain the specifications used to verify CGAs. The
verification procedures defined in such specifications must be used
for Sec values other than 0,1, or 2.
7.1. Verification That a Particular HBA Address Corresponds to a Given
CGA Parameter Data Structure
HBAs are constructed as a CGA Extension, so a properly formatted HBA
and its correspondent CGA Parameter Data Structure will successfully
finish the verification process described in Section 5 of . Such
verification is useful when the goal is the verification of the
binding between the public key and the HBA.
7.2. Verification That a Particular HBA Address Belongs to the HBA Set
Associated with a Given CGA Parameter Data Structure
For multihoming applications, it is also relevant that the receiver
of the HBA information verifies if a given HBA address belongs to a
certain HBA set. An HBA set is identified by a CGA Parameter Data
structure that contains a Multi-Prefix extension. So, the receiver
needs to verify if a given HBA belongs to the HBA set defined by a
CGA Parameter Data Structure. It should be noted that the receiver
may need to verify if an HBA belongs to the HBA set defined by the
CGA Parameter Data Structure of another HBA of the set. If this is
the case, HBAs will fail to pass the CGA verification process defined
in , because the prefix included in the Subnet Prefix field of the
CGA Parameter Data Structure will not match the prefix of the HBA
that is being verified. To verify if an HBA belongs to an HBA set
associated with another HBA, verify that the HBA prefix is included
in the prefix set defined in the Multi-Prefix extension, and if this
is the case, then substitute the prefix included in the Subnet Prefix
field by the prefix of the HBA, and then perform the CGA verification
process defined in .
So, the process to verify that an HBA belongs to an HBA set
determined by a CGA Parameter Data Structure is called HBA
verification and it is the following:
The inputs to the HBA verification process are:
o An HBA
o A CGA Parameter Data Structure
The steps of the HBA verification process are the following:
1. Verify that the 64-bit HBA prefix is included in the prefix set of
the Multi-Prefix extension. If it is not included, the
verification fails. If it is included, replace the prefix
contained in the Subnet Prefix field of the CGA Parameter Data
Structure by the 64-bit HBA prefix.
2. Run the verification process described in Section 5 of  with
the HBA and the new CGA Parameters Data Structure (including the
Multi-Prefix extension) as inputs. The steps of the process are
included below, extracted from :
2.1. Check that the collision count in the CGA Parameter Data
Structure is 0, 1, or 2. The CGA verification fails if the
collision count is out of the valid range.
2.2. Check that the subnet prefix in the CGA Parameter Data
Structure is equal to the subnet prefix (i.e., the leftmost 64
bits) of the address. The CGA verification fails if the prefix
values differ. Note: This step always succeeds because of the
action taken in step 1.
2.3. Execute the SHA-1 algorithm on the CGA Parameter Data
Structure. Take the 64 leftmost bits of the SHA-1 hash value.
The result is Hash1.
2.4. Compare Hash1 with the interface identifier (i.e., the
rightmost 64 bits) of the address. Differences in the three
leftmost bits and in bits 6 and 7 (i.e., the "u" and "g" bits)
are ignored. If the 64-bit values differ (other than in the
five ignored bits), the CGA verification fails.
2.5. Read the security parameter Sec from the three leftmost bits
of the 64-bit interface identifier of the address. (Sec is an
unsigned 3-bit integer.)
2.6. Concatenate from left to right the Modifier, 9 zero octets,
the public key, and any extension fields (in this case, the
Multi-Prefix extension will be included, at least) that follow
the public key in the CGA Parameter Data Structure. Execute
the SHA-1 algorithm on the concatenation. Take the 112
leftmost bits of the SHA-1 hash value. The result is Hash2.
2.7. Compare the 16*Sec leftmost bits of Hash2 with zero. If any
one of them is non-zero, the CGA verification fails.
Otherwise, the verification succeeds. (If Sec=0, the CGA
verification never fails at this step.)
8. Example of HBA Application in a Multihoming Scenario
In this section, we will describe a possible application of the HBA
technique to IPv6 multihoming.
We will consider the following scenario: a multihomed site obtains
Internet connectivity through two providers: ISPA and ISPB. Each
provider has delegated a prefix to the multihomed site (PrefA::/nA
and PrefB::/nb, respectively). In order to benefit from multihoming,
the hosts within the multihomed site will configure multiple IP
addresses, one per available prefix. The resulting configuration is
depicted in the next figure.
| Host2 |
| ISPA | | ISPB |
| | | |
| multihomed site |
| PA::/nA |
| PB::/nB +------+ |
| |Host1 | |
| +------+ |
We assume that both Host1 and Host2 support the Shim6 protocol.
Host2 is not located in a multihomed site, so there is no need for it
to create HBAs (it must be able to verify them though, in order to
support the Shim6 protocol, as we will describe next).
Host1 is located in the multihomed site, so it will generate its
addresses as HBAs. In order to do that, it needs to execute the
HBA-set generation process as detailed in Section 6 of this memo.
The inputs of the HBA-set generation process will be: a prefix vector
containing the two prefixes available in its link, i.e., PA:LA::/64
and PB:LB::/64, a Sec parameter value, and optionally a public key.
In this case, we will assume that a public key is provided so that we
can also illustrate how a renumbering event can be supported when
HBA/CGA addresses are used (see the sub-section referring to dynamic
address set support). So, after executing the HBA-set generation
process, Host1 will have: an HBA-set consisting in two addresses,
i.e., PA:LA:iidA and PB:LB:iidB with their respective CGA Parameter
Data Structures, i.e., CGA_PDS_A and CGA_PDS_B. Note that iidA and
iidB are different but both contain information about the prefix set
available in the multihomed site.
We will next consider a communication between Host1 and Host2.
Assume that both ISPs of the multihomed site are working properly, so
any of the available addresses in Host1 can be used for the
communication. Suppose then that the communication is established
using PA:LA:iidA and IPHost2 for Host1 and Host2, respectively. So
far, no special Shim6 support has been required, and PA:LA:iidA is
used as any other global IP address.
Suppose that at a certain moment, one of the hosts involved in the
communication decides that multihoming support is required in this
communication (this basically means that one of the hosts involved in
the communication desires enhanced fault-tolerance capabilities for
this communication, so that if an outage occurs, the communication
can be re-homed to an alternative provider).
At this moment, the Shim6 protocol Host-Pair Context establishment
exchange will be performed between the two hosts (see ). In this
exchange, Host1 will send CGA_PDS_A to Host2.
After the reception of CGA_PDS_A, Host2 will verify that the received
CGA Parameter Data Structure corresponds to the address being used in
the communication PA:LA:iidA. This means that Host2 will execute the
HBA verification process described in Section 7 of this memo with PA:
LA:iidA and CGA_PDS_A as inputs. In this case, the verification will
succeed since the CGA Parameter Data Structure and the addresses used
in the verification match.
As long as there are no outages affecting the communication path
through ISPA, packets will continue flowing. If a failure affects
the path through ISPA, Host1 will attempt to re-home the
communication to an alternative address, i.e., PB:LB:iidB. In order
to accomplish this, after detecting the outage, Host1 will inform
Host2 about the alternative address. Host2 will verify that the new
address belongs to the HBA set of the initial address. In order to
accomplish this, Host2 will execute the HBA verification process with
the CGA Parameter Data Structure of the original address (i.e.,
CGA_PDS_A) and the new address (i.e., PB:LB:iidB) as inputs. The
verification process will succeed because PB:LB::/64 has been
included in the Multi-Prefix extension during the HBA-set generation
process. Additional verifications may be required to prevent
flooding attacks (see the comments about flooding attacks prevention
in the Security Considerations section of this memo).
Once the new address is verified, it can be used as an alternative
locator to re-home the communication, while preserving the original
address (PA:LA:iidA) as an identifier for the upper layers. This
means that following packets will be addressed to/from this new
address. Note that no additional HBA verification is required for
the following packets, since the new valid address can be stored in
In this example, only the HBA capabilities of the Host1 addresses
were used. In other words, neither the public key included in the
CGA Parameter Data Structure nor its correspondent private key was
used in the protocol. In the following section, we will consider a
case where its usage is required.
8.1. Dynamic Address Set Support
In the previous section, we have presented the mechanisms that allow
a host to use different addresses of a predetermined set to exchange
packets of a communication. The set of addresses involved was
predetermined and known when the communication was initiated. To
achieve such functionality, only HBA functionalities of the addresses
were needed. In this section, we will explore the case where the
goal is to exchange packets using additional addresses that were not
known when the communication was established. An example of such a
situation is when a new prefix is available in a site after a
renumbering event. In this case, the hosts that have the new address
available may want to use it in communications that were established
before the renumbering event. In this case, HBA functionalities of
the addresses are not enough and CGA capabilities are to be used.
Consider then the previous case of the communication between Host1
and Host2. Suppose that the communication is up and running, as
described earlier. Host1 is using PA:LA:iidA and Host2 is using
IPHost2 to exchange packets. Now suppose that a new address, PC:LC:
addC is available in Host1. Note that this address is just a regular
IPv6 address, and it is neither an HBA nor a CGA. Host1 wants to use
this new address in the existent communication with Host2. It should
be noted that the HBA mechanism described in the previous section
cannot be used to verify this new address, since this address does
not belong to the HBA set (since the prefix was not available at the
moment of the generation of the HBA set). This means that
alternative verification mechanisms will be needed.
In order to verify this new address, CGA capabilities of PA:LA:iidA
are used. Note that the same address is used, only that the
verification mechanism is different. So, if Host1 wants to use PC:
LC:addC to exchange packets in the established communication, it will
use the UPDATE message defined in the Shim6 protocol , conveying
the new address, PC:LC:addC, and this message will be signed using
the private key corresponding to the public key contained in
CGA_PDS_A. When Host2 receives the message, it will verify the
signature using the public key contained in the CGA Parameter Data
Structure associated with the address used for establishing the
communication, i.e., CGA_PDS_A and PA:LA:iidA, respectively. Once
that the signature is verified, the new address (PC:LC:addC) can be
used in the communication.
In any case, a renumbering event has an impact on a site that is
using the HBA technique. In particular, the new prefix added will
not be included in the existing HBA set, so it is only possible to
use the new prefix with the existing HBA set if CGA capabilities are
used. While this is acceptable for the short term, in the long run,
the site will need to renumber its HBA addresses. In order to do
that, it will need to re-generate the HBA sets assigned to hosts
including the new prefix in the prefix set, which will result in
different addresses, not only because we need to add a new address
with the new prefix, but also because the addresses with the existing
prefixes will also change because of the inclusion of a new prefix in
the prefix set. Moreover, since HBA addresses need to be generated
locally, once these are generated after the renumbering event, the
new address information needs to be conveyed to the DNS manager in
case that such address information is to be published in the DNS (see
DNS considerations section for more details).
9. DNS Considerations
HBA sets can be generated using any prefix set. Actually, the only
particularity of the HBA is that they contain information about the
prefix set in the interface identifier part of the address in the
form of a hash, but no assumption about the properties of prefixes
used for the HBA generation is made. This basically means that
depending on the prefixes used for the HBA set generation, it may or
may not be recommended to publish the resulting (HBA) addresses in
the DNS. For instance, when Unique Local Address (ULA) prefixes 
are included in the HBA generation process, specific DNS
considerations related to the local nature of the ULA should be taken
into account and proper recommendations related to publishing such
prefixes in the DNS should followed. Moreover, among its addresses,
a given host can have some HBAs and some other IPv6 addresses. The
consequence from this is that only HBA addresses will be bound
together by the HBA technique, while other addresses would not be
bound to the HBA set. This would basically mean that if one of the
other addresses is used for initiating a Shim6 communication, it
won't be possible to use the HBA technique to bind the address used
with the HBA set. Furthermore, since HBA addresses are
indistinguishable from other IPv6 addresses in their format, an
initiator will not be able to distinguish, by merely looking at the
different addresses, which ones belong to the HBA set and which ones
do not, so alternative means would be required the initiator is
supposed to use only HBA for establishing communications in the
presence of non-HBA addresses in the DNS.
In addition, it should be noted that the actual HBA values are a
result of the HBA generation procedure, meaning that they cannot be
arbitrarily chosen. This has an implication with respect to DNS
management, because the party that generates the HBA address set
needs to convey the address information to the DNS manager, so that
the addresses are published and not the other way around. The
situation is similar to regular CGA addresses and even to the case
where stateless address autoconfiguration is used. In order to do
that, it is possible to use Dynamic DNS updates  or other
proprietary tools. A similar consideration applies when the host
wants to publish reverse-DNS entries. Since the host needs to
generate its HBA addresses, it will need to convey the address
information to the DNS manager so the proper reverse-DNS entry is
populated in case it is needed. It should be noted that neither the
Shim6 protocol nor the HBA technique rely on the reverse DNS for its
proper functioning and the general reasons for requiring reverse-DNS
population apply as for any other regular IPv6 address.
10. IANA Considerations
This document defines a new CGA Extension, the Multi-Prefix
extension. This extension has been assigned the CGA Extension Type
11. Security Considerations
The goal of HBAs is to create a group of addresses that are securely
bound, so that they can be used interchangeably when communicating
with a node. If there is no secure binding between the different
addresses of a node, a number of attacks are enabled, as described in
. In particular, it would be possible for an attacker to
redirect the communications of a victim to an address selected by the
attacker, hijacking the communication. When using HBAs, only the
addresses belonging to an HBA set can be used interchangeably,
limiting the addresses that can be used to redirect the communication
to a predetermined set that belongs to the original node involved in
the communication. So, when using HBAs, a node that is communicating
using address A can redirect the communication to a new address B if
and only if B belongs to the same HBA set as A.
This means that if an attacker wants to redirect communications
addressed to address HBA1 to an alternative address IPX, the attacker
will need to create a CGA Parameter Data Structure that generates an
HBA set that contains both HBA1 and IPX.
In order to generate the required HBA set, the attacker needs to find
a CGA Parameter Data Structure that fulfills the following
o the prefix of HBA1 and the prefix of IPX are included in the
o HBA1 is included in the HBA set generated.
Note: this assumes that it is acceptable for the attacker to redirect
HBA1 to any address of the prefix of IPX.
The remaining fields that can be changed at will by the attacker in
order to meet the above conditions are: the Modifier, other prefixes
in the Multi-Prefix extension, and other extensions. In any case, in
order to obtain the desired HBA set, the attacker will have to use a
brute-force attack, which implies the generation of multiple HBA sets
with different parameters (for instance with a different Modifier)
until the desired conditions are meet. The expected number of times
that the generation process will have to be repeated until the
desired HBA set is found is exponentially related with the number of
bits containing hash information included in the interface identifier
of the HBA. Since 59 of the 64 bits of the interface identifier
contain hash bits, then the expected number of generations that will
have to be performed by the attacker are O(2^59). Note: We assume
brute force is the best attack against HBA/CGAs. Also, note that the
assumption that the Sec tool defined in  multiplies the attack
factor holds for brute-force attacks but may not hold for other
The protection against brute-force attacks can be improved by
increasing the Sec parameter. A non-zero Sec parameter implies that
steps 3-4 of the generation process will be repeated O(2^(16*Sec))
times (expected number of times). If we assimilate the cost of
repeating the steps 3-4 to the cost of generating the HBA address, we
can estimate the number of times that the generation is to be
repeated in O(2^(59+16*Sec)), in the case of Sec values of 1 and 2.
For other Sec values, Sec protection mechanisms will be defined by
the specifications pointed by the CGA SEC registry defined in RFC
11.1. Security Considerations When Using HBAs in the Shim6 Protocol
In this section, we will analyze the security provided by HBAs in the
context of a Shim6 protocol as described in Section 8 of this memo.
First of all, it must be noted that HBAs cannot prevent
man-in-the-middle (hereafter MITM) attacks. This means that in the
scenario described in Section 8, if an attacker is located along the
path between Host1 and Host2 during the lifetime of the
communication, the attacker will be able to change the addresses used
for the communication. This means that he will be able to change the
addresses used in the communication, adding or removing prefixes at
his will. However, the attacker must make sure that the CGA
Parameter Data Structure and the HBA set is changed accordingly.
This essentially means that the attacker will have to change the
interface identifier part of the addresses involved, since a change
in the prefix set will result in different interface identifiers of
the addresses of the HBA set, unless the appropriate Modifier value
is used (which would require O(2(59+16*Sec)) attempts). So, HBA
doesn't provide MITM attacks protection, but a MITM attacker will
have to change the address used in the communication in order to
change the prefix set valid for the communication.
HBAs provide protection against time shifting attacks , . In
the multihoming context, an attacker would perform a time shifted
attack in the following way: an attacker placed along the path of the
communication will modify the packets to include an additional
address as a valid address for the communication. Then the attacker
would leave the on-path location, but the effects of the attack would
remain (i.e., the address would still be considered as a valid
address for that communication). Next we will present how HBAs can
be used to prevent such attacks.
If the attacker is not on-path when the initial CGA Parameter Data
Structure is exchanged, his only possibility to launch a redirection
attack is to fake the signature of the message for adding new
addresses using CGA capabilities of the addresses. This implies
discovering the public key used in the CGA Parameter Data Structure
and then cracking the key pair, which doesn't seem feasible. So in
order to launch a redirection attack, the attacker needs to be
on-path when the CGA Parameter Data Structure is exchanged, so he can
modify it. Now, in order to launch the redirection attack, the
attacker needs to add his own prefix in the prefix set of the CGA
Parameter Data Structure. We have seen in the previous section that
there are two possible approaches for this:
1. Find the right Modifier value, so that the address initially used
in the communication is contained in the new HBA set. The cost of
this attack is O(2(59+16*Sec)) iterations of the generation
process, so it is deemed unfeasible.
2. Use any Modifier value, so that the address initially used in the
communication is probably not included in the HBA set. In this
case, the attacker must remain on-path, since he needs to rewrite
the address carried in the packets (if not, the endpoints will
notice a change in the address used in the communication). This
essentially means that the attacker cannot launch a time shifted
attack, but he must be a full-time man-in-the-middle.
So, the conclusion is that HBAs provide protection against time
HBAs do not provide complete protection against flooding attacks,
and, as a result, the SHIM6 protocol has other means to deal with
them. However, HBAs make it very difficult to launch a flooding
attack towards a specific address. It is possible though, to launch
a flooding attack against a prefix. And of course, the protection
that HBA offers applies only to nodes that employ it; HBA provides no
solution for general-purpose flooding-attack protection for other
Suppose that an attacker has easy access to a prefix PX::/nX and that
he wants to launch a flooding attack on a host located in the address
P:iid. The attack would consist of establishing communication with a
server S and requesting a heavy flow from it. Then simply
redirecting the flow to P:iid, flooding the target. In order to
perform this attack, the attacker needs to generate an HBA set
including P and PX in the prefix set, and be sure that the resulting
HBA set contains P:iid. In order to do this, the attacker needs to
find the appropriate Modifier value. The expected number of attempts
required to find such Modifier value is O(2(59+16*Sec)), as presented
earlier. So, we can conclude that such attack is not feasible.
However, the target of a flooding attack is not limited to specific
hosts, but it can also be launched against other elements of the
infrastructure, such as router or access links. In order to do that,
the attacker can establish a communication with a server S and
request a download of a heavy flow. Then, the attacker redirects the
communication to any address of the target network. Even if the
target address is not assigned to any host, the flow will flood the
access link of the target site, and the site access router will also
suffer the overload. Such attack cannot be prevented using HBAs,
since the attacker can easily generate an HBA set using his own
prefix and the target network prefix. In order to prevent such
attacks, additional mechanisms are required, such as reachability
11.2. Privacy Considerations
HBAs can be used as RFC 4941  addresses. If a node wants to use
temporary addresses, it will need to periodically generate new HBA
sets. The effort required for this operation depends on the Sec
parameter value. If Sec=0, then the cost of generating a new HBA set
is similar to the cost of generating a random number, i.e., one
iteration of the HBA set generation procedure. However, if Sec>0,
then the cost of generating an HBA set is significantly increased,
since it required O(2(16*Sec)) iterations of the generation process.
In this case, depending on the frequency of address change required,
the support for RFC 4941 address may be more expensive.
11.3. SHA-1 Dependency Considerations
Recent attacks on 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.
The aforementioned analysis for CGAs and their extensions (including
HBAs) is performed in RFC 4982 . The conclusion of the analysis
is that the security of the protocols using CGAs and their extensions
are not affected by the recently available attacks against hash
functions. In spite of that, the CGA specification  was updated
by RFC 4982  to enable the support of alternative hash functions.
11.4. DoS Attack Considerations
In order to use the HBA technique, the owner of the HBA set must
inform its peer about the CGA Parameter Data Structure in order to
allow the peer to verify that the different HBAs belong to the same
HBA set. Such information must then be stored by the peer to verify
alternative addresses in the future. This can be a vector for DoS
attacks, since the peer must commit resources (in this particular
case memory) to be able to use the HBA technique for address
verification. It is then possible for an attacker to launch a DoS
attack by conveying HBA information to a victim, imposing on the
victim to use memory for storing HBA related state, and eventually
running out of memory for other genuine operations. In order to
prevent such an attack, protocols that use the HBA technique should
implement proper DoS prevention techniques.
For instance, the Shim6 protocol  includes a 4-way handshake to
establish the Shim6 context and, in particular, to establish the HBA-
related state. In this 4-way handshake, the receiver remains
stateless during the first 2 messages, while the initiator must keep
state throughout the exchange of the 4 messages so that the cost of
the context establishment is higher in memory terms for the initiator
(i.e., the potential attacker) than for the receiver (i.e., the
potential victim). In addition to that, the 4-way handshake prevents
the usage of spoofed addresses from off-path attacker, since the
initiator must be able to receive information through the address it
has used as source address, enabling the tracking of the location
from which the attack was launched.
This document was originally produced by a MULTI6 design team
consisting of (in alphabetical order): Jari Arkko, Marcelo Bagnulo,
Iljitsch van Beijnum, Geoff Huston, Erik Nordmark, Margaret
Wasserman, and Jukka Ylitalo.
The initial discussion about HBA benefited from contributions from
Alberto Garcia-Martinez, Tuomas Aura, and Arturo Azcorra.
The HBA-set generation and HBA verification processes described in
this document contain several steps extracted from .
Jari Arkko, Matthew Ford, Francis Dupont, Mohan Parthasarathy, Pekka
Savola, Brian Carpenter, Eric Rescorla, Robin Whittle, Matthijs
Mekking, Hannes Tschofenig, Spencer Dawkins, Lars Eggert, Tim Polk,
Peter Koch, Niclas Comstedt, David Ward, and Sam Hartman have
reviewed this document and provided valuable comments.
The text included in Section 3.2 was provided by Eric Rescorla.
The author would also like to thank Francis Dupont for providing the
first implementation of HBA.
14.1. 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.
 Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
Neighbor Discovery (SEND)", RFC 3971, March 2005.
 Bassham, L., Polk, W., and R. Housley, "Algorithms and
Identifiers for the Internet X.509 Public Key Infrastructure
Certificate and Certificate Revocation List (CRL) Profile",
RFC 3279, April 2002.
 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 5280, May 2008.
 Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
 Narten, T., Draves, R., and S. Krishnan, "Privacy Extensions
for Stateless Address Autoconfiguration in IPv6", RFC 4941,
 Bagnulo, M. and J. Arkko, "Cryptographically Generated
Addresses (CGA) Extension Field Format", RFC 4581,
 Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming Shim
Protocol for IPv6", RFC 5533, June 2009.
 Bagnulo, M. and J. Arkko, "Support for Multiple Hash Algorithms
in Cryptographically Generated Addresses (CGAs)", RFC 4982,
14.2. Informative References
 Nordmark, E. and T. Li, "Threats Relating to IPv6 Multihoming
Solutions", RFC 4218, October 2005.
 Nikander, P., Arkko, J., Aura, T., Montenegro, G., and E.
Nordmark, "Mobile IP Version 6 Route Optimization Security
Design Background", RFC 4225, December 2005.
 Arkko, J., Vogt, C., and W. Haddad, "Enhanced Route
Optimization for Mobile IPv6", RFC 4866, May 2007.
 Hoffman, P. and B. Schneier, "Attacks on Cryptographic Hashes
in Internet Protocols", RFC 4270, November 2005.
 Bellovin, S. and E. Rescorla, "Deploying a New Hash Algorithm",
 Nordmark, E., "Multi6 Application Referral Issues", Work
in Progress, October 2004.
 Bagnulo, M., Garcia-Martinez, A., and A. Azcorra, "Efficient
Security for IPv6 Multihoming", ACM Computer Communications
Review Vol 35 n 2, April 2005.
 Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, October 2005.
 Vixie, P., Thomson, S., Rekhter, Y., and J. Bound, "Dynamic
Updates in the Domain Name System (DNS UPDATE)", RFC 2136,
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