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Network Working Group R. Bush
Internet-Draft Arrcus & IIJ
Intended status: Standards Track R. Austein
Expires: November 8, 2020 Arrcus
May 7, 2020
Layer 3 Discovery and Liveness Signing
draft-ymbk-lsvr-l3dl-signing-01
Abstract
The Layer 3 Discovery and Liveness protocol OPEN PDU may contain a
key and a certificate, which can be used to verify signatures on
subsequent PDUs. This document describes two mechanisms based on
digital signatures, one that is Trust On First Use (TOFU), and one
that uses certificates to provide authentication as well as session
integrity.
Requirements Language
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.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 8, 2020.
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Copyright Notice
Copyright (c) 2020 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 . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Trust On First Use Method . . . . . . . . . . . . . . . . . . 3
2.1. Signing a PDU . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Verifying the OPEN PDU . . . . . . . . . . . . . . . . . 4
2.3. Verifying Other PDUs . . . . . . . . . . . . . . . . . . 4
3. Public Key Infrastructure Method . . . . . . . . . . . . . . 5
3.1. Signing OPEN PDU with PKI . . . . . . . . . . . . . . . . 5
3.2. Verifying OPEN PDU with PKI . . . . . . . . . . . . . . . 5
4. Local Policy . . . . . . . . . . . . . . . . . . . . . . . . 6
5. NEWKEY, Key Roll . . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
9. Normative References . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
This draft is being published without incorporating changes from an
excellent security review. This is being done so a couple of other
drafts can reference it. While all comments will, of course, be
appreciated, readers may want to wait for the -01 version.
The Layer 3 Discovery and Liveness protocol [I-D.ietf-lsvr-l3dl] OPEN
PDU contains an algorithm specifier, a key, and a certificate, which
can be used to verify signatures on subsequent PDUs. This document
describes two methods of key generation and signing for use by L3DL,
Trust On First Use (TOFU) and a PKI-based mechanism to provide
authentication as well as session integrity.
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The Key in the OPEN PDU SHOULD be the public key of an asymmetric key
pair. The sender signs with the private key, of course. The device
sending the OPEN may use one key for all links, a different key for
each link, or some aggregation(s) thereof.
In the TOFU method the OPEN key is generated on the sending device,
believed without question by the receiver, and used to verify all
subsequent PDUs from the same sender with the same Key Type.
With the PKI-mechanism, an enrollment step is performed. The public
key is put into a certificate [RFC5280], which is signed by the the
operational environment's trust anchor. In this way, the relying
party can be confident that the public key is under control of the
identified L3DL protocol entity.
To the receiver verifying signatures on PDUs, the two methods are
indistinguishable; the key provided in the OPEN PDU is used to verify
the signatures of subsequent PDUs. The difference that PKI-based
keys may be verified against the trust anchor when the OPEN PDU is
received.
In the PKI method the OPEN key MUST be verified against the trust
anchor for the operational domain. It is then used to verify all
subsequent PDUs in the session.
2. Trust On First Use Method
There are three parts to using a key: signing PDUs, verifying the
OPEN PDU, and verifying subsequent PDUs.
2.1. Signing a PDU
All signed PDUs are generated in the same way:
o Compose the PDU, with all fields including "Sig Type" and
"Signature Length" set, but omitting the trailing "Signature"
field itself. The Certificate Length should be zero and the
Certificate field should be empty. This is the "message to be
signed" for purposes of the signature algorithm.
o Generate the signature as specified for the chosen signature
suite, using the private member of the asymmetric key pair. In
general this will involve first hashing the "message to be signed"
then signing the hash, but the precise details may vary with the
specific algorithm. The result will be a sequence of octets, the
length of which MUST be equal to the setting of the "Signature
Length" field.
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o Construct the complete message by appending the signature octets
to the otherwise complete message composed above.
In the case of the OPEN PDU, the message to be signed will include
the public member of the asymmetric keypair, but as far as the
signature algorithm is concerned that's just payload, no different
from any other PDU content.
2.2. Verifying the OPEN PDU
The process for verifying an OPEN PDU is slightly different from the
process for verifying other PDU types, because the OPEN PDU also
establishes the session key.
o Verify that the PDU is syntactically correct, and extract the Auth
Type, Key, Sig Type, and Signature fields.
o Verify that Auth Type and Sig Type refer to the same algorithm
suite, and that said algorithm suite is one that the
implementation understands.
o Construct the "message to be verified" by truncating the PDU to
remove the Signature field (in practice this should not require
copying any data, just subtract the signature length from the PDU
length).
o Verify the message constructed above against the public key using
the rules for the specific signature suite.
o Record Auth Type and Key as this sessions's authentication type
and session key, for use in verifying subseuqent PDUs.
If any of the above verification steps fail, generate an error using
error code 2 ("Authorization failure in OPEN").
2.3. Verifying Other PDUs
The process for verifying non-OPEN PDUs is slightly simpler, but
follows the same basic pattern as for OPEN PDUs.
o Verify that the PDU is syntactically correct, and extract the Sig
Type and Signature fields.
o Verify that Sig Type refers to the same algorithm suite as the
Auth Type recorded during verification of the OPEN PDU.
o Construct the "message to be verified" by truncating the PDU to
remove the Signature field.
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o Verify the message constructed above against the recorded session
key using the rules for the specific signature suite.
If any of the above verification steps fail, generate an error using
error code 3 ("Signature failure in PDU").
3. Public Key Infrastructure Method
Using a PKI, [RFC5280], is almost the same as using TOFU, but with
one additional step: during verification of an OPEN PDU, after
extracting the Key field from the PDU but before attempting to use it
to verify the PDU's signature, the receiver MUST verify the received
key against the PKI to confirm that it's an authorized key.
Generating an OPEN PDU using the PKI method requires a certificate,
which must be supplied via out of band configuration. The
certificate is a signature of the public key to be sent in the Key
field of the OPEN PDU, signed by the trust anchor private key.
Verifying an OPEN PDU using the PKI method requires the public key of
the trust anchor, which the receiver uses to verify the certificate,
thereby demonstrating that the supplied is represents an authorized
L3DL speaker in this administrative domain.
We use the term "certificate" here in the generic sense. These are
not X.509 certificates: X.509 is much more complicated than we need
for L3DL. The certificates used here are just signatures of one key
(the session key supplied in the Key field of the OPEN PDU) by
another key (the trust anchor).
3.1. Signing OPEN PDU with PKI
Generating and signing the OPEN PDU with the PKI method is almost the
same as in Section 2.1. The only difference is that the PKI method
MUST supply the appropriate certificate in the Certificate field.
Note that the Auth Type field applies to both the Key and Certificate
fields. That is: the certificate uses the same certificate suite as
the session keys, L3DL does not support cross-algorithm-suite
certification.
3.2. Verifying OPEN PDU with PKI
Verifying the OPEN PDU with PKI is similar to verifying with TOFU as
described in Section 2.2, but includes one critical extra step:
After extracting the Key field from the PDU but before verifying the
Signature, extract the Certificate field and verfiy that the
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Certificate is a valid signature of the Key field, according to the
rules for the signature suite specified by Auth Type. If this step
fails, handle as in Section 2.2.
4. Local Policy
Whether to use TOFU, PKI, or no signatures at all is a matter of
local policy, to be decided by the operator. The useful policy
combinations for Key and Certificate are probably:
o Not signing: sender need not sign, receiver does not check.
o Require TOFU: sender MUST supply key and receiver MUST check,
certificate not needed and ignored if sent.
o Allow TOFU: sender must supply key and receiver MUST check,
receiver SHOULD check certificate if supplyed by sender.
o Require PKI: sender must supply key and certificate, receiver must
check both.
5. NEWKEY, Key Roll
Modern key management allows for agility in 'rolling' to a new key or
even algorithm in case of key expiry, key compromise, or merely
prudence. Declaring a new key with an L3DL OPEN PDU would cause
serious churn in topology as a new OPEN may cause a withdraw of
previously announced encapsulations. Therefore, a gentler rekeying
is needed.
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 = 8 | Payload Length | New Key Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| New Key Length | New Key ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | New Cert Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| New Certificate ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Old Sig Type | Old Signature Length | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Old Signature ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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The New Key Type, New Key Length, New Key, New Cert Length, and New
Certificate field declare the replacement algorithm suite, key, and
certificate.
The NEWKEY PDU is signed using the current (soon to be old) algorithm
suite and key.
The sender and the receiver should be cautious of algorithm suite
downgrade attacks.
To avoid possible race conditions, the receiver SHOULD accept
signatures using either the new or old key for a configurable time
(default 30 seconds). This is intended to accommodate situations
such as senders with high peer out-degree and a single per-device
asymmetric key.
If the sender does not receive an ACK in the normal window, including
retransmission, then the sender MAY choose to allow a session reset
by either issuing a new OPEN or by letting the receiver eventually
have a signature failure (error code 3) on a PDU.
The rekeying operation changes the session key and algorithm suite
described in Section 2.3. The NEWKEY PDU itself is verified using
the old algorithm and session key, subsequent PDUs are verified with
the new algorithm and session key recorded after the NEWKEY PDU has
been accepted.
6. Security Considerations
The TOFU method requires a leap of faith to accept the key in the
OPEN PDU, as it can not be verified against any authority. Hence it
is jokingly referred to as Married On First Date. The assurance it
does provide is that subsequent signed PDUs are from the same peer.
And data integrity is a positive side effect of the signature
covering the payload.
The PKI-based method offers assurance that the certificate, and hence
the keying material, provided in the OPEN PDU are authorized by a
central authority, e.g. the network's network security team. The
onward assurance of talking to the same peer and data integrity are
the same as in the TOFU method.
With the PKI-based method, automated device provisioning could
restrict which certificates are allowed from which peers on a per
interface basis. This would complicate key rolls. Where one draws
the line between rigidity, flexibility, and security varies.
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The REKEY PDU is open to abuse to create an algorithm suite downgrade
attack.
7. IANA Considerations
This document requests the IANA create a new entry in the L3DL PDU
Type registry as follows:
PDU
Code PDU Name
---- -------------------
8 NEWKEY
This document requests the IANA add a registry entry for "TOFU -
Trust On First Use" to the L3DL-Signature-Type registry as follows:
Number Name
------ -------------------
1 TOFU - Trust On First Use
2 PKI
8. Acknowledgments
The authors than Russ Housley for advice and review.
9. Normative References
[I-D.ietf-lsvr-l3dl]
Bush, R., Austein, R., and K. Patel, "Layer 3 Discovery
and Liveness", draft-ietf-lsvr-l3dl-03 (work in progress),
November 2019.
[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>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[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>.
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Authors' Addresses
Randy Bush
Arrcus & IIJ
5147 Crystal Springs
Bainbridge Island, WA 98110
United States of America
Email: randy@psg.com
Rob Austein
Arrcus, Inc.
Email: sra@hactrn.net
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