HMAC authentication for the
Babel routing protocolIRIF, University of Paris-Diderot75205 Paris Cedex 13Franceclarado_perso@yahoo.frIRIF, University of Paris-Diderot75205 Paris Cedex 13Franceweronika.kolodziejak@gmail.comIRIF, University of Paris-DiderotCase 701475205 Paris Cedex 13Francejch@irif.frThis document describes a cryptographic authentication mechanism for
the Babel routing protocol that has provisions for replay avoidance. This
document updates RFC 6126bis and obsoletes RFC 7298.By default, the Babel routing protocol 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 smaller
metric, a larger seqno, 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 , 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.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.The correctness of the protocol relies on the following assumptions:
that the Hashed Message Authentication Code (HMAC) being used is
invulnerable to pre-image attacks, i.e., that an attacker is unable to
generate a packet with a correct HMAC;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 HMAC being used. The second
assumption can be met either by using a robust random number generator
and sufficiently large indices and nonces, by
using a reliable hardware clock, or by rekeying whenever a collision
becomes likely.If the assumptions above are met, the protocol described in this
document has the following properties:
it is invulnerable to spoofing: any 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 a packet from C as valid, then a node B will only
accept a copy of that packet as authentic if B has accepted an older
packet from C and B has received no later packet from C.While this protocol makes serious efforts to mitigate the effects of
a denial of service attack, it does not fully protect against such
attacks.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
when, and only when,
they appear in all capitals, as shown here.When a node B sends out a Babel packet through an interface that is
configured for HMAC cryptographic protection, it computes one or more
HMACs which it appends to the packet. When a node A receives a packet
over an interface that requires HMAC cryptographic protection, it
independently computes a set of HMACs and compares them to the HMACs
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 HMACs 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, A discards the packet and challenges B to prove that
it knows the HMAC 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. 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
HMAC 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 HMAC 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. 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 above).Every Babel node maintains a set of conceptual data structures
described in Section 3.2 of . This protocol
extends these data structures as follows.Every Babel node maintains an interface table, as described in Section
3.2.3 . Implementations of this protocol MUST
allow each interface to be provisioned with a set of one or more HMAC keys
and the associated HMAC algorithms (see ). In order to allow incremental deployment of
this protocol (see ),
implementations SHOULD allow an interface to be configured in a mode in
which it participates in the HMAC authentication protocol but accepts
packets that are not authentified.This protocol extends each entry in this table that is associated with
an interface on which HMAC authentication has been configured with two new
pieces of data:
a set of one or more HMAC keys, each associated with a given HMAC
algorithm ; the length of each key is exactly the hash size of the
associated HMAC algorithm (i.e., the key is not subject to the
preprocessing described in Section 2 of ); 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, and the PC is initialised to an arbitrary value
(typically 0).Every Babel node maintains a neighbour table, as described in
Section 3.2.4 of . 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 are managed as described in
. The Nonce and expiry timer are
initially undefined, and used as described in
.A Babel node computes the HMAC of a Babel packet as follows.First, the node builds a pseudo-header that will participate in HMAC
computation but will not be sent. If the packet was carried over IPv6,
the pseudo-header has the following format:
If the packet was carried over IPv4, the pseudo-header has the following
format:
Fields :
The source IP address of the packet.The source UDP port number of the packet.The destination IP address of the packet.The destination UDP port number of the packet.The node takes the concatenation of the pseudo-header and the packet
including the packet header but excluding the packet trailer (from octet
0 inclusive up to (Body Length + 4) exclusive) and computes an
HMAC with one of the implemented hash algorithms. Every implementation
MUST implement HMAC-SHA256 as defined in and
Section 2 of , SHOULD implement keyed BLAKE2s
, and MAY implement other HMAC algorithms.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 ). For an
interface on which HMAC protection is configured, the TLV aggregation
logic MUST take into account the overhead due to PC TLVs (one in each
packet) and HMAC 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
);for each key configured on the interface, an HMAC is computed as
specified in above, and stored in an
HMAC TLV that MUST be appended to the packet trailer (see Section 4.2 of
).When a packet is received on an interface that is configured for HMAC
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 HMAC 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 HMAC of the packet. It then
compares every generated HMAC against every HMAC included in the packet;
if there is at least one match, the packet passes the HMAC 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 HMAC test has passed
successfully. The HMAC of the packet MUST NOT be computed for each HMAC
TLV contained in the packet, but only once for each configured key.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
TLVs, Challenge Requests and Challenge Replies. When a PC TLV is
encountered, the enclosed PC and Index are saved for later processing; if
multiple PCs are found, 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 . If a Challenge Reply is encountered, it
is tested for validity as described in
and a note is made of the result of the test.The preparse phase above has yielded 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 may need to be created at this stage),
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
, 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.
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.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.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.Since a challenge may be prompted by a packet replayed by an attacker,
a node MUST impose a rate limitation to the challenges it sends; the limit
SHOULD default to one challenge request every 300ms, and MAY be
configurable.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 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.A challenge sent to a multicast address MUST be silently ignored.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; otherwise, the challenge has failed.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 ) 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 being used, the timer SHOULD default
to a value of 5 min, and MAY be configurable.Fields :
Set to 16 to indicate an HMAC TLV.The length of the body, in octets, exclusive of the
Type and Length fields. The length of the body depends on the HMAC
algorithm being used.The body contains the HMAC of the packet, computed as
described in .This TLV is allowed in the packet trailer (see Section 4.2 of
), and MUST be ignored if it is found in the
packet body.Fields :
Set to 17 to indicate a PC TLV.The length of the body, in octets, exclusive of the
Type and Length fields.The Packet Counter (PC), a 32-bit (4 octet) unsigned
integer which is increased with every packet sent over this interface.
A fresh index (as defined in ) MUST be
generated whenever the PC overflows.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 size strictly larger than 32 octets.Fields :
Set to 18 to indicate a Challenge Request TLV.The length of the body, in octets, exclusive of the
Type and Length fields.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.Fields :
Set to 19 to indicate a Challenge Reply TLV.The length of the body, in octets, exclusive of the
Type and Length fields.A copy of the nonce contained in the corresponding
challenge request.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 featureful security mechanism such as
the one described in .This mechanism relies on two assumptions, as described in . First, it assumes that the hash being
used is invulnerable to pre-image attacks (Section 1.1 of ); at the time of writing, SHA-256, which is mandatory
to implement (), 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 HMAC 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 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.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 HMAC creates no local state whatsoever (). Reception of a replayed packet with correct
hash, on the other hand, causes a challenge to be sent; this is mitigated
somewhat by requiring that challenges be rate-limited.At first sight, sending a challenge requires retaining enough
information to validate the challenge reply. However, the nonce included
in a challenge request and echoed in the challenge reply can be fairly
large (up to 192 octets), which should in principle permit encoding the
per-challenge state as a secure "cookie" within the nonce itself.IANA has allocated the following values in the Babel TLV Types
registry:TypeNameReference16HMACthis document17PCthis document18Challenge Requestthis document19Challenge Replythis documentThe protocol described in this document is based on the original HMAC
protocol defined by Denis Ovsienko . 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 Donald Eastlake, Toke Hoiland-Jorgensen, Florian Horn, and
Dave Taht.Key words for use in RFCs to Indicate Requirement LevelsAmbiguity of Uppercase vs Lowercase in RFC 2119 Key WordsThe Babel Routing ProtocolHMAC: Keyed-Hashing for Message AuthenticationUS Secure Hash Algorithms (SHA and SHA-based HMAC and HKDF)The BLAKE2 Cryptographic Hash and Message Authentication Code (MAC)Babel Hashed Message Authentication Code (HMAC) Cryptographic
AuthenticationBabel Routing Protocol over Datagram Transport Layer Security
Issues with Existing Cryptographic Protection Methods for Routing Protocols
Randomness Requirements for SecurityThis protocol supports incremental deployment (transitioning from an
insecure network to a secured network with no service interruption) and
key rotation (transitioning from a set of keys to a different set of
keys).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 incremental deployment and key rotation,
implementations SHOULD support an interface configuration in which they
send authenticated packets but accept all packets, and SHOULD allow
changing the set of keys associated with an interface without
a restart.[RFC Editor: please remove this section before publication.]Changed the title.Removed the appendix about the packet trailer, this is now in
rfc6126bis.Removed the appendix with implicit indices.Clarified the definitions of acronyms.Limited the size of nonces and indices.Made BLAKE2s a recommended HMAC algorithm.Added requirement to expire per-neighbour crypto state.Clarified that PCs are 32-bit unsigned integers.Clarified that indices and nonces are of arbitrary size.Added reference to RFC 4086.Use the TLV values allocated by IANA.Fixed an issue with packets that contain a successful challenge
reply: they should be accepted before checking the PC value.Clarified that keys are the exact value of the HMAC hash size, and not
subject to preprocessing; this makes management more deterministic.Use normative language in more places.