Internet Engineering Task Force (IETF) V. Smyslov

Internet Engineering Task Force (IETF) V. Smyslov
Request for Comments: 9593 ELVIS-PLUS
Category: Standards Track July 2024
ISSN: 2070-1721

Announcing Supported Authentication Methods in the Internet Key Exchange
Protocol Version 2 (IKEv2)

Abstract

This specification defines a mechanism that allows implementations of
the Internet Key Exchange Protocol Version 2 (IKEv2) to indicate the
list of supported authentication methods to their peers while
establishing IKEv2 Security Associations (SAs). This mechanism
improves interoperability when IKEv2 partners are configured with
multiple credentials of different types for authenticating each
other.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9593.

Copyright Notice

Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
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in the Revised BSD License.

Table of Contents

1. Introduction
2. Terminology and Notation
3. Protocol Details
3.1. Exchanges
3.2. SUPPORTED_AUTH_METHODS Notify Message Type
3.2.1. 2-Octet Announcement
3.2.2. 3-Octet Announcement
3.2.3. Multi-octet Announcement
4. Interaction with IKEv2 Extensions concerning Authentication
5. IANA Considerations
6. Security Considerations
7. References
7.1. Normative References
7.2. Informative References
Appendix A. Examples of Announcing Supported Authentication
Methods
A.1. No Need to Use the IKE_INTERMEDIATE Exchange
A.2. With Use of the IKE_INTERMEDIATE Exchange
Acknowledgments
Author's Address

1. Introduction

The Internet Key Exchange Protocol Version 2 (IKEv2), defined in
[RFC7296], performs authenticated key exchange in IPsec. IKEv2,
unlike its predecessor IKEv1, defined in [RFC2409], doesn't include a
mechanism to negotiate an authentication method that the peers would
use to authenticate each other. It is assumed that each peer selects
whichever authentication method it thinks is appropriate, depending
on authentication credentials it has.

This approach generally works well when there is no ambiguity in
selecting authentication credentials. SA establishment failure
between peers may occur when there are several credentials of
different types configured on one peer, while only some of them are
supported on the other peer. Another problem situation is when a
single credential may be used to produce different types of
authentication tokens (e.g., signatures of different formats). Since
IKEv2 requires that each peer use exactly one authentication method,
and it doesn't provide means for peers to indicate to the other side
which authentication methods they support, the peer that supports a
wider range of authentication methods (or authentication token
formats) could improperly select a method (or format) that is not
supported by the other side.

Emerging post-quantum signature algorithms may bring additional
challenges for implementations, especially if so-called hybrid
schemes are used (e.g., see [COMPOSITE-SIGS]).

This specification defines an extension to the IKEv2 protocol that
allows peers to announce their supported authentication methods, thus
decreasing risks of SA establishment failure in situations when there
are several ways for the peers to authenticate themselves.

2. Terminology and Notation

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.

3. Protocol Details

When establishing an IKE SA, each party may send to its peer a list
of the authentication methods it supports and is configured to use.
For this purpose, this specification introduces a new Notify Message
Type SUPPORTED_AUTH_METHODS. The Notify payload with this Notify
Message Type is utilized to convey the supported authentication
methods of the party sending it. The sending party may additionally
specify that some of the authentication methods are only for use with
the particular trust anchors. The receiving party may take this
information into consideration when selecting an algorithm for its
authentication (i.e., the algorithm used for calculation of the AUTH
payload) if several alternatives are available. To simplify the
receiver's task of linking the announced authentication methods with
the trust anchors, the protocol ensures that the
SUPPORTED_AUTH_METHODS notification is always co-located with the
CERTREQ payload in the same message.

3.1. Exchanges

The initiator starts the IKE_SA_INIT exchange as usual. If the
responder is willing to use this extension, it includes a new
notification SUPPORTED_AUTH_METHODS in the IKE_SA_INIT response
message. This notification contains a list of authentication methods
supported by the responder, ordered by their preference.

Initiator Responder
----------- -----------
HDR, SAi1, KEi, Ni -->
<-- HDR, SAr1, KEr, Nr, [CERTREQ,]
[N(SUPPORTED_AUTH_METHODS)(...)]

Figure 1: The IKE_SA_INIT Exchange

If the initiator doesn't support this extension, it ignores the
received notification as an unknown status notify.

Regardless of whether the notification is received, if the initiator
supports and is willing to use this extension, it includes the
SUPPORTED_AUTH_METHODS notification in the IKE_AUTH request message,
with a list of authentication methods supported by the initiator,
ordered by their preference.

Initiator Responder
----------- -----------
HDR, SK {IDi, [CERT,] [CERTREQ,]
[IDr,] AUTH, SAi2, TSi, TSr,
[N(SUPPORTED_AUTH_METHODS)(...)] } -->
<-- HDR, SK {IDr, [CERT,]
AUTH, SAr2, TSi, TSr }

Figure 2: The IKE_AUTH Exchange

Because the responder sends the SUPPORTED_AUTH_METHODS notification
in the IKE_SA_INIT exchange, it must take into account that the
response message could grow so much that the IP fragmentation might
take place.

* the SUPPORTED_AUTH_METHODS notification to be included is so
large, that the responder suspects that IP fragmentation of the
resulting IKE_SA_INIT response message may happen;

* both peers support the IKE_INTERMEDIATE exchange, defined in
[RFC9242] (i.e., the responder has received and is going to send
the INTERMEDIATE_EXCHANGE_SUPPORTED notification);

then the responder MAY choose not to send an actual list of the
supported authentication methods in the IKE_SA_INIT exchange and
instead ask the initiator to start the IKE_INTERMEDIATE exchange for
the list to be sent in. This would allow using IKE fragmentation
[RFC7383] for long messages (which cannot be used in the IKE_SA_INIT
exchange), thus avoiding IP fragmentation. In this case, the
responder includes a SUPPORTED_AUTH_METHODS notification containing
no data in the IKE_SA_INIT response.

If the initiator receives the empty SUPPORTED_AUTH_METHODS
notification in the IKE_SA_INIT exchange, it means that the responder
is going to send the list of the supported authentication methods in
the IKE_INTERMEDIATE exchange. If this exchange is to be initiated
anyway for some other reason, then the responder MAY use it to send
the SUPPORTED_AUTH_METHODS notification. Otherwise, the initiator
MAY start the IKE_INTERMEDIATE exchange for this sole purpose by
sending an empty IKE_INTERMEDIATE request. The initiator MAY also
indicate its identity (and possibly the perceived responder's
identity too) by including the IDi payload (possibly along with the
IDr payload) in the IKE_INTERMEDIATE request. This information could
help the responder to send back only those authentication methods
that are configured to be used for authentication of this particular
initiator. If these payloads are sent, they MUST be identical to the
IDi/IDr payloads sent later in the IKE_AUTH request.

If the responder has sent any CERTREQ payload in the IKE_SA_INIT,
then it SHOULD resend the same payload(s) in the IKE_INTERMEDIATE
response containing the SUPPORTED_AUTH_METHODS notification if any of
the included Announcements has a non-zero Cert Link field (see
Sections 3.2.2 and 3.2.3). This requirement allows peers to have a
list of Announcements and a list of CAs in the same message, which
simplifies their linking. Note that this requirement is always
fulfilled for the IKE_SA_INIT and IKE_AUTH exchanges. However, if
for any reason the responder doesn't resend CERTREQ payload(s) in the
IKE_INTERMEDIATE exchange, then the initiator MUST NOT abort
negotiation. Instead, the initiator MAY either link the
Announcements to the CAs received in the IKE_SA_INIT response, or it
MAY ignore the Announcements containing links to CAs.

If multiple IKE_INTERMEDIATE exchanges take place during IKE SA
establishments, it is RECOMMENDED that the responder use the last
IKE_INTERMEDIATE exchange (the one just before IKE_AUTH) to send the
list of supported authentication methods. However, it is not always
possible for the responder to know how many IKE_INTERMEDIATE
exchanges the initiator will use. In this case the responder MAY
send the list in any IKE_INTERMEDIATE exchange. If the initiator
sends IDi/IDr in an IKE_INTERMEDIATE request, then it is RECOMMENDED
that the responder sends back the list of authentication methods in
the response.

Initiator Responder
----------- -----------
HDR, SAi1, KEi, Ni -->
<-- HDR, SAr1, KEr, Nr, [CERTREQ,]
[N(SUPPORTED_AUTH_METHODS)()]

HDR, SK {..., [IDi, [IDr,]]} -->
<-- HDR, SK {..., [CERTREQ,]
[N(SUPPORTED_AUTH_METHODS)(...)] }

HDR, SK {IDi, [CERT,] [CERTREQ,]
[IDr,] AUTH, SAi2, TSi, TSr,
[N(SUPPORTED_AUTH_METHODS)(...)] } -->
<-- HDR, SK {IDr, [CERT,]
AUTH, SAr2, TSi, TSr }

Figure 3: Using the IKE_INTERMEDIATE Exchange for Sending
Authentication Methods

Note that sending the SUPPORTED_AUTH_METHODS notification and using
information obtained from it are optional for both the initiator and
the responder. If multiple SUPPORTED_AUTH_METHODS notifications are
included in a message, all their announcements form a single ordered
list, unless overridden by other extension (see Section 4).

3.2. SUPPORTED_AUTH_METHODS Notify Message Type

The format of the SUPPORTED_AUTH_METHODS Notify payload is shown
below.

1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Payload |C| RESERVED | Payload Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Protocol ID | SPI Size | Notify Message Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ List of Supported Auth Methods Announcements ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 4: SUPPORTED_AUTH_METHODS Notify Payload Format

The Notify payload format is defined in Section 3.10 of [RFC7296].
When a Notify payload of type SUPPORTED_AUTH_METHODS is sent, the
Protocol ID field is set to 0, the SPI Size is set to 0 (meaning
there is no SPI field), and the Notify Message Type is set to 16443.

Notification data contains the list of supported authentication
methods announcements. Each individual announcement is a variable-
size data blob whose format depends on the announced authentication
method. The blob always starts with an octet containing the length
of the blob followed by an octet containing the authentication
method. Authentication methods are represented as values from the
"IKEv2 Authentication Method" registry defined in [IKEV2-IANA]. The
meaning of the remaining octets of the blob, if any, depends on the
authentication method. Note that, for the currently defined
authentication methods, the length octet fully defines both the
format and the semantics of the blob.

If more authentication methods are defined in the future, the
corresponding documents must describe the semantics of the
announcements for these methods. Implementations MUST ignore
announcements whose semantics they don't understand.

3.2.1. 2-Octet Announcement

If the announcement contains an authentication method that is not
concerned with public key cryptography, then the following format is
used.

1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length (=2) | Auth Method |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 5: 2-Octet Announcement Format

Length: Length of the blob in octets; must be 2 for this case.

Auth Method: Announced authentication method.

This format is applicable for the authentication methods "Shared Key
Message Integrity Code" (2) and "NULL Authentication" (13). Note
that the authentication method "Generic Secure Password
Authentication Method" (12) would also fall in this category;
however, it is negotiated separately (see [RFC6467]), and for this
reason there is no point to announce it via this mechanism. See also
Section 4.

3.2.2. 3-Octet Announcement

If the announcement contains an authentication method that is
concerned with public key cryptography, then the following format is
used. This format allows linking the announcement with a particular
trust anchor from the Certificate Request payload.

1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length (=3) | Auth Method | Cert Link |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 6: 3-Octet Announcement Format

Length: Length of the blob in octets; must be 3 for this case.

Auth Method: Announced authentication method.

Cert Link: Links this announcement with a particular CA.

If the Cert Link field contains a non-zero value N, it means that the
announced authentication method is intended to be used only with the
N-th trust anchor (CA certificate) from the Certificate Request
payload(s) sent by this peer. If it is zero, then this
authentication method may be used with any CA. If multiple CERTREQ
payloads were sent, the CAs from all of them are treated as a single
list for the purpose of the linking. If no Certificate Request
payload were received, the content of this field MUST be ignored and
treated as zero.

This format is applicable for the authentication methods "RSA Digital
Signature" (1), "DSS Digital Signature" (3), "ECDSA with SHA-256 on
the P-256 curve" (9), "ECDSA with SHA-384 on the P-384 curve" (10)
and "ECDSA with SHA-512 on the P-521 curve" (11). Note, however,
that these authentication methods are currently superseded by the
"Digital Signature" (14) authentication method, which has a different
announcement format, described below.

3.2.3. Multi-octet Announcement

The following format is currently used only with the "Digital
Signature" (14) authentication method.

1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length (>3) | Auth Method | Cert Link | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
~ AlgorithmIdentifier ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 7: Multi-octet Announcement Format

Length: Length of the blob in octets; must be greater than 3 for
this case.

Auth Method: Announced authentication method. At the time of
writing this document, only value 14 ("Digital Signature") is
allowed.

Cert Link: Links this announcement with a particular CA; see
Section 3.2.2 for details.

AlgorithmIdentifier: The variable-length ASN.1 object that is
encoded using Distinguished Encoding Rules (DER) [X.690] and
identifies the signature algorithm (see Section 4.1.1.2 of
[RFC5280]).

The "Digital Signature" authentication method, defined in [RFC7427],
supersedes previously defined signature authentication methods. In
this case, the real authentication algorithm is identified via
AlgorithmIdentifier ASN.1 object. Appendix A of [RFC7427] contains
examples of commonly used ASN.1 objects.

4. Interaction with IKEv2 Extensions concerning Authentication

Generally in IKEv2 each party independently determines the way it
authenticates itself to the peer. In other words, authentication
methods selected by the peers need not be the same. However, some
IKEv2 extensions break this rule.

The prominent example is "Secure Password Framework for Internet Key
Exchange Version 2" [RFC6467], which defines a framework for using
secure password authentication in IKEv2. With this framework, peers
negotiate using one of the secure password methods in the IKE_SA_INIT
exchange -- the initiator sends a list of supported methods in the
request, and the responder picks one of them and sends it back in the
response.

If peers negotiate secure password authentication, then the selected
method is used by both initiator and responder, and no other
authentication methods are involved. For this reason, there is no
point to announce supported authentication methods in this case.
Thus, if the peers choose to go with secure password authentication,
they MUST NOT send the SUPPORTED_AUTH_METHODS notification.

In the situation when peers are going to use Multiple Authentication
Exchanges [RFC4739], they MAY include multiple SUPPORTED_AUTH_METHODS
notifications (instead of one), each containing authentication
methods appropriate for each authentication round. The notifications
are included in the order of the preference of performing
authentication rounds.

5. IANA Considerations

This document defines a new type in the "IKEv2 Notify Message Status
Types" registry:

+=======+============================+===========+
| Value | Notify Message Status Type | Reference |
+=======+============================+===========+
| 16443 | SUPPORTED_AUTH_METHODS | RFC 9593 |
+-------+----------------------------+-----------+

Table 1

6. Security Considerations

Security considerations for the IKEv2 protocol are discussed in
[RFC7296]. Security properties of different authentication methods
vary. Refer to corresponding documents, listed in the "IKEv2
Authentication Method" registry on [IKEV2-IANA] for discussion of
security properties of each authentication method.

Announcing authentication methods gives an eavesdropper additional
information about peers' capabilities. If a peer advertises "NULL
Authentication" along with other methods, then an active on-path
attacker can encourage peers to use NULL authentication by removing
all other announcements. Note that this is not a real "downgrade"
attack, since authentication methods in IKEv2 are not negotiated, and
in this case NULL authentication should be allowed by local security
policy.

Similarly, if an on-path attacker can break some of the announced
authentication methods online, then the attacker can encourage peers
to use one of these weaker methods by removing all other
announcements, and if this succeeds, then perform a person-in-the-
middle attack.

7. References

7.1. Normative References

[IKEV2-IANA]
IANA, "Internet Key Exchange Version 2 (IKEv2)
Parameters",
<https://www.iana.org/assignments/ikev2-parameters/>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.

[RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
Kivinen, "Internet Key Exchange Protocol Version 2
(IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
2014, <https://www.rfc-editor.org/info/rfc7296>.

[RFC7427] Kivinen, T. and J. Snyder, "Signature Authentication in
the Internet Key Exchange Version 2 (IKEv2)", RFC 7427,
DOI 10.17487/RFC7427, January 2015,
<https://www.rfc-editor.org/info/rfc7427>.

[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>.

[RFC9242] Smyslov, V., "Intermediate Exchange in the Internet Key
Exchange Protocol Version 2 (IKEv2)", RFC 9242,
DOI 10.17487/RFC9242, May 2022,
<https://www.rfc-editor.org/info/rfc9242>.

[X.690] ITU-T, "Information Technology - ASN.1 encoding rules:
Specification of Basic Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished Encoding Rules
(DER)", ISO/IEC 8825-1:2021 (E), ITU-T
Recommendation X.690, February 2021.

7.2. Informative References

[COMPOSITE-SIGS]
Ounsworth, M., Gray, J., Pala, M., and J. Klaussner,
"Composite Signatures For Use In Internet PKI", Work in
Progress, Internet-Draft, draft-ietf-lamps-pq-composite-
sigs-01, 24 May 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-lamps-
pq-composite-sigs-01>.

[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, DOI 10.17487/RFC2409, November 1998,
<https://www.rfc-editor.org/info/rfc2409>.

[RFC4739] Eronen, P. and J. Korhonen, "Multiple Authentication
Exchanges in the Internet Key Exchange (IKEv2) Protocol",
RFC 4739, DOI 10.17487/RFC4739, November 2006,
<https://www.rfc-editor.org/info/rfc4739>.

[RFC6467] Kivinen, T., "Secure Password Framework for Internet Key
Exchange Version 2 (IKEv2)", RFC 6467,
DOI 10.17487/RFC6467, December 2011,
<https://www.rfc-editor.org/info/rfc6467>.

[RFC7383] Smyslov, V., "Internet Key Exchange Protocol Version 2
(IKEv2) Message Fragmentation", RFC 7383,
DOI 10.17487/RFC7383, November 2014,
<https://www.rfc-editor.org/info/rfc7383>.

Appendix A. Examples of Announcing Supported Authentication Methods

This appendix shows some examples of announcing authentication
methods. This appendix is purely informative; if it disagrees with
the body of this document, the other text is considered correct.
Note that some payloads that are not relevant to this specification
may be omitted for brevity.

A.1. No Need to Use the IKE_INTERMEDIATE Exchange

This example illustrates the situation when the
SUPPORTED_AUTH_METHODS Notify payload fits into the IKE_SA_INIT
message, and thus the IKE_INTERMEDIATE exchange is not needed. In
this scenario, the responder announces that it supports the "Shared
Key Message Integrity Code" and the "NULL Authentication"
authentication methods. The initiator informs the responder that it
supports only the "Shared Key Message Integrity Code" authentication
method.

Initiator Responder
----------- -----------
IKE_SA_INIT
HDR, SAi1, KEi, Ni -->
<-- HDR, SAr1, KEr, Nr,
N(SUPPORTED_AUTH_METHODS(
PSK, NULL))

IKE_AUTH
HDR, SK {IDi,
AUTH, SAi2, TSi, TSr,
N(SUPPORTED_AUTH_METHODS(PSK))} -->
<-- HDR, SK {IDr,
AUTH, SAr2, TSi, TSr}

A.2. With Use of the IKE_INTERMEDIATE Exchange

This example illustrates the situation when the IKE_INTERMEDIATE
exchange is used. In this scenario, the responder announces that it
supports the "Digital signature" authentication method using the
RSASSA-PSS algorithm with CA1 and CA2 and the same method using the
ECDSA algorithm with CA3. The initiator supports only the "Digital
signature" authentication method using the RSASSA-PSS algorithm with
no link to a particular CA.

Initiator Responder
----------- -----------
IKE_SA_INIT
HDR, SAi1, KEi, Ni,
N(SIGNATURE_HASH_ALGORITHMS) -->
<-- HDR, SAr1, KEr, Nr,
CERTREQ(CA1, CA2, CA3),
N(SIGNATURE_HASH_ALGORITHMS),
N(SUPPORTED_AUTH_METHODS())

IKE_INTERMEDIATE
HDR, SK {..., IDi]} -->
<-- HDR, SK {...,
CERTREQ(CA1, CA2, CA3),
N(SUPPORTED_AUTH_METHODS(
SIGNATURE(RSASSA-PSS:1),
SIGNATURE(RSASSA-PSS:2),
SIGNATURE(ECDSA:3)))}

IKE_AUTH
HDR, SK {IDi, CERT, CERTREQ(CA2),
AUTH, SAi2, TSi, TSr,
N(SUPPORTED_AUTH_METHODS(
SIGNATURE(RSASSA-PSS:0)))} -->
<-- HDR, SK {IDr, CERT,
AUTH, SAr2, TSi, TSr}

Acknowledgments

The author would like to thank Paul Wouters for his valuable comments
and proposals. The author is also grateful to Daniel Van Geest, who
made proposals for protocol improvement. Reese Enghardt and Rifaat
Shekh-Yusef contributed to the clarity of the document.

Author's Address

Valery Smyslov
ELVIS-PLUS
PO Box 81
Moscow (Zelenograd)
124460
Russian Federation
Phone: +7 495 276 0211
Email: [email protected]