Signature Algorithm Agility in the Secure Neighbor Discovery (SEND) Protocol
draft-cheneau-csi-send-sig-agility-00

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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).

Table of Contents

1.  Introduction
2.  Overview
    2.1.  Compatibility with existing specifications
        2.1.1.  Classification of SEND nodes
        2.1.2.  Principal Scenarios
    2.2.  Agility Requirements
    2.3.  Mechanism for Agility Support of CGA and SEND
3.  Supported Signature Algorithm Option
    3.1.  Neighbor Cache interactions
    3.2.  Processing Rules for Senders
    3.3.  Processing Rules for Receivers
4.  SEND Universal Signature Option
    4.1.  Processing Rules for Senders
    4.2.  Processing Rules for Receivers
5.  Basic negotiation
    5.1.  Overview
    5.2.  Sending Unsolicited Messages
6.  Security Considerations
7.  IANA Considerations
8.  Acknowledgments
9.  References
    9.1.  Normative References
    9.2.  Informative References
Appendix A.  On the number of Public Keys supported per CGA
§  Authors' Addresses

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] (Arkko, J., Kempf, J., Zill, B., and P. Nikander, “SEcure Neighbor Discovery (SEND),” March 2005.) and [RFC3972] (Aura, T., “Cryptographically Generated Addresses (CGA),” March 2005.) is also addressed.

2.  Overview

2.1.  Compatibility with existing specifications

The current SEND protocol specification, [RFC3971] (Arkko, J., Kempf, J., Zill, B., and P. Nikander, “SEcure Neighbor Discovery (SEND),” March 2005.), 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 deployment of [RFC3971] (Arkko, J., Kempf, J., Zill, B., and P. Nikander, “SEcure Neighbor Discovery (SEND),” March 2005.)-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.

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‑csi‑cga‑pk‑agility] (Cheneau, T., Laurent-Maknavicius, M., Shen, S., and M. Vanderveen, “Support for Multiple Signature Algorithms in Cryptographically Generated Addresses (CGAs),” October 2009.)). 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:

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] (Arkko, J., Kempf, J., Zill, B., and P. Nikander, “SEcure Neighbor Discovery (SEND),” March 2005.) 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.

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.

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:

• 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.
• 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 Signature Algorithm and hash algorithm.
• 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‑csi‑cga‑pk‑agility] (Cheneau, T., Laurent-Maknavicius, M., Shen, S., and M. Vanderveen, “Support for Multiple Signature Algorithms in Cryptographically Generated Addresses (CGAs),” October 2009.).

This document proposes an update to [RFC3971] (Arkko, J., Kempf, J., Zill, B., and P. Nikander, “SEcure Neighbor Discovery (SEND),” March 2005.) to allow two SEND nodes to choose an appropriate signature algorithm. This solution encompasses the following:

• 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;
• A modification of the "RSA Signature" option defined in the SEND specification;

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.

Note that the ECC support for SEND is described in document [cheneau‑csi‑ecc‑sig‑agility] (Cheneau, T., Laurent-Maknavicius, M., Shen, S., and M. Vanderveen, “ECC public key and signature support in Cryptographically Generated Addresses (CGA) and in the Secure Neighbor Discovery (SEND),” October 2009.).

3.  Supported Signature Algorithm Option

The Supported Signature Algorithm (SSA) 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 (Supported Signature Algorithm option):

 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  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

NDP option type, TBA. See Section 7 (IANA Considerations).

Length

The length of the option (including the Type, Length fields), in octets. 8-bit unsigned integer, the values lower than 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 Signature Algorithm algorithm.

If the first leftmost bit, bit 0, is set to 0, it 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 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 (compatible with [RFC3971] (Arkko, J., Kempf, J., Zill, B., and P. Nikander, “SEcure Neighbor Discovery (SEND),” March 2005.))
• Value 1 is RSA/SHA-256
• Section 5 of document [cheneau‑csi‑ecc‑sig‑agility] (Cheneau, T., Laurent-Maknavicius, M., Shen, S., and M. Vanderveen, “ECC public key and signature support in Cryptographically Generated Addresses (CGA) and in the Secure Neighbor Discovery (SEND),” October 2009.) provides values for ECDSA signature algorithm

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 Signature Algorithm field(s) 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:

• When a 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 one of the algorithms among the set in order to generate a Universal Signature Option.
• If a message passes 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.
• If a 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.

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                             .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

Same value as in [RFC3971] (Arkko, J., Kempf, J., Zill, B., and P. Nikander, “SEcure Neighbor Discovery (SEND),” March 2005.): 12.

Length

The length of the option (including the Type, Length, Reserved, Key Hash, Digital Signature, and Padding fields) in units of 8 octets.

Key Position

An 8-bit field indicating which Public Key in the CGA Parameters data structure (carried in the CGA option) has been used to compute the Digital Signature and is hashed in the Key Hash field. 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‑csi‑cga‑pk‑agility] (Cheneau, T., Laurent-Maknavicius, M., Shen, S., and M. Vanderveen, “Support for Multiple Signature Algorithms in Cryptographically Generated Addresses (CGAs),” October 2009.)]). 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 and the Key Position field is set to 0, the Digital Signature field is computed the same way as the Digital Signature field of the RSA Signature Option described in [RFC3971] (Arkko, J., Kempf, J., Zill, B., and P. Nikander, “SEcure Neighbor Discovery (SEND),” March 2005.). These values are compatible with [RFC3971] (Arkko, J., Kempf, J., Zill, B., and P. Nikander, “SEcure Neighbor Discovery (SEND),” March 2005.).
• If the value of the Signature Type Identifier field is 0 and the Key Position field is different than 0, this Digital Signature field is computed the same way as the Digital Signature field of the RSA Signature Option described in [RFC3971] (Arkko, J., Kempf, J., Zill, B., and P. Nikander, “SEcure Neighbor Discovery (SEND),” March 2005.) except that the associated Public Key in the Key Hash field is pointed by the value of the Key Position field.
• If the value of the Signature Type Identifier field is 1, then this Digital Signature field is computed the same way as the Digital Signature field of the RSA Signature Option described in [RFC3971] (Arkko, J., Kempf, J., Zill, B., and P. Nikander, “SEcure Neighbor Discovery (SEND),” March 2005.) except that the signature is computed with the RSASSA-PKCS1-v1_5 algorithm (as defined in [PKCS1] (RSA Laboratories, “RSA Encryption Standard, Version 2.1,” November 2002.)) and the SHA-256 hash function. The Key Position field indicates the position of the Public Key signed in the Key Hash field.
• Values for ECDSA signature algorithm are defined in Section 6 of [cheneau‑csi‑ecc‑sig‑agility] (Cheneau, T., Laurent-Maknavicius, M., Shen, S., and M. Vanderveen, “ECC public key and signature support in Cryptographically Generated Addresses (CGA) and in the Secure Neighbor Discovery (SEND),” October 2009.).

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 (On the number of Public Keys supported per CGA).

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) 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 Parameters 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:

• 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.
• The message is then constructed in its entirety, without any of the Universal Signature options.
• The Universal Signature option(s) is (are) added as the last option in the message.
• The data to be signed is constructed as explained in [RFC3971] (Arkko, J., Kempf, J., Zill, B., and P. Nikander, “SEcure Neighbor Discovery (SEND),” March 2005.), under the description of the Digital Signature field.
• 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 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] (Arkko, J., Kempf, J., Zill, B., and P. Nikander, “SEcure Neighbor Discovery (SEND),” March 2005.).

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:

• 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.)
• The Key Hash field MUST correspond to a known public key, either one learned from the CGA option in the same message and by the position indicated in the Key Position field message, or one known by other means.
• The Digital Signature field MUST have correct encoding and MUST not exceed the length of the Universal Signature option minus the Padding.
• The Digital Signature verification MUST show that the signature has been calculated as specified in the previous section.
• If the use of a trust anchor has been configured, a valid certification path (see Section 6.3 of [RFC3971] (Arkko, J., Kempf, J., Zill, B., and P. Nikander, “SEcure Neighbor Discovery (SEND),” March 2005.)) 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).

5.  Basic negotiation

5.1.  Overview

This section describes different configurations 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 (Basic negotiation - Case 1) 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

Case 2: two nodes sharing at least one common Signing Algorithm must be able to securely communicate. Figure 4 (Basic negotiation - Case 2) 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)}
                       <--------
                                   (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

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 (Basic negotiation - Case 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 verif only)}
                      -------->
                               NA
                               {CGA option,
                               ECC Signature option
                               Supported-Signature-Algo option
                               (ECC sign & verif, RSA verif only)}
                     <--------

IPv6 traffic         <------->  IPv6 traffic

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 (Basic negotiation - Case 4) 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

Upon receiving the Neighbor Solicitation message, node B determines, through the Supported Signature Algorithm option, that node A will not be able to verify any of its signature algorithm. However, based on their local policy, node B may answer and node A might decide to trust the unsecure Neighbor Discovery (thus being vulnerable), see Section 4.2 (Processing Rules for Receivers).

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:

• 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.
• 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.
• 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.

6.  Security Considerations

Section 4 (SEND Universal Signature Option) 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). See also section 4 of the companion draft [cheneau‑csi‑cga‑pk‑agility] (Cheneau, T., Laurent-Maknavicius, M., Shen, S., and M. Vanderveen, “Support for Multiple Signature Algorithms in Cryptographically Generated Addresses (CGAs),” October 2009.).

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 has been proved to be flawed in the light of recent attacks [Recent_SHA‑1_Attack] (McDonald, C., Haukes, P., and J. Pieprzyk, “SHA-1 collisions now 2^52,” May 2009.) [NIST‑st] (National Institute of Standards and Technology, “NIST Comments on Cryptanalytic Attacks on SHA-1,” .).

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.

7.  IANA Considerations

Section 3 (Supported Signature Algorithm Option) 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.

8.  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.

9.  References

9.1. Normative References

9.2. Informative References

Appendix A.  On the number of Public Keys supported per CGA

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] (Deering, S. and R. Hinden, “Internet Protocol, Version 6 (IPv6) Specification,” December 1998.)). 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:

• The IPv6 header is 40 bytes long. Described in [RFC2460] (Deering, S. and R. Hinden, “Internet Protocol, Version 6 (IPv6) Specification,” December 1998.).
• The bare Router Advertisement message (without any option) is 16 bytes long. Described in [RFC4861] (Narten, T., Nordmark, E., Simpson, W., and H. Soliman, “Neighbor Discovery for IP version 6 (IPv6),” September 2007.).
• A Prefix Information Option (can appear more than once) is 32 bytes long. Described in [RFC4861] (Narten, T., Nordmark, E., Simpson, W., and H. Soliman, “Neighbor Discovery for IP version 6 (IPv6),” September 2007.).
• A Source Link-Layer Option, when a IEEE 802 address is used, is 8 bytes long. Described in [RFC4861] (Narten, T., Nordmark, E., Simpson, W., and H. Soliman, “Neighbor Discovery for IP version 6 (IPv6),” September 2007.).
• A MTU Option is 8 bytes long. Described in [RFC4861] (Narten, T., Nordmark, E., Simpson, W., and H. Soliman, “Neighbor Discovery for IP version 6 (IPv6),” September 2007.).
• The CGA Option is the size of the CGA Parameters data structure plus 4 bytes rounded up to the closest multiple of 8 value. This option is defined in [RFC3971] (Arkko, J., Kempf, J., Zill, B., and P. Nikander, “SEcure Neighbor Discovery (SEND),” March 2005.). The CGA Parameters data structure (defined in [RFC3972] (Aura, T., “Cryptographically Generated Addresses (CGA),” March 2005.) 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 1 (Common sizes for DER-encoded RSA Public Key) provides size of the commonly used DER-encoded RSA Public Keys.
  • Extension(s): variable size. Public Key Extension field defined in [cheneau‑csi‑cga‑pk‑agility] (Cheneau, T., Laurent-Maknavicius, M., Shen, S., and M. Vanderveen, “Support for Multiple Signature Algorithms in Cryptographically Generated Addresses (CGAs),” October 2009.) is 4 bytes plus the size of the Public Key long. Public Key size are defined in Table 1 (Common sizes for DER-encoded RSA Public Key).
• The Timestamp Option is 16 bytes long. Defined in [RFC3971] (Arkko, J., Kempf, J., Zill, B., and P. Nikander, “SEcure Neighbor Discovery (SEND),” March 2005.).
• The Nonce Option minimum size is 8 bytes long. Defined in [RFC3971] (Arkko, J., Kempf, J., Zill, B., and P. Nikander, “SEcure Neighbor Discovery (SEND),” March 2005.).
• 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 2 (Common sizes of the Digital Signature field when using RSA) presents common sizes for usual Digital Signature field when using RSA. 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.

Authors' Addresses