Secure BFD Sequence Numbers
draft-ietf-bfd-secure-sequence-numbers-08
Network Working Group M. Jethanandani
Internet-Draft Kloud Services
Updates: 5880 (if approved) S. Agarwal
Intended status: Standards Track Cisco Systems, Inc
Expires: September 9, 2021 A. Mishra
O3b Networks
A. Saxena
Ciena Corporation
A. Dekok
Network RADIUS SARL
March 8, 2021
Secure BFD Sequence Numbers
draft-ietf-bfd-secure-sequence-numbers-08
Abstract
This document describes a security enhancement for the sequence
number used in BFD control packets. This document updates RFC 5880.
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 September 9, 2021.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 2
3. Theory of operation . . . . . . . . . . . . . . . . . . . . . 2
4. Impact of using a hash . . . . . . . . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
6. Security Considerations . . . . . . . . . . . . . . . . . . . 5
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
8.1. Normative References . . . . . . . . . . . . . . . . . . 5
8.2. Informative References . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
BFD [RFC5880] section 6.7 describes the use of monotonically
incrementing 32-bit sequence numbers for use in authentication of BFD
packets. While this method protects against simple replay attacks,
the monotonically incrementing sequence numbers are predictable and
vulnerable to more complex attack vectors. This document proposes
the use of non-monotonically-incrementing sequence numbers in the BFD
authentication section to enhance the security of BFD sessions.
Specifically, the document presents a method to generate pseudo-
random sequence numbers on the frame by algorithmically hashing
monotonically increasing sequence numbers. Since the monotonically
increasing sequence number does not appear on the wire, it is
difficult for a third party to launch a replay attack.
2. Requirements Language
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 RFC 2119 [RFC2119].
3. Theory of operation
Instead of inserting a monotonically, sometimes occasionally,
increasing sequence number in BFD control packets, the ciphertext
result from a symmetric key algorithm operation (Symmetric-key
algorithms require both the sender and the recipient of a message to
have the same shared secret key) is inserted. The result is
computed, using a shared key, on the sequence number. That
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ciphertext result is then inserted into the sequence number field of
the packet. In case of BFD Authentication
[I-D.ietf-bfd-optimizing-authentication], the sequence number used in
computing an authenticated packet would be this new computed
ciphertext. Even though the BFD Authentication
[I-D.ietf-bfd-optimizing-authentication] sequence number is
independent of this enhancement, it would benefit by using the
computed ciphertext.
As currently defined in BFD [RFC5880], a BFD packet with
authentication will undergo the following steps, where:
[O]: original RFC 5880 packet with monotonically increasing sequence
number
[S]: pseudo random sequence number
[A]: Authentication
Sender Receiver
[O] [S] [A] ------------- [A] [S] [O]
This document proposes that for enhanced security in sequence number
encoding, the sender would identify a symmetric key algorithm that
would create a 32 bit ciphertext. The symmetric key is provisioned
securely on the sender and receiver of the BFD session. The
mechanism of provisioning such a key is outside the scope of this
document. This key SHOULD be different from the symmetric key used
to to authenticate the packet. Instead of sending the sequence
number, the sender encrypts the sequence number using it as input to
the symmetric algorithm to produce the ciphertext, which is then
inserted in place of the sequence number.
Upon receiving the BFD Control packet, the receiver decrypts the
ciphertext using the same provisioned shared key to produce the
received sequence number. It compares the received sequence number
against the expected sequence number. The mechanism used for
comparing is an implementation detail (implementations may pre-
calculate the expected sequence number, or decrypt the received
sequence number before comparing against expected value). To
tolerate dropped frames, the receiver must compare the received
sequence number against the current expected sequence number. BFD
[RFC5880] mentions that received sequence number should be between
(bfd.RcvAuthSeq(+1) to bfd.RcvAuthSeq+(3*Detect Mult) inclusive.
Note: The first sequence number can be obtained using the same
principles stated in BFD [RFC5880] i.e. (using bfd.AuthSeqKnown and
bfd.RcvAuthSeq)
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K: symmetric key
S: sequence number
S': encrypted sequence number OR ciphertext result
O: original RFC 5880 packet with monotonically increasing sequence
number
f(S, K) = S', where f is a symmetric encryption algorithm
f(S', K) = S, where f is a symmetric decryption algorithm
Sender Receiver
[O] [S'] [A] -------- [A] [S] [O]
The above diagram describes how the sender encrypts and receiver
decrypts the sequence number. The sender starts by taking the
monotonically increasing (but non linear) sequence number and
encrypting it using a symmetric encryption algorithm. The resulting
ciphertext replaces the sequence number. As per BFD [RFC5880], it
then calculates the hash for the entire packet and appends the hash
value to the end of the packet, before transmitting it.
The receiver hashes the entire packet as part of receiver
authentication. On successful authentication, it decrypts the
ciphertext with the same key used to encrypt it, in order to obtain
the original sequence number. If it is greater than the previously
received monotonically increasing sequence number, then the receiver
knows it's a valid sequence number.
4. Impact of using a hash
Under this proposal, every packet's sequence number is encoded in
ciphertext. Therefore, there is some impact on the system and its
performance while doing encryption/decryption. As security measures
go, this enhancement greatly increases the security of the packet
with or without authentication of the entire packet.
5. IANA Considerations
This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an
RFC.
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6. Security Considerations
In a symmetric key algorithm, the key is shared between the two
systems. Distribution of this key to all the systems at the same
time can be quite a cumbersome task. BFD sessions running a fast
rate will require these keys to be refreshed often, which poses a
further challenge. Therefore, it is difficult to change the keys
during the operation of a BFD session without affecting the stability
of the BFD session. Therefore, it is recommended to administratively
disable the BFD session before changing the keys. If the keys are
not changed frequently, an attacker can try to guess the key to
launch a replay attack.
This method allows the BFD end-points to detect a malicious packet
(the decrypted sequence number will not be in sequence). The
behavior of the session, when such a packet is detected, is based on
the implementation. A flood of such malicious packets may cause a
BFD session to be operationally down.
The symmetric algorithm and key size will determine the difficulty
for an attacker to decipher the key from the transmitted BFD frames.
The sequential nature of the payload (sequence numbers) simplifies
the decoding of the key. It is, therefore, recommended to use longer
keys or more secure symmetric algorithms.
7. Acknowledgements
The authors would like to thank Jeff Haas and Reshad Rahman for their
reviews of and suggestions for the document.
8. References
8.1. Normative References
[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>.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
<https://www.rfc-editor.org/info/rfc5880>.
8.2. Informative References
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[I-D.ietf-bfd-optimizing-authentication]
Jethanandani, M., Mishra, A., Saxena, A., and M. Bhatia,
"Optimizing BFD Authentication", draft-ietf-bfd-
optimizing-authentication-11 (work in progress), July
2020.
Authors' Addresses
Mahesh Jethanandani
Kloud Services
Email: mjethanandani@gmail.com
Sonal Agarwal
Cisco Systems, Inc
170 W. Tasman Drive
San Jose, CA 95070
USA
Email: agarwaso@cisco.com
URI: www.cisco.com
Ashesh Mishra
O3b Networks
Email: mishra.ashesh@gmail.com
Ankur Saxena
Ciena Corporation
3939 North First Street
San Jose, CA 95134
USA
Email: ankurpsaxena@gmail.com
Alan DeKok
Network RADIUS SARL
100 Centrepointe Drive #200
Ottowa, ON K2G 6B1
Canada
Email: aland@freeradius.org
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