Network Working Group C. Do
Internet-Draft W. Kolodziejak
Obsoletes: 7298 (if approved) J. Chroboczek
Intended status: Standards Track IRIF, University of Paris-Diderot
Expires: February 26, 2021 August 25, 2020

MAC authentication for the Babel routing protocol


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 Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on February 26, 2021.

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Table of Contents

1. Introduction

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:

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 [I-D.ietf-babel-dtls], 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: [RFC4086] and sufficiently large indices and nonces, by using a reliable hardware clock, or by rekeying often enough that collisions are unlikely.

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

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

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) which 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, 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. 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. 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 above).

3. Data Structures

Every Babel node maintains a set of conceptual data structures described in Section 3.2 of [RFC6126bis]. 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 [RFC6126bis]. 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 entry in this table that is associated with an interface on which MAC authentication has been configured with two new pieces of data: Section 7 for suggested sizes), and the PC is initialised to an arbitrary value (typically 0).

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

3.2. The Neighbour table

Every Babel node maintains a neighbour table, as described in Section 3.2.4 of [RFC6126bis]. This protocol extends each entry in this table with two new pieces of data: Section 4.3. The Nonce and expiry timer are initially undefined, and used as described in Section

The Index and PC are initially undefined, and are managed as described in

4. Protocol Operation

4.1. MAC computation

A Babel node computes the MAC of a Babel packet 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
|                                                               |
+                                                               +
|                                                               |
+                          Src address                          +
|                                                               |
+                                                               +
|                                                               |
|           Src port            |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
+                                                               +
|                         Dest address                          |
+                                                               +
|                                                               |
+                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                               |           Dest port           |
 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           |

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:

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], 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 [RFC6126bis]). 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:

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:

In the algorithm described above, challenge requests are processed and challenges are sent before the PC/Index 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 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 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's 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. 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 300ms, 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. 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 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. 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 300ms and MAY be configurable. 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; otherwise, the challenge has failed.

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 [RFC6126bis]) 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.

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 :

Set to 16 to indicate a MAC TLV.
The length of the body, in octets, exclusive of the Type and Length fields. The length depends on the MAC algorithm being used.
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 [RFC6126bis]), 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 :

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 Section 3.1) 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 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 :

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. Nonces of length 0 are valid: if a node has reliable stable storage, then it may use a sequential counter for generating nonces which 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 :

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.

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 featureful security mechanism such as the one described in [I-D.ietf-babel-dtls].

This mechanism relies on two assumptions, as described in Section 1.2. First, it assumes that the MAC being used is invulnerable to pre-image attacks (Section 1.1 of [RFC6039]); at the time of writing, SHA-256, 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 [RFC2898], 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 administator'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

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 [RFC6126bis], 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 [RFC6126bis] 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 responses 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 this document
17 PC this document
18 Challenge Request this document
19 Challenge Reply this document

9. 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 Donald Eastlake, Toke Hoiland-Jorgensen, Florian Horn, Benjamin Kaduk, Dave Taht and Martin Vigoureux.

10. References

10.1. Normative References

[RFC2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, DOI 10.17487/RFC2104, February 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC6126bis] Chroboczek, J. and D. Schinazi, "The Babel Routing Protocol", Internet-Draft draft-ietf-babel-rfc6126bis-06, October 2018.
[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.
[RFC7693] Saarinen, M-J. and J-P. Aumasson, "The BLAKE2 Cryptographic Hash and Message Authentication Code (MAC)", RFC 7693, DOI 10.17487/RFC7693, November 2015.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017.

10.2. Informational References

[BCRYPT] Niels, P. and D. Mazieres, "A Future-Adaptable Password Scheme", 1999.

In Proceedings of the 1999 USENIX Annual Technical Conference.

[I-D.ietf-babel-dtls] Decimo, A., Schinazi, D. and J. Chroboczek, "Babel Routing Protocol over Datagram Transport Layer Security", Internet-Draft draft-ietf-babel-dtls-07, July 2019.
[RFC2898] Kaliski, B., "PKCS #5: Password-Based Cryptography Specification Version 2.0", RFC 2898, DOI 10.17487/RFC2898, September 2000.
[RFC4086] Eastlake 3rd, D., Schiller, J. and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, DOI 10.17487/RFC4086, June 2005.
[RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007.
[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.
[RFC7298] Ovsienko, D., "Babel Hashed Message Authentication Code (HMAC) Cryptographic Authentication", RFC 7298, DOI 10.17487/RFC7298, July 2014.
[RFC7914] Percival, C. and S. Josefsson, "The scrypt Password-Based Key Derivation Function", RFC 7914, DOI 10.17487/RFC7914, August 2016.

Appendix A. Changes from previous versions

[RFC Editor: please remove this section before publication.]

A.1. Changes since draft-ietf-babel-hmac-00

A.2. Changes since draft-ietf-babel-hmac-01

A.3. Changes since draft-ietf-babel-hmac-02

A.4. Changes since draft-ietf-babel-hmac-03

A.5. Changes since draft-ietf-babel-hmac-04

A.6. Changes since draft-ietf-babel-hmac-05

A.7. Changes since draft-ietf-babel-hmac-06

A.8. Changes since draft-ietf-babel-hmac-07

A.8.1. Changes since draft-ietf-babel-hmac-08

A.8.2. Changes since draft-ietf-babel-hmac-09

A.8.3. Changes since draft-ietf-babel-hmac-10

Authors' Addresses

Clara Do IRIF, University of Paris-Diderot 75205 Paris Cedex 13, France EMail:
Weronika Kolodziejak IRIF, University of Paris-Diderot 75205 Paris Cedex 13, France EMail:
Juliusz Chroboczek IRIF, University of Paris-Diderot Case 7014 75205 Paris Cedex 13, France EMail: