EDHOC PSK authentication
draft-lopez-lake-edhoc-psk-02
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| Authors | Elsa Lopez-Perez , Göran Selander , John Preuß Mattsson , Rafael Marin-Lopez | ||
| Last updated | 2024-12-09 (Latest revision 2024-10-21) | ||
| Replaced by | draft-ietf-lake-edhoc-psk | ||
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draft-lopez-lake-edhoc-psk-02
LAKE Working Group E. Lopez-Perez
Internet-Draft Inria
Intended status: Informational G. Selander
Expires: 21 April 2025 J. P. Mattsson
Ericsson
R. Marin-Lopez
University of Murcia
18 October 2024
EDHOC PSK authentication
draft-lopez-lake-edhoc-psk-02
Abstract
This document specifies the Pre-Shared Key (PSK) authentication
method for the Ephemeral Diffie-Hellman Over COSE (EDHOC) key
exchange protocol. It describes the authentication processes,
message flows, and security considerations of this authentication
method.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at
https://elsalopez133.github.io/draft-lopez-lake-edhoc-psk/#go.draft-
lopez-lake-edhoc-psk.html. Status information for this document may
be found at https://datatracker.ietf.org/doc/draft-lopez-lake-edhoc-
psk/.
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Source for this draft and an issue tracker can be found at
https://github.com/ElsaLopez133/draft-lopez-lake-psk.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Credentials . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Message flow of PSK . . . . . . . . . . . . . . . . . . . 5
4. Key derivation . . . . . . . . . . . . . . . . . . . . . . . 5
5. Message formatting and processing. Differences with respect to
RFC9528 . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.1. Message 1 . . . . . . . . . . . . . . . . . . . . . . . . 6
5.2. Message 2 . . . . . . . . . . . . . . . . . . . . . . . . 6
5.3. Message 3 . . . . . . . . . . . . . . . . . . . . . . . . 7
5.4. Message 4 . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6.1. Identity protection . . . . . . . . . . . . . . . . . . . 8
6.2. Number of messages . . . . . . . . . . . . . . . . . . . 8
6.3. External Authorization Data . . . . . . . . . . . . . . . 9
6.4. Attacks . . . . . . . . . . . . . . . . . . . . . . . . . 9
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 9
8. Unified Approach and Recommendations . . . . . . . . . . . . 9
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
10. Normative References . . . . . . . . . . . . . . . . . . . . 9
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
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1. Introduction
1.1. Motivation
Pre-shared key (PSK) authentication method provides a balance between
security and computational efficiency. This authentication method
was proposed in the first I-Ds of Ephemeral Diffie-Hellman Over COSE
(EDHOC) [RFC9528], and was ruled out to speed out the development
process. However, there is now a renewed effort to reintroduce PSK
authentication, making this draft an update to the [RFC9528].
EDHOC with PSK authentication could be beneficial for existing
systems where two nodes have been provided with a PSK from other
parties out of band. This allows the nodes to perform ephemeral
Diffie-Hellman to achieve Perfect Forward Secrecy (PFS), ensuring
that past communications remain secure even if the PSK is
compromised. The authentication provided by EDHOC prevents
eavesdropping by on-path attackers, as they would need to be active
participants in the communication to intercept and potentially tamper
with the session. Examples could be Generic Bootstrapping
Architecture (GBA) and Authenticated Key Management Architecture
(AKMA) in mobile systems, or Peer and Authenticator in EAP.
Another prominent use case of PSK authentication in the EDHOC
protocol is session resumption. This allows previously connected
parties to quickly reestablish secure communication using pre-shared
keys from their earlier session, reducing the overhead of full key
exchange. This efficiency is beneficial in scenarios where frequent
key updates are needed, such in resource-constrained environments or
applications requiring high-frequency secure communications. The use
of PSK authentication in EDHOC ensures that session key can be
refreshed without heavy computational overhead, typically associated
with public key operations, thus optimizing both performance and
security.
1.2. Assumptions
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 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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3. Protocol
In this method, the Pre-Shared Key identifier (ID_CRED_PSK) is sent
in message_3. The ID_CRED_PSK allows retrieval of CRED_PSK, a COSE
object that contains the PSK. Through this document we will refer to
the Pre-Shared Key authentication method as EDHOC-PSK.
3.1. Credentials
Initiator and Responder are assumed to have a PSK with good amount of
randomness and the requirements that:
* Only the Initiator and the Responder have access to the PSK.
* The Responder is able to retrieve the PSK using ID_CRED_PSK.
where:
* ID_CRED_PSK is a COSE header map containing header parameters that
can identify a pre-shared key. For example:
ID_CRED_PSK = {4 : h'lf' }
* CRED_PSK is a COSE_Key compatible credential, encoded as a CCS or
CWT. For example:
{ /CCS/
2 : "mydotbot", /sub/
8 : { /cnf/
1 : { /COSE_Key/
1 : 4, /kty/
2 : h'32', /kid/
-1 : h'50930FF462A77A3540CF546325DEA214' /k/
}
}
}
The purpose of ID_CRED_PSK is to facilitate the retrieval of the PSK.
It is RECOMMENDED that it uniquely identifies the CRED_PSK as the
recipient might otherwise have to try several keys. If ID_CRED_PSK
contains a single 'kid' parameter, then the compact encoding is
applied; see Section 3.5.3.2 of [RFC9528]. The authentication
credential CRED_PSK substitutes CRED_I and CRED_R specified in
[RFC9529], and, when applicable, MUST follow the same guidelines
described in Sections 3.5.2 and 3.5.3 of [RFC9528].
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3.2. Message flow of PSK
The ID_CRED_PSK is sent in message_3, encrypted using a key derived
from the ephemeral shared secret, G_XY. The Responder authenticates
the Initiator first. Figure 1 shows the message flow of PSK
authentication method.
Initiator Responder
| METHOD, SUITES_I, G_X, C_I, EAD_1 |
+------------------------------------------------------------------>|
| message_1 |
| |
| G_Y, Enc( C_R, EAD_2 ) |
|<------------------------------------------------------------------+
| message_2 |
| |
| Enc( ID_CRED_PSK ), AEAD( EAD_3 ) |
+------------------------------------------------------------------>|
| message_3 |
| |
| AEAD( EAD_4 ) |
|<------------------------------------------------------------------+
| message_4 |
Figure 1: Overview of message flow of PSK.
This approach provides protection against passive attackers for both
Initiator and Responder. message_4 remains optional, but is needed to
to authenticate the Responder and achieve mutual authentication in
EDHOC if not relaying on external applications, such as OSCORE. With
this fourth message, the protocol achieves both explicit key
confirmation and mutual authentication.
4. Key derivation
The pseudorandom keys (PRKs) used for PSK authentication method in
EDHOC are derived using EDHOC_Extract, as done in [RFC9528].
PRK = EDHOC_Extract( salt, IKM )
where the salt and input keying material (IKM) are defined for each
key. The definition of EDHOC_Extract depends on the EDHOC hash
algorithm selected in the cipher suite.
Figure 2 lists the key derivations that differ from those specified
in Section 4.1.2 of [RFC9528].
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PRK_3e2m = PRK_2e
PRK_4e3m = EDHOC_Extract( SALT_4e3m, CRED_PSK )
KEYSTREAM_3 = EDHOC_KDF( PRK_3e2m, TBD, TH_3, key_length )
K_3 = EDHOC_KDF( PRK_4e3m, TBD, TH_3, key_length )
IV_3 = EDHOC_KDF( PRK_4e3m, TBD, TH_3, iv_length )
Figure 2: Key derivation of EDHOC PSK authentication method.
where:
* KEYSTREAM_3 is used to encrypt the ID_CRED_PSK in message_3.
* TH_3 = H( TH_2, PLAINTEXT_2, CRED_PSK )
* TH_4 = H( TH_3, ID_CRED_PSK, ? EAD_3, CRED_PSK )
5. Message formatting and processing. Differences with respect to
[RFC9528]
This section specifies the differences on the message formatting
compared to [RFC9528].
5.1. Message 1
Same as message_1 of EDHOC, described in Section 5.2.1 of [RFC9528].
5.2. Message 2
message_2 SHALL be a CBOR sequence, defined as:
message_2 = (
G_Y_CIPHERTEXT_2 : bstr,
)
where:
* G_Y_CIPHERTEXT_2 is the concatenation of G_Y (i.e., the ephemeral
public key of the Responder) and CIPHERTEXT_2.
* CIPHERTEXT_2 is calculated with a binary additive stream cipher,
using KEYSTREAM_2 and the following plaintext:
- PLAINTEXT_2 = ( C_R, / bstr / -24..23, ? EAD_2 )
- CIPHERTEXT_2 = PLAINTEXT_2 XOR KEYSTREAM_2
Contrary to [RFC9528], MAC_2 is not used.
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5.3. Message 3
message_3 SHALL be a CBOR Sequence, as defined below:
message_3 = (
CIPHERTEXT_3: bstr,
)
where:
* CIPHERTEXT_3 is a concatenation of two different ciphertexts:
- CIPHERTEXT_3A is bit string calculated with a binary additive
stream cipher, using a KESYSTREAM_3 generated with EDHOC_Expand
and the following plaintext:
o PLAINTEXT_3A = ( ID_CRED_PSK )
- CIPHERTEXT_3B is a COSE_Encrypt0 object as defined in Sections
5.2 and 5.3 of [RFC9052], with the EDHOC AEAD algorithm of the
selected cipher suite, using the encryption key K_3, the
initialization vector IV_3 (if used by the AEAD algorithm), the
parameters described in Section 5.2 of [RFC9528], plaintext
PLAINTEXT_3B and the following parameters as input:
o protected = h''
o external_aad = << Enc(ID_CRED_PSK), TH_3 >>
o K_3 and IV_3 as defined in Section 5.2
o PLAINTEXT_3B = ( ? EAD_3 )
The Initiator computes TH_4 = H( TH_3, ID_CRED_PSK, PLAINTEXT_3,
CRED_PSK ), defined in Section 5.2.
5.4. Message 4
message_4 is mandatory and is a CBOR sequence, defined as:
message_4 = (
CIPHERTEXT_4 : bstr,
)
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A fourth message is mandatory for Responder's authentication. The
Initiator MUST NOT persistently store PRK_out or application keys
until the Initiator has verified message_4 or a message protected
with a derived application key, such as an OSCORE message, from the
Responder and the application has authenticated the Responder.
6. Security Considerations
When evaluating the security considerations, it is important to
differentiate between the initial handshake and session resumption
phases.
1. *Initial Handshake*: a fresh CRED_PSK is used to establish a
secure connection.
2. *Session Resumption*: the same PSK identifier (ID_CRED_PSK) is
reused each time EDHOC is executed. While this enhances
efficiency and reduces the overhead of key exchanges, it presents
privacy risks if not managed properly. Over multiple resumption
sessions, initiating a full EDHOC session changes the resumption
PSK, resulting in a new ID_CRED_PSK. The periodic renewal of the
CRED_PSK and ID_CRED_PSK helps mitigate long-term privacy risks
associated with static key identifiers.
6.1. Identity protection
The current EDHOC methods protect the Initiator’s identity against
active attackers and the Responder’s identity against passive
attackers (See Section 9.1 of [RFC9528]). With EDHOC-PSK
authentication method, both the Initiator's and Responder's
identities are protected against passive attackers, but not against
active attackers.
6.2. Number of messages
The current EDHOC protocol consists of three mandatory messages and
an optional fourth message. In the case of EDHOC-PSK authentication
method, message_4 remains optional, but mutual authentication is not
guaranteed without it, or an OSCORE message or any application data
that confirms that the Responder owns the PSK. Additionally, with
this fourth message the protocol achieves explicit key confirmation
in addition to mutual authentication.
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6.3. External Authorization Data
The Initiator and Responder can send information in EAD_3 and EAD_4
or in OSCORE messages in parallel with message_3 and message_4. This
is possible because the Initiator knows that only the Responder with
access to the CRED_PSK can decrypt the information.
6.4. Attacks
EDHOC-PSK authentication method offers privacy and resistance to
passive attacks but might be vulnerable to certain active attacks due
to delayed authentication.
7. Privacy Considerations
8. Unified Approach and Recommendations
For use cases involving the transmission of application data,
application data can be sent concurrently with message_3, maintaining
the protocol's efficiency. In applications such as EAP-EDHOC, where
application data is not sent, message_4 is mandatory. Thus, EDHOC-
PSK authentication method doe snot include any extra messages. Other
implementations may continue using OSCORE in place of EDHOC
message_4, with a required change in the protocol's language to: The
Initiator SHALL NOT persistently store PRK_out or application keys
until the Initiator has verified message_4 or a message protected
with a derived application key, such as an OSCORE message.
This change ensures that key materials are only stored once their
integrity and authenticity are confirmed, thereby enhancing privacy
by preventing early storage of potentially compromised keys.
Lastly, whether the Initiator or Responder authenticates first is not
relevant when using symmetric keys. This consideration was important
for the privacy properties when using asymmetric authentication but
is not significant in the context of symmetric key usage.
9. IANA Considerations
This document has no IANA actions.
10. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
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[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/rfc/rfc8174>.
[RFC9052] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Structures and Process", STD 96, RFC 9052,
DOI 10.17487/RFC9052, August 2022,
<https://www.rfc-editor.org/rfc/rfc9052>.
[RFC9528] Selander, G., Preuß Mattsson, J., and F. Palombini,
"Ephemeral Diffie-Hellman Over COSE (EDHOC)", RFC 9528,
DOI 10.17487/RFC9528, March 2024,
<https://www.rfc-editor.org/rfc/rfc9528>.
[RFC9529] Selander, G., Preuß Mattsson, J., Serafin, M., Tiloca, M.,
and M. Vučinić, "Traces of Ephemeral Diffie-Hellman Over
COSE (EDHOC)", RFC 9529, DOI 10.17487/RFC9529, March 2024,
<https://www.rfc-editor.org/rfc/rfc9529>.
Acknowledgments
TODO acknowledge.
Authors' Addresses
Elsa Lopez-Perez
Inria
Email: elsa.lopez-perez@inria.fr
Göran Selander
Ericsson
Email: goran.selander@ericsson.com
John Preuß Mattsson
Ericsson
Email: john.mattsson@ericsson.com
Rafael Marin-Lopez
University of Murcia
Email: rafa@um.es
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