Internet Engineering Task Force (IETF) C. Dô

Internet Engineering Task Force (IETF) C. Dô
Request for Comments: 8967 W. Kolodziejak
Obsoletes: 7298 J. Chroboczek
Category: Standards Track IRIF, University of Paris-Diderot
ISSN: 2070-1721 January 2021

MAC Authentication for the Babel Routing Protocol

Abstract

This document describes a cryptographic authentication mechanism for
the Babel routing protocol that has provisions for replay avoidance.
This document obsoletes RFC 7298.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8967.

Copyright Notice

Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction
1.1. Applicability
1.2. Assumptions and Security Properties
1.3. Specification of Requirements
2. Conceptual Overview of the Protocol
3. Data Structures
3.1. The Interface Table
3.2. The Neighbour Table
4. Protocol Operation
4.1. MAC Computation
4.2. Packet Transmission
4.3. Packet Reception
4.4. Expiring Per-Neighbour State
5. Incremental Deployment and Key Rotation
6. Packet Format
6.1. MAC TLV
6.2. PC TLV
6.3. Challenge Request TLV
6.4. Challenge Reply TLV
7. Security Considerations
8. IANA Considerations
9. References
9.1. Normative References
9.2. Informational References
Acknowledgments
Authors' Addresses

1. Introduction

By default, the Babel routing protocol [RFC8966] trusts the
information contained in every UDP datagram that it receives on the
Babel port. An attacker can redirect traffic to itself or to a
different node in the network, causing a variety of potential issues.
In particular, an attacker might:

* spoof a Babel packet and redirect traffic by announcing a route
with a smaller metric, a larger sequence number, or a longer
prefix;

* spoof a malformed packet, which could cause an insufficiently
robust implementation to crash or interfere with the rest of the
network;

* replay a previously captured Babel packet, which could cause
traffic to be redirected or otherwise interfere with the network.

Protecting a Babel network is challenging due to the fact that the
Babel protocol uses both unicast and multicast communication. One
possible approach, used notably by the Babel over Datagram Transport
Layer Security (DTLS) protocol [RFC8968], is to use unicast
communication for all semantically significant communication, and
then use a standard unicast security protocol to protect the Babel
traffic. In this document, we take the opposite approach: we define
a cryptographic extension to the Babel protocol that is able to
protect both unicast and multicast traffic and thus requires very few
changes to the core protocol. This document obsoletes [RFC7298].

1.1. Applicability

The protocol defined in this document assumes that all interfaces on
a given link are equally trusted and share a small set of symmetric
keys (usually just one, and two during key rotation). The protocol
is inapplicable in situations where asymmetric keying is required,
where the trust relationship is partial, or where large numbers of
trusted keys are provisioned on a single link at the same time.

This protocol supports incremental deployment (where an insecure
Babel network is made secure with no service interruption), and it
supports graceful key rotation (where the set of keys is changed with
no service interruption).

This protocol does not require synchronised clocks, it does not
require persistently monotonic clocks, and it does not require
persistent storage except for what might be required for storing
cryptographic keys.

1.2. Assumptions and Security Properties

The correctness of the protocol relies on the following assumptions:

* that the Message Authentication Code (MAC) being used is
invulnerable to forgery, i.e., that an attacker is unable to
generate a packet with a correct MAC without access to the secret
key;

* that a node never generates the same index or nonce twice over the
lifetime of a key.

The first assumption is a property of the MAC being used. The second
assumption can be met either by using a robust random number
generator [RFC4086] and sufficiently large indices and nonces, by
using a reliable hardware clock, or by rekeying often enough that
collisions are unlikely.

If the assumptions above are met, the protocol described in this
document has the following properties:

* it is invulnerable to spoofing: any Babel packet accepted as
authentic is the exact copy of a packet originally sent by an
authorised node;

* locally to a single node, it is invulnerable to replay: if a node
has previously accepted a given packet, then it will never again
accept a copy of this packet or an earlier packet from the same
sender;

* among different nodes, it is only vulnerable to immediate replay:
if a node A has accepted an authentic packet from C, then a node B
will only accept a copy of that packet if B has accepted an older
packet from C, and B has received no later packet from C.

While this protocol makes efforts to mitigate the effects of a denial
of service attack, it does not fully protect against such attacks.

1.3. Specification of Requirements

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

2. Conceptual Overview of the Protocol

When a node B sends out a Babel packet through an interface that is
configured for MAC cryptographic protection, it computes one or more
MACs (one per key) that it appends to the packet. When a node A
receives a packet over an interface that requires MAC cryptographic
protection, it independently computes a set of MACs and compares them
to the MACs appended to the packet; if there is no match, the packet
is discarded.

In order to protect against replay, B maintains a per-interface
32-bit integer known as the "packet counter" (PC). Whenever B sends
a packet through the interface, it embeds the current value of the PC
within the region of the packet that is protected by the MACs and
increases the PC by at least one. When A receives the packet, it
compares the value of the PC with the one contained in the previous
packet received from B, and unless it is strictly greater, the packet
is discarded.

By itself, the PC mechanism is not sufficient to protect against
replay. Consider a peer A that has no information about a peer B
(e.g., because it has recently rebooted). Suppose that A receives a
packet ostensibly from B carrying a given PC; since A has no
information about B, it has no way to determine whether the packet is
freshly generated or a replay of a previously sent packet.

In this situation, peer A discards the packet and challenges B to
prove that it knows the MAC key. It sends a "Challenge Request", a
TLV containing a unique nonce, a value that has never been used
before and will never be used again. Peer B replies to the Challenge
Request with a "Challenge Reply", a TLV containing a copy of the
nonce chosen by A, in a packet protected by MAC and containing the
new value of B's PC. Since the nonce has never been used before, B's
reply proves B's knowledge of the MAC key and the freshness of the
PC.

By itself, this mechanism is safe against replay if B never resets
its PC. In practice, however, this is difficult to ensure, as
persistent storage is prone to failure, and hardware clocks, even
when available, are occasionally reset. Suppose that B resets its PC
to an earlier value and sends a packet with a previously used PC n.
Peer A challenges B, B successfully responds to the challenge, and A
accepts the PC equal to n + 1. At this point, an attacker C may send
a replayed packet with PC equal to n + 2, which will be accepted by
A.

Another mechanism is needed to protect against this attack. In this
protocol, every PC is tagged with an "index", an arbitrary string of
octets. Whenever B resets its PC, or whenever B doesn't know whether
its PC has been reset, it picks an index that it has never used
before (either by drawing it randomly or by using a reliable hardware
clock) and starts sending PCs with that index. Whenever A detects
that B has changed its index, it challenges B again.

With this additional mechanism, this protocol is invulnerable to
replay attacks (see Section 1.2).

3. Data Structures

Every Babel node maintains a set of conceptual data structures
described in Section 3.2 of [RFC8966]. This protocol extends these
data structures as follows.

3.1. The Interface Table

Every Babel node maintains an interface table, as described in
Section 3.2.3 of [RFC8966]. Implementations of this protocol MUST
allow each interface to be provisioned with a set of one or more MAC
keys and the associated MAC algorithms (see Section 4.1 for suggested
algorithms and Section 7 for suggested methods for key generation).
In order to allow incremental deployment of this protocol (see
Section 5), implementations SHOULD allow an interface to be
configured in a mode in which it participates in the MAC
authentication protocol but accepts packets that are not
authenticated.

This protocol extends each table entry associated with an interface
on which MAC authentication has been configured with two new pieces
of data:

* a set of one or more MAC keys, each associated with a given MAC
algorithm;

* a pair (Index, PC), where Index is an arbitrary string of 0 to 32
octets, and PC is a 32-bit (4-octet) integer.

We say that an index is fresh when it has never been used before with
any of the keys currently configured on the interface. The Index
field is initialised to a fresh index, for example, by drawing a
random string of sufficient length (see Section 7 for suggested
sizes), and the PC is initialised to an arbitrary value (typically
0).

3.2. The Neighbour Table

Every Babel node maintains a neighbour table, as described in
Section 3.2.4 of [RFC8966]. This protocol extends each entry in this
table with two new pieces of data:

* a pair (Index, PC), where Index is a string of 0 to 32 octets, and
PC is a 32-bit (4-octet) integer;

* a Nonce, which is an arbitrary string of 0 to 192 octets, and an
associated challenge expiry timer.

The Index and PC are initially undefined, and they are managed as
described in Section 4.3. The Nonce and challenge expiry timer are
initially undefined, and they are used as described in
Section 4.3.1.1.

4. Protocol Operation

4.1. MAC Computation

A Babel node computes the MAC of a Babel packet as follows.

First, the node builds a pseudo-header that will participate in MAC
computation but will not be sent. If the packet is carried over
IPv6, the pseudo-header has the following format:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Src address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Src port | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
+ +
| Dest address |
+ +
| |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Dest port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

If the packet is carried over IPv4, the pseudo-header has the
following format:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Src address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Src port | Dest address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Dest port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields:

Src address The source IP address of the packet.

Src port The source UDP port number of the packet.

Dest address The destination IP address of the packet.

Src port The destination UDP port number of the packet.

The node takes the concatenation of the pseudo-header and the Babel
packet including the packet header but excluding the packet trailer
(from octet 0 inclusive up to (Body Length + 4) exclusive) and
computes a MAC with one of the implemented algorithms. Every
implementation MUST implement HMAC-SHA256 as defined in [RFC6234] and
Section 2 of [RFC2104], SHOULD implement keyed BLAKE2s [RFC7693] with
128-bit (16-octet) digests, and MAY implement other MAC algorithms.

4.2. Packet Transmission

A Babel node might delay actually sending TLVs by a small amount, in
order to aggregate multiple TLVs in a single packet up to the
interface MTU (Section 4 of [RFC8966]). For an interface on which
MAC protection is configured, the TLV aggregation logic MUST take
into account the overhead due to PC TLVs (one in each packet) and MAC
TLVs (one per configured key).

Before sending a packet, the following actions are performed:

* a PC TLV containing the PC and Index associated with the outgoing
interface MUST be appended to the packet body;

- the PC MUST be incremented by a strictly positive amount
(typically just 1);

- if the PC overflows, a fresh index MUST be generated (as
defined in Section 3.1);

a node MUST NOT include multiple PC TLVs in a single packet;

* for each key configured on the interface, a MAC is computed as
specified in Section 4.1 and stored in a MAC TLV that MUST be
appended to the packet trailer (see Section 4.2 of [RFC8966]).

4.3. Packet Reception

When a packet is received on an interface that is configured for MAC
protection, the following steps are performed before the packet is
passed to normal processing:

* First, the receiver checks whether the trailer of the received
packet carries at least one MAC TLV; if not, the packet MUST be
immediately dropped and processing stops. Then, for each key
configured on the receiving interface, the receiver computes the
MAC of the packet. It then compares every generated MAC against
every MAC included in the packet; if there is at least one match,
the packet passes the MAC test; if there is none, the packet MUST
be silently dropped and processing stops at this point. In order
to avoid memory exhaustion attacks, an entry in the neighbour
table MUST NOT be created before the MAC test has passed
successfully. The MAC of the packet MUST NOT be computed for each
MAC TLV contained in the packet, but only once for each configured
key.

* If an entry for the sender does not exist in the neighbour table,
it MAY be created at this point (or, alternatively, its creation
can be delayed until a challenge needs to be sent, see below).

* The packet body is then parsed a first time. During this
"preparse" phase, the packet body is traversed and all TLVs are
ignored except PC, Challenge Request, and Challenge Reply TLVs.
When a PC TLV is encountered, the enclosed PC and Index are saved
for later processing. If multiple PCs are found (which should not
happen, see Section 4.2), only the first one is processed, the
remaining ones MUST be silently ignored. If a Challenge Request
is encountered, a Challenge Reply MUST be scheduled, as described
in Section 4.3.1.2. If a Challenge Reply is encountered, it is
tested for validity as described in Section 4.3.1.3, and a note is
made of the result of the test.

* The preparse phase above yields two pieces of data: the PC and
Index from the first PC TLV, and a bit indicating whether the
packet contains a successful Challenge Reply. If the packet does
not contain a PC TLV, the packet MUST be dropped, and processing
stops at this point. If the packet contains a successful
Challenge Reply, then the PC and Index contained in the PC TLV
MUST be stored in the neighbour table entry corresponding to the
sender (which already exists in this case), and the packet is
accepted.

* Otherwise, if there is no entry in the neighbour table
corresponding to the sender, or if such an entry exists but
contains no Index, or if the Index it contains is different from
the Index contained in the PC TLV, then a challenge MUST be sent
as described in Section 4.3.1.1, the packet MUST be dropped, and
processing stops at this stage.

* At this stage, the packet contains no successful Challenge Reply,
and the Index contained in the PC TLV is equal to the Index in the
neighbour table entry corresponding to the sender. The receiver
compares the received PC with the PC contained in the neighbour
table; if the received PC is smaller or equal than the PC
contained in the neighbour table, the packet MUST be dropped and
processing stops (no challenge is sent in this case, since the
mismatch might be caused by harmless packet reordering on the
link). Otherwise, the PC contained in the neighbour table entry
is set to the received PC, and the packet is accepted.

In the algorithm described above, Challenge Requests are processed
and challenges are sent before the (Index, PC) pair is verified
against the neighbour table. This simplifies the implementation
somewhat (the node may simply schedule outgoing requests as it walks
the packet during the preparse phase) but relies on the rate limiting
described in Section 4.3.1.1 to avoid sending too many challenges in
response to replayed packets. As an optimisation, a node MAY ignore
all Challenge Requests contained in a packet except the last one, and
it MAY ignore a Challenge Request in the case where it is contained
in a packet with an Index that matches the one in the neighbour table
and a PC that is smaller or equal to the one contained in the
neighbour table. Since it is still possible to replay a packet with
an obsolete Index, the rate limiting described in Section 4.3.1.1 is
required even if this optimisation is implemented.

The same is true of Challenge Replies. However, since validating a
Challenge Reply has minimal additional cost (it is just a bitwise
comparison of two strings of octets), a similar optimisation for
Challenge Replies is not worthwhile.

After the packet has been accepted, it is processed as normal, except
that any PC, Challenge Request, and Challenge Reply TLVs that it
contains are silently ignored.

4.3.1. Challenge Requests and Replies

During the preparse stage, the receiver might encounter a mismatched
Index, to which it will react by scheduling a Challenge Request. It
might encounter a Challenge Request TLV, to which it will reply with
a Challenge Reply TLV. Finally, it might encounter a Challenge Reply
TLV, which it will attempt to match with a previously sent Challenge
Request TLV in order to update the neighbour table entry
corresponding to the sender of the packet.

4.3.1.1. Sending Challenges

When it encounters a mismatched Index during the preparse phase, a
node picks a nonce that it has never used with any of the keys
currently configured on the relevant interface, for example, by
drawing a sufficiently large random string of bytes or by consulting
a strictly monotonic hardware clock. It MUST then store the nonce in
the entry of the neighbour table associated to the neighbour (the
entry might need to be created at this stage), initialise the
neighbour's challenge expiry timer to 30 seconds, and send a
Challenge Request TLV to the unicast address corresponding to the
neighbour.

A node MAY aggregate a Challenge Request with other TLVs; in other
words, if it has already buffered TLVs to be sent to the unicast
address of the neighbour, it MAY send the buffered TLVs in the same
packet as the Challenge Request. However, it MUST arrange for the
Challenge Request to be sent in a timely manner, as any packets
received from that neighbour will be silently ignored until the
challenge completes.

A node MUST impose a rate limitation to the challenges it sends; the
limit SHOULD default to one Challenge Request every 300 ms and MAY be
configurable. This rate limiting serves two purposes. First, since
a challenge may be sent in response to a packet replayed by an
attacker, it limits the number of challenges that an attacker can
cause a node to send. Second, it limits the number of challenges
sent when there are multiple packets in flight from a single
neighbour.

4.3.1.2. Replying to Challenges

When it encounters a Challenge Request during the preparse phase, a
node constructs a Challenge Reply TLV by copying the Nonce from the
Challenge Request into the Challenge Reply. It MUST then send the
Challenge Reply to the unicast address from which the Challenge
Request was sent. A challenge sent to a multicast address MUST be
silently ignored.

A node MAY aggregate a Challenge Reply with other TLVs; in other
words, if it has already buffered TLVs to be sent to the unicast
address of the sender of the Challenge Request, it MAY send the
buffered TLVs in the same packet as the Challenge Reply. However, it
MUST arrange for the Challenge Reply to be sent in a timely manner
(within a few seconds) and SHOULD NOT send any other packets over the
same interface before sending the Challenge Reply, as those would be
dropped by the challenger.

Since a Challenge Reply might be caused by a replayed Challenge
Request, a node MUST impose a rate limitation to the Challenge
Replies it sends; the limit SHOULD default to one Challenge Reply for
each peer every 300 ms and MAY be configurable.

4.3.1.3. Receiving Challenge Replies

When it encounters a Challenge Reply during the preparse phase, a
node consults the neighbour table entry corresponding to the
neighbour that sent the Challenge Reply. If no challenge is in
progress, i.e., if there is no Nonce stored in the neighbour table
entry or the challenge timer has expired, the Challenge Reply MUST be
silently ignored, and the challenge has failed.

Otherwise, the node compares the Nonce contained in the Challenge
Reply with the Nonce contained in the neighbour table entry. If the
two are equal (they have the same length and content), then the
challenge has succeeded and the nonce stored in the neighbour table
for this neighbour SHOULD be discarded; otherwise, the challenge has
failed (and the nonce is not discarded).

4.4. Expiring Per-Neighbour State

The per-neighbour (Index, PC) pair is maintained in the neighbour
table, and is normally discarded when the neighbour table entry
expires. Implementations MUST ensure that an (Index, PC) pair is
discarded within a finite time since the last time a packet has been
accepted. In particular, unsuccessful challenges MUST NOT prevent an
(Index, PC) pair from being discarded for unbounded periods of time.

A possible implementation strategy for implementations that use a
Hello history (Appendix A of [RFC8966]) is to discard the (Index, PC)
pair whenever the Hello history becomes empty. Another
implementation strategy is to use a timer that is reset whenever a
packet is accepted and to discard the (Index, PC) pair whenever the
timer expires. If the latter strategy is used, the timer SHOULD
default to a value of 5 minutes and MAY be configurable.

5. Incremental Deployment and Key Rotation

In order to perform incremental deployment, the nodes in the network
are first configured in a mode where packets are sent with
authentication but not checked on reception. Once all the nodes in
the network are configured to send authenticated packets, nodes are
reconfigured to reject unauthenticated packets.

In order to perform key rotation, the new key is added to all the
nodes. Once this is done, both the old and the new key are sent in
all packets, and packets are accepted if they are properly signed by
either of the keys. At that point, the old key is removed.

In order to support the procedures described above, implementations
of this protocol SHOULD support an interface configuration in which
packets are sent authenticated but received packets are accepted
without verification, and they SHOULD allow changing the set of keys
associated with an interface without a restart.

6. Packet Format

6.1. MAC TLV

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 = 16 | Length | MAC...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

Fields:

Type Set to 16 to indicate a MAC TLV.

Length The length of the body, in octets, exclusive of the Type
and Length fields. The length depends on the MAC algorithm
being used.

MAC The body contains the MAC of the packet, computed as
described in Section 4.1.

This TLV is allowed in the packet trailer (see Section 4.2 of
[RFC8966]) and MUST be ignored if it is found in the packet body.

6.2. PC TLV

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 = 17 | Length | PC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Index...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

Fields:

Type Set to 17 to indicate a PC TLV.

Length The length of the body, in octets, exclusive of the Type
and Length fields.

PC The Packet Counter (PC), a 32-bit (4-octet) unsigned
integer that is increased with every packet sent over this
interface. A fresh index (as defined in Section 3.1) MUST
be generated whenever the PC overflows.

Index The sender's Index, an opaque string of 0 to 32 octets.

Indices are limited to a size of 32 octets: a node MUST NOT send a
TLV with an index of size strictly larger than 32 octets, and a node
MAY ignore a PC TLV with an index of length strictly larger than 32
octets. Indices of length 0 are valid: if a node has reliable stable
storage and the packet counter never overflows, then only one index
is necessary, and the value of length 0 is the canonical choice.

6.3. Challenge Request TLV

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 = 18 | Length | Nonce...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

Fields:

Type Set to 18 to indicate a Challenge Request TLV.

Length The length of the body, in octets, exclusive of the Type
and Length fields.

Nonce The nonce uniquely identifying the challenge, an opaque
string of 0 to 192 octets.

Nonces are limited to a size of 192 octets: a node MUST NOT send a
Challenge Request TLV with a nonce of size strictly larger than 192
octets, and a node MAY ignore a nonce that is of size strictly larger
than 192 octets. Nonces of length 0 are valid: if a node has
reliable stable storage, then it may use a sequential counter for
generating nonces that get encoded in the minimum number of octets
required; the value 0 is then encoded as the string of length 0.

6.4. Challenge Reply TLV

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 = 19 | Length | Nonce...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

Fields:

Type Set to 19 to indicate a Challenge Reply TLV.

Length The length of the body, in octets, exclusive of the Type
and Length fields.

Nonce A copy of the nonce contained in the corresponding
Challenge Request.

7. Security Considerations

This document defines a mechanism that provides basic security
properties for the Babel routing protocol. The scope of this
protocol is strictly limited: it only provides authentication (we
assume that routing information is not confidential), it only
supports symmetric keying, and it only allows for the use of a small
number of symmetric keys on every link. Deployments that need more
features, e.g., confidentiality or asymmetric keying, should use a
more feature-rich security mechanism such as the one described in
[RFC8968].

This mechanism relies on two assumptions, as described in
Section 1.2. First, it assumes that the MAC being used is
invulnerable to forgery (Section 1.1 of [RFC6039]); at the time of
writing, HMAC-SHA256, which is mandatory to implement (Section 4.1),
is believed to be safe against practical attacks.

Second, it assumes that indices and nonces are generated uniquely
over the lifetime of a key used for MAC computation (more precisely,
indices must be unique for a given (key, source) pair, and nonces
must be unique for a given (key, source, destination) triple). This
property can be satisfied either by using a cryptographically secure
random number generator to generate indices and nonces that contain
enough entropy (64-bit values are believed to be large enough for all
practical applications) or by using a reliably monotonic hardware
clock. If uniqueness cannot be guaranteed (e.g., because a hardware
clock has been reset), then rekeying is necessary.

The expiry mechanism mandated in Section 4.4 is required to prevent
an attacker from delaying an authentic packet by an unbounded amount
of time. If an attacker is able to delay the delivery of a packet
(e.g., because it is located at a Layer 2 switch), then the packet
will be accepted as long as the corresponding (Index, PC) pair is
present at the receiver. If the attacker is able to cause the
(Index, PC) pair to persist for arbitrary amounts of time (e.g., by
repeatedly causing failed challenges), then it is able to delay the
packet by arbitrary amounts of time, even after the sender has left
the network, which could allow it to redirect or blackhole traffic to
destinations previously advertised by the sender.

This protocol exposes large numbers of packets and their MACs to an
attacker that is able to capture packets; it is therefore vulnerable
to brute-force attacks. Keys must be chosen in a manner that makes
them difficult to guess. Ideally, they should have a length of 32
octets (both for HMAC-SHA256 and BLAKE2s), and be chosen randomly.
If, for some reason, it is necessary to derive keys from a human-
readable passphrase, it is recommended to use a key derivation
function that hampers dictionary attacks, such as PBKDF2 [RFC8018],
bcrypt [BCRYPT], or scrypt [RFC7914]. In that case, only the derived
keys should be communicated to the routers; the original passphrase
itself should be kept on the host used to perform the key generation
(e.g., an administrator's secure laptop computer).

While it is probably not possible to be immune against denial of
service (DoS) attacks in general, this protocol includes a number of
mechanisms designed to mitigate such attacks. In particular,
reception of a packet with no correct MAC creates no local Babel
state (Section 4.3). Reception of a replayed packet with correct
MAC, on the other hand, causes a challenge to be sent; this is
mitigated somewhat by requiring that challenges be rate limited
(Section 4.3.1.1).

Receiving a replayed packet with an obsolete index causes an entry to
be created in the neighbour table, which, at first sight, makes the
protocol susceptible to resource exhaustion attacks (similarly to the
familiar "TCP SYN Flooding" attack [RFC4987]). However, the MAC
computation includes the sender address (Section 4.1), and thus the
amount of storage that an attacker can force a node to consume is
limited by the number of distinct source addresses used with a single
MAC key (see also Section 4 of [RFC8966], which mandates that the
source address is a link-local IPv6 address or a local IPv4 address).

In order to make this kind of resource exhaustion attacks less
effective, implementations may use a separate table of uncompleted
challenges that is separate from the neighbour table used by the core
protocol (the data structures described in Section 3.2 of [RFC8966]
are conceptual, and any data structure that yields the same result
may be used). Implementers might also consider using the fact that
the nonces included in Challenge Requests and Replies can be fairly
large (up to 192 octets), which should in principle allow encoding
the per-challenge state as a secure "cookie" within the nonce itself;
note, however, that any such scheme will need to prevent cookie
replay.

8. IANA Considerations

IANA has allocated the following values in the Babel TLV Types
registry:

+======+===================+===========+
| Type | Name | Reference |
+======+===================+===========+
| 16 | MAC | RFC 8967 |
+------+-------------------+-----------+
| 17 | PC | RFC 8967 |
+------+-------------------+-----------+
| 18 | Challenge Request | RFC 8967 |
+------+-------------------+-----------+
| 19 | Challenge Reply | RFC 8967 |
+------+-------------------+-----------+

Table 1

9. References

9.1. Normative References

[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997,
<https://www.rfc-editor.org/info/rfc2104>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234,
DOI 10.17487/RFC6234, May 2011,
<https://www.rfc-editor.org/info/rfc6234>.

[RFC7693] Saarinen, M-J., Ed. and J-P. Aumasson, "The BLAKE2
Cryptographic Hash and Message Authentication Code (MAC)",
RFC 7693, DOI 10.17487/RFC7693, November 2015,
<https://www.rfc-editor.org/info/rfc7693>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8966] Chroboczek, J. and D. Schinazi, "The Babel Routing
Protocol", RFC 8966, DOI 10.17487/RFC8966, January 2021,
<https://www.rfc-editor.org/info/rfc8966>.

9.2. Informational References

[BCRYPT] Niels, P. and D. Mazières, "A Future-Adaptable Password
Scheme", Proceedings of the FREENIX Track: 1999 USENIX
Annual Technical Conference, June 1999.

[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/info/rfc4086>.

[RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common
Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007,
<https://www.rfc-editor.org/info/rfc4987>.

[RFC6039] Manral, V., Bhatia, M., Jaeggli, J., and R. White, "Issues
with Existing Cryptographic Protection Methods for Routing
Protocols", RFC 6039, DOI 10.17487/RFC6039, October 2010,
<https://www.rfc-editor.org/info/rfc6039>.

[RFC7298] Ovsienko, D., "Babel Hashed Message Authentication Code
(HMAC) Cryptographic Authentication", RFC 7298,
DOI 10.17487/RFC7298, July 2014,
<https://www.rfc-editor.org/info/rfc7298>.

[RFC7914] Percival, C. and S. Josefsson, "The scrypt Password-Based
Key Derivation Function", RFC 7914, DOI 10.17487/RFC7914,
August 2016, <https://www.rfc-editor.org/info/rfc7914>.

[RFC8018] Moriarty, K., Ed., Kaliski, B., and A. Rusch, "PKCS #5:
Password-Based Cryptography Specification Version 2.1",
RFC 8018, DOI 10.17487/RFC8018, January 2017,
<https://www.rfc-editor.org/info/rfc8018>.

[RFC8968] Décimo, A., Schinazi, D., and J. Chroboczek, "Babel
Routing Protocol over Datagram Transport Layer Security",
RFC 8968, DOI 10.17487/RFC8968, January 2021,
<https://www.rfc-editor.org/info/rfc8968>.

Acknowledgments

The protocol described in this document is based on the original HMAC
protocol defined by Denis Ovsienko [RFC7298]. The use of a pseudo-
header was suggested by David Schinazi. The use of an index to avoid
replay was suggested by Markus Stenberg. The authors are also
indebted to Antonin Décimo, Donald Eastlake, Toke Høiland-Jørgensen,
Florian Horn, Benjamin Kaduk, Dave Taht, and Martin Vigoureux.

Authors' Addresses

Clara Dô
IRIF, University of Paris-Diderot
75205 Paris CEDEX 13
France

Email: [email protected]

Weronika Kolodziejak
IRIF, University of Paris-Diderot
75205 Paris CEDEX 13
France

Email: [email protected]

Juliusz Chroboczek
IRIF, University of Paris-Diderot
Case 7014
75205 Paris CEDEX 13
France

Email: [email protected]