Network Working Group M. Green
Internet-Draft Cryptography Engineering LLC
Intended status: Standards Track R. Droms
Expires: January 4, 2018 Interisle Consulting
R. Housley
Vigil Security, LLC
P. Turner
Venafi
S. Fenter
July 3, 2017
Data Center use of Static Diffie-Hellman in TLS 1.3
draft-green-tls-static-dh-in-tls13-01
Abstract
Unlike earlier versions of TLS, current drafts of TLS 1.3 have
instead adopted ephemeral-mode Diffie-Hellman and elliptic-curve
Diffie-Hellman as the primary cryptographic key exchange mechanism
used in TLS. This document describes an optional configuration for
TLS servers that allows for the use of a static Diffie-Hellman
private key for all TLS connections made to the server. Passive
monitoring of TLS connections can be enabled by installing a
corresponding copy of this key in each monitoring device.
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 http://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 4, 2018.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. ASN.1 . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Enterprise Out-of-band TLS Decryption Architecture . . . . . 4
3. Enterprise Requirements for Passive (out-of-band) TLS
Decryption . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Summary of the Existing Diffie-Hellman Handshake . . . . . . 6
5. Using static (EC)DHE on the server . . . . . . . . . . . . . 7
6. Key Representation . . . . . . . . . . . . . . . . . . . . . 7
7. TLS Static DH Key (TSK) Protocol . . . . . . . . . . . . . . 8
7.1. Key Push . . . . . . . . . . . . . . . . . . . . . . . . 10
7.2. Key Request . . . . . . . . . . . . . . . . . . . . . . . 10
8. Alternative Solutions for Enterprise Monitoring and
Troubleshooting . . . . . . . . . . . . . . . . . . . . . . . 11
9. Weaknesses of Alternative Solutions . . . . . . . . . . . . . 11
10. Security considerations . . . . . . . . . . . . . . . . . . . 12
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13
13. Normative References . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
Unlike earlier versions of TLS, current drafts of TLS 1.3
[I-D.ietf-tls-tls13] do not provide support for the RSA handshake --
and have instead adopted ephemeral-mode Diffie-Hellman (DHE) and
elliptic-curve Diffie-Hellman (ECDHE) as the primary cryptographic
key exchange mechanism used in TLS.
While ephemeral (EC) Diffie-Hellman is in nearly all ways an
improvement over the TLS RSA handshake, the use of these mechanisms
complicates certain enterprise settings. Specifically, the use of
ephemeral ciphersuites is not compatible with current enterprise
network monitoring tools such as Intrusion Detection Systems (IDS)
and application monitoring systems, which leverage the current TLS
RSA handshake passively monitor intranet TLS connections made between
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endpoints under the enterprise's control. This traffic includes TLS
connections made from enterprise network security devices (firewalls)
and load balancers at the edge of the enterprise network to internal
enterprise TLS servers. It does not include TLS connections
traveling over the external Internet.
Such monitoring of the enterprise network is ubiquitous and
indispensable in some industries. This monitoring is required for
effective and safe operation of enterprise networks. Loss of this
capability may slow adoption of TLS 1.3.
This document describes an optional configuration for TLS servers
that is compatible with the TLS 1.3 ephemeral ciphersuites without
precluding enterprise network monitoring. This configuration allows
for the use of a static (EC) Diffie-Hellman private key for all TLS
connections made to the server. Passive monitoring of TLS
connections can be enabled by installing a corresponding copy of this
key in each authorized monitoring device.
An advantage of this proposal is that it can be implemented using
software modifications to the TLS server and enterprise network
monitoring tools, without the need to make changes to TLS client
implementations.
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This document introduces the term "static (elliptic curve) Diffie-
Hellman ephemeral", generally written as "static (EC)DHE", to refer
to long-lived finite field or elliptic curve Diffie-Hellman keys or
key pairs that will be used with the TLS 1.3 ephemeral ciphersuites
to negotiate traffic keys for multiple TLS sessions.
For clarity, this document also introduces the term "ephemeral
(elliptic curve) Diffie-Hellman ephemeral", generally written as
"ephemeral (EC)DHE", to denote finite field or elliptic curve Diffie-
Hellman keys or key pairs that will be used with the TLS 1.3
ephemeral ciphersuites to negotiate traffic keys for a single TLS
sessions.
1.2. ASN.1
The Cryptographic Message Syntax (CMS) [RFC5652] and asymmetric key
packages [RFC5958] are generated using ASN.1 [X680], which uses the
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Basic Encoding Rules (BER) and the Distinguished Encoding Rules (DER)
[X690].
2. Enterprise Out-of-band TLS Decryption Architecture
This document describes the use of a static (elliptic-curve) Diffie-
Hellman (static (EC)DHE) private key by servers for use in TLS 1.3
sessions internal to an enterprise network where network monitoring
is required. In Figure 1, the Web Servers use a static (EC)DHE key
pair with the standard TLS 1.3 handshake for connections from the
Load Balancer, and the Back-End Services use static (EC)DHE for
connections from the Web Servers. The Load Balancer uses ephemeral
(EC)DHE key pairs with the standard TLS 1.3 handshake for connections
from external Browsers over the Internet, to provide Forward Secrecy
on those connections that are exposed to third-party monitoring.
Internally, the static (EC)DHE keys are provided to authorized TLS
Decrypter devices, such as intrusion detection systems, application
monitoring systems or network packet capture devices.
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********************************
* *
* +--------+ *
* TLS | Web | *
* Termination + Server + *
* | /| |\ *
+---------+ +----------+ * +----|-----+/ +--------+ \+----------+ *
| | | | * | Load + + Back-end | *
| Browser +--+ Internet |-*-+ Balancer | | Server | *
| | | | * | | + + | *
+---------+ +----------+ * +----------+\ +--------+ /+----------+ *
* | .\| Web |/. *
* . + Server + . *
* . | | . *
* . +--------+ . *
* . . *
* . -------- . *
* . / TLS \ . *
* | Decrypter| *
* \ / *
* -------- *
* *
*** Enterprise Network Boundary **
|
<------ Ephemeral (EC)DHE ------>|<-------- Static (EC)DHE -------->
|
Figure 1: Enterprise TLS Decryption Architecture
3. Enterprise Requirements for Passive (out-of-band) TLS Decryption
Enterprise networks based on this architecture have operational
requirements for traffic monitoring and ex post facto analysis for
purposes of:
o Application troubleshooting and performance analysis
o Fraud monitoring
o Security, including intrusion detection, malware detection,
confidential data exfiltration and layer 7 DDoS protection
o Audit compliance
o Customer Experience Monitoring
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Specific requirements to meet the listed operational requirements
include:
o TLS decryption for network security monitoring tools must be done
in real time with no gaps in decryption.
o The solution must be able to decrypt passively captured pcap
traces.
o The solution must scale to handle thousands of TLS sessions/sec.
o Key material must be preserved for back-in-time analysis. The
period for key retention depends upon local policy, reflecting
operational, security and compliance requirements.
o Key material must be encrypted during network transit
o The solution must not negatively impact the enterprise
infrastructure (servers, network, etc.)
o The solution must be able to decrypt the session when a TLS
session is reused. This may involve the use of a TLS decryption
appliance.
o The solution must be able to decrypt in a physical data center, in
a virtual environment, and in a cloud.
4. Summary of the Existing Diffie-Hellman Handshake
In TLS 1.3, servers exchange keys using two primary modes, DHE and
ECDHE. In a simplified view of the full handshake, the following
steps occur:
1. The client generates an ephemeral public and private key, and
transmits the public key within a "key_share" message, along with
a random nonce (ClientHello.random).
2. The server generates an ephemeral public and private key, and
transmits the public key within a "key_share" message, along with
a random nonce (ServerHello.random).
3. The two parties now calculate a shared (EC)DHE secret by
combining the other party's ephemeral public key with their own
ephemeral private key.
4. A series of traffic and handshake keys is derived by combining
this shared secret with various inputs from the handshake,
including the ClientHello.random and ServerHello.random.
5. Data encryption is performed using the shared secret.
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5. Using static (EC)DHE on the server
The proposal embodied in this draft modifies the standard TLS
handshake summarized above in the following ways:
For each elliptic curve (and FF-DH parameter length) supported by
the server, the server is provisioned with a static (EC)DHE
private/public key pair. This key pair may be either:
* generated at server installation, and rotated at periodic
intervals appropriate for any long-term server key,
* generated at a central key management server and distributed
(in a secure encrypted form) to the appropriate endpoint
servers.
All steps of the original handshake proceed as above, with the
following modification to server behavior. Step (2) proceeds as
follows:
2. The server transmits the static public key within a "key_share"
message, along with a random nonce (ServerHello.random).
6. Key Representation
The Asymmetric Key Package [RFC5958] MUST be used to transfer the
centrally managed Diffie-Hellman key pair. The key package contains
at least one Diffie-Hellman key pair. Each Diffie-Hellman key pair
is associated with a set of attributes, including the key validity
period for that Diffie-Hellman key pair.
OneAsymmetricKey is defined in Section 2 of [RFC5958]. The fields
are used as follows:
o version MUST be set to v2, which has an integer value of 1.
o privateKeyAlgorithm MUST be set to the algorithm identifier of the
Diffie-Hellman key pair. For convenience, some popular algorithm
identifiers are listed in Figure 2.
o privateKey MUST be set to the Diffie-Hellman private key encoded
as an OCTET STRING.
o attributes MUST be included even though the field is optional.
The set of attributes MUST include the key validity period
attribute defined in Section 15 of [RFC7906]. Other attributes
MAY be included as well.
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o publicKey MUST be included even though the field is optional. It
MUST be set to the Diffie-Hellman public key, encoded as a BIT
STRING. This is the same BIT STRING that would be included in an
X.509 certificate [RFC5280] for this public key.
+-------------------------------------------------------------------+
| |
| Finite Field Diffie-Hellman |
| object identifier: { 1 2 840 10046 2 1 } |
| parameter encoding: DomainParameters, Section 2.3.3 of [RFC3279] |
| private key encoding: INTEGER |
| public key encoding: INTEGER |
| |
| Elliptic Curve Diffie-Hellman |
| object identifier: { 1 3 132 1 12 } |
| parameter encoding: ECParameters, Section 2.1.2 of [RFC5480] |
| (MUST use the namedCurve CHOICE) |
| private key encoding: ECPrivateKey, Section 3 of [RFC5915] |
| public key encoding: ECPoint, Section 2.2 of [RFC5480] |
| |
+-------------------------------------------------------------------+
Figure 2: Popular Diffie-Hellman Algorithm Identifiers
The CMS protecting content types [RFC5652][RFC5083] can be used to
provide authentication and confidentiality protection for the
Asymmetric Key Package:
o SignedData can be used to apply a digital signature to the
Asymmetric Key Package.
o EncryptedData can be used to encrypt the Asymmetric Key Package
with previously distributed symmetric encryption key.
o EnvelopedData can be used to encrypt the Asymmetric Key Package,
where the sender and the receiver establish a symmetric encryption
key using Diffie-Hellman key agreement.
o AuthEnvelopedData can be used to protect the Asymmetric Key
Package where the sender and the receiver establish a symmetric
authenticated encryption key using Diffie-Hellman key agreement.
7. TLS Static DH Key (TSK) Protocol
The TLS Static DH Key (TSK) Protocol is used in cases where the
Diffie-Hellman keys are centrally managed. The two main roles in the
TSK protocol are "key manager" and "key consumer". Key consumers can
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be TLS servers or TLS decrypters. The key manager generates,
distributes, and tracks static (EC)DHE keys used by key consumers.
TSK messaging is based on HTTPS [RFC2818]. Keys are transferred as
Asymmetric Key Packages [RFC5958], using the profile in Section 6 of
this document.
-------------- -----------------
| TLS server |-------| key manager |
-------------- -----------------
| |
| |
| |
| -----------------
|------------>| TLS decrypter |
| -----------------
|
|
--------------
| TLS client |
--------------
Figure 3: TSK protocol components
The key manager can push keys to key consumers:
TLS server key manager TLS decrypter
| | |
| |-- |
| | \ Generate |
| | / key pair |
| |<- |
| | |
| |----------------------->|
| | Push key pair |
|<------------------------| |
| Push key pair | |
Figure 4: TSK protocol push model
Alternatively, key consumers can request (or pull) keys from the key
manager.
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TLS server key manager TLS decrypter
| | |
| |-- |
| | \ Generate |
| | / key pair |
| |<- |
| | |
| |<-----------------------|
| | Request key pair |
|------------------------>| |
| Request key pair | |
Figure 5: TSK protocol pull model
7.1. Key Push
An HTTPS-based TSK push is composed of the appropriate HTTP headers,
followed by the binary value of the BER (Basic Encoding Rules)
encoding of the Asymmetric Key Package.
The Content-Type header MUST be application/cms [RFC7193] if the
Asymmetric Key Package is encrypted with CMS [RFC6032]. The Content-
Type header MUST be application/pkcs8 if the Asymmetric Key Package
is transferred in plain text (within the encrypted HTTPS stream).
7.2. Key Request
A key consumer may request a key by providing a fingerprint [RFC6234]
of the public key. The key manager is responsible for determining if
the key consumer is authorized to receive a copy of the key being
requested.
Example with plain text Asymmetric Key Package:
GET /tsk/key/PublicKeyFingerprint
Accept: application/pkcs8
Example with CMS encrypted and/or signed Asymmetric Key Package:
GET /tsk/key/PublicKeyFingerprint
Accept: application/cms
The response to the TSK push is composed of the appropriate HTTP
headers, followed by the binary value of the BER (Basic Encoding
Rules) encoding of the Asymmetric Key Package.
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The Content-Type header MUST be application/cms [RFC7193] if the
Asymmetric Key Package is encrypted with CMS [RFC6032]. The Content-
Type header MUST be application/pkcs8 if the Asymmetric Key Package
is transferred in plain text (within the encrypted HTTPS stream).
8. Alternative Solutions for Enterprise Monitoring and Troubleshooting
o Export of ephemeral keys
o Export of decrypted traffic from TLS proxy devices at the edge of
the enterprise network
o Placement of TLS proxies in the enterprise network
o Reliance on TCP/IP headers not encrypted by TLS
o Reliance on application/server logs
o Doing troubleshooting and malware analysis at the endpoint.
o Adding a TCP or UDP extension to provide the information needed to
do packet analysis.
9. Weaknesses of Alternative Solutions
Export of ephemeral keys: Scale - In a large enterprise there will
be billions of ephemeral keys to export and manage. There will
also be difficulty in transporting these keys to real time
tools that need decrypted packets. The complexity of the
solution is a problem that adds risk.
Export of decrypted traffic from TLS proxy devices: Decrypted
traffic at only the edge of the network is not adequate for the
enterprise requirements listed above (troubleshooting, network
security monitoring, etc...)
TLS proxies in the network: Inline TLS proxies will not scale to the
number of decryption points needed within an enterprise. Each
inline proxy adds cost, latency, and production risk.
Reliance on TCP/IP headers: IP and/or TCP headers are not adequate
for the enterprise requirements listed above. Troubleshooters
must be able to find transactions in a pcap trace, identified
by markers like userids, session ids, URLs, and time stamps.
Threat Detection teams must be able to look for Indicators of
Compromise in the payload of packets, etc.
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Reliance on Application/server logs: Logging is not adequate for the
enterprise requirements listed above. Code developers cannot
anticipate every possible problem and put a log message in just
the right place. There are billions of lines of code in a data
center, and it's not scalable to try and improve logging.
Troubleshooting and malware analysis at the endpoint: Endpoints
don't have the robustness to do their own workload and handle
the burden of the various enterprise requirements listed above.
These requirements would include always-on full packet capture
at the endpoint with no packet drops.
Adding TCP/UDP extensions: An important part of troubleshooting,
network security monitoring, etc. is analysis of the
application-specific payload of the packet. It is not possible
to anticipate ahead of time, among thousands of unique
applications, which fields in the application payload will be
important.
10. Security considerations
We now consider the security implications of the change described
above:
1. The shift from fully-ephemeral (EC)HDE to static (EC)DHE affects
the security properties offered by the TLS 1.3 handshake by
eliminating the Forward Secrecy property provided by the server.
If a server is compromised and the private key is stolen, then an
attacker who observes any TLS handshake (even one that occurred
prior to the compromise) performed with this static (EC)DHE key
pair will be able to recover session traffic encryption keys and
will be able to decrypt traffic.
2. As long as the server static secret key is not compromised, the
resulting protocol will provide strong cryptographic security, as
long as the Diffie-Hellman parameters (e.g., finite-field group
or elliptic curve) are correctly generated and provide security
at a sufficient cryptographic security level.
3. A flaw in the generation of finite-field Diffie-Hellman
parameters or the use of an insecure implementation could leak
some bits of the static secret key over time. This risk is not
present in ephemeral DH implementations. Implementers should use
care to avoid such pitfalls.
Thus the modification described in Section 10 represents a deliberate
weakening of some security properties. Implementers who choose to
include this capability should carefully consider the risks to their
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infrastructure of using a handshake without Forward Secrecy. Static
(EC)DHE key pairs should be rotated regularly.
11. IANA Considerations
This document contains no actions for IANA.
12. Acknowledgements
This modification to TLS was initially suggested by Hugo Krawczyk.
13. Normative References
[I-D.ietf-tls-tls13]
Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-20 (work in progress),
April 2017.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000,
<http://www.rfc-editor.org/info/rfc2818>.
[RFC3279] Bassham, L., Polk, W., and R. Housley, "Algorithms and
Identifiers for the Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 3279, DOI 10.17487/RFC3279, April
2002, <http://www.rfc-editor.org/info/rfc3279>.
[RFC5083] Housley, R., "Cryptographic Message Syntax (CMS)
Authenticated-Enveloped-Data Content Type", RFC 5083,
DOI 10.17487/RFC5083, November 2007,
<http://www.rfc-editor.org/info/rfc5083>.
[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,
<http://www.rfc-editor.org/info/rfc5280>.
[RFC5480] Turner, S., Brown, D., Yiu, K., Housley, R., and T. Polk,
"Elliptic Curve Cryptography Subject Public Key
Information", RFC 5480, DOI 10.17487/RFC5480, March 2009,
<http://www.rfc-editor.org/info/rfc5480>.
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[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, DOI 10.17487/RFC5652, September 2009,
<http://www.rfc-editor.org/info/rfc5652>.
[RFC5915] Turner, S. and D. Brown, "Elliptic Curve Private Key
Structure", RFC 5915, DOI 10.17487/RFC5915, June 2010,
<http://www.rfc-editor.org/info/rfc5915>.
[RFC5958] Turner, S., "Asymmetric Key Packages", RFC 5958,
DOI 10.17487/RFC5958, August 2010,
<http://www.rfc-editor.org/info/rfc5958>.
[RFC6032] Turner, S. and R. Housley, "Cryptographic Message Syntax
(CMS) Encrypted Key Package Content Type", RFC 6032,
DOI 10.17487/RFC6032, December 2010,
<http://www.rfc-editor.org/info/rfc6032>.
[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,
<http://www.rfc-editor.org/info/rfc6234>.
[RFC7193] Turner, S., Housley, R., and J. Schaad, "The application/
cms Media Type", RFC 7193, DOI 10.17487/RFC7193, April
2014, <http://www.rfc-editor.org/info/rfc7193>.
[RFC7906] Timmel, P., Housley, R., and S. Turner, "NSA's
Cryptographic Message Syntax (CMS) Key Management
Attributes", RFC 7906, DOI 10.17487/RFC7906, June 2016,
<http://www.rfc-editor.org/info/rfc7906>.
[X680] ITU-T, "Information technology -- Abstract Syntax Notation
One (ASN.1): Specification of basic notation",
ITU-T Recommendation X.680, 2015.
[X690] ITU-T, "Information technology -- ASN.1 encoding rules:
Specification of Basic Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished Encoding Rules
(DER)", ITU-T Recommendation X.690, 2015.
Authors' Addresses
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Matthew Green
Cryptography Engineering LLC
4506 Roland Ave
Baltimore, MD 21210
USA
Email: mgreen@cryptographyengineering.com
Ralph Droms
Interisle Consulting
Email: rdroms.ietf@gmail.com
Russ Housley
Vigil Security, LLC
918 Spring Knoll Drive
Herndon, VA 20170
USA
Email: housley@vigilsec.com
Paul Turner
Venafi
175 East 400 South, Suite 300
Salt Lake City, UT 84111
USA
Email: paul.turner@venafi.com
Steve Fenter
Email: steven.fenter58@gmail.com
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