Internet DRAFT - draft-ietf-curdle-gss-keyex-sha2
draft-ietf-curdle-gss-keyex-sha2
Internet Engineering Task Force S. Sorce
Internet-Draft H. Kario
Updates: 4462 (if approved) Red Hat, Inc.
Intended status: Standards Track Jul 22, 2019
Expires: January 23, 2020
GSS-API Key Exchange with SHA2
draft-ietf-curdle-gss-keyex-sha2-10
Abstract
This document specifies additions and amendments to RFC4462. It
defines a new key exchange method that uses SHA-2 for integrity and
deprecates weak DH groups. The purpose of this specification is to
modernize the cryptographic primitives used by GSS Key Exchanges.
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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 23, 2020.
Copyright Notice
Copyright (c) 2019 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
Provisions Relating to IETF Documents
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Document Conventions . . . . . . . . . . . . . . . . . . . . 2
4. New Diffie-Hellman Key Exchange methods . . . . . . . . . . . 3
5. New Elliptic Curve Diffie-Hellman Key Exchange methods . . . 4
5.1. Generic GSS-API Key Exchange with ECDH . . . . . . . . . 4
5.2. ECDH Key Exchange Methods . . . . . . . . . . . . . . . . 8
6. Deprecated Algorithms . . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8.1. New Finite Field DH mechanisms . . . . . . . . . . . . . 10
8.2. New Elliptic Curve DH mechanisms . . . . . . . . . . . . 10
8.3. GSSAPI Delegation . . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1. Normative References . . . . . . . . . . . . . . . . . . 11
9.2. Informative References . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
SSH GSS-API Methods [RFC4462] allows the use of GSSAPI [RFC2743] for
authentication and key exchange in SSH. It defines three exchange
methods all based on DH groups and SHA-1. This document updates
RFC4462 with new methods intended to support environments that desire
to use the SHA-2 cryptographic hash functions.
2. Rationale
Due to security concerns with SHA-1 [RFC6194] and with MODP groups
with less than 2048 bits [NIST-SP-800-131Ar1] we propose the use of
hashes based on SHA-2 [RFC6234] with DH group14, group15, group16,
group17 and group18 [RFC3526]. Additionally we add support for key
exchange based on Elliptic Curve Diffie Hellman with the NIST P-256,
P-384 and P-521 [SEC2v2] as well as the X25519 and X448 [RFC7748]
curves. Following the practice of [RFC8268] only SHA-256 and SHA-512
hashes are used for DH groups. For NIST curves the same curve-to-
hashing algorithm pairing used in [RFC5656] is adopted for
consistency.
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.
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4. New Diffie-Hellman Key Exchange methods
This document adopts the same naming convention defined in [RFC4462]
to define families of methods that cover any GSS-API mechanism used
with a specific Diffie-Hellman group and SHA-2 Hash combination.
+--------------------------+--------------------------------+
| Key Exchange Method Name | Implementation Recommendations |
+--------------------------+--------------------------------+
| gss-group14-sha256-* | SHOULD/RECOMMENDED |
| gss-group15-sha512-* | MAY/OPTIONAL |
| gss-group16-sha512-* | SHOULD/RECOMMENDED |
| gss-group17-sha512-* | MAY/OPTIONAL |
| gss-group18-sha512-* | MAY/OPTIONAL |
+--------------------------+--------------------------------+
Table 1: New key exchange algorithms
Each key exchange method prefix is registered by this document. The
IESG is the change controller of all these key exchange methods; this
does NOT imply that the IESG is considered to be in control of the
corresponding GSS-API mechanism.
Each method in any family of methods (Table 2) specifies GSS-API-
authenticated Diffie-Hellman key exchanges as described in
Section 2.1 of [RFC4462]. The method name for each method (Table 1)
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 corresponding GSS-API mechanism's OID.
Base64 encoding is described in Section 4 of [RFC4648].
+---------------------+-------------+-------------+-----------------+
| Family Name prefix | Hash | Group | Reference |
| | Function | | |
+---------------------+-------------+-------------+-----------------+
| gss-group14-sha256- | SHA-256 | 2048-bit | Section 3 of |
| | | MODP | [RFC3526] |
| gss-group15-sha512- | SHA-512 | 3072-bit | Section 4 of |
| | | MODP | [RFC3526] |
| gss-group16-sha512- | SHA-512 | 4096-bit | Section 5 of |
| | | MODP | [RFC3526] |
| gss-group17-sha512- | SHA-512 | 6144-bit | Section 6 of |
| | | MODP | [RFC3526] |
| gss-group18-sha512- | SHA-512 | 8192-bit | Section 7 of |
| | | MODP | [RFC3526] |
+---------------------+-------------+-------------+-----------------+
Table 2: Family method references
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5. New Elliptic Curve Diffie-Hellman Key Exchange methods
In [RFC5656] new SSH key exchange algorithms based on Elliptic Curve
Cryptography are introduced. We reuse much of section 4 of [RFC5656]
to define GSS-API-authenticated ECDH Key Exchanges.
Additionally, we also utilize the curves defined in
[I-D.ietf-curdle-ssh-curves] to complement the three classic NIST-
defined curves required by [RFC5656].
5.1. Generic GSS-API Key Exchange with ECDH
This section reuses much of the scheme defined in Section 2.1 of
[RFC4462] and combines it with the scheme defined in Section 4 of
[RFC5656]; in particular, all checks and verification steps
prescribed in Section 4 of [RFC5656] apply here as well.
Key-agreement schemes ECDHE-Curve25519 and ECDHE-Curve448 perform the
Diffie-Helman protocol using the functions X25519 and X448,
respectively. Implementations MUST compute these functions using the
algorithms described in [RFC7748]. When they do so, implementations
MUST check whether the computed Diffie-Hellman shared secret is the
all-zero value and abort if so, as described in Section 6 of
[RFC7748]. Alternative implementations of these functions SHOULD
abort when either input forces the shared secret to one of a small
set of values, as discussed in Section 7 of [RFC7748].
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 each generate an ephemeral key pair, according to
Section 3.2.1 of [SEC1v2]. Keys are verified upon receipt by the
parties according to Section 3.2.3.1 of [SEC1v2].
For NIST Curves the keys use the uncompressed point representation
and MUST be converted using the algorithm in Section 2.3.4 of
[SEC1v2]. If the conversion fails or the point is transmitted using
the compressed representation, the key exchange MUST fail.
A GSS Context is established according to Section 4 of [RFC5656]; The
client initiates the establishment using GSS_Init_sec_context() and
the server responds to it using GSS_Accept_sec_context(). For the
negotiation, the client MUST set mutual_req_flag and integ_req_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
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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".
The client MUST include its public key with the first message it
sends to the server during this process; if the server receives more
than one key or none at all, the key exchange MUST fail.
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.
Once a party receives the peer's public key it proceeds to compute a
shared secret K. For NIST Curves the computation is done according
to Section 3.3.1 of [SEC1v2] and the resulting value z is converted
to the octet string K using the conversion defined in Section 2.3.5
of [SEC1v2]. For curve25519 and curve448 the algorithms in Section 6
of [RFC7748] are used instead.
To verify the integrity of the handshake, peers use the Hash Function
defined by the selected Key Exchange method to calculate H:
H = hash(V_C || V_S || I_C || I_S || K_S || Q_C || Q_S || K).
The GSS_GetMIC() call is used by the server with H as the payload and
generates a MIC. The GSS_VerifyMIC() call is used by the client to
verify the MIC.
If any GSS_Init_sec_context() or GSS_Accept_sec_context() returns a
major_status other than GSS_S_COMPLETE or GSS_S_CONTINUE_NEEDED, or
any other GSS-API call returns a major_status other than
GSS_S_COMPLETE, the key exchange MUST fail. The same recommendations
expressed in Section 2.1 of [RFC4462] are followed with regards to
error reporting.
The following is an overview of the key exchange process:
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Client Server
------ ------
Generate ephemeral key pair.
Calls GSS_Init_sec_context().
SSH_MSG_KEXGSS_INIT --------------->
Verify received key is valid.
(Optional) <------------- SSH_MSG_KEXGSS_HOSTKEY
(Loop)
| Calls GSS_Accept_sec_context().
| <------------ SSH_MSG_KEXGSS_CONTINUE
| Calls GSS_Init_sec_context().
| SSH_MSG_KEXGSS_CONTINUE ------------>
Calls GSS_Accept_sec_context().
Generate ephemeral key pair.
Compute shared secret.
Computes hash H.
Calls GSS_GetMIC( H ) = MIC.
<------------ SSH_MSG_KEXGSS_COMPLETE
Verify received key is valid.
Compute shared secret.
Compute hash = H
Calls GSS_VerifyMIC( MIC, H )
This is implemented with the following messages:
The client sends:
byte SSH_MSG_KEXGSS_INIT
string output_token (from GSS_Init_sec_context())
string Q_C, client's ephemeral public key octet string
The server may respond with:
byte SSH_MSG_KEXGSS_HOSTKEY
string server public host key and certificates (K_S)
The server sends:
byte SSH_MSG_KEXGSS_CONTINUE
string output_token (from GSS_Accept_sec_context())
Each time the client receives the message described above, it makes
another call to GSS_Init_sec_context().
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The client sends:
byte SSH_MSG_KEXGSS_CONTINUE
string output_token (from GSS_Init_sec_context())
As the final message the server sends either:
byte SSH_MSG_KEXGSS_COMPLETE
string Q_S, server's ephemeral public key octet string
string mic_token (MIC of H)
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 Q_S, server's ephemeral public key octet string
string mic_token (MIC of H)
boolean FALSE
The hash H is 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 K_S, server's public host key
string Q_C, client's ephemeral public key octet string
string Q_S, server's ephemeral public key octet string
mpint K, shared secret
This value is called the exchange hash, and it is used to
authenticate the key exchange. The exchange hash SHOULD be kept
secret. If no SSH_MSG_KEXGSS_HOSTKEY message has been sent by the
server or received by the client, then the empty string is used in
place of K_S when computing the exchange hash.
Since this key exchange method does not require the host key to be
used for any encryption operations, the SSH_MSG_KEXGSS_HOSTKEY
message is OPTIONAL. If the "null" host key algorithm described in
Section 5 of [RFC4462] is used, this message MUST NOT be sent.
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.
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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.
5.2. ECDH Key Exchange Methods
+--------------------------+--------------------------------+
| Key Exchange Method Name | Implementation Recommendations |
+--------------------------+--------------------------------+
| gss-nistp256-sha256-* | SHOULD/RECOMMENDED |
| gss-nistp384-sha384-* | MAY/OPTIONAL |
| gss-nistp521-sha512-* | MAY/OPTIONAL |
| gss-curve25519-sha256-* | SHOULD/RECOMMENDED |
| gss-curve448-sha512-* | MAY/OPTIONAL |
+--------------------------+--------------------------------+
Table 3: New key exchange methods
Each key exchange method prefix is registered by this document. The
IESG is the change controller of all these key exchange methods; this
does NOT imply that the IESG is considered to be in control of the
corresponding GSS-API mechanism.
Each method in any family of methods (Table 4) specifies GSS-API-
authenticated Elliptic Curve Diffie-Hellman key exchanges as
described in Section 5.1. The method name for each method (Table 3)
is the concatenation of the family method name with the Base64
encoding of the MD5 hash [RFC1321] of the ASN.1 DER encoding
[ISO-IEC-8825-1] of the corresponding GSS-API mechanism's OID.
Base64 encoding is described in Section 4 of [RFC4648].
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+------------------------+----------+---------------+---------------+
| Family Name prefix | Hash | Parameters / | Definition |
| | Function | Function Name | |
+------------------------+----------+---------------+---------------+
| gss-nistp256-sha256- | SHA-256 | secp256r1 | Section 2.4.2 |
| | | | of [SEC2v2] |
| gss-nistp384-sha384- | SHA-384 | secp384r1 | Section 2.5.1 |
| | | | of [SEC2v2] |
| gss-nistp521-sha512- | SHA-512 | secp521r1 | Section 2.6.1 |
| | | | of [SEC2v2] |
| gss-curve25519-sha256- | SHA-256 | X22519 | Section 5 of |
| | | | [RFC7748] |
| gss-curve448-sha512- | SHA-512 | X448 | Section 5 of |
| | | | [RFC7748] |
+------------------------+----------+---------------+---------------+
Table 4: Family method refences
6. Deprecated Algorithms
Because they have small key lengths and are no longer strong in the
face of brute-force attacks, the algorithms in the following table
are considered deprecated and SHOULD NOT be used.
Deprecated Algorithms
+--------------------------+--------------------------------+
| Key Exchange Method Name | Implementation Recommendations |
+--------------------------+--------------------------------+
| gss-group1-sha1-* | SHOULD NOT |
| gss-group14-sha1-* | SHOULD NOT |
| gss-gex-sha1-* | SHOULD NOT |
+--------------------------+--------------------------------+
7. IANA Considerations
This document augments the SSH Key Exchange Method Names in
[RFC4462].
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IANA is requested to update the SSH Protocol Parameters
[IANA-KEX-NAMES] registry with the following entries:
+--------------------------+------------+
| Key Exchange Method Name | Reference |
+--------------------------+------------+
| gss-group1-sha1-* | This draft |
| gss-group14-sha1-* | This draft |
| gss-gex-sha1-* | This draft |
| gss-group14-sha256-* | This draft |
| gss-group15-sha512-* | This draft |
| gss-group16-sha512-* | This draft |
| gss-group17-sha512-* | This draft |
| gss-group18-sha512-* | This draft |
| gss-nistp256-sha256-* | This draft |
| gss-nistp384-sha384-* | This draft |
| gss-nistp521-sha512-* | This draft |
| gss-curve25519-sha256-* | This draft |
| gss-curve448-sha512-* | This draft |
+--------------------------+------------+
8. Security Considerations
8.1. New Finite Field DH mechanisms
Except for the use of a different secure hash function and larger DH
groups, no significant changes has been made to the protocol
described by [RFC4462]; therefore all the original Security
Considerations apply.
8.2. New Elliptic Curve DH mechanisms
Although a new cryptographic primitive is used with these methods the
actual key exchange closely follows the key exchange defined in
[RFC5656]; therefore all the original Security Considerations as well
as those expressed in [RFC5656] apply.
8.3. 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 mechanism
implementations (such as 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 result in the user silently delegating credentials to the
attacker, who can then impersonate the user at will.
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9. References
9.1. Normative References
[I-D.ietf-curdle-ssh-curves]
Adamantiadis, A., Josefsson, S., and M. Baushke, "Secure
Shell (SSH) Key Exchange Method using Curve25519 and
Curve448", draft-ietf-curdle-ssh-curves-08 (work in
progress), June 2018.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
DOI 10.17487/RFC1321, April 1992,
<https://www.rfc-editor.org/info/rfc1321>.
[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>.
[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743,
DOI 10.17487/RFC2743, January 2000,
<https://www.rfc-editor.org/info/rfc2743>.
[RFC3526] Kivinen, T. and M. Kojo, "More Modular Exponential (MODP)
Diffie-Hellman groups for Internet Key Exchange (IKE)",
RFC 3526, DOI 10.17487/RFC3526, May 2003,
<https://www.rfc-editor.org/info/rfc3526>.
[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>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>.
[RFC5656] Stebila, D. and J. Green, "Elliptic Curve Algorithm
Integration in the Secure Shell Transport Layer",
RFC 5656, DOI 10.17487/RFC5656, December 2009,
<https://www.rfc-editor.org/info/rfc5656>.
[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>.
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[RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
for Security", RFC 7748, DOI 10.17487/RFC7748, January
2016, <https://www.rfc-editor.org/info/rfc7748>.
[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>.
[SEC1v2] Certicom Research, "SEC 1: Elliptic Curve Cryptography",
Standards for Efficient Cryptography SEC 1, Version 2.0,
2009.
[SEC2v2] Certicom Research, "SEC 2: Recommended Elliptic Curve
Domain Parameters", Standards for Efficient
Cryptography SEC 2, Version 2.0, 2010.
9.2. 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>.
[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>.
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[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>.
[RFC8268] Baushke, M., "More Modular Exponentiation (MODP) Diffie-
Hellman (DH) Key Exchange (KEX) Groups for Secure Shell
(SSH)", RFC 8268, DOI 10.17487/RFC8268, December 2017,
<https://www.rfc-editor.org/info/rfc8268>.
Authors' Addresses
Simo Sorce
Red Hat, Inc.
140 Broadway
24th Floor
New York, NY 10025
USA
Email: simo@redhat.com
Hubert Kario
Red Hat, Inc.
Purkynova 115
Brno 612 00
Czech Republic
Email: hkario@redhat.com
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