tls D. Benjamin
Internet-Draft Google, LLC.
Intended status: Experimental C. Wood
Expires: November 15, 2019 Apple, Inc.
May 14, 2019
Importing External PSKs for TLS
draft-ietf-tls-external-psk-importer-00
Abstract
This document describes an interface for importing external PSK (Pre-
Shared Key) into TLS 1.3.
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 November 15, 2019.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 2
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
4. Key Import . . . . . . . . . . . . . . . . . . . . . . . . . 3
5. Label Values . . . . . . . . . . . . . . . . . . . . . . . . 5
6. Deprecating Hash Functions . . . . . . . . . . . . . . . . . 5
7. Backwards Compatibility and Incremental Deployment . . . . . 5
8. Security Considerations . . . . . . . . . . . . . . . . . . . 5
9. Privacy Considerations . . . . . . . . . . . . . . . . . . . 6
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
11.1. Normative References . . . . . . . . . . . . . . . . . . 7
11.2. Informative References . . . . . . . . . . . . . . . . . 8
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
TLS 1.3 [RFC8446] supports pre-shared key (PSK) authentication,
wherein PSKs can be established via session tickets from prior
connections or externally via some out-of-band mechanism. The
protocol mandates that each PSK only be used with a single hash
function. This was done to simplify protocol analysis. TLS 1.2
[RFC5246], in contrast, has no such requirement, as a PSK may be used
with any hash algorithm and the TLS 1.2 PRF. This means that
external PSKs could possibly be re-used in two different contexts
with the same hash functions during key derivation. Moreover, it
requires external PSKs to be provisioned for specific hash functions.
To mitigate these problems, external PSKs can be bound to a specific
hash function when used in TLS 1.3, even if they are associated with
a different KDF (and hash function) when provisioned. This document
specifies an interface by which external PSKs may be imported for use
in a TLS 1.3 connection to achieve this goal. In particular, it
describes how KDF-bound PSKs can be differentiated by different hash
algorithms to produce a set of candidate PSKs, each of which are
bound to a specific hash function. This expands what would normally
have been a single PSK identity into a set of PSK identities.
However, it requires no change to the TLS 1.3 key schedule.
2. Conventions and Definitions
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
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14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Overview
Intuitively, key importers mirror the concept of key exporters in TLS
in that they diversify a key based on some contextual information
before use in a connection. In contrast to key exporters, wherein
differentiation is done via an explicit label and context string, the
key importer defined herein uses a label and set of hash algorithms
to differentiate an external PSK into one or more PSKs for use.
Imported keys do not require negotiation for use, as a client and
server will not agree upon identities if not imported correctly.
Thus, importers induce no protocol changes with the exception of
expanding the set of PSK identities sent on the wire. Endpoints may
incrementally deploy PSK importer support by offering non-imported
keys for TLS versions prior to TLS 1.3. (Negotiation and use of
imported PSKs requires both endpoints support the importer API
described herein.)
3.1. Terminology
o External PSK (EPSK): A PSK established or provisioned out-of-band,
i.e., not from a TLS connection, which is a tuple of (Base Key,
External Identity, KDF). The associated KDF (and hash function)
may be undefined.
o Base Key: The secret value of an EPSK.
o External Identity: The identity of an EPSK.
o Imported Identity: The identity of a PSK as sent on the wire.
4. Key Import
A key importer takes as input an EPSK with external identity
'external_identity' and base key 'epsk', as defined in Section 3.1,
along with an optional label, and transforms it into a set of PSKs
and imported identities for use in a connection based on supported
HashAlgorithms. In particular, for each supported HashAlgorithm
'hash', the importer constructs an ImportedIdentity structure as
follows:
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struct {
opaque external_identity<1...2^16-1>;
opaque label<0..2^8-1>;
HashAlgorithm hash;
} ImportedIdentity;
[[TODO: An alternative design might combine label and hash into the
same field so that future protocols which don't have a notion of
HashAlgorithm don't need this field.]]
ImportedIdentity.label MUST be bound to the protocol for which the
key is imported. Thus, TLS 1.3 and QUICv1 [I-D.ietf-quic-transport]
MUST use "tls13" as the label. Similarly, TLS 1.2 and all prior TLS
versions should use "tls12" as ImportedIdentity.label, as well as
SHA256 as ImportedIdentity.hash. Note that this means future
versions of TLS will increase the number of PSKs derived from an
external PSK.
A unique and imported PSK (IPSK) with base key 'ipskx' bound to this
identity is then computed as follows:
epskx = HKDF-Extract(0, epsk)
ipskx = HKDF-Expand-Label(epskx, "derived psk",
Hash(ImportedIdentity), Hash.length)
[[TODO: The length of ipskx MUST match that of the corresponding and
supported ciphersuites.]]
The hash function used for HKDF [RFC5869] is that which is associated
with the external PSK. It is not bound to ImportedIdentity.hash. If
no hash function is specified, SHA-256 MUST be used. Differentiating
epsk by ImportedIdentity.hash ensures that each imported PSK is only
used with at most one hash function, thus satisfying the requirements
in [RFC8446]. Endpoints MUST import and derive an ipsk for each hash
function used by each ciphersuite they support. For example,
importing a key for TLS_AES_128_GCM_SHA256 and TLS_AES_256_GCM_SHA384
would yield two PSKs, one for SHA256 and another for SHA384. In
contrast, if TLS_AES_128_GCM_SHA256 and TLS_CHACHA20_POLY1305_SHA256
are supported, only one derived key is necessary.
The resulting IPSK base key 'ipskx' is then used as the binder key in
TLS 1.3 with identity ImportedIdentity. With knowledge of the
supported hash functions, one may import PSKs before the start of a
connection.
EPSKs may be imported for early data use if they are bound to
protocol settings and configurations that would otherwise be required
for early data with normal (ticket-based PSK) resumption. Minimally,
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that means ALPN, QUIC transport settings, etc., must be provisioned
alongside these EPSKs.
5. Label Values
For clarity, the following table specifies PSK importer labels for
varying instances of the TLS handshake.
+----------------------------------+----------+
| Protocol | Label |
+----------------------------------+----------+
| TLS 1.3 [RFC8446] | "tls13" |
| | |
| QUICv1 [I-D.ietf-quic-transport] | "tls13" |
| | |
| TLS 1.2 [RFC5246] | "tls12" |
| | |
| DTLS 1.2 [RFC6347] | "dtls12" |
| | |
| DTLS 1.3 [I-D.ietf-tls-dtls13] | "dtls13" |
+----------------------------------+----------+
6. Deprecating Hash Functions
If a client or server wish to deprecate a hash function and no longer
use it for TLS 1.3, they may remove this hash function from the set
of hashes used during while importing keys. This does not affect the
KDF operation used to derive concrete PSKs.
7. Backwards Compatibility and Incremental Deployment
Recall that TLS 1.2 permits computing the TLS PRF with any hash
algorithm and PSK. Thus, an external PSK may be used with the same
KDF (and underlying HMAC hash algorithm) as TLS 1.3 with importers.
However, critically, the derived PSK will not be the same since the
importer differentiates the PSK via the identity and hash function.
Thus, PSKs imported for TLS 1.3 are distinct from those used in TLS
1.2, and thereby avoid cross-protocol collisions. Note that this
does not preclude endpoints from using non-imported PSKs for TLS 1.2.
Indeed, this is necessary for incremental deployment.
8. Security Considerations
This is a WIP draft and has not yet seen significant security
analysis.
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9. Privacy Considerations
DISCLAIMER: This section contains a sketch of a design for protecting
external PSK identities. It is not meant to be implementable as
written.
External PSK identities are typically static by design so that
endpoints may use them to lookup keying material. For some systems
and use cases, this identity may become a persistent tracking
identifier. One mitigation to this problem is encryption. Future
drafts may specify a way for encrypting PSK identities using a
mechanism similar to that of the Encrypted SNI proposal
[I-D.ietf-tls-esni]. Another approach is to replace the identity
with an unpredictable or "obfuscated" value derived from the
corresponding PSK. One such proposal, derived from a design outlined
in [I-D.ietf-dnssd-privacy], is as follows. Let ipskx be the
imported PSK with identity ImportedIdentity, and N be a unique nonce
of length equal to that of ImportedIdentity.hash. With these values,
construct the following "obfuscated" identity:
struct {
opaque nonce[hash.length];
opaque obfuscated_identity<1..2^16-1>;
HashAlgorithm hash;
} ObfuscatedIdentity;
ObfuscatedIdentity.nonce carries N,
ObfuscatedIdentity.obfuscated_identity carries HMAC(ipskx, N), where
HMAC is computed with ImportedIdentity.hash, and
ObfuscatedIdentity.hash is ImportedIdentity.hash.
Upon receipt of such an obfuscated identity, a peer must lookup the
corresponding PSK by exhaustively trying to compute
ObfuscatedIdentity.obfuscated_identity using ObfuscatedIdentity.nonce
and each of its known imported PSKs. If N is chosen in a predictable
fashion, e.g., as a timestamp, it may be possible for peers to
precompute these obfuscated identities to ease the burden of trial
decryption.
10. IANA Considerations
This document makes no IANA requests.
11. References
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11.1. Normative References
[I-D.ietf-quic-transport]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-20 (work
in progress), April 2019.
[I-D.ietf-tls-dtls13]
Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", draft-ietf-tls-dtls13-31 (work in progress), March
2019.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[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>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008, <https://www.rfc-
editor.org/info/rfc5246>.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010, <https://www.rfc-
editor.org/info/rfc5869>.
[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, <https://www.rfc-
editor.org/info/rfc6234>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.
[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>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
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11.2. Informative References
[I-D.ietf-dnssd-privacy]
Huitema, C. and D. Kaiser, "Privacy Extensions for DNS-
SD", draft-ietf-dnssd-privacy-05 (work in progress),
October 2018.
[I-D.ietf-tls-esni]
Rescorla, E., Oku, K., Sullivan, N., and C. Wood,
"Encrypted Server Name Indication for TLS 1.3", draft-
ietf-tls-esni-03 (work in progress), March 2019.
Appendix A. Acknowledgements
The authors thank Eric Rescorla and Martin Thomson for discussions
that led to the production of this document, as well as Christian
Huitema for input regarding privacy considerations of external PSKs.
John Mattsson provided input regarding PSK importer deployment
considerations.
Authors' Addresses
David Benjamin
Google, LLC.
Email: davidben@google.com
Christopher A. Wood
Apple, Inc.
Email: cawood@apple.com
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