Internet DRAFT - draft-kario-gss-qr-kex
draft-kario-gss-qr-kex
Internet Engineering Task Force H. Kario
Internet-Draft Red Hat, Inc.
Updates: 4462 (if approved) Sep 30, 2020
Intended status: Standards Track
Expires: April 3, 2021
Quantum-Resistant GSS-API Key Exchange for SSH
draft-kario-gss-qr-kex-00
Abstract
This document specifies additions and amendments to RFC4462. It
defines a new key exchange method that uses GSS-API in a way to
provide key exchange method that is resistant to attacks by quantum
computers. The purpose of this specification is to provide an easy-
to-implement upgrade to environments that require resistance against
quantum computers before widely accepted post-quantum cryptography
algorithms are established.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 3, 2021.
Copyright Notice
Copyright (c) 2020 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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publication of this document. Please review these documents
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to this document. Code Components extracted from this document must
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Document Conventions . . . . . . . . . . . . . . . . . . . . 3
4. New Quantum Resistant Key Exchange Methods . . . . . . . . . 3
4.1. Generic Quantum Resistant GSS-API key Exchange . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6.1. Symmetric cipher security . . . . . . . . . . . . . . . . 7
6.2. User authentication . . . . . . . . . . . . . . . . . . . 8
6.3. Used GSSAPI Mechanisms . . . . . . . . . . . . . . . . . 8
6.4. GSSAPI Delegation . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . 8
7.2. Informative References . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
SSH GSS-API Methods [RFC4462] allows the use of GSSAPI for
authentication and key exchange in SSH. Unfortunately for resistance
against quantum computers all of the methods in RFC 4462 as well as
all of the new methods introduced in SSH GSS-API SHA-2 Methods
[RFC8732] derive their security from Finite-Field Diffie-Hellman or
Elliptic Curve Diffie-Hellman key exchanges. Both FFDH and ECDH are
believed to be vulnerable to Shor's algorithm running on quantum
computers. This document updates RFC4462 with new methods intended
for use in environments where use of quantum resistant algorithms is
more important that the forward secrecy provided by FFDH and ECDH.
2. Rationale
Due to security concerns with FFDH and ECDH against attacks using
quantum computers, we propose a new key exchange method that does not
use FFDH or ECDH to agree on a shared secret to derive later
encryption keys but rather uses GSS-API as a secure communication
channel to exchange secrets that are then used to derive encryption
keys.
To provide resistance against quantum computer attacks the connection
needs to also carefully select encryption ciphers, and host
authentication methods.
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3. Document Conventions
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 [RFC2119] RFC8174 [RFC8174] when, and only when, they
appear in all capitals, as shown here.
4. New Quantum Resistant Key Exchange Methods
This document adopts the same naming convention as defined in
[RFC4462] to define families of methods that cover any GSS-API
mechanism used with a specific SHA-2 Hash. It also reuses much of
the the scheme defined in Section 2.1 of [RFC4462].
The following new key exchange algorithms are defined:
+--------------------------+--------------------------------+
| Key Exchange Method Name | Implementation Recommendations |
+--------------------------+--------------------------------+
| gss-qr-sha256-* | SHOULD/RECOMMENDED |
| gss-qr-sha512-* | MAY/OPTIONAL |
+--------------------------+--------------------------------+
Each key exchange method is implicitly registered by this document.
The IESG is considered to be the owner of all these key exchange
methods; this does NOT imply that the IESG is considered to be the
owner of the underlying GSS-API mechanism.
Each method in any family of methods specifies GSS-API-authenticated
exchanges as described in Section 2.1 of [RFC4462]. The method name
for each method is the concatenation of the family name prefix with
the Base64 encoding of the MD5 hash [RFC1321] of the ASN.1 DER
encoding [ISO-IEC-8825-1] of the underlying GSS-API mechanism's OID.
Base64 encoding is described in Section 6.8 of [RFC2045].
Family method references
+--------------------+---------------+
| Family Name prefix | Hash Function |
+--------------------+---------------+
| gss-qr-sha256- | SHA-256 |
| gss-qr-sha512- | SHA-512 |
+--------------------+---------------+
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4.1. Generic Quantum Resistant GSS-API key Exchange
This section reuses much of the scheme defined in Section 2.1 of
[RFC4462] though it does not transport FFDH key shares in the
exchanged messages.
This section defers to [RFC7546] as the source of information on GSS-
API context establishment operations, Section 3 being the most
relevant. All security considerations described in [RFC7546] apply
here too.
The parties generate nonces in the key exchange. The generated
nonces MUST be at least 256 bits long and come from a quantum safe
CSPRNG. The nonces MUST NOT be reused in other key exchanges.
The client initiates negotiation by calling GSS_Init_sec_context()
and the server responds to it by calling GSS_Accept_sec_context().
For the negotiation, client MUST set the mutual_req_flag,
conf_req_flag, and integ_req_flag flag to "true". In addition,
deleg_req_flag MAY be set to "true" to request access delegation, if
requested by the user. Since the key exchange process authenticates
only the host, the setting of anon_req_flag is immaterial to this
process. If the client does not support the "gssapi-keyex" user
authentication method described in Section 4 of [RFC4462], or does
not intend to use that method in conjunction with the GSS-API context
established during key exchange, then anon_req_flag SHOULD be set to
"true". Otherwise, this flag MAY be set to true if the client wishes
to hide its identity. This key exchange process will exchange only a
single message token once the context has been established;
therefore, the replay_det_req_flag and sequence_req_flag SHOULD be
set to "false".
During GSS context establishment, multiple tokens may be exchanged by
the client and the server. When the GSS context is established
(major_status is GSS_S_COMPLETE), the parties check that mutual_state
and integ_avail are both "true". If not, the key exchange MUST fail.
To verify the integrity of the handshake both peers use the Hash
Function defined by the selected Key Exchange method to calculate the
running hash of exchanged messages, H_S and H_C.
H_S = hash(V_C || V_S || I_C || KC_S || ... || KC_C).
H_C = hash(V_C || V_S || I_C || KC_S || ... || KC_C || KC).
The GSS_wrap() call is used by the server and client to encrypt the
calculated hash and the selected nonce. The peers use the
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GSS_unwrap() to decrypt the value used to check if the other peer has
received the same messages and to get the nonce it selected.
Peers MUST verify if the length of the selected nonce is not shorter
than 32 octets. If the received nonce is shorter, the key exchange
MUST fail.
The following is an overview of the key exchange process:
Client Server
------ ------
Calls GSS_Init_sec_context().
SSH_MSG_KEXGSS_INIT --------------->
(Loop)
| Calls GSS_Accept_sec_context().
| <------------ SSH_MSG_KEXGSS_CONTINUE
| Calls GSS_Init_sec_context().
| SSH_MSG_KEXGSS_CONTINUE ------------>
Calls GSS_Accept_sec_context().
Generates ephemeral nonce.
Computes hash H_S.
Calls GSS_wrap( H_S || nonce_S ).
<------------ SSH_MSG_KEXGSS_COMPLETE
Computes hash H_S.
Calls GSS_unwrap().
Verifies that computed H_S matches received value.
Computes hash H_C.
Generates ephemeral nonce.
Calls GSS_wrap( H_C || nonce_C ).
SSH_MSG_KEXGSS_COMPLETE ------------>
Computes hash H_C.
Calls GSS_unwrap().
Verifies that computed H_C matches received value.
This is implemented with the following messages:
The client sends:
byte SSH_MSG_KEXGSS_INIT
string output_token (from GSS_Init_sec_context())
The server sends:
byte SSH_MSG_KEXGSS_CONTINUE
string output_token (from GSS_Accept_sec_context())
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Each time the client receives the message described above, it makes
another call to GSS_Init_sec_context().
The client sends:
byte SSH_MSG_KEXGSS_CONTINUE
string output_token (from GSS_Init_sec_context())
The final server message is either:
byte SSH_MSG_KEXGSS_COMPLETE
string enc_nonce (GSS_wrap() of H_S and nonce_S)
boolean TRUE
string output_token (from GSS_Accept_sec_context())
Or the following if no output_token is available:
byte SSH_MSG_KEXGSS_COMPLETE
string enc_nonce (GSS_wrap() of H_S and nonce_S)
boolean FALSE
As the final message the client sends either:
byte SSH_MSG_KEXGSS_COMPLETE
string enc_nonce (GSS_wrap() of H_C and nonce_C)
boolean TRUE
string output_token (from GSS_Accept_sec_context())
Or the following if no output_token is available:
byte SSH_MSG_KEXGSS_COMPLETE
string enc_nonce (GSS_wrap() of H_C and nonce_C)
boolean FALSE
The hashes H_S and H_C are computed as the HASH hash of the
concatenation of the following:
string V_C, the client's version string (CR, NL excluded)
string V_S, server's version string (CR, NL excluded)
string I_C, payload of the client's SSH_MSG_KEXINIT
string I_S, payload of the server's SSH_MSG_KEXINIT
string KC_S, payload of the server's SSH_MSG_KEXGSS_CONTINUE
string KC_C, payload of the client's SSH_MSG_KEXGSS_CONTINUE
string KC_S, payload of the server's second SSH_MSG_KEXGSS_CONTINUE
string KC_C, payload of the client's second SSH_MSG_KEXGSS_CONTINUE
...
string KC, payload of the server's SSH_MSG_KEXGSS_COMPLETE
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Those values are called exchange hashes, and they are used to
authenticate the key exchange. The exchange hashes SHOULD be kept
secret. If no SSH_MSG_KEXGSS_CONTINUE messages have been sent by the
server or received by the client, then an empty string is used in
place of KC_S and KC_C when computing the exchange hash. When
multiple SSH_MSG_KEXGSS_CONTINUE messages have been sent by either
side, then they should all be included in the exchange hash, in order
they have been processed by both sides of the connection. For the
H_S hash, the KC is an empty string.
Once a party has both the server nonce (nonce_S) and the client nonce
(nonce_C) it concatenates them, in this order, to compute the used
shared secret K:
K = nonce_S || nonce_C
If the client receives a SSH_MSG_KEXGSS_CONTINUE message after a call
to GSS_Init_sec_context() has returned a major_status code of
GSS_S_COMPLETE, a protocol error has occurred and the key exchange
MUST fail.
If the client receives a SSH_MSG_KEXGSS_COMPLETE message and a call
to GSS_Init_sec_context() does not result in a major_status code of
GSS_S_COMPLETE, a protocol error has occurred and the key exchange
MUST fail.
5. IANA Considerations
This document augments the SSH Key Exchange Method Names in
[RFC4462].
IANA is requested to update the SSH Protocol Parameters
[IANA-KEX-NAMES] registry with the following entries:
+--------------------------+------------+
| Key Exchange Method Name | Reference |
+--------------------------+------------+
| gss-qr-sha256-* | This draft |
| gss-qr-sha512-* | This draft |
+--------------------------+------------+
6. Security Considerations
6.1. Symmetric cipher security
Current understanding of quantum computer capabilities suggest that
symmetric ciphers with keys smaller than 256 bits will require less
than the current recommended minimal work factor of 2^128 operations.
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As such, connections that use this key exchange methods MUST use
ciphers with at least 256 bit keys to retain quantum resistance.
6.2. User authentication
For the connection to remain resistant against quantum computers, the
user authentication needs to also use quantum resistant algorithms.
In particular, it's RECOMMENDED that connections use gssapi-keyex for
client authentication. The publickey mechanism MUST NOT be used
unless the asymmetric keys used for it use post-quantum algorithms.
DSA, ECDSA, and RSA keys MUST NOT be used.
6.3. Used GSSAPI Mechanisms
The security of the key exchange depends on the security of the used
GSSAPI mechanism. The described key exchange will be quantum
resistant only in case the used GSSAPI mechanism is quantum
resistant.
For example, the Kerberos 5 mechanism is quantum resistant only when
it's used together with algorithms and key sizes that are quantum
resistant. Quantum safe algorithm SHOULD be used throught the
kerberos infrastructure, both for authentication and encryption.
Currently aes256-cts-hmac-sha384-192 mechanism defined in [RFC8009]
for encryption is an example of such an algorithm.
6.4. GSSAPI Delegation
Some GSSAPI mechanisms can act on a request to delegate credentials
to the target host when the deleg_req_flag is set. In this case,
extra care must be taken to ensure that the acceptor being
authenticated matches the target the user intended. Some mechanisms
implementations (like commonly used krb5 libraries) may use insecure
DNS resolution to canonicalize the target name; in these cases
spoofing a DNS response that points to an attacker-controlled machine
may results in the user silently delegating credentials to the
attacker, who can then impersonate the user at will.
7. References
7.1. Normative References
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
DOI 10.17487/RFC1321, April 1992,
<https://www.rfc-editor.org/info/rfc1321>.
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[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
<https://www.rfc-editor.org/info/rfc2045>.
[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>.
[RFC4462] Hutzelman, J., Salowey, J., Galbraith, J., and V. Welch,
"Generic Security Service Application Program Interface
(GSS-API) Authentication and Key Exchange for the Secure
Shell (SSH) Protocol", RFC 4462, DOI 10.17487/RFC4462, May
2006, <https://www.rfc-editor.org/info/rfc4462>.
[RFC7546] Kaduk, B., "Structure of the Generic Security Service
(GSS) Negotiation Loop", RFC 7546, DOI 10.17487/RFC7546,
May 2015, <https://www.rfc-editor.org/info/rfc7546>.
[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>.
[RFC8732] Sorce, S. and H. Kario, "Generic Security Service
Application Program Interface (GSS-API) Key Exchange with
SHA-2", RFC 8732, DOI 10.17487/RFC8732, February 2020,
<https://www.rfc-editor.org/info/rfc8732>.
7.2. Informative References
[IANA-KEX-NAMES]
Internet Assigned Numbers Authority, "Secure Shell (SSH)
Protocol Parameters: Key Exchange Method Names", June
2005, <https://www.iana.org/assignments/ssh-parameters/
ssh-parameters.xhtml#ssh-parameters-16>.
[ISO-IEC-8825-1]
International Organization for Standardization /
International Electrotechnical Commission, "ASN.1 encoding
rules: Specification of Basic Encoding Rules (BER),
Canonical Encoding Rules (CER) and Distinguished Encoding
Rules (DER)", ISO/IEC 8825-1, November 2015,
<http://standards.iso.org/ittf/PubliclyAvailableStandards/
c068345_ISO_IEC_8825-1_2015.zip>.
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[NIST-SP-800-131Ar1]
National Institute of Standards and Technology,
"Transitions: Recommendation for Transitioning of the Use
of Cryptographic Algorithms and Key Lengths", NIST Special
Publication 800-131A Revision 1, November 2015,
<http://nvlpubs.nist.gov/nistpubs/SpecialPublications/
NIST.SP.800-131Ar1.pdf>.
[RFC6194] Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security
Considerations for the SHA-0 and SHA-1 Message-Digest
Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011,
<https://www.rfc-editor.org/info/rfc6194>.
[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>.
[RFC8009] Jenkins, M., Peck, M., and K. Burgin, "AES Encryption with
HMAC-SHA2 for Kerberos 5", RFC 8009, DOI 10.17487/RFC8009,
October 2016, <https://www.rfc-editor.org/info/rfc8009>.
Author's Address
Hubert Kario
Red Hat, Inc.
Purkynova 115
Brno 612 00
Czech Republic
Email: hkario@redhat.com
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