| 6lo | P. Thubert, Ed. |
| Internet-Draft | Cisco |
| Updates: 6775 (if approved) | B. Sarikaya |
| Intended status: Standards Track | |
| Expires: April 21, 2019 | M. Sethi |
| Ericsson | |
| R. Struik | |
| Struik Security Consultancy | |
| October 18, 2018 |
Address Protected Neighbor Discovery for Low-power and Lossy Networks
draft-ietf-6lo-ap-nd-08
This document specifies an extension to 6LoWPAN Neighbor Discovery (ND) defined in RFC6775 and updated in [I-D.ietf-6lo-rfc6775-update]. The new extension is called Address Protected Neighbor Discovery (AP-ND) and it protects the owner of an address against address theft and impersonation attacks in a low-power and lossy network (LLN). Nodes supporting this extension compute a cryptographic identifier (Crypto-ID) and use it with one or more of their Registered Addresses. The Crypto-ID identifies the owner of the Registered Address and can be used to provide proof of ownership of the Registered Addresses. Once an address is registered with the Crypto-ID and a proof-of-ownership is provided, only the owner of that address can modify the registration information, thereby enforcing Source Address Validation.
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Neighbor Discovery Optimizations for 6LoWPAN networks [RFC6775] (6LoWPAN ND) adapts the original IPv6 neighbor discovery (NDv6) protocols defined in [RFC4861] and [RFC4862] for constrained low-power and lossy network (LLN). In particular, 6LoWPAN ND introduces a unicast host address registration mechanism that reduces the use of multicast. 6LoWPAN ND defines a new Address Registration Option (ARO) that is carried in the unicast Neighbor Solicitation (NS) and Neighbor Advertisement (NA) messages exchanged between a 6LoWPAN Node (6LN) and a 6LoWPAN Router (6LR). It also defines the Duplicate Address Request (DAR) and Duplicate Address Confirmation (DAC) messages between the 6LR and the 6LoWPAN Border Router (6LBR). In LLN networks, the 6LBR is the central repository of all the registered addresses in its domain.
The registration mechanism in 6LoWPAN ND prevents the use of an address if that address is already registered in the subnet (first come first serve). In order to validate address ownership, the registration mechanism enables the 6LR and 6LBR to validate the association between the registered address of a node, and its Registration Ownership Verifier (ROVR). ROVR is defined in [I-D.ietf-6lo-rfc6775-update] and it can be derived from the MAC address of the device (using the 64-bit Extended Unique Identifier EUI-64 address format specified by IEEE). However, the EUI-64 can be spoofed, and therefore, any node connected to the subnet and aware of a registered-address-to-ROVR mapping could effectively fake the ROVR. This would allow the an attacker to steal the address and redirect traffic for that address. [I-D.ietf-6lo-rfc6775-update] defines an Extended Address Registration Option (EARO) option that allows to transport alternate forms of ROVRs, and is a pre-requisite for this specification.
In this specification, a 6LN generates a cryptographic ID (Crypto-ID) and places it in the ROVR field during the registration of one (or more) of its addresses with the 6LR(s). Proof of ownership of the Crypto-ID is passed with the first registration exchange to a new 6LR, and enforced at the 6LR. The 6LR validates ownership of the cryptographic ID before it creates any new registration state, or changes existing information.
The protected address registration protocol proposed in this document enables Source Address Validation (SAVI) [RFC7039]. This ensures that only the actual owner uses a registered address in the IPv6 source address field. A 6LN can only use a 6LR for forwarding packets only if it has previously registered the address used in the source field of the IPv6 packet.
The 6lo adaptation layer in [RFC4944] and [RFC6282] requires a device to form its IPv6 addresses based on its Layer-2 address to enable a better compression. This is incompatible with Secure Neighbor Discovery (SeND) [RFC3971] and Cryptographically Generated Addresses (CGAs) [RFC3972], since they derive the Interface ID (IID) in IPv6 addresses with cryptographic keys.
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 [RFC2119].
Terms and concepts from the following documents are used in this specification:
This document uses the following acronyms:
This specification defines a cryptographic identifier (Crypto-ID) that can be used as a replacement to the MAC address in the ROVR field of the EARO option; the computation of the Crypto-ID is detailed in Section 4.1. A node in possession of the necessary cryptographic primitives SHOULD use Crypto-ID by default as ROVR in its registration. Whether a ROVR is a Crypto-ID is indicated by a new "C" flag in the NS(EARO) message.
In order to prove its ownership of a Crypto-ID, the registering node needs to supply certain parameters including a nonce and a signature that will prove that the node has the private-key corresponding to the public-key used to build the Crypto-ID. This specification adds the capability to carry new options in the NS(EARO) and the NA(EARO). The NS(EARO) carries a variation of the CGA Option (Section 4.3), a Nonce option and a variation of the RSA Signature option (Section 4.5) in the NS(EARO). The NA(EARO) carries a Nonce option.
In order to avoid the need for new ND option types, this specification reuses/ extends options defined in SEND [RFC3971] and 6LoWPAN ND [RFC6775] [I-D.ietf-6lo-rfc6775-update]. This applies in particular to the CGA option and the RSA Signature Option. This specification provides aliases for the specific variations of those options as used in this document. The presence of the EARO option in the NS/NA messages indicates that the options are to be processed as specified in this document, and not as defined in SEND [RFC3971].
Each 6LN using this specification for address registration MUST support Elliptic Curve Crytpograhy (ECC) and a hash function. The choice of elliptic curves and hash function currently defined in this specification are listed in Section 8.2.
The Crypto-ID is computed by a 6LN as follows:
This specification updates the EARO option as follows:
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 | Status | Opaque |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Rsvd |C| I |R|T| TID | Registration Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
... Registration Ownership Verifier (ROVR) ...
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Enhanced Address Registration Option
6LoWPAN ND. No other new Status values are defined.
This specification uses Status values "Validation Requested" and "Validation Failed", which are defined in
This specification defines the Crypto-ID Parameters Option (CIPO), as a variation of the CGA Option that carries the parameters used to form a Crypto-ID. In order to provide cryptographic agility [RFC7696], AP-ND supports two possible elliptic curves, indicated by a Crypto-Type field. NIST P-256 [FIPS186-4] MUST be supported by all implementations. The Edwards-Curve Digital Signature Algorithm (EdDSA) curve Ed25519ph (pre-hashing) [RFC8032] MAY be supported as an alternate.
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 | Pad Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Crypto-Type | Modifier | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
. .
. Public Key (variable length) .
. .
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Padding .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Crypto-ID Parameters Option
This document reuses the Nonce Option defined in section 5.3.2. of SEND without a change.
This document reuses the RSA Signature Option (RSAO) defined in section 5.2. of SEND. Admittedly, the name is ill-chosen since the option is extended for non-RSA Signatures and this specification defines an alias to avoid the confusion.
The description of the operation on the option detailed in section 5.2. of SEND apply, but for the following changes:
The scope of the protocol specified here is a 6LoWPAN Low Power Lossy Network (LLN), typically a stub network connected to a larger IP network via a Border Router called a 6LBR per [RFC6775]. A 6LBR has sufficient capability to satisfy the needs of duplicate address detection.
The 6LBR maintains registration state for all devices in its attached LLN. Together with the first-hop router (the 6LR), the 6LBR assures uniqueness and grants ownership of an IPv6 address before it can be used in the LLN. This is in contrast to a traditional network that relies on IPv6 address auto-configuration [RFC4862], where there is no guarantee of ownership from the network, and each IPv6 Neighbor Discovery packet must be individually secured [RFC3971].
---+-------- ............
| External Network
|
+-----+
| | 6LBR
+-----+
o o o
o o o o
o o LLN o o o
o o o (6LR)
o (6LN)
Figure 3: Basic Configuration
In a mesh network, the 6LR is directly connected to the host device. This specification mandates that the peer-wise layer-2 security is deployed so that all the packets from a particular host are securely identifiable by the 6LR. The 6LR may be multiple hops away from the 6LBR. Packets are routed between the 6LR and the 6LBR via other 6LRs. This specification mandates that a chain of trust is established so that a packet that was validated by the first 6LR can be safely routed by other on-path 6LRs to the 6LBR.
The 6LR/6LBR ensures first-come/first-serve by storing the EARO information including the Crypto-ID associated to the node being registered. The node can claim any address as long as it is the first to make such a claim. After a successful registration, the node becomes the owner of the registered address and the address is bound to the Crypto-ID in the 6LR/6LBR registry.
This specification enables the 6LR to verify the ownership of the binding at any time assuming that the "C" flag is set. The verification prevents other nodes from stealing the address and trying to attract traffic for that address or use it as their source address.
A node may use multiple IPv6 addresses at the same time. The node may use a same Crypto-ID, to prove the ownership of multiple IPv6 addresses. The separation of the address and the cryptographic material avoids the constrained device to compute multiple keys for multiple addresses. The registration process allows the node to use the same Crypto-ID for all of its addresses.
A 6LN registers to a 6LR that is one hop away from it with the "C" flag set in the EARO, indicating that the ROVR field contains a Crypto-ID. The Target Address in the NS message indicates the IPv6 address that the 6LN is trying to register. The on-link (local) protocol interactions are shown in Figure 4. If the 6LR does not have a state with the 6LN that is consistent with the NS(EARO), then it replies with a challenge NA (EARO, status=Validation Requested) that contains a Nonce Option (shown as NonceLR in Figure 4). The Nonce option MUST contain a random Nonce value that was never used with this device.
The 6LN replies to the challenge with an NS(EARO) that includes a new Nonce option (shown as NonceLN in Figure 4), the CIPO (Section 4.3), and the NDPSO containing the signature. The information associated to a Crypto-ID stored by the 6LR on the first NS exchange where it appears. The 6LR MUST store the CIPO parameters associated with the Crypto-ID so it can be used for more than one address.
6LN 6LR
| |
|<------------------------- RA -------------------------|
| | ^
|---------------- NS with EARO (Crypto-ID) ------------>| |
| | option
|<- NA with EARO (status=Validation Requested), NonceLR-| |
| | v
|------- NS with EARO, CIPO, NonceLN and NDPSO -------->|
| |
|<------------------- NA with EARO ---------------------|
| |
...
| |
|--------------- NS with EARO (Crypto-ID) ------------->|
| |
|<------------------- NA with EARO ---------------------|
| |
...
| |
|--------------- NS with EARO (Crypto-ID) ------------->|
| |
|<------------------- NA with EARO ---------------------|
| |
Figure 4: On-link Protocol Operation
The steps for the registration to the 6LR are as follows:
The signature generated by the 6LN to provide proof-of-ownership of the private-key is carried in the NDP Signature Option (NDPSO). It is generated by the 6LN as follows:
The 6LR on receiving the NDPSO and CIPO options first hashes the JWK encoded public-key in the CIPO option to make sure that the leftmost bits up to the size of the ROVR match. Only if the check is successful, it tries to verify the signature in the NDPSO option using the following.
In a multihop 6LoWPAN, the registration with Crypto-ID is propagated to 6LBR as described in this section. If the 6LR and the 6LBR maintain a security association, then there is no need to propagate the proof of ownership to the 6LBR.
A new device that joins the network auto-configures an address and performs an initial registration to a neighboring 6LR with an NS message that carries an Address Registration Option (EARO) [RFC6775]. The 6LR validates the address with an 6LBR using a DAR/DAC exchange, and the 6LR confirms (or denies) the address ownership with an NA message that also carries an Address Registration Option.
Figure 5 illustrates a registration flow all the way to a 6LowPAN Backbone Router (6BBR).
6LN 6LR 6LBR 6BBR
| | | |
| NS(EARO) | | |
|--------------->| | |
| | Extended DAR | |
| |-------------->| |
| | | |
| | | proxy NS(EARO) |
| | |--------------->|
| | | | NS(DAD)
| | | | ------>
| | | |
| | | | <wait>
| | | |
| | | proxy NA(EARO) |
| | |<---------------|
| | Extended DAC | |
| |<--------------| |
| NA(EARO) | | |
|<---------------| | |
| | | |
Figure 5: (Re-)Registration Flow
In a multihop 6LoWPAN, a 6LBR sends RAs with prefixes downstream and the 6LR receives and relays them to the nodes. 6LR and 6LBR communicate using ICMPv6 Duplicate Address Request (DAR) and Duplicate Address Confirmation (DAC) messages. The DAR and DAC use the same message format as NS and NA, but have different ICMPv6 type values.
In AP-ND we extend DAR/DAC messages to carry cryptographically generated ROVR. In a multihop 6LoWPAN, the node exchanges the messages shown in Figure 5. The 6LBR must identify who owns an address (EUI-64) to defend it, if there is an attacker on another 6LR.
Observations regarding the following threats to the local network in [RFC3971] also apply to this specification.
The threats discussed in 6LoWPAN ND [RFC6775] and its update [I-D.ietf-6lo-rfc6775-update] also apply here. Compared with SeND, this specification saves about 1Kbyte in every NS/NA message. Also, this specification separates the cryptographic identifier from the registered IPv6 address so that a node can have more than one IPv6 address protected by the same cryptographic identifier. SeND forces the IPv6 address to be cryptographic since it integrates the CGA as the IID in the IPv6 address. This specification frees the device to form its addresses in any fashion, thereby enabling not only 6LoWPAN compression which derives IPv6 addresses from Layer-2 addresses but also privacy addresses.
A collision of Registration Ownership Verifiers (ROVR) (i.e., the Crypto-ID in this specification) is possible, but it is a rare event. The formula for calculating the probability of a collision is 1 - e^{-k^2/(2n)} where n is the maximum population size (2^64 here, 1.84E19) and K is the actual population (number of nodes). If the Crypto-ID is 64-bits (the least possible size allowed), the chance of a collision is 0.01% when the network contains 66 million nodes. Moreover, the collision is only relevant when this happens within one stub network (6LBR). In the case of such a collision, an attacker may be able to claim the registered address of an another legitimate node. However for this to happen, the attacker would also need to know the address which was registered by the legitimate node. This registered address is never broadcasted on the network and therefore providing an additional 64-bits that an attacker must correctly guess. To prevent address disclosure, it is RECOMMENDED that nodes derive the address being registered independently of the ROVR.
This document defines a new 128-bit value under the CGA Message Type [RFC3972] namespace, 0x8701 55c8 0cca dd32 6ab7 e415 f148 84d0.
IANA is requested to create a new subregistry "Crypto-Type Subregistry" in the "Internet Control Message Protocol version 6 (ICMPv6) Parameters". The registry is indexed by an integer 0..255 and contains a Signature Algorithm and a Hash Function as shown in Table 1. The following Crypto-Type values are defined in this document:
| Crypto-Type value | Signature Algorithm | Hash Function | Defining Specification |
|---|---|---|---|
| 0 | NIST P-256 [FIPS186-4] | SHA-256 [RFC6234] | RFC THIS |
| 1 | Ed25519ph [RFC8032] | SHA-512 [RFC6234] | RFC THIS |
As is evident from the table above, although the two curves provide similar security, they however rely on different hash functions. Supporting multiple hash functions on constrained devices is not ideal. [I-D.struik-lwig-curve-representations] provides information on how to represent Montgomery curves and (twisted) Edwards curves as curves in short-Weierstrass form and illustrates how this can be used to implement elliptic curve computations using existing implementations that already implement, e.g., ECDSA and ECDH using NIST [FIPS186-4] prime curves. New Crypto-Type values providing similar or better security (with less code) can be defined in future.
Assignment of new values for new Crypto-Type MUST be done through IANA with "Specification Required" and "IESG Approval" as defined in [RFC8126].
Many thanks to Charlie Perkins for his in-depth review and constructive suggestions. We are also especially grateful to Robert Moskowitz for his comments that lead to many improvements.
In this section we state requirements of a secure neighbor discovery protocol for low-power and lossy networks.