| 6lo | P. Thubert, Ed. |
| Internet-Draft | Cisco |
| Updates: 6775 (if approved) | B. Sarikaya |
| Intended status: Standards Track | |
| Expires: August 3, 2018 | M. Sethi |
| Ericsson | |
| January 30, 2018 |
Address Protected Neighbor Discovery for Low-power and Lossy Networks
draft-ietf-6lo-ap-nd-05
This document defines an extension to 6LoWPAN Neighbor Discovery (ND)[I-D.ietf-6lo-rfc6775-update] called Address Protected ND (AP-ND); AP-ND protects the owner of an address against address theft and impersonation inside a low-power and lossy network (LLN). Nodes supporting this extension compute a cryptographic Owner Unique Interface ID and associate it with one or more of their Registered Addresses. The Cryptographic ID uniquely identifies the owner of the Registered Address and can be used for proof-of-ownership. It is used in 6LoWPAN ND in place of the EUI-64-based unique ID that is associated with the registration. Once an address is registered with a Cryptographic ID, only the owner of that ID can modify the anchor state information of the Registered Address, and Source Address Validation can be enforced.
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This Internet-Draft will expire on August 3, 2018.
Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved.
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"Neighbor Discovery Optimizations for 6LoWPAN networks" (6LoWPAN ND) adapts the classical IPv6 ND protocol [RFC4861][RFC4862] (IPv6 ND) for operations over a constrained low-power and lossy network (LLN). In particular, 6LoWPAN ND introduces a unicast host address registration mechanism that contributes to reduce the use of multicast messages that are present in the classical IPv6 ND protocol. 6LoWPAN ND defines a new Address Registration Option (ARO) that is carried in the unicast Neighbor Solicitation (NS) and Neighbor Advertisement (NA) messages between the 6LoWPAN Node (6LN) and the 6LoWPAN Router (6LR). Additionally, 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 present in the subnet (first come first serve). In order to validate address ownership, the registration mechanism enables the 6LR and 6LBR to validate claims for a registered address with an associated Owner Unique Interface IDentifier (OUID). 6LoWPAN ND specifies that the OUID is derived from the MAC address of the device (using the 64-bit Extended Unique Identifier EUI-64 address format specified by IEEE), which can be spoofed. Therefore, any node connected to the subnet and aware of a registered-address-to-OUID mapping could effectively fake the OUID, steal the address and redirect traffic for that address towards a different 6LN. The "Update to 6LoWPAN ND" defines an Extended ARO (EARO) option that allows to transport alternate forms of OUIDs, and is a prerequisite for this specification.
According to this specification, a 6LN generates a cryptographic ID (Crypto-ID) and places it in the OUID field in the registration of one (or more) of its addresses with the 6LR(s) that the 6LN uses as default router(s). Proof of ownership of the cryptographic ID (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 can create a registration state, or a change the anchor information, that is the Link-Layer Address and associated parameters, in an existing registration state.
The protected address registration protocol proposed in this document enables the enforcement of Source Address Validation (SAVI) [RFC7039], which ensures that only the correct owner uses a registered address in the source address field in IPv6 packets. Consequently, a 6LN that sources a packet has to use a 6LR to which the source address of the packet is registered to forward the packet. The 6LR maintains state information for the registered addressed, including the MAC address, and a link-layer cryptographic key associated with the 6LN. In SAVI-enforcement mode, the 6LR allows only packets from a connected Host if the connected Host owns the registration of the source address of the packet.
The 6lo adaptation layer framework ([RFC4944], [RFC6282]) expects that a device forms its IPv6 addresses based on Layer-2 address, so as to enable a better compression. This is incompatible with "Secure Neighbor Discovery (SeND)" and "Cryptographically Generated Addresses (CGAs)", which derive the Interface ID (IID) in the IPv6 addresses from cryptographic material. "Privacy Considerations for IPv6 Address Generation Mechanisms" places additional recommendations on the way addresses should be formed and renewed.
This document specifies that a device may form and register addresses at will, without a constraint on the way the address is formed or the number of addresses that are registered in parallel. It enables to protect multiple addresses with a single cryptographic material and to send the proof only once to a given 6LR for multiple addresses and refresher registrations.
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].
Readers are expected to be familiar with all the terms and concepts that are discussed in [RFC3971], [RFC3972], [RFC4861], [RFC4919], [RFC6775], and [I-D.ietf-6lo-backbone-router] which proposes an evolution of [RFC6775] for wider applicability.
This document defines Crypto-ID as an identifier of variable size which in most cases is 64 bits long. It is generated using cryptographic means explained later in this document Section 4.2. "Elliptic Curves for Security" [RFC7748] and "Edwards-Curve Digital Signature Algorithm (EdDSA)" [RFC8032] provides information on Elliptic Curve Cryptography (ECC) and a (twisted) Edwards curve, Ed25519, which can be used with this specification. "Alternative Elliptic Curve Representations" [I-D.struik-lwig-curve-representations] provides additional 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 [FIPS-186-4] prime curves.
The document also conforms to the terms and models described in [RFC5889] and uses the vocabulary and the concepts defined in [RFC4291] for the IPv6 Architecture. Finally, common terminology related to Low power And Lossy Networks (LLN) defined in [RFC7102] is also used.
This specification defines a cryptographic identifier (Crypto-ID) that can be used as a replacement to the MAC address in the OUID field of the EARO option; the computation of the Crypto-ID is detailed in Section 4.2. A node in possession of the necessary cryptographic material SHOULD use Crypto-ID by default as OUID in its registration. Whether a OUID 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 produce the parameters that where used to build it, as well as a nonce and a signature that will prove that it has the private key that corresponds to the public key that was used to build the Crypto-ID. This specification adds the capability to carry new options in the NS(EARO) and the NBA(EARO). These options are a variation of the CGA Option Section 4.4, a Nonce option and a variation of the RSA Signature option Section 4.6 in the NS(EARO) and a Nonce option in the NA(EARO).
In order to avoid an inflation of 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 AP-ND. The presence of the EARO option in the NS/NA messages indicates that the options are to be understood as specified in this document. A router that would receive a NS(EARO) and try to process it as a SEND message will find that the signature does not match and drop the packet.
Public Key is the most important parameter in CGA Parameters (sent by 6LN in an NS message). ECC Public Key could be in uncompressed form or in compressed form where the first octet of the OCTET STRING is 0x04 and 0x02 or 0x03, respectively. Point compression can further reduce the key size by about 32 octets.
Elliptic Curve Cryptography (ECC) is used to calculate the Crypto-ID. Each 6LN using a Crypto-ID for registration MUST have a public/private key pair. The digital signature is constructed by using the 6LN's private key over its EUI-64 (MAC) address. The signature value is computed using the ECDSA signature algorithm and the hash function used is SHA-256 [RFC6234].
NIST P-256 [FIPS186-4] that MUST be supported by all implementations. To support cryptographic algorithm agility [RFC7696], Edwards-Curve Digital Signature Algorithm (EdDSA) curve Ed25519ph (pre-hashing) [RFC8032] MAY be supported as an alternate.
The Crypto-ID is computed as follows:
With this specification, only 64 bits are retained, but it could be expanded to more bits in the future by increasing the size of the OUID field.
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 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |C|T| TID | Registration Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Owner Unique ID (EUI-64 or Crypto-ID) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Enhanced Address Registration Option
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, AP-ND supports two possible signature algorithms, indicated by a Crypto-Type field. A value of 0 indicates that NIST P-256 is used for the signature operation and SHA-256 as the hash algorithm. NIST P-256 MUST be supported by all implement A value of 1 indicates that Ed25519ph is used for the signature operation and SHA-256 as the hash algorithm.
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 present work 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].
The 6LBR maintains a registration state for all devices in the attached LLN, and, in conjunction with the first-hop router (the 6LR), is in a position to validate uniqueness and grant ownership of an IPv6 address before it can be used in the LLN. This is a fundamental difference with a classical network that relies on IPv6 address auto-configuration [RFC4862], where there is no guarantee of ownership from the network, and any 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 expects 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 expects that a chain of trust is established so that a packet that was validated by the first 6LR can be safely routed by the next 6LRs to the 6LBR.
The 6LR/6LBR ensures first-come/first-serve by storing the EARO information including the Crypto-ID correlated to the node being registered. The node is free to claim any address it likes 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 to verify the ownership of the binding at any time assuming that the "C" flag is set. If it is not set, then the verification methods presented in this specification cannot be applied. 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, or multiple crypto-IDs derived from a same key pair, to protect 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 bind all of its addresses to the same Crypto-ID.
A 6LN registers to a 6LR that is one hop away from it with the "C" flag set in the EARO, indicating that the Owner Unique ID field contains a Crypto-ID. 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. The Nonce option MUST contain a Nonce value that was never used with this device.
The 6LN replies to the challenge with a proof-of-ownership NS(EARO) that includes the echoed Nonce option, the CIPO with all the parameters that where used to build EARO with a Crypto-ID, and as the last option the NDPSO with the signature. The information associated to a crypto-ID is passed to and stored by the 6LR on the first NS exchange where it appears. The 6LR SHOULD store the CIPO information 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), Nonce --| |
| | v
|-------- NS with EARO, CIPO, Nonce 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:
In a multihop 6LoWPAN, the registration with Crypto-ID is propagated to 6LBR as described in Section 6.2. If a chain of trust is present between the 6LR and the 6LBR, then there is no need to propagate the proof of ownership to the 6LBR. All the 6LBR needs to know is that this particular OUID is randomly generated, so as to enforce that any update via a different 6LR is also random.
A new device that joins the network auto-configures an address and performs an initial registration to an on-link 6LR with an NS message that carries an Address Registration Option (EARO) [RFC6775]. The 6LR validates the address with the central 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 OUID. 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.
Occasionally, a 6LR might miss the node's OUID (that it received in ARO). 6LR should be able to ask for it again. This is done by restarting the exchanges shown in Figure 4. The result enables 6LR to refresh the information that was lost. The 6LR MUST send DAR message with ARO to 6LBR. The 6LBR replies with a DAC message with the information copied from the DAR, and the Status field is set to zero. With this exchange, the 6LBR can (re)validate and store the information to make sure that the 6LR is not a fake.
In some cases, the 6LBR may use a DAC message to solicit a Crypto-ID from a 6LR and also requests 6LR to verify the EUI-64 6LR received from 6LN. This may happen when a 6LN node is compromised and a fake node is sending the Crypto-ID as if it is the node's EUI-64. Note that the detection in this case can only be done by 6LBR not by 6LR.
The observations regarding the threats to the local network in [RFC3971] also apply to this specification. Considering RFC3971 security section subsection by subsection:
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, so as to enable the classical 6LoWPAN compression which derives IPv6 addresses from Layer-2 addresses, as well as privacy addresses. The threats discussed in Section 9.2 of [RFC3971] are countered by the protocol described in this document as well.
Collisions of Owner Unique Interface IDentifier (OUID) (which is the Crypto-ID in this specification) is a possibility that needs to be considered. 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-bit long, then the chance of finding a collision is 0.01% when the network contains 66 million nodes. It is important to note that the collision is only relevant when this happens within one stub network (6LBR). A collision of Crypto-ID is a rare event. In the case of 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 however never broadcasted on the network and therefore it provides an additional entropy of 64-bits that an attacker must correctly guess. To prevent such a scenario, it is RECOMMENDED that nodes derive the address being registered independently of the OUID.
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-256 [RFC6234] | RFC THIS |
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 Rene Struik and Robert Moskowitz for their comments that lead to many improvements to this document, in particular WRT ECC computation and references.
In this section we state requirements of a secure neighbor discovery protocol for low-power and lossy networks.