Internet DRAFT - draft-cheneau-send-sig-agility
draft-cheneau-send-sig-agility
CGA & SEND maintenance T. Cheneau
Internet-Draft M. Maknavicius
Updates: RFC3971 TMSP
(if approved) S. Shen
Expires: December 7, 2009 Huawei
M. Vanderveen
Qualcomm
June 5, 2009
Signature Algorithm Agility in the Secure Neighbor Discovery (SEND)
Protocol
draft-cheneau-send-sig-agility-01
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Abstract
This draft describes a mechanism to enable the Secure Neighbor
Discovery (SEND) protocol to select between different signature
algorithms to use with Cryptographically Generated Addresses (CGA).
It also provides optional support for interoperability between nodes
that do not share any common signature algorithms.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Compatibility with existing specifications . . . . . . . . 4
2.1.1. Classification of SEND nodes . . . . . . . . . . . . . 4
2.1.2. Principal Scenarios . . . . . . . . . . . . . . . . . 6
2.2. Agility Requirements . . . . . . . . . . . . . . . . . . . 7
2.3. Mechanism for Agility Support of CGA and SEND . . . . . . 8
3. Supported Signature Algorithm Option . . . . . . . . . . . . . 9
3.1. Neighbor Cache interactions . . . . . . . . . . . . . . . 10
3.2. Processing Rules for Senders . . . . . . . . . . . . . . . 10
3.3. Processing Rules for Receivers . . . . . . . . . . . . . . 10
4. SEND Universal Signature Option . . . . . . . . . . . . . . . 12
4.1. Processing Rules for Senders . . . . . . . . . . . . . . . 14
4.2. Processing Rules for Receivers . . . . . . . . . . . . . . 15
5. Basic negotiation . . . . . . . . . . . . . . . . . . . . . . 17
5.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 17
5.2. Sending Unsolicited Messages . . . . . . . . . . . . . . . 20
6. Router-as-a-notary function . . . . . . . . . . . . . . . . . 21
6.1. Signature Check Request Message . . . . . . . . . . . . . 21
6.2. Signature Status Message . . . . . . . . . . . . . . . . . 23
6.3. Using notary for DAD procedure . . . . . . . . . . . . . . 25
7. Security Considerations . . . . . . . . . . . . . . . . . . . 26
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 29
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
10.1. Normative References . . . . . . . . . . . . . . . . . . . 30
10.2. Informative References . . . . . . . . . . . . . . . . . . 30
Appendix A. On the number of Public Keys supported per CGA . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 36
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1. Introduction
The usage scenarios associated with neighbor discovery have recently
been extended to include environments with mobile or nomadic nodes.
Many of these nodes have limited battery power and computing
resources. Therefore, heavy public key signing algorithms like RSA
are not feasible to support on such constrained nodes. Fortunately,
more lightweight yet secure signing algorithms do exist and have been
standardized, e.g. Elliptic Curve based algorithms.
It is then a worthwhile goal to extend secure neighbor discovery to
support signing and corresponding hashing algorithm agility. Besides
accommodating power-constrained nodes, signing and hashing algorithm
agility is also desired as a safety measure over time, to offer
alternatives when cryptanalysis of one type of algorithm makes
significant progress.
The aim of this memo is to outline options for allowing public key
signing algorithm and hashing algorithm agility for nodes configured
to perform secure neighbor discovery operations. The extent to which
these options impact existing specifications [RFC3971] and [RFC3972]
is also addressed.
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2. Overview
2.1. Compatibility with existing specifications
The current SEND protocol specification, [RFC3971], mandates the use
of the RSA signature algorithm. Since the time of its writing,
different signature algorithms have been shown to be secure and have
been adopted by other protocols in an effort to reduce key length,
signature generation and verification time, and increase security
level. This shift in signature algorithm adoption particularly
benefits lightweight devices, which are power and memory-limited but
in need of secure signing algorithms support. For these reasons, we
feel that the restriction on the signature algorithm for SEND is no
longer warranted.
2.1.1. Classification of SEND nodes
At the time of this writing, there are no known large-scale or even
small-scale deployments of [RFC3971]-compatible devices. However, in
the interest of caution, we assume that there exist nodes that
support only the RSA algorithm and that are configured to perform
secure neighbor discovery. Such nodes may not be updated in the near
term or for the foreseeable future. On the other hand, it appears
that there will be deployments of nodes that support only Elliptic
Curve Cryptography as their public key algorithm, i.e. ECDSA as a
signature algorithm, rather than traditional RSA.
To ensure that all possible network/link configurations are
considered when designing a signature agility solution, we categorize
nodes (hosts and routers) according to their support for different
signature algorithms, as follows:
Type H1 host:
A host that only supports one type of signature algorithm and has
a CGA generated with the public key of this algorithm.
Examples of this type of hosts: an old host that does not support
signature agility, i.e. only supports RSA signature algorithm; or,
a host that only supports ECDSA signature.
Type H2 host:
A host that supports multiple signature algorithms and has a CGA
generated with only one key selected from among its supported
algorithms.
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Examples of this type of hosts: (1) a host that supports RSA and
ECDSA signature algorithms, but only has a CGA derived with an RSA
public key; (2) a host that supports RSA and ECDSA signature
algorithms, but only has a CGA derived with an ECC public key.
Type H3 host:
A host that supports multiple signature algorithms and has a CGA
generated with multiple keys of different supported algorithms.
Such CGA generation is made possible by the introduction of a new
CGA extension (see companion draft [cheneau-cga-pk-agility]).
Such hosts can be compatible with hosts of other types for secure
neighbor discovery.
Type H4 host:
A host that supports multiple signature algorithms and has
multiple CGAs, each of which is associated with a single key of
one supported algorithm. For simplicity, we do not consider hosts
that have multiple CGAs, one or more of which are generated from
multiple public keys.
A node MUST select and settle on one CGA when building a trust
relationship with another device via SeND (more below). In such
cases, a destination node may be reached at a CGA associated with
a signature algorithm that the originating node cannot verify.
The destination node will need to securely redirect the
originating node to one of its other CGA(s) (presumably with a
common signature algorithm). The need for a method to secure the
binding between the two CGAs of the destination node is still an
open problem.
Based on this reasoning, consideration of H4 type nodes is left
for future work.
Routers are more likely to possess the resources necessary to support
multiple signature and hashing algorithms. It is also more feasible
that routers employ certificates. However, for a basic signature
agility solution, we do not mandate that routers support multiple
signature and hashing algorithms.
Possible router devices with different signature algorithm support
ability are:
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Type R1 router:
A router that only supports one type of signature algorithm and
has a CGA and Certificate with a public key of this algorithm.
Such routers are expected to be commonplace, as compliance with
[RFC3971] suffices for them.
Type R2 router:
A router that supports multiple types of signature algorithms and
has one CGA and Certificate with a public key of one of the
algorithm types.
This type of router can sign and verify signatures of the type of
certificate it owns, and additionally, it can verify signatures of
other algorithm types.
Type R3 router:
A router that supports multiple types of signature algorithms and
has one CGA composed of multiple Publics Keys and multiple
certificates containing each a Public Key.
Type R4 router:
A router that supports multiple types of signature algorithms and
has multiple CGAs and Certificates with public key of several
different algorithm types.
This type of router can sign and verify signatures of multiple
types. Such routers may not be attractive to build and deploy due
to increased requirements on its resources. Moreover using
multiple CGAs (with no bindings) may make that routers appear as
having multiple identities.
Note that all types of router presented above can be configured to
use SEND over multiple interfaces or to have multiple addresses on
the same interface. In this case, the router will use separate CGAs.
Such configuration is treated in this draft as if the different
addresses refer to separate entities.
2.1.2. Principal Scenarios
Based on the discussion above, a SEND agility solution should at
least properly deal with the communication between devices of type
H1, H2, H3, R1, R2 and R3.
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An H1 or R1 node interacting with an H2 or R2 node: i.e., a node
supporting only RSA (for example, an old non-agility node which
only supports RFC3971) and a node supporting both RSA and ECDSA
(or other new algorithms). These two nodes may be able to perform
secure neighbor discovery.
An H1 or R1 node interacting with another H1 or R1 node, but their
algorithms differ: e.g., a node supporting only RSA (for example,
an old non-agility node which only supports RFC3971) and a node
supporting only ECDSA (or other new algorithms). In this case,
implementations supporting SEND signature agility solution may
likely realize the incompatibility, while older implementations
may not.
A node of any type (H1, H2, H3, R1, R2, R3) interacting with
another node, their algorithms differ but there is a 3rd party
willing/able to help: this is an optional solution applicable to
the previous scenario, where two nodes that support SEND but do
not have any signature algorithms in common can talk through a
third party (router). In this case they should be able to perform
facilitated secure neighbor discovery.
An H2, H3 or R2 node interacting with another H2, H3, or R2 node:
e.g., two nodes that support at least one signature algorithms in
common will be able to perform secure neighbor discovery.
An additional rule for H2, H3 or R2, R3 node interacting with
another H2, H3, or R2, R3 node applies: two nodes that support two
or more signature algorithms in common (one of which is likely
preferred over the other), will be able to perform secure neighbor
discovery with any of these signature algorithms.
2.2. Agility Requirements
We hold the following to be requirements on a signing algorithm
agility solution for SEND:
o A Signature-Algorithm-Agility-Node should be able to communicate
with a Non-Signature-Algorithm-Agility-Node, but not necessarily
employ SEND. Traditional ND should suffice, to accommodate nodes
that only support one type of Signature Algorithm, which may not
be RSA. Local policy MAY disable this behavior, namely the use of
unsecured ND messages when communicating with a node that does not
share any common signature algorithm.
o Two Signature-Algorithm-Agility nodes that support a common
Signature Algorithm and hashing algorithm should be able to
communicate using SEND and sign messages using the common
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Signature Algorithm and hash algorithm.
o The current SEND/CGA specifications should incur as few changes as
possible.
2.3. Mechanism for Agility Support of CGA and SEND
To achieve signature agility for SEND, it must be possible for a CGA
to be generated from and to be securely associated with multiple
public keys corresponding to different signature algorithms. This
capability is described in the companion draft
[cheneau-cga-pk-agility].
This document proposes an update to [RFC3971] to allow two SEND nodes
to choose an appropriate signature algorithm. This solution
encompasses the following:
o A "Supported Signature Algorithm" Neighbor Discovery Protocol
option which contains a list of signing and hashing algorithms
that the sender node supports for SEND purposes and its
interaction with the Neighbor Cache;
o A modification of the "RSA Signature" option defined in the SEND
specification;
o An optional solution to support secure communication through a
router acting as a third party when nodes don't share any common
Signature Algorithm.
We define the aforementioned options format and provide processing
rules for both senders and receivers of SEND messages employing the
new options, as well as example negotiation message flows.
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3. Supported Signature Algorithm Option
The Supported Signature Algorithm NDP option contains a list of
signing and hashing algorithm pairs that the sender node supports.
The format of this option is described in Figure 1:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sig. Alg. 1 | Sig. Alg. 2 | Sig. Alg 3. | Sig. Alg 4. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
| ... |
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... | Sig. Alg. N |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Supported Signature Algorithm option
Type
NDP option type, TBA. See Section 8.
Length
The length of the option (including the Type, Length fields), in
octets. 8-bit unsigned integer, the values lower that 2 are
invalid.
Reserved
Reserved for future use. This 16-bit field MUST be set to zero by
the sender, and MUST be ignored by the receiver.
Signature Algorithm
A one-octet long field indicating a signature algorithm and the
corresponding hash algorithm that this node supports; this support
implies at least ability to verify signatures of this PK
algorithm.
The first leftmost bit, bit 0, if set to 0, indicates that the
emitter is able to perform signature checks only (i.e. no
signature generation with this type of signature algorithm). If
this bit is set to 1, it indicates that the emitter has a public
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key of this type and can generate signatures. Bit 1 and 2 are
reserved. Bit 3 to 7 are named Signature Type Identifier subfield
and encode an identifier for the signature algorithm and
corresponding hash algorithm. Default values for the Signature
Type Identifier subfield defined in this document are taken in
part from the IANA-defined numbers for the IKEv2 protocol, i.e.
IANA registry named "IKEv2 Authentication Method":
* Value 0 is RSA/SHA-1
* Value 1 is RSA/SHA-256
* Value 9 is ECDSA with SHA-256 on the P-256 curve
* Value 10 is ECDSA with SHA-384 on the P-384 curve
* Value 11 is ECDSA with SHA-512 on the P-521 curve
The Signature/hash Algorithm combinations SHOULD be included in
order of preference.
A SSA option MAY be built to respect a Local Policy. However, the
SSA option MUST not indicate Signature Algorithm(s) that the
emitting node's CGA does not support and MUST contain at least one
Signature Algorithm with the first bit on (i.e. this Signature
Algorithm is available for signature generation).
3.1. Neighbor Cache interactions
Neighbor Cache MUST have the ability to store Supported Signature
Algorithm information for each entry (i.e. IPv6 address). Supported
Signature Algorithm information for an entry MAY be empty (e.g. entry
created by a RFC 3971 node or an unverifiable message).
3.2. Processing Rules for Senders
If a node has been configured to use SEND, then all Neighbor
Solicitation, Neighbor Advertisement, Router Solicitation, Router
Advertisement, and Redirect messages it sends MUST contain the
Supported Signature Algorithm option. This option MUST contain in
the Signing Algorithm field all the signature algorithms it is
willing to use in signature generation and verification.
3.3. Processing Rules for Receivers
Upon receiving a SEND packet with a Supported Signature Algorithm
Option, a receiver performs the following operations:
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o when the message is a Neighbor Solicitation or a Router
Solicitation, the receiving node computes the intersection between
the set of Supported Signature Algorithm indicated by the option
and its own. If the set is empty, this means the node will not be
able to use a Signature Algorithm that the initiating node can
check. Given the local policy, a receiver node MAY still respond
to the received message using its "preferred" Signature Algorithm
(even if the node knows the receiver will not be able to verify
the Signature Algorithm). If the set is not empty, the receiving
node will choose among the set one of the algorithms in order to
generate a Universal Signature Option.
o If the message pass the SEND verifications (CGA verification,
Timestamp, Nonce, Universal Signature Option verification) and
contains a Supported Signature Algorithm Option, the information
of the Supported Signature Algorithm in the Neighbor Cache is
updated by the information contained in the Supported Signature
Option attached to the message.
o If the message does not pass the SEND verifications because of a
unverifiable RSA Signature Option or Universal Signature Option,
if it contains a Supported Signature Algorithm Option, and the
Neighbor Cache entry associated to that node does not contain any
information about the Supported Signature Algorithm, the Neighbor
Cache entry SHOULD be updated with the information contained in
the Supported Signature Algorithm Option.
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4. SEND Universal Signature Option
We propose replacing the RSA Signature Option by a new algorithm-
independent signature option. The "Universal Signature Option" is an
updated version of the RSA Signature Option, that allows a node to
specify which of its potential multiple keys it is using. To achieve
this, we use the 16-bit reserved field of the RSA Signature Option,
and define a new 8-bit field that contains the position of the Public
Key associated with the signature and a new 5-bit Signature Type
Identifier field that details the type of algorithms used to generate
the Digital Signature.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Key Position | Res.| Sig ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Key Hash |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Digital Signature .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Padding .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Universal Signature Option format
Type
Same value as in [RFC3971]: 12.
Length
The length of the option (including the Type, Length, Reserved,
Key Hash, Digital Signature, and Padding fields) in units of 8
octets.
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Key Position
An 8-bit field indicating which Public Key in the CGA parameter
structure (carried in the CGA option) has been used to compute the
Digital Signature. The index starts at 0, meaning the key is the
one in the Public Key field. Values greater than 1 refer to
Public Key found in the CGA Extension field (as defined in the
companion document [cheneau-cga-pk-agility]]). Value 255 is a
reserved value that indicates no CGA option in the message
contains the Public Key.
Reserved
A 3-bit field reserved for future use. The value MUST be set to
zero by the sender and MUST be ignored by the receiver.
Signature Type Identifier
Signature Type Identifier is a 5-bit field. It corresponds to the
Signature Type Identifier subfield (bits 3 to 7 of the Signature
Algorithm field) in the Supported Signature Algorithm option . It
indicates the type of signature contained in the Digital Signature
field.
Key Hash
A 128-bit field containing the most significant (leftmost) 128
bits of a hash of the public key used for constructing the
signature. It is computed using the same hash function as used in
generating digital signature (indicated in Signature Type
Identifier). The hash value is computed over the presentation
used in the Public Key field of the CGA Parameters data structure
carried in the CGA option. Its purpose is to associate the
signature with a particular key known by the receiver. Such a key
can either be stored in the certificate cache of the receiver or
be received in the CGA option in the same message.
Digital Signature
A variable-length field containing a signature constructed by
using the sender's private key associated to the public key
pointed by the Key Position field. The signature type is
determined from the value of the Signature Type Identifier field.
If the value of the Signature Type Identifier field is 0, then the
Key Position field must be set to 0 and this Digital Signature
field is computed the same way as the Digital Signature field of
the RSA Signature Option described in [RFC3971]. If the value of
the Signature Type Identifier field is 1, then this Digital
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Signature field is computed the same way as the Digital Signature
field of the RSA Signature Option described in [RFC3971] except
that the signature is computed with the RSASSA-PKCS1-v1_5
algorithm as defined in [PKCS1] and hash function is SHA-256. If
the value of the Signature Type Identifier field is 9, 10 or 11,
then this Digital Signature field is computed using the ECDSA
signature algorithm (as defined on [SEC1]) and hash function
defined in Signature Type Identifier on the following data:
1. The 128-bit CGA Message Type tag [RFC3972] value for SEND,
0x086F CA5E 10B2 00C9 9C8C E001 6427 7C08. (The tag value has
been generated randomly by the editor of the [RFC3971]
specification.).
2. The 128-bit Source Address field from the IP header.
3. The 128-bit Destination Address field from the IP header.
4. The 8-bit Type, 8-bit Code, and 16-bit Checksum fields from
the ICMP header.
5. The NDP message header, starting from the octet after the ICMP
Checksum field and continuing up to but not including NDP
options.
6. All NDP options preceding, but not including, any of the
Universal Signature options.
This field starts after the Key Hash field. The length of the
Digital Signature field is determined by the length of the
Universal Signature option minus the length of the other fields
(including the variable length Pad field).
Padding This variable-length field contains padding, as many bytes
long as remain after the end of the signature.
A Neighbor Solicitation/Advertisement, Router Solicitation/
Advertisement and Redirect message MAY contain more than one
Universal Signature Option, as long as it does not exceed the MTU.
This is particularly useful for routers operating in heterogeneous
networks, where hosts have a disjoint set of supported signature
algorithms. For information on how to compute the message size, see
Appendix A.
4.1. Processing Rules for Senders
When sending a SEND message spontaneously, an emitter node CAN choose
a signature algorithm of its preference (defined by its local policy)
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among the corresponding Public Keys carried in the CGA option. Using
this signature algorithm, the node computes the Digital Signature and
fills the Key Position field with the position of the key in the CGA
parameter data structure.
If the node has been configured to use SEND, then all Neighbor
Solicitation, Neighbor Advertisement, Router Advertisement, and
Redirect messages MUST contain at least one Universal Signature
option. Router Solicitation messages not sent with the unspecified
source address MUST contain the Universal Signature option.
A node sending a message with one or more Universal Signature
option(s) MUST construct the message as follows:
o If the node has previously received hints (e.g. a NDP message with
a Supported Signature Algorithm option or an entry in the Neighbor
Cache) on the type of Signature Algorithm it should use, it MUST
restrict its choice on those Signature Algorithms.
o The message is then constructed in its entirety, without any of
the Universal Signature options.
o The Universal Signature option(s) is (are) added as the last
option in the message.
o The data to be signed is constructed as explained in [RFC3971],
under the description of the Digital Signature field.
o The message, in the form defined above, is signed by using the
configured private key associated to the selected Signature
Algorithm, and the result signature is is encapsulated into the
Digital Signature field.
4.2. Processing Rules for Receivers
Neighbor Solicitation, Neighbor Advertisement, Router Advertisement,
and Redirect messages without any Universal Signature option or with
an unverifiable Universal Signature option MUST be treated as
unsecured (i.e., processed in the same way as NDP messages sent by a
non-SEND node). See Section 8 of [RFC3971].
Router Solicitation messages without any Universal Signature option
MUST also be treated as unsecured, unless the source address of the
message is the unspecified address.
Redirect, Neighbor Solicitation, Neighbor Advertisement, Router
Solicitation, and Router Advertisement messages containing one or
more Universal Signature option MUST be checked as follows:
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o The receiver MUST ignore any options that come after the first
Universal Signature option. (The options are ignored for both
signature verification and NDP processing purposes.)
o The Key Hash field MUST correspond to a known public key, either
one learned from the CGA option in the same message by the
position indicated in the Key Position field message, or one known
by other means.
o The Digital Signature field MUST have correct encoding and MUST
not exceed the length of the Universal Signature option minus the
Padding.
o The Digital Signature verification MUST show that the signature
has been calculated as specified in the previous section.
o If the use of a trust anchor has been configured, a valid
certification path (see Section 6.3 of [RFC3971]) between the
receiver's trust anchor and the sender's public key MUST be known.
Messages that do not pass all the above tests MUST be silently
discarded if the host has been configured to accept only secured ND
messages. The messages MAY be accepted if the host has been
configured to accept both secured and unsecured messages but MUST be
treated as unsecured messages. The receiver MAY also otherwise
silently discard packets (e.g., as a response to an apparent CPU
exhausting DoS attack).
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5. Basic negotiation
5.1. Overview
This section describes different configuration of SEND-enabled nodes
with varying signing capabilities and their interaction during the
negotiation phase.
Case 1: when both nodes support the same two Signature Algorithms,
they can pick the Signature Algorithm they prefer for signing and are
able to verify each others signature. Figure 3 is an example of such
a message flow.
Node A Node B
NS
{CGA option,
RSA Signature option.
Supported-Signature-Algo option
(RSA sign & verif, ECC sign & verif)}
-------->
NA
{CGA option,
ECC Signature option
Supported-Signature-Algo option
(ECC sign & verif, RSA sign & verif)}
<--------
IPv6 traffic <-------> IPv6 traffic
Figure 3: Basic negotiation - Case 1
Case 2: two nodes sharing at least one common Signing Algorithm must
be able to securely communicate. Figure 4 is an example of such a
message flow.
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Node A Node B
NS
{CGA option,
RSA Signature option.
Supported-Signature-Algo option
(RSA sign & verif, ECC sign & verif)}
-------->
NA
{CGA option,
ECC Signature option
Supported-Signature-Algo option
(ECC sign & verif)}
<--------
(At this point, Node B could not
authenticate Node A's Neighbor
Solicitation)
--------> (unidirectionnal) IPv6 traffic
NS
{CGA option,
ECC Signature option
Supported-Signature-Algo option
(ECC sign & verif)}
<--------
NA
{CGA option,
ECC Signature option.
Supported-Signature-Algo option
(RSA sign & verif, ECC sign & verif)}
-------->
IPv6 traffic <-------> IPv6 traffic
Figure 4: Basic negotiation - Case 2
Case 3: when two nodes have a disjoint set of Signature Algorithm
support for signing, but the two nodes are able to verify each
others, a full negotiation is possible. Figure 5 is an example of
such a message flow.
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Node A Node B
NS
{CGA option,
RSA Signature option.
Supported-Signature-Algo option
(RSA sign & verif, ECC verif only)}
-------->
NA
{CGA option,
ECC Signature option
Supported-Signature-Algo option
(ECC sign & verif, RSA verif only)}
<--------
IPv6 traffic <-------> IPv6 traffic
Figure 5: Basic negotiation - Case 3
Case 4: when two nodes have a disjoint set of Signature Algorithm
support for signing, but one node is able to verify, a partial
negotiation is possible. Figure 6 is an example of such a message
flow.
Node A Node B
NS
{CGA option,
RSA Signature option.
Supported-Signature-Algo option
(RSA sign & verif)}
-------->
NA
{CGA option,
ECC Signature option
Supported-Signature-Algo option
(ECC sign & verif, RSA verif only)}
<--------
(...depending on local policies...)
IPv6 traffic <-------> IPv6 traffic
Figure 6: Basic negotiation - Case 4
Section 6 describes an optional functionality that allow nodes in
Case 4 to perform a trustful complete negotiation.
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5.2. Sending Unsolicited Messages
When sending unsolicited message, a node MAY have to rely on the
entries of its Neighbor Cache. The Neighbor Cache will provide hints
concerning the Signature Algorithm supported by the neighbors.
Neighbor Cache can assist the node in the Signature Algorithm
selection process when:
o A router advertises unsolicited Router Advertisement message to
the All-Nodes multicast address (e.g. to indicate a prefix
lifetime is going down to 0). The router needs to know which
signature algorithm(s) to use in order to send verifiable messages
to hosts. To do so, the router MAY rely on the Neighbor Cache and
compute an intersection of the set of all Supported Signature
Algorithms. The router will then be able to advertise a Router
Advertisement signed multiple times with the resulting subset of
Supported Signature Algorithms or advertise multiple Router
Advertisements, each signed with a single Signature Algorithm part
of the intersection.
o A node sends unsolicited Neighbor Advertisement (e.g. when
changing its Link-Layer address). This is similar to the previous
problem and can also be solved using the Neighbor Cache the same
way.
o A router sends a Redirect message to a host. Choosing a supported
signature algorithm without probing the node can be difficult.
However, Neighbor Cache will most likely contain an entry for the
host, prior to the decision to send a Redirect message, because of
the Address Resolution process. This entry should contain
information on the Supported Signature Algorithm(s) and thus
provide hints concerning the Signature Algorithm to choose to sign
the Redirect messages.
Note that the information on the neighbors with which a communication
has occurred recently or is ongoing are in the Neighbor Cache and are
maintained up to date through the Neighbor Unreachability Detection
procedure.
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6. Router-as-a-notary function
This optional functionality enhances backward compatibility by
introducing a new entity. This new entity, named "notary", certifies
the authenticity of a node's message. This improves communication
when, for example, two nodes have a disjoint set of supported
Signature Algorithm types and still require secure neighbor
discovery.
In this specification, the notary function is offered by routers,
although other nodes may offer this capability in the future
specification. Authorization for the router to act as a notary is
shown through router's certificate in a KeyPurposeID as defined in
[krishnan-cgaext-send-cert-eku] and provided by the trust anchor.
The notary function requires the two specific ICMP messages:
signature check request message and signature status message.
6.1. Signature Check Request Message
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Packet Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Request ID. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ +
| SEND secured packet |
~ (NDP packets should fit completely) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-
Signature Check Request Message format
Type
TBA.
Code
TBA.
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Packet Length
Packet length is the size of the SEND secured packet
Checksum
Checksum is a CRC-16 of the whole packet. During the CRC-16
computation, this field is set to 0. The purpose of this field is
to quickly invalidate transmission errors.
Reserved
This 16-bit field is reserved. MUST be set to 0 by senders and
ignored by receivers.
Request Identifier
Request Identifier helps matching a signature check request and
the signature status (response) messages. Request Identifier
field is randomly generated.
SEND secured packet
SEND secured packet is the packet that the node was not able to
verify on his own, subject of the verification. Note that the
encapsulated packet MUST not make the whole Signature Check
Request message exceed the MTU (as no fragmentation support is
available).
Options
This field contains one or more NDP options. Currently, only one
option is mandatory in this field. It is the Supported Signature
Algorithm option, that allows the notary to choose a correct
signature algorithm to sign the Signature Status message.
Note that this message MAY be protected by usual SEND NDP options
(CGA option, Timestamp, Nonce, Universal Signature Option). In this
case, the Universal Signature Option contains the whole packet that
the node wants to be checked on the router (so packet may not be
tampered with).
A router acting as a notary processes the packet as follows:
if the packet is protected with SEND options, the notary:
* Verifies the CGA of the emitter
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* Verifies the Universal Signature Option of the message (linked
to CGA of the source address). If more than one Universal
Signature Options are in the message, the notary can decide to
check any of them.
* Verifies the CGA and signature of the SEND secured packet
(inner packet).
* Responds with a Signature status message (defined in the
following section) indicating the status of the SEND secured
packet Universal Signature Option.
if the packet is not protected, the notary:
* verifies the CGA and signature of the SEND secured packet
(inner packet). If more than one Universal Signature Option
are in the message, the notary can decide to check any of them.
* Responds with a Signature status message (defined in the
following section) indicating the status of the SEND secured
packet Universal Signature Option.
6.2. Signature Status Message
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Status |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Request ID. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Hash |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-
Signature Status Message format
Type
TBA.
Code
TBA.
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Status
The 16-bit status field can be set to any of the following values:
0: all validation checks passed
1: Signature Check Request message checksum failed
2: inner packet CGA verification check failed
3: inner packet signature verification check failed
4: unsupported hash algorithm (to compute Hash1/Hash2)
5: unsupported Public Key algorithm
6: ask later (router is busy)
Request Identifier
The Request Identifier helps matching a signature check request
and the signature status (response) message. The Request
Identifier is copied from the Signature Check Request message.
Hash
The Hash field contains the result of a hash function applied on
the Request ID field and on the Send Secured Packet field of the
Signature Check Request message. The hash function is the same as
the one in the Key Hash field of the Universal Signature Option
that will protect this message.
Options
This field contains one or more NDP options. Mandatory options
are CGA Option, Timestamp Option and Universal Signature Option.
Universal Signature Option MUST be the last option.
This message is a response to a Signature Check Request message and
is protected by SEND options generated using a public key contained
in a certificate of the router authorized to act as notary. If the
Signature Check Request message is protected by the Nonce option,
this option MUST be copied in the Signature Status message.
On reception of this message, a requesting node performs CGA
verification, Nonce (if included in the initial request) and
Timestamp checks, and Universal Signature Option check. If any of
those test fails, the packet is dropped and an error MAY be logged.
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Then, if the status message is 0, that node can treat the original
packet that created the need for a Notary Signature Check Request
message as a secured packet. On a status value different from 0, the
packet will be considered as unsecure and be treated as such. Status
value MAY be loged for further diagnosis.
6.3. Using notary for DAD procedure
When performing the DAD procedure, a node can receive Neighbor
Solicitation or Neighbor Advertisement that are protected by a
Universal Signature Option the node can not check. In this specific
case, the node can ask the notary to check the signature for him.
However, the node, while performing DAD, MUST send the Signature
Check Request message using the unspecified address as source
address. The notary MUST respond with a Signature Status message
directed to the All-Node multicast address.
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7. Security Considerations
+-----------------------+-----------------------+
| RSA key length (bits) | ECC key length (bits) |
+-----------------------+-----------------------+
| 3072 | 256 |
| | |
| 7680 | 384 |
| | |
| 15380 | 512 |
+-----------------------+-----------------------+
Equivalence between Elliptic Curves and RSA security levels
Table 1: Security level equivalence between ECC and RSA
Section 4 presents a new Universal Signature Option. A recommended
use of this option is to allow signatures of equivalent security
level (i.e. Public Keys with equivalent key lengths) as shown in
Table 1. See also section 4 of the companion draft
[cheneau-cga-pk-agility].
Usage of SHA-1 for signature is strongly NOT RECOMMENDED, and when
available should be preferred by the usage of SHA-256. SHA-1
security is been proved to be flawed in the light of recent attacks
[Recent SHA-1 Attack] [NIST-st].
The Universal Signature Option is vulnerable to downgrade attacks.
That is, given that a node can employ multiple signature types, an
attacker may choose to use a flawed one. To mitigate this issue,
nodes are allowed, on a local policy, to refuse to check certain
types of signature (i.e. those which are know to be flawed) and will
treat the associated messages as unsecured. When trying to
completely mitigate downgrade attacks, an administrator MAY deploy
SEND-secured nodes only authorizing a single signature algorithm
scheme. This comes at a price of a reduced interoperability.
Section 6 introduces an optional notary functionality that offers to
nodes to check messages on their behalf, involving heavy
cryptographic computation. This can lead to flooding attacks and
Denial of Services. However, Neighbor Discovery Protocol [RFC4861]
and Secure Neighbor Discovery Protocol [RFC3971] are already prone to
flooding attacks. One possible solution is to use rate limiting on
Signature Check Request messages.
Notary functionality is also vulnerable to "Good Router Goes Bad"
attacks (as described in [RFC3756]). Notary can make node trust
unsecured packets and drop valid ones. This issue can be mitigated
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when multiple notaries are present on a link. The node can use a
round-robin algorithm to load-balance the Signature Check Request
message, thus reducing the risk of cache poisoning by a compromised
notary.
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8. IANA Considerations
This document requests IANA to allocate types for the two new notary
ICMP messages.
Section 3 defines a Signature Type Identifier subfield containing new
values corresponding to different Signature Algorithm. This document
requests creation of a new registry to the IANA.
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9. Acknowledgments
The authors gratefully acknowledge the contributions of Marcelo
Bagnulo, Gabriel Montenegro, Greg Daley, Dave Thaler, Steve Kent,
Jari Arko, and Francis Dupont for their helpful feedback.
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10. References
10.1. Normative References
[RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)",
RFC 3972, March 2005.
[RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
Neighbor Discovery (SEND)", RFC 3971, March 2005.
[RFC4982] Bagnulo, M. and J. Arkko, "Support for Multiple Hash
Algorithms in Cryptographically Generated Addresses
(CGAs)", RFC 4982, July 2007.
[cheneau-cga-pk-agility]
Cheneau, T., Laurent-Maknavicius, M., Shen, S., and M.
Vanderveen, "Support for Multiple Signature Algorithms in
Cryptographically Generated Addresses (CGAs)",
draft-cheneau-cga-pk-agility-01 (work in progress),
June 2009.
10.2. Informative References
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC3756] Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor
Discovery (ND) Trust Models and Threats", RFC 3756,
May 2004.
[RFC4581] Bagnulo, M. and J. Arkko, "Cryptographically Generated
Addresses (CGA) Extension Field Format", RFC 4581,
October 2006.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
[NIST-st] National Institute of Standards and Technology, "NIST
Comments on Cryptanalytic Attacks on SHA-1",
<http://csrc.nist.gov/groups/ST/hash/statement.html>.
[krishnan-cgaext-send-cert-eku]
Krishnan, S., Kukec, A., and K. Ahmed, "Certificate
profile and certificate management for SEND",
draft-krishnan-cgaext-send-cert-eku-03 (work in progress),
March 2009.
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[FIPS-186-3]
National Institute of Standards and Technology, "Draft
Digital Signature Standard", FIPS PUB 186-3, March 2006.
[PKCS1] RSA Laboratories, "RSA Encryption Standard, Version 2.1",
PKCS 1, November 2002.
[FIPS.180-2]
National Institute of Standards and Technology, "Secure
Hash Standard", FIPS PUB 180-2, August 2002, <http://
csrc.nist.gov/publications/fips/fips180-2/fips180-2.pdf>.
[SEC1] Standards for Efficient Cryptography Group, "SEC 1:
Elliptic Curve Cryptography", September 2000,
<http://secg.org>.
[Recent SHA-1 Attack]
McDonald, C., Haukes, P., and J. Pieprzyk, "SHA-1
collisions now 2^52", May 2009, <http://
eurocrypt2009rump.cr.yp.to/
837a0a8086fa6ca714249409ddfae43d.pdf>.
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Appendix A. On the number of Public Keys supported per CGA
+------------------+--------------+---------------------------------+
| RSA key length | Public | Size of the DER-encoded Public |
| (bits) | exponent | Key (bytes) |
+------------------+--------------+---------------------------------+
| 384 | 3 or 17 | 76 |
| | | |
| 384 | 65537 | 78 |
| | | |
| 512 | 3 or 17 | 92 |
| | | |
| 512 | 65537 | 94 |
| | | |
| 1024 | 3 or 17 | 160 |
| | | |
| 1024 | 65537 | 162 |
| | | |
| 2048 | 3 or 17 | 292 |
| | | |
| 2048 | 65537 | 294 |
| | | |
| 3072 | 3 or 17 | 420 |
| | | |
| 3072 | 65537 | 422 |
| | | |
| 7680 | 3 or 17 | 996 |
| | | |
| 7680 | 65537 | 998 |
| | | |
| 15360 | 3 or 17 | 1956 |
| | | |
| 15360 | 65537 | 1958 |
+------------------+--------------+---------------------------------+
Table 2: Common sizes for DER-encoded RSA Public Key
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+----------------------+--------------------------------------------+
| RSA Key Length (in | Size of the Digital Signature field |
| bits) | without padding |
+----------------------+--------------------------------------------+
| 384 | 48 |
| | |
| 512 | 64 |
| | |
| 1024 | 128 |
| | |
| 2048 | 256 |
| | |
| 3072 | 384 |
| | |
| 7680 | 960 |
| | |
| 15360 | 1920 |
+----------------------+--------------------------------------------+
Table 3: Common sizes of the Digital Signature field when using RSA
+--------------------------+----------------------------------------+
| Name of the elliptic | Size of the DER-encoded Public Key |
| curve | (bytes) |
+--------------------------+----------------------------------------+
| P-256 | 88 |
| | |
| P-384 | 120 |
| | |
| P-521 | 158 |
+--------------------------+----------------------------------------+
Table 4: Common sizes for DER-encoded ECC Public Key
+-----------------------+-------------------------------------------+
| Name of the elliptic | Size of the Digital Signature field |
| curve | (without padding) |
+-----------------------+-------------------------------------------+
| P-256 | 71 |
| | |
| P-384 | 104 |
| | |
| P-521 | 139 |
+-----------------------+-------------------------------------------+
Table 5: Common sizes of the Digital Signature field when using ECDSA
(+ DER encoding)
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When using multiple public keys to form a CGA, one may reach the
maximum number of possible public keys before each Neighbor Discovery
Message exceed the Maximum Transfer Unit (which must be at least 1280
octets according to [RFC2460]). This section aims to approximate
this limit.
Numerous factors (presence and number of option, size of public keys,
etc) influence the size of the Neighbor Discovery message. For
example, when sending a SEND-secured Router Advertisement message:
o The IPv6 header is 40 bytes long. Described in [RFC2460].
o The bare Router Advertisement message (without any option) is 16
bytes long. Described in [RFC4861].
o A Prefix Information Option (can appear more than once) is 32
bytes long. Described in [RFC4861].
o A Source Link-Layer Option, when a IEEE 802 address is used, is 8
bytes long. Described in [RFC4861].
o A MTU Option is 8 bytes long. Described in [RFC4861].
o The CGA Option is the size of the CGA Parameter Data Structure
plus 4 bytes rounded up to the closest multiple of 8 value. This
option is defined in [RFC3971]. The CGA Parameter Data Structure
(defined in [RFC3972] size depends on the following fields:
* Modifier: 16 bytes long.
* Subnet Prefix: 8 bytes long.
* Collision Count: 1 byte long.
* Public Key: variable size. Table 2 provides size of the
commonly used DER-encoded RSA Public Keys. Table 4 provides
size for the commonly used DER-encoded ECC Public Keys.
* Extension(s): variable size. Public Key Extension field
defined in [cheneau-cga-pk-agility] is 4 bytes plus the size of
the Public Key long. Public Key size are defined in Table 2
and Table 4.
o The Timestamp Option is 16 bytes long. Defined in [RFC3971].
o The Nonce Option minimum size is 8 bytes long. Defined in
[RFC3971].
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o The Universal Signature Option depends on the size of the Digital
Signature. The fixed part of the option is 20 bytes long. This
option is updated in this document. Table 3 presents common sizes
for usual Digital Signature field when using RSA. Table 5
presents common sizes for Digital Signature field when using
ECDSA. This option size must be a multiple of 8 bytes.
A Router Advertisement message, carrying a Prefix Information Option
and a Source Link-Layer Option, without Nonce, with one 1024-bits
long RSA Public Key and a Public Exponent of 3 in the CGA Option is
456 bytes long. Using the same RSA Public Key, adding one ECC P-521
key to CGA Option, the same message, signed with a Universal
Signature option generated by RSA and a Universal Signature Option
signed by ECDSA, is 768 bytes long. Note that EC P-521 and 1024-bits
RSA keys should not be used together because they do not present the
same security level (see Section 7) and are shown here to indicate
sizes of messages with "big" keys.
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Authors' Addresses
Tony Cheneau
Institut TELECOM, TELECOM SudParis, CNRS SAMOVAR UMR 5157
9 rue Charles Fourier
Evry 91011
France
Email: tony.cheneau@it-sudparis.eu
Maryline Laurent-Maknavicius
Institut TELECOM, TELECOM SudParis, CNRS SAMOVAR UMR 5157
9 rue Charles Fourier
Evry 91011
France
Email: maryline.maknavicius@it-sudparis.eu
Sean Shen
Huawei
No. 9 Xinxi Road
Beijing 100085
China
Email: sshen@huawei.com
Michaela Vanderveen
Qualcomm
Email: mvandervn@gmail.com
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