530 lines
17 KiB
XML
530 lines
17 KiB
XML
<?xml version="1.0" encoding="utf-8"?>
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<!DOCTYPE rfc SYSTEM "rfc2629.dtd">
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<?rfc comments="yes"?>
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<?rfc compact="yes"?>
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<?rfc subcompact="no"?>
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<?rfc inline="yes"?>
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<?rfc sortrefs="yes"?>
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<?rfc symrefs="yes"?>
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<?rfc toc="yes"?>
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<?rfc tocdepth="6"?>
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<?rfc tocindent="yes"?>
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<?rfc tocompact="yes"?>
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<rfc category="std" docName="draft-ymbk-lsvr-l3dl-signing-01" ipr="trust200902">
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<front>
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<title>Layer 3 Discovery and Liveness Signing</title>
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<author fullname="Randy Bush" initials="R." surname="Bush">
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<organization>Arrcus & IIJ</organization>
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<address>
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<postal>
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<street>5147 Crystal Springs</street>
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<city>Bainbridge Island</city>
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<region>WA</region>
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<code>98110</code>
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<country>United States of America</country>
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</postal>
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<email>randy@psg.com</email>
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</address>
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</author>
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<!--
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<author fullname="Russ Housley" initials="R" surname="Housley">
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<organization abbrev="Vigil Security">Vigil Security, LLC</organization>
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<address>
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<postal>
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<street>516 Dranesville Road</street>
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<city>Herndon</city>
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<region>VA</region>
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<code>20170</code>
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<country>USA</country>
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</postal>
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<email>housley@vigilsec.com</email>
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</address>
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</author>
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-->
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<author initials="R." surname="Austein" fullname="Rob Austein">
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<organization abbrev="Arrcus">Arrcus, Inc.</organization>
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<address>
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<email>sra@hactrn.net</email>
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</address>
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</author>
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<date />
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<abstract>
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<t>The Layer 3 Discovery and Liveness protocol OPEN PDU may contain
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a key and a certificate, which can be used to verify signatures on
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subsequent PDUs. This document describes two mechanisms based on
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digital signatures, one that is Trust On First Use (TOFU), and one
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that uses certificates to provide authentication as well as session
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integrity.</t>
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</abstract>
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<note title="Requirements Language">
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<t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
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"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
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"OPTIONAL" in this document are to be interpreted as described in
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BCP 14 <xref target="RFC2119"/> <xref target="RFC8174"/> when,
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and only when, they appear in all capitals, as shown here.</t>
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</note>
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</front>
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<middle>
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<section anchor="intro" title="Introduction">
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<t>This draft is being published without incorporating changes from
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an excellent security review. This is being done so a couple of
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other drafts can reference it. While all comments will, of course,
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be appreciated, readers may want to wait for the -01 version.</t>
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<t>The Layer 3 Discovery and Liveness protocol <xref
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target="I-D.ietf-lsvr-l3dl"/> OPEN PDU contains an algorithm
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specifier, a key, and a certificate, which can be used to verify
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signatures on subsequent PDUs. This document describes two methods
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of key generation and signing for use by L3DL, Trust On First Use
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(TOFU) and a PKI-based mechanism to provide authentication as well
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as session integrity.</t>
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<t>The Key in the OPEN PDU SHOULD be the public key of an asymmetric
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key pair. The sender signs with the private key, of course. The
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device sending the OPEN may use one key for all links, a different
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key for each link, or some aggregation(s) thereof.</t>
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<t>In the TOFU method the OPEN key is generated on the sending
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device, believed without question by the receiver, and used to
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verify all subsequent PDUs from the same sender with the same Key
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Type.</t>
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<t>With the PKI-mechanism, an enrollment step is performed. The
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public key is put into a certificate <xref target="RFC5280"/>, which
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is signed by the the operational environment's trust anchor. In
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this way, the relying party can be confident that the public key is
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under control of the identified L3DL protocol entity.</t>
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<t>To the receiver verifying signatures on PDUs, the two methods are
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indistinguishable; the key provided in the OPEN PDU is used to
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verify the signatures of subsequent PDUs. The difference that
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PKI-based keys may be verified against the trust anchor when the
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OPEN PDU is received.</t>
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<t>In the PKI method the OPEN key MUST be verified against the trust
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anchor for the operational domain. It is then used to verify all
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subsequent PDUs in the session.</t>
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</section>
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<section anchor="tofu" title="Trust On First Use Method">
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<t>There are three parts to using a key: signing PDUs, verifying
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the OPEN PDU, and verifying subsequent PDUs.</t>
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<section anchor="tofu-pdu-signing" title="Signing a PDU">
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<t>All signed PDUs are generated in the same way:</t>
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<t>
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<list style="symbols">
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<t>
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Compose the PDU, with all fields including "Sig Type" and
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"Signature Length" set, but omitting the trailing
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"Signature" field itself. The Certificate Length should
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be zero and the Certificate field should be empty. This
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is the "message to be signed" for purposes of the
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signature algorithm.
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</t>
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<t>
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Generate the signature as specified for the chosen signature
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suite, using the private member of the asymmetric key pair.
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In general this will involve first hashing the "message to
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be signed" then signing the hash, but the precise details
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may vary with the specific algorithm. The result will be a
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sequence of octets, the length of which MUST be equal to the
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setting of the "Signature Length" field.
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</t>
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<t>
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Construct the complete message by appending the signature
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octets to the otherwise complete message composed above.
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</t>
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</list>
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</t>
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<t>
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In the case of the OPEN PDU, the message to be signed will
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include the public member of the asymmetric keypair, but as
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far as the signature algorithm is concerned that's just
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payload, no different from any other PDU content.
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</t>
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</section>
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<section anchor="tofu-open-verifying" title="Verifying the OPEN PDU">
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<t>
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The process for verifying an OPEN PDU is slightly different
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from the process for verifying other PDU types, because the
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OPEN PDU also establishes the session key.
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</t>
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<t>
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<list style="symbols">
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<t>
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Verify that the PDU is syntactically correct, and extract
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the Auth Type, Key, Sig Type, and Signature fields.
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</t>
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<t>
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Verify that Auth Type and Sig Type refer to the same
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algorithm suite, and that said algorithm suite is one that
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the implementation understands.
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</t>
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<t>
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Construct the "message to be verified" by truncating the PDU
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to remove the Signature field (in practice this should not
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require copying any data, just subtract the signature length
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from the PDU length).
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</t>
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<t>
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Verify the message constructed above against the public key
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using the rules for the specific signature suite.
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</t>
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<t>
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Record Auth Type and Key as this sessions's authentication
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type and session key, for use in verifying subseuqent PDUs.
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</t>
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</list>
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</t>
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<t>
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If any of the above verification steps fail, generate an error
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using error code 2 ("Authorization failure in OPEN").
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</t>
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<!--
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Why are we using a different error code for failures in OPEN
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PDUs than we do in other PDUs? We don't want to provide an
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oracle, so we want to return the same error code for any
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verification failure for a particular PDU, so the only effect
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would be to have all failures in OPEN PDUs return a different
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single error code than all failures in any other PDU would
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use, which doens't seem useful.
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-->
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</section>
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<section anchor="tofu-other-verifying" title="Verifying Other PDUs">
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<t>
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The process for verifying non-OPEN PDUs is slightly simpler,
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but follows the same basic pattern as for OPEN PDUs.
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</t>
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<t>
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<list style="symbols">
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<t>
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Verify that the PDU is syntactically correct, and extract
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the Sig Type and Signature fields.
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</t>
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<t>
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Verify that Sig Type refers to the same algorithm suite as
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the Auth Type recorded during verification of the OPEN PDU.
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</t>
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<t>
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Construct the "message to be verified" by truncating the PDU
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to remove the Signature field.
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</t>
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<t>
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Verify the message constructed above against the recorded
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session key using the rules for the specific signature
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suite.
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</t>
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</list>
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</t>
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<t>
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If any of the above verification steps fail, generate an error
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using error code 3 ("Signature failure in PDU").
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</t>
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<!--
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See note in previous section regarding error codes.
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-->
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</section>
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</section>
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<section anchor="pki" title="Public Key Infrastructure Method">
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<t>
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Using a PKI is almost the same as using TOFU, but with one
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additional step: during verification of an OPEN PDU, after
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extracting the Key field from the PDU but before attempting to
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use it to verify the PDU's signature, the receiver MUST verify
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the received key against the PKI to confirm that it's an
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authorized key.
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</t>
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<t>
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Generating an OPEN PDU using the PKI method requires a
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certificate, which must be supplied via out of band
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configuration. The certificate is a signature of the public key
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to be sent in the Key field of the OPEN PDU, signed by the trust
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anchor private key.
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</t>
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<t>
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Verifying an OPEN PDU using the PKI method requires the public
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key of the trust anchor, which the receiver uses to verify the
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certificate, thereby demonstrating that the supplied is
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represents an authorized L3DL speaker in this administrative
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domain.
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</t>
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<t>
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We use the term "certificate" here in the generic sense. These
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are not X.509 certificates: X.509 is much more complicated than
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we need for L3DL. The certificates used here are just
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signatures of one key (the session key supplied in the Key field
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of the OPEN PDU) by another key (the trust anchor).
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</t>
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<section anchor="pki-open-signing" title="Signing OPEN PDU with PKI">
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<t>
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Generating and signing the OPEN PDU with the PKI method is
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almost the same as in <xref target="tofu-pdu-signing"/>. The
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only difference is that the PKI method MUST supply the
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appropriate certificate in the Certificate field.
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</t>
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<t>
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Note that the Auth Type field applies to both the Key and
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Certificate fields. That is: the certificate uses the same
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certificate suite as the session keys, L3DL does not support
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cross-algorithm-suite certification.
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</t>
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</section>
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<section anchor="verify-pki-open" title="Verifying OPEN PDU with PKI">
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<t>
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Verifying the OPEN PDU with PKI is similar to verifying with
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TOFU as described in <xref target="tofu-open-verifying"/>, but
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includes one critical extra step:
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</t>
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<t>
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After extracting the Key field from the PDU but before
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verifying the Signature, extract the Certificate field and
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verfiy that the Certificate is a valid signature of the Key
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field, according to the rules for the signature suite
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specified by Auth Type. If this step fails, handle as in
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<xref target="tofu-open-verifying"/>.
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</t>
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</section>
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</section>
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<section anchor="local-policy" title="Local Policy">
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<t>
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Whether to use TOFU, PKI, or no signatures at all is a matter of
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local policy, to be decided by the operator. The useful
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policy combinations for Key and Certificate are probably:
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</t>
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<t>
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<list style="symbols">
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<t>
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Not signing: sender need not sign, receiver does not check.
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</t>
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<t>
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Require TOFU: sender MUST supply key and receiver MUST
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check, certificate not needed and ignored if sent.
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</t>
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<t>
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Allow TOFU: sender must supply key and receiver MUST check,
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receiver SHOULD check certificate if supplyed by sender.
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</t>
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<t>
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Require PKI: sender must supply key and certificate,
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receiver must check both.
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</t>
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</list>
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</t>
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</section>
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<section anchor="roll" title="NEWKEY, Key Roll">
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<t>Modern key management allows for agility in 'rolling' to a new
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key or even algorithm in case of key expiry, key compromise, or
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merely prudence. Declaring a new key with an L3DL OPEN PDU would
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cause serious churn in topology as a new OPEN may cause a withdraw
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of previously announced encapsulations. Therefore, a gentler
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rekeying is needed.</t>
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<!--
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protocol "Type = 8:8,Payload Length:16,New Key Type:8,New Key Length:16,New Key ...:32,New Cert Length:16,New Certificate ...:32,Old Sig Type:8,Old Signature Length:16,Old Signature ...:40"
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-->
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<figure>
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<artwork>
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0 1 2 3
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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
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Type = 8 | Payload Length | New Key Type |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| New Key Length | New Key ... |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| | New Cert Length |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| New Certificate ... |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Old Sig Type | Old Signature Length | |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
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| Old Signature ... |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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</artwork>
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</figure>
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<t>The New Key Type, New Key Length, New Key, New Cert Length, and
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New Certificate field declare the replacement algorithm suite,
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key, and certificate.</t>
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<t>The NEWKEY PDU is signed using the current (soon to be old)
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algorithm suite and key.</t>
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<t>The sender and the receiver should be cautious of algorithm suite
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downgrade attacks.</t>
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<t>To avoid possible race conditions, the receiver SHOULD accept
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signatures using either the new or old key for a configurable time
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(default 30 seconds). This is intended to accommodate situations
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such as senders with high peer out-degree and a single per-device
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asymmetric key.</t>
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<t>If the sender does not receive an ACK in the normal window,
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including retransmission, then the sender MAY choose to allow a
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session reset by either issuing a new OPEN or by letting the
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receiver eventually have a signature failure (error code 3) on a
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PDU.</t>
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<t>
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The rekeying operation changes the session key and algorithm
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suite described in <xref target="tofu-other-verifying"/>. The
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NEWKEY PDU itself is verified using the old algorithm and
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session key, subsequent PDUs are verified with the new algorithm
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and session key recorded after the NEWKEY PDU has been accepted.
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</t>
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</section>
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<section anchor="security" title="Security Considerations">
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<t>The TOFU method requires a leap of faith to accept the key in the
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OPEN PDU, as it can not be verified against any authority. Hence it
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is jokingly referred to as Married On First Date. The assurance it
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does provide is that subsequent signed PDUs are from the same peer.
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And data integrity is a positive side effect of the signature
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covering the payload.</t>
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<t>The PKI-based method offers assurance that the certificate, and
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hence the keying material, provided in the OPEN PDU are authorized
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by a central authority, e.g. the network's network security team. The
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onward assurance of talking to the same peer and data integrity are
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the same as in the TOFU method.</t>
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<t>With the PKI-based method, automated device provisioning could
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restrict which certificates are allowed from which peers
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on a per interface basis. This would complicate key rolls. Where
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one draws the line between rigidity, flexibility, and security
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varies.</t>
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<t>The REKEY PDU is open to abuse to create an algorithm suite
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downgrade attack.</t>
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</section>
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<section anchor="iana" title="IANA Considerations">
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<t>This document requests the IANA create a new entry in the L3DL PDU
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Type registry as follows:</t>
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<figure>
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<artwork>
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PDU
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Code PDU Name
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---- -------------------
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8 NEWKEY
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</artwork>
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</figure>
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<t>This document requests the IANA add a registry entry for "TOFU -
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Trust On First Use" to the L3DL-Signature-Type registry as follows:</t>
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<figure>
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<artwork>
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Number Name
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------ -------------------
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1 TOFU - Trust On First Use
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2 PKI
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</artwork>
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</figure>
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</section>
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<section anchor="acks" title="Acknowledgments">
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<t>The authors than Russ Housley for advice and review.</t>
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</section>
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</middle>
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<back>
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|
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<references title="Normative References">
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<?rfc include="reference.RFC.2119"?>
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<?rfc include="reference.RFC.5280"?>
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|
<?rfc include="reference.RFC.8174"?>
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<?rfc include="reference.I-D.ietf-lsvr-l3dl"?>
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</references>
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<!--
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<references title="Informative References">
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</references>
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-->
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</back>
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</rfc>
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