draft-ietf-tls-external-psk-importer-00.txt   draft-ietf-tls-external-psk-importer-01.txt 
tls D. Benjamin tls D. Benjamin
Internet-Draft Google, LLC. Internet-Draft Google, LLC.
Intended status: Experimental C. Wood Intended status: Standards Track C. Wood
Expires: November 15, 2019 Apple, Inc. Expires: April 4, 2020 Apple, Inc.
May 14, 2019 October 02, 2019
Importing External PSKs for TLS Importing External PSKs for TLS
draft-ietf-tls-external-psk-importer-00 draft-ietf-tls-external-psk-importer-01
Abstract Abstract
This document describes an interface for importing external PSK (Pre- This document describes an interface for importing external PSK (Pre-
Shared Key) into TLS 1.3. Shared Key) into TLS 1.3.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on November 15, 2019. This Internet-Draft will expire on April 4, 2020.
Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 2 2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 3.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
4. Key Import . . . . . . . . . . . . . . . . . . . . . . . . . 3 4. Key Import . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Label Values . . . . . . . . . . . . . . . . . . . . . . . . 5 5. Deprecating Hash Functions . . . . . . . . . . . . . . . . . 5
6. Deprecating Hash Functions . . . . . . . . . . . . . . . . . 5 6. Incremental Deployment . . . . . . . . . . . . . . . . . . . 5
7. Backwards Compatibility and Incremental Deployment . . . . . 5 7. Security Considerations . . . . . . . . . . . . . . . . . . . 6
8. Security Considerations . . . . . . . . . . . . . . . . . . . 5 8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 6
9. Privacy Considerations . . . . . . . . . . . . . . . . . . . 6 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 6 10.1. Normative References . . . . . . . . . . . . . . . . . . 6
11.1. Normative References . . . . . . . . . . . . . . . . . . 7 10.2. Informative References . . . . . . . . . . . . . . . . . 7
11.2. Informative References . . . . . . . . . . . . . . . . . 8
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 8 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 8
Appendix B. Addressing Selfie . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction 1. Introduction
TLS 1.3 [RFC8446] supports pre-shared key (PSK) authentication, TLS 1.3 [RFC8446] supports pre-shared key (PSK) authentication,
wherein PSKs can be established via session tickets from prior wherein PSKs can be established via session tickets from prior
connections or externally via some out-of-band mechanism. The connections or externally via some out-of-band mechanism. The
protocol mandates that each PSK only be used with a single hash protocol mandates that each PSK only be used with a single hash
function. This was done to simplify protocol analysis. TLS 1.2 function. This was done to simplify protocol analysis. TLS 1.2
[RFC5246], in contrast, has no such requirement, as a PSK may be used [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 with any hash algorithm and the TLS 1.2 PRF. This means that
external PSKs could possibly be re-used in two different contexts external PSKs could possibly be re-used in two different contexts
with the same hash functions during key derivation. Moreover, it with the same hash functions during key derivation. Moreover, it
requires external PSKs to be provisioned for specific hash functions. requires external PSKs to be provisioned for specific hash functions.
To mitigate these problems, external PSKs can be bound to a specific 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 KDF and hash function when used in TLS 1.3, even if they are
a different KDF (and hash function) when provisioned. This document associated with a different hash function when provisioned. This
specifies an interface by which external PSKs may be imported for use document specifies an interface by which external PSKs may be
in a TLS 1.3 connection to achieve this goal. In particular, it imported for use in a TLS 1.3 connection to achieve this goal. In
describes how KDF-bound PSKs can be differentiated by different hash particular, it describes how KDF-bound PSKs can be differentiated by
algorithms to produce a set of candidate PSKs, each of which are the target (D)TLS protocol version and KDF for which the PSK will be
bound to a specific hash function. This expands what would normally used. This produces a set of candidate PSKs, each of which are bound
have been a single PSK identity into a set of PSK identities. to a specific target protocol and KDF. This expands what would
However, it requires no change to the TLS 1.3 key schedule. normally have been a single PSK identity into a set of PSK
identities. However, importantly, it requires no change to the TLS
1.3 key schedule.
2. Conventions and Definitions 2. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
3. Overview 3. Overview
Intuitively, key importers mirror the concept of key exporters in TLS Key importers mirror the concept of key exporters in TLS in that they
in that they diversify a key based on some contextual information diversify a key based on some contextual information before use in a
before use in a connection. In contrast to key exporters, wherein connection. In contrast to key exporters, wherein differentiation is
differentiation is done via an explicit label and context string, the done via an explicit label and context string, the key importer
key importer defined herein uses a label and set of hash algorithms defined herein uses an optional context string along with a target
to differentiate an external PSK into one or more PSKs for use. protocol and KDF identifier to differentiate an external PSK into one
or more PSKs for use.
Imported keys do not require negotiation for use, as a client and Imported keys do not require negotiation for use, as a client and
server will not agree upon identities if not imported correctly. server will not agree upon identities if not imported correctly.
Thus, importers induce no protocol changes with the exception of Thus, importers induce no protocol changes with the exception of
expanding the set of PSK identities sent on the wire. Endpoints may expanding the set of PSK identities sent on the wire. Endpoints may
incrementally deploy PSK importer support by offering non-imported incrementally deploy PSK importer support by offering non-imported
keys for TLS versions prior to TLS 1.3. (Negotiation and use of keys for TLS versions prior to TLS 1.3. Non-imported and imported
imported PSKs requires both endpoints support the importer API PSKs are distinct since their identities are different on the wire.
described herein.) See Section 6 for more details.
Clients which import external keys TLS MUST NOT use these keys for
any other purpose. Moreover, each external PSK MUST be associated
with at most one hash function.
3.1. Terminology 3.1. Terminology
o External PSK (EPSK): A PSK established or provisioned out-of-band, 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, i.e., not from a TLS connection, which is a tuple of (Base Key,
External Identity, KDF). The associated KDF (and hash function) External Identity, Hash).
may be undefined.
o Base Key: The secret value of an EPSK. o Base Key: The secret value of an EPSK.
o External Identity: The identity of an EPSK. o External Identity: The identity of an EPSK.
o Imported Identity: The identity of a PSK as sent on the wire. o Target protocol: The protocol for which a PSK is imported for use.
o Target KDF: The KDF for which a PSK is imported for use.
o Imported PSK (IPSK): A PSK derived from an EPSK, external
identity, optional context string, and target protocol and KDF.
o Imported Identity: The identity of an Imported PSK as sent on the
wire.
4. Key Import 4. Key Import
A key importer takes as input an EPSK with external identity A key importer takes as input an EPSK with external identity
'external_identity' and base key 'epsk', as defined in Section 3.1, "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 along with an optional context, and transforms it into a set of PSKs
and imported identities for use in a connection based on supported and imported identities for use in a connection based on supported
HashAlgorithms. In particular, for each supported HashAlgorithm (target) protocols and KDFs. In particular, for each supported
'hash', the importer constructs an ImportedIdentity structure as target protocol "target_protocol" and KDF "target_kdf", the importer
follows: constructs an ImportedIdentity structure as follows:
struct { struct {
opaque external_identity<1...2^16-1>; opaque external_identity<1...2^16-1>;
opaque label<0..2^8-1>; opaque context<0..2^16-1>;
HashAlgorithm hash; uint16 target_protocol;
} ImportedIdentity; uint16 target_kdf;
} ImportedIdentity;
[[TODO: An alternative design might combine label and hash into the The list of "target_kdf" values is maintained by IANA as described in
same field so that future protocols which don't have a notion of Section 9. External PSKs MUST NOT be imported for versions of (D)TLS
HashAlgorithm don't need this field.]] 1.2 or prior versions. See Section 6 for discussion on how imported
PSKs for TLS 1.3 and non-imported PSKs for earlier versions co-exist
for incremental deployment.
ImportedIdentity.label MUST be bound to the protocol for which the ImportedIdentity.context MUST include the context used to derive the
key is imported. Thus, TLS 1.3 and QUICv1 [I-D.ietf-quic-transport] EPSK, if any exists. For example, ImportedIdentity.context may
MUST use "tls13" as the label. Similarly, TLS 1.2 and all prior TLS include information about peer roles or identities to mitigate
versions should use "tls12" as ImportedIdentity.label, as well as Selfie-style reflection attacks. See Appendix B for more details.
SHA256 as ImportedIdentity.hash. Note that this means future If the EPSK is a key derived from some other protocol or sequence of
versions of TLS will increase the number of PSKs derived from an protocols, ImportedIdentity.context MUST include a channel binding
external PSK. for the deriving protocols [RFC5056].
A unique and imported PSK (IPSK) with base key 'ipskx' bound to this ImportedIdentity.target_protocol MUST be the (D)TLS protocol version
identity is then computed as follows: for which the PSK is being imported. For example, TLS 1.3 [RFC8446]
and QUICv1 [QUIC] use 0x0304. Note that this means future versions
of TLS will increase the number of PSKs derived from an external PSK.
An Imported PSK derived from an EPSK with base key 'epsk' bound to
this identity is then computed as follows:
epskx = HKDF-Extract(0, epsk) epskx = HKDF-Extract(0, epsk)
ipskx = HKDF-Expand-Label(epskx, "derived psk", ipskx = HKDF-Expand-Label(epskx, "derived psk",
Hash(ImportedIdentity), Hash.length) Hash(ImportedIdentity), L)
[[TODO: The length of ipskx MUST match that of the corresponding and L is corresponds to the KDF output length of
supported ciphersuites.]] ImportedIdentity.target_kdf as defined in Section 9. For hash-based
KDFs, such as HKDF_SHA256(0x0001), this is the length of the hash
function output, i.e., 32 octets. This is required for the IPSK to
be of length suitable for supported ciphersuites.
The identity of 'ipskx' as sent on the wire is ImportedIdentity.
The hash function used for HKDF [RFC5869] is that which is associated The hash function used for HKDF [RFC5869] is that which is associated
with the external PSK. It is not bound to ImportedIdentity.hash. If with the EPSK. It is not the hash function associated with
no hash function is specified, SHA-256 MUST be used. Differentiating ImportedIdentity.target_kdf. If no hash function is specified,
epsk by ImportedIdentity.hash ensures that each imported PSK is only SHA-256 MUST be used. Diversifying EPSK by
used with at most one hash function, thus satisfying the requirements ImportedIdentity.target_kdf ensures that an IPSK is only used as
in [RFC8446]. Endpoints MUST import and derive an ipsk for each hash input keying material to at most one KDF, thus satisfying the
function used by each ciphersuite they support. For example, requirements in [RFC8446].
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 Endpoints generate a compatible ipskx for each target ciphersuite
contrast, if TLS_AES_128_GCM_SHA256 and TLS_CHACHA20_POLY1305_SHA256 they offer. For example, importing a key for TLS_AES_128_GCM_SHA256
are supported, only one derived key is necessary. and TLS_AES_256_GCM_SHA384 would yield two PSKs, one for HKDF-SHA256
and another for HKDF-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 The resulting IPSK base key 'ipskx' is then used as the binder key in
TLS 1.3 with identity ImportedIdentity. With knowledge of the TLS 1.3 with identity ImportedIdentity. With knowledge of the
supported hash functions, one may import PSKs before the start of a supported KDFs, one may import PSKs before the start of a connection.
connection.
EPSKs may be imported for early data use if they are bound to EPSKs may be imported for early data use if they are bound to
protocol settings and configurations that would otherwise be required protocol settings and configurations that would otherwise be required
for early data with normal (ticket-based PSK) resumption. Minimally, for early data with normal (ticket-based PSK) resumption. Minimally,
that means ALPN, QUIC transport settings, etc., must be provisioned that means ALPN, QUIC transport settings, etc., must be provisioned
alongside these EPSKs. alongside these EPSKs.
5. Label Values 5. Deprecating Hash Functions
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 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 use it for TLS 1.3, they remove the corresponding KDF from the set of
of hashes used during while importing keys. This does not affect the target KDFs used for importing keys. This does not affect the KDF
KDF operation used to derive concrete PSKs. operation used to derive Imported PSKs.
7. Backwards Compatibility and Incremental Deployment 6. Incremental Deployment
Recall that TLS 1.2 permits computing the TLS PRF with any hash 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 algorithm and PSK. Thus, an EPSK may be used with the same KDF (and
KDF (and underlying HMAC hash algorithm) as TLS 1.3 with importers. underlying HMAC hash algorithm) as TLS 1.3 with importers. However,
However, critically, the derived PSK will not be the same since the critically, the derived PSK will not be the same since the importer
importer differentiates the PSK via the identity and hash function. differentiates the PSK via the identity, target protocol, and target
Thus, PSKs imported for TLS 1.3 are distinct from those used in TLS KDF. Thus, PSKs imported for TLS 1.3 are distinct from those used in
1.2, and thereby avoid cross-protocol collisions. Note that this 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. does not preclude endpoints from using non-imported PSKs for TLS 1.2.
Indeed, this is necessary for incremental deployment. Indeed, this is necessary for incremental deployment.
8. Security Considerations 7. Security Considerations
This is a WIP draft and has not yet seen significant security
analysis.
9. Privacy Considerations DISCLAIMER: This is a WIP draft and has not yet seen significant
security analysis.
DISCLAIMER: This section contains a sketch of a design for protecting 8. Privacy Considerations
external PSK identities. It is not meant to be implementable as
written.
External PSK identities are typically static by design so that External PSK identities are typically static by design so that
endpoints may use them to lookup keying material. For some systems endpoints may use them to lookup keying material. However, for some
and use cases, this identity may become a persistent tracking systems and use cases, this identity may become a persistent tracking
identifier. One mitigation to this problem is encryption. Future identifier.
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 { 9. IANA Considerations
opaque nonce[hash.length];
opaque obfuscated_identity<1..2^16-1>;
HashAlgorithm hash;
} ObfuscatedIdentity;
ObfuscatedIdentity.nonce carries N, This specification introduces a new registry for TLS KDF identifiers
ObfuscatedIdentity.obfuscated_identity carries HMAC(ipskx, N), where and defines the following target KDF values:
HMAC is computed with ImportedIdentity.hash, and
ObfuscatedIdentity.hash is ImportedIdentity.hash.
Upon receipt of such an obfuscated identity, a peer must lookup the +-------------+-----+ | Description | Value | +-------------+-----+ |
corresponding PSK by exhaustively trying to compute Reserved | 0x0000 | | | | | HKDF_SHA256 | 0x0001 | | | | |
ObfuscatedIdentity.obfuscated_identity using ObfuscatedIdentity.nonce HKDF_SHA384 | 0x0002 | +-------------+-----+
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 New target KDF values are allocated according to the following
process:
This document makes no IANA requests. o Values in the range 0x0000-0xfeff are assigned via Specification
Required [RFC8126].
11. References o Values in the range 0xff00-0xffff are reserved for Private Use
11.1. Normative References [RFC8126].
[I-D.ietf-quic-transport] 10. References
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] 10.1. Normative References
Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version [QUIC] Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
1.3", draft-ietf-tls-dtls13-31 (work in progress), March and Secure Transport", draft-ietf-quic-transport-23 (work
2019. in progress), September 2019.
[RFC1035] Mockapetris, P., "Domain names - implementation and [RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>. November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, <https://www.rfc- DOI 10.17487/RFC2119, March 1997,
editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC5056] Williams, N., "On the Use of Channel Bindings to Secure
Channels", RFC 5056, DOI 10.17487/RFC5056, November 2007,
<https://www.rfc-editor.org/info/rfc5056>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, (TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008, <https://www.rfc- DOI 10.17487/RFC5246, August 2008,
editor.org/info/rfc5246>. <https://www.rfc-editor.org/info/rfc5246>.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand [RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869, Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010, <https://www.rfc- DOI 10.17487/RFC5869, May 2010,
editor.org/info/rfc5869>. <https://www.rfc-editor.org/info/rfc5869>.
[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms [RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234, (SHA and SHA-based HMAC and HKDF)", RFC 6234,
DOI 10.17487/RFC6234, May 2011, <https://www.rfc- DOI 10.17487/RFC6234, May 2011,
editor.org/info/rfc6234>. <https://www.rfc-editor.org/info/rfc6234>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, Writing an IANA Considerations Section in RFCs", BCP 26,
January 2012, <https://www.rfc-editor.org/info/rfc6347>. RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>. <https://www.rfc-editor.org/info/rfc8446>.
11.2. Informative References 10.2. Informative References
[I-D.ietf-dnssd-privacy] [CCB] Bhargavan, K., Delignat-Lavaud, A., and A. Pironti,
Huitema, C. and D. Kaiser, "Privacy Extensions for DNS- "Verified Contributive Channel Bindings for Compound
SD", draft-ietf-dnssd-privacy-05 (work in progress), Authentication", Proceedings 2015 Network and Distributed
October 2018. System Security Symposium, DOI 10.14722/ndss.2015.23277,
2015.
[I-D.ietf-tls-esni] [Selfie] Drucker, N. and S. Gueron, "Selfie: reflections on TLS 1.3
Rescorla, E., Oku, K., Sullivan, N., and C. Wood, with PSK", 2019, <https://eprint.iacr.org/2019/347.pdf>.
"Encrypted Server Name Indication for TLS 1.3", draft-
ietf-tls-esni-03 (work in progress), March 2019.
Appendix A. Acknowledgements Appendix A. Acknowledgements
The authors thank Eric Rescorla and Martin Thomson for discussions The authors thank Eric Rescorla and Martin Thomson for discussions
that led to the production of this document, as well as Christian that led to the production of this document, as well as Christian
Huitema for input regarding privacy considerations of external PSKs. Huitema for input regarding privacy considerations of external PSKs.
John Mattsson provided input regarding PSK importer deployment John Mattsson provided input regarding PSK importer deployment
considerations. considerations.
Appendix B. Addressing Selfie
The Selfie attack [Selfie] relies on a misuse of the PSK interface.
The PSK interface makes the implicit assumption that each PSK is
known only to one client and one server. If multiple clients or
multiple servers with distinct roles share a PSK, TLS only
authenticates the entire group. A node successfully authenticates
its peer as being in the group whether the peer is another node or
itself.
Applications which require authenticating finer-grained roles while
still configuring a single shared PSK across all nodes can resolve
this mismatch either by exchanging roles over the TLS connection
after the handshake or by incorporating the roles of both the client
and server into the IPSK context string. For instance, if an
application identifies each node by MAC address, it could use the
following context string.
struct {
opaque client_mac<0..2^16-1>;
opaque server_mac<0..2^16-1>;
} Context;
If an attacker then redirects a ClientHello intended for one node to
a different node, the receiver will compute a different context
string and the handshake will not complete.
Note that, in this scenario, there is still a single shared PSK
across all nodes, so each node must be trusted not to impersonate
another node's role.
Authors' Addresses Authors' Addresses
David Benjamin David Benjamin
Google, LLC. Google, LLC.
Email: davidben@google.com Email: davidben@google.com
Christopher A. Wood Christopher A. Wood
Apple, Inc. Apple, Inc.
Email: cawood@apple.com Email: cawood@apple.com
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