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Versions: (draft-miller-posh) 00 01 02 03 04
05 06 RFC 7711
XMPP Working Group M. Miller
Internet-Draft Cisco Systems, Inc.
Intended status: Standards Track P. Saint-Andre
Expires: April 13, 2015 &yet
October 10, 2014
PKIX over Secure HTTP (POSH)
draft-ietf-xmpp-posh-02
Abstract
Experience has shown that it is extremely difficult to deploy proper
PKIX certificates for TLS in multi-tenanted environments, since
certification authorities will not issue certificates for hosted
domains to hosting services, hosted domains do not want hosting
services to hold their private keys, and hosting services wish to
avoid liability for holding those keys. As a result, domains hosted
in multi-tenanted environments often deploy non-HTTP applications
such as email and instant messaging using certificates that identify
the hosting service, not the hosted domain. Such deployments force
end users and peer services to accept a certificate with an improper
identifier, resulting in obvious security implications. This
document defines two methods that make it easier to deploy
certificates for proper server identity checking in non-HTTP
application protocols. The first method enables the TLS client
associated with a user agent or peer application server to obtain the
end-entity certificate of a hosted domain over secure HTTP as an
alternative to standard PKIX techniques. The second method enables a
hosted domain to securely delegate a non-HTTP application to a
hosting service using redirects provided by HTTPS itself or by a
pointer in a file served over HTTPS at the hosted domain. While this
approach was developed for use in the Extensible Messaging and
Presence Protocol (XMPP) as a Domain Name Association prooftype, it
can be applied to any non-HTTP application protocol.
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
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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 April 13, 2015.
Copyright Notice
Copyright (c) 2014 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
(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
to this document. Code Components extracted from this document must
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. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Obtaining Verification Materials . . . . . . . . . . . . . . 4
3.1. Source Domain Possesses PKIX Certificate Information . . 5
3.2. Source Domain References PKIX Certificate . . . . . . . . 6
3.3. Performing Verification . . . . . . . . . . . . . . . . . 7
4. Secure Delegation . . . . . . . . . . . . . . . . . . . . . . 8
5. Order of Operations . . . . . . . . . . . . . . . . . . . . . 8
6. Caching Results . . . . . . . . . . . . . . . . . . . . . . . 9
7. Alternates and Roll-over . . . . . . . . . . . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
9. Security Considerations . . . . . . . . . . . . . . . . . . . 11
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
10.1. Normative References . . . . . . . . . . . . . . . . . . 12
10.2. Informative References . . . . . . . . . . . . . . . . . 13
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
We start with a thought experiment.
Imagine that you work on the operations team of a hosting company
that provides the "foo" service (or email or instant messaging or
social networking service) for ten thousand different customer
organizations. Each customer wants their service to be identified by
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the customer's domain name (e.g., foo.example.com), not the hosting
company's domain name (e.g., hosting.example.net).
In order to properly secure each customer's "foo" service via
Transport Layer Security (TLS) [RFC5246], you need to obtain PKIX
certificates [RFC5280] containing identifiers such as
foo.example.com, as explained in the "CertID" specification
[RFC6125]. Unfortunately, you can't obtain such certificates
because:
o Certification authorities won't issue such certificates to you
because you work for the hosting company, not the customer
organization.
o Customers won't obtain such certificates and then give them (plus
the associated private keys) to you because their legal department
is worried about liability.
o You don't want to install such certificates (plus the associated
private keys) on your servers anyway because your legal department
is worried about liability, too.
Given your inability to deploy public keys / certificates containing
the right identifiers, your back-up approach has always been to use a
certificate containing hosting.example.net as the identifier.
However, more and more customers and end users are complaining about
warning messages in user agents and the inherent security issues
involved with taking a "leap of faith" to accept the identity
mismatch between the source domain (foo.example.com) and the
delegated domain (hosting.example.net).
This situation is both insecure and unsustainable. You have
investigated the possibility of using DNS Security [RFC4033] and DNS-
Based Authentication of Named Entities (DANE) [RFC6698] to solve the
problem. However, your customers and your operations team have told
you that it will be several years before they will be able to deploy
DNSSEC and DANE for all of your customers (because of tooling
updates, slow deployment of DNSSEC at some top-level domains, etc.).
The product managers in your company are pushing you to find a method
that can be deployed more quickly to overcome the lack of proper
server identity checking for your hosted customers.
One possible approach that your team has investigated is to ask each
customer to provide the public key / certificate for the "foo"
service at a special HTTPS URL on their website
("https://foo.example.com/.well-known/posh.foo.json" is one
possibility). This could be a public key that you generate for the
customer, but because the customer hosts it via HTTPS, any user agent
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can find that public key and check it against the public key you
provide during TLS negotiation for the "foo" service (as one added
benefit, the customer never needs to hand you a private key).
Alternatively, the customer can redirect requests for that special
HTTPS URL to an HTTPS URL at your own website, thus making it
explicit that they have delegated the "foo" service to you.
The approach sketched out above, called POSH ("PKIX Over Secure
HTTP"), is explained in the remainder of this document. While this
approach was developed for use in the Extensible Messaging and
Presence Protocol (XMPP) as a prooftype for Domain Name Associations
(DNA) [I-D.ietf-xmpp-dna], it can be applied to any non-HTTP
application protocol.
2. Terminology
This document inherits security terminology from [RFC5280]. The
terms "source domain", "derived domain", "reference identifier", and
"presented identifier" are used as defined in the "CertID"
specification [RFC6125].
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
[RFC2119].
3. Obtaining Verification Materials
Server identity checking (see [RFC6125]) involves three different
aspects:
1. A proof of the TLS server's identity (in PKIX, this takes the
form of a PKIX certificate [RFC5280]).
2. Rules for checking the certificate (which vary by application
protocol, although [RFC6125] attempts to harmonize those rules).
3. The materials that a TLS client uses to verify the TLS server's
identity or check the TLS server's proof (in PKIX, this takes the
form of chaining the end-entity certificate back to a trusted
root and performing all validity checks as described in
[RFC5280], [RFC6125], and the relevant application protocol
specification).
When POSH is used, the first two aspects remain the same: the TLS
server proves it identity by presenting a PKIX certificate [RFC5280]
and the certificate is checked according to the rules defined in the
appropriate application protocol specification (such as [RFC6120] for
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XMPP). However, the TLS client obtains the materials it will use to
verify the server's proof by retrieving a JSON document [RFC7159]
containing hashes of the PKIX certificate over HTTPS ([RFC7230] and
[RFC2818]) from a well-known URI [RFC5785].
The process for retrieving a PKIX certificate over secure HTTP is as
follows.
1. The TLS client performs an HTTPS GET at the source domain to the
path "/.well-known/posh.{servicedesc}.json". The value of
"{servicedesc}" is application-specific; see Section 8 of this
document for more details. For example, if the application
protocol is some hypothetical "Foo" service, then "{servicedesc}"
could be "foo"; thus if a Foo client were to use POSH to verify a
Foo server for the domain "foo.example.com", the HTTPS GET
request would be as follows:
GET /.well-known/posh.foo.json HTTP/1.1
Host: foo.example.com
2. The source domain HTTPS server responds in one of three ways:
* If it possesses PKIX certificate information for the requested
path, it responds as detailed in Section 3.1.
* If it has a reference to where the PKIX certificate
information can be obtained, it responds as detailed in
Section 3.2.
* If it does not have any PKIX certificate information or a
reference to such information for the requested path, it
responds with an HTTP client error status code (e.g., 404).
3.1. Source Domain Possesses PKIX Certificate Information
If the source domain HTTPS server possesses the certificate
information, it responds to the HTTPS GET with a success status code
and the message body set to a JSON document [RFC7159]; the document
is a JSON object which MUST have the following:
o A "fingerprints" field whose value is a JSON array of fingerprint
descriptors.
o An "expires" field whose value is a JSON number specifying the
number of seconds after which the TLS client ought to consider the
key information to be stale (further explained under Section 6).
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Each included fingerprint descriptor is a JSON object, where each
member name is the textual name of a hash function (as listed in
[HASH-NAMES]) and its associated value is the base 64 encoded
fingerprint hash generated using the named hash function (where the
encoding adheres to the definition in Section 4 of [RFC4648] and
where the padding bits are set to zero). Each fingerprint descriptor
MUST possess at least one named hash function.
The fingerprint hash for a given hash algorithm is generated by
performing the named hash function over the DER encoding of the PKIX
X.509 certifiate; for example, a "sha-1" fingerprint is generated by
performing the SHA-1 hash function over the DER encoding of the PKIX
certificate.
The following example illustrates the usage described above.
Example Content Response
HTTP/1.1 200 OK
Content-Type: application/json
Content-Length: 134
{
"fingerprints": [
{
"sha-1":"UpjRI/A3afKE8/AIeTZ5o1dECTY=",
"sha-256":"4/mggdlVx8A3pvHAWW5sD+qJyMtUHgiRuPjVC48N0XQ="
}
],
"expires": 604800
}
The "expires" value is a hint regarding the expiration of the keying
materials. It MUST be a non-negative integer. If no "expires" field
is included or its value is equal to 0, a TLS client SHOULD consider
these verification materials invalid. See Section 6 for how to
reconcile this "expires" field with the reference's "expires" field.
3.2. Source Domain References PKIX Certificate
If the source domain HTTPS server has a reference to the certificate
information, it responds to the HTTPS GET with a success status code
and message body set to a JSON document. The document is a JSON
object which MUST contain the following:
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o A "url" field whose value is a JSON string specifying the HTTPS
URL where TLS clients can obtain the actual certificate
information.
o An "expires" field whose value is a JSON number specifying the
number of seconds after which the TLS client ought to consider the
delegation to be stale (further explained under Section 6).
Example Reference Response
HTTP/1.1 200 Ok
Content-Type: application/json
Content-Length: 79
{
"url":"https://hosting.example.net/.well-known/posh.foo.json",
"expires":86400
}
The client performs an HTTPS GET for the URL specified in the "url"
field value. The HTTPS server for the URL to which the client has
been redirected responds to the request with a JSON document
containing fingerprints as described in Section 3.1. The content
retrieved from the "url" location MUST NOT itself be a reference
(i.e., containing a "url" field instead of a "fingerprints" field),
in order to prevent circular delegations.
Note: The JSON document returned by the source domain HTTPS server
MUST contain either a reference or a fingerprints document, but
MUST NOT contain both.
Note: See Section 9 for discussion about HTTPS redirects.
The "expires" value is a hint regarding the expiration of the source
domain's delegation of service to the delegated domain. It MUST be a
non-negative integer. If no "expires" field is included or its value
is equal to 0, a TLS client SHOULD consider the delegation invalid.
See Section 6 for guidelines about reconciling this "expires" field
with the "expires" field of the fingerprints document.
3.3. Performing Verification
The TLS client compares the PKIX information obtained from the TLS
server against each fingerprint descriptor object in the POSH
results, until a match is found or the collection of POSH
verification materials is exhausted. If none of the fingerprint
descriptor objects match the TLS server PKIX information, the TLS
client SHOULD reject the connection (however, the TLS client might
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still accept the connection if other verification schemes are
successful).
4. Secure Delegation
The delegation from the source domain to the delegated domain can be
considered secure if the certificate offered by the TLS server
matches the POSH certificate, regardless of how the POSH certificate
is obtained.
5. Order of Operations
In order for the TLS client to perform verification of reference
identifiers without potentially compromising data, POSH processes
MUST be complete before any application-level data is exchanged for
the source domain. The TLS client SHOULD perform all POSH retrievals
before opening any socket connections to the application protocol
server. For application protocols that use DNS SRV (including
queries for TLSA records in concert with SRV records as described in
[I-D.ietf-dane-srv]), the POSH processes ideally ought to be done in
parallel with resolving the SRV records and the addresses of any
targets, similar to the "happy eyeballs" approach for IPv4 and IPv6
[RFC6555].
The following diagram illustrates the possession flow:
Client Domain Server
------ ------ ------
| | |
| Request POSH | |
|------------------------->| |
| | |
| Return POSH fingerprints | |
|<-------------------------| |
| | |
| Service TLS Handshake |
|<===================================================>|
| | |
| Service Data |
|<===================================================>|
| | |
Figure 1: Order of Events for Possession Flow
While the following diagram illustrates the reference flow:
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Client Domain Server
------ ------ ------
| | |
| Request POSH | |
|------------------------->| |
| | |
| Return POSH url | |
|<-------------------------| |
| | |
| Request POSH |
|---------------------------------------------------->|
| | |
| Return POSH fingerprints |
|<----------------------------------------------------|
| | |
| Service TLS Handshake |
|<===================================================>|
| | |
| Service Data |
|<===================================================>|
| | |
Figure 2: Order of Events for Reference Flow
6. Caching Results
The TLS client MUST NOT cache results (reference or fingerprints)
indefinitely. If the source domain returns a reference, the TLS
client MUST use the lower of the two "expires" values when
determining how long to cache results (i.e., if the reference
"expires" value is lower than the fingerprints "expires" value, honor
the reference "expires" value). Once the TLS client considers the
results stale, it needs to perform the entire POSH process again
starting with the HTTPS GET to the source domain. The TLS client MAY
use a lower value than any provided in the "expires" field(s), or not
cache results at all.
The TLS client SHOULD NOT rely on HTTP caching mechanisms, instead
using the expiration hints provided in the POSH reference document or
fingerprints documents. To that end, the HTTPS servers for source
domains and derived domains SHOULD specify a 'Cache-Control' header
indicating a very short duration (e.g., max-age=60) or "no-cache" to
indicate that the response (redirect, reference, or content) is not
appropriate to cache at the HTTP level.
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7. Alternates and Roll-over
To indicate alternate PKIX certificates (such as when an existing
certificate will soon expire), the returned fingerprints document MAY
contain multiple fingerprint descriptors. The fingerprints SHOULD be
ordered with the most relevant certificate first as determined by the
application service operator (e.g., the renewed certificate),
followed by the next most relevant certificate (e.g., the certificate
soonest to expire). Here is an example:
{
"fingerprints": [
{
"sha-1":"UpjRI/A3afKE8/AIeTZ5o1dECTY=",
"sha-256":"4/mggdlVx8A3pvHAWW5sD+qJyMtUHgiRuPjVC48N0XQ"
},
{
"sha-1":"T29tGO9d7kxbfWnUaac8+5+ICLM=",
"sha-256":"otyLADSKjRDjVpj8X7/hmCAD5C7Qe+PedcmYV7cUncE="
}
],
"expires": 806400
}
8. IANA Considerations
This document registers a well-known URI [RFC5785] for protocols that
use POSH. The completed template follows.
URI suffix: posh.
Change controller: IETF
Specification document: [[ this document ]]
Related information: Because the "posh." string is merely a
prefix, protocols that use POSH need to register particular
URIs that are prefixed with the "posh." string.
Note that the registered URI is "posh." (with a trailing dot). This
is merely a prefix to be placed at the front of well-known URIs
[RFC5785] registered by protocols that use POSH, which themselves are
responsible for the relevant registrations with the IANA. The URIs
registered by such protocols SHOULD match the URI template [RFC6570]
path "/.well-known/posh.{servicedesc}.json"; that is, begin with
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"posh." and end with ".json" (indicating a media type of application/
json [RFC7159]).
For POSH-using protocols that rely on DNS SRV records [RFC2782], the
"{servicedesc}" part of the well-known URI SHOULD be
"{service}.{proto}", where the "{service}" is the DNS SRV "Service"
prepended by the underscore character "_" and the "{proto}" is the
DNS SRV "Proto" also prepended by the underscore character "_". As
an example, the well-known URI for XMPP server-to-server connections
would be "posh._xmpp-server._tcp.json" since XMPP [RFC6120] registers
a service name of "xmpp-server" and uses TCP as the underlying
transport protocol.
For other POSH-using protocols, the "{servicedesc}" part of the well-
known URI can be any unique string or identifier for the protocol,
which might be a service name registered with the IANA in accordance
with [RFC6335] or which might be an unregistered name. As an
example, the well-known URI for the mythical "Foo" service could be
"posh.foo.json".
Note: As explained in [RFC5785], the IANA registration policy
[RFC5226] for well-known URIs is Specification Required.
9. Security Considerations
This document supplements but does not supersede the security
considerations provided in specifications for application protocols
that decide to use POSH (e.g., [RFC6120] and [RFC6125] for XMPP).
Specifically, the security of requests and responses sent via HTTPS
depends on checking the identity of the HTTP server in accordance
with [RFC2818]. Additionally, the security of POSH can benefit from
other HTTP hardening protocols, such as HSTS [RFC6797] and key
pinning [I-D.ietf-websec-key-pinning], especially if the TLS client
shares some information with a common HTTPS implementation (e.g.,
platform-default web browser).
Note well that POSH is used by a TLS client to obtain the public key
of a TLS server to which it might connect for a particular
application protocol such as IMAP or XMPP. POSH does not enable a
hosted domain to transfer private keys to a hosting service via
HTTPS. POSH also does not enable a TLS server to engage in
certificate enrollment with a certification authority via HTTPS, as
is done in Enrollment over Secure Transport [RFC7030].
A web server at the source domain might redirect an HTTPS request to
another URL. The location provided in the redirect response MUST
specify an HTTPS URL. Source domains SHOULD use only temporary
redirect mechanisms, such as HTTP status codes 302 (Found) and 307
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(Temporary Redirect). Clients MAY treat any redirect as temporary,
ignoring the specific semantics for 301 (Moved Permanently) and 308
(Permanent Redirect) [RFC7238]. To protect against circular
references, clients MUST NOT follow an infinite number of redirects.
It is RECOMMENDED that clients follow no more than 10 redirects,
although applications or implementations can require that fewer
redirects be followed.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[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, May 2008.
[RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
Uniform Resource Identifiers (URIs)", RFC 5785, April
2010.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, March 2011.
[RFC7159] Bray, T., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, March 2014.
[RFC7230] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
(HTTP/1.1): Message Syntax and Routing", RFC 7230, June
2014.
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10.2. Informative References
[I-D.ietf-dane-srv]
Finch, T., Miller, M., and P. Saint-Andre, "Using DNS-
Based Authentication of Named Entities (DANE) TLSA Records
with SRV Records", draft-ietf-dane-srv-06 (work in
progress), June 2014.
[I-D.ietf-websec-key-pinning]
Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
Extension for HTTP", draft-ietf-websec-key-pinning-14
(work in progress), June 2014.
[I-D.ietf-xmpp-dna]
Saint-Andre, P. and M. Miller, "Domain Name Associations
(DNA) in the Extensible Messaging and Presence Protocol
(XMPP)", draft-ietf-xmpp-dna-05 (work in progress),
February 2014.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", RFC
4033, March 2005.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, March 2011.
[RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
Cheshire, "Internet Assigned Numbers Authority (IANA)
Procedures for the Management of the Service Name and
Transport Protocol Port Number Registry", BCP 165, RFC
6335, August 2011.
[RFC6555] Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
Dual-Stack Hosts", RFC 6555, April 2012.
[RFC6570] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
and D. Orchard, "URI Template", RFC 6570, March 2012.
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[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, August 2012.
[RFC6797] Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
Transport Security (HSTS)", RFC 6797, November 2012.
[RFC7030] Pritikin, M., Yee, P., and D. Harkins, "Enrollment over
Secure Transport", RFC 7030, October 2013.
[RFC7238] Reschke, J., "The Hypertext Transfer Protocol Status Code
308 (Permanent Redirect)", RFC 7238, June 2014.
[HASH-NAMES]
"Hash Function Textual Names",
<http://www.iana.org/assignments/hash-function-text-names/
hash-function-text-names.xhtml>.
Appendix A. Acknowledgements
Many thanks to Thijs Alkemade, Philipp Hancke, Joe Hildebrand, and
Tobias Markmann for their implementation feedback. Thanks also to
Dave Cridland, Chris Newton, Max Pritikin, and Joe Salowey for their
input on the specification.
Authors' Addresses
Matthew Miller
Cisco Systems, Inc.
1899 Wynkoop Street, Suite 600
Denver, CO 80202
USA
Email: mamille2@cisco.com
Peter Saint-Andre
&yet
P.O. Box 787
Parker, CO 80134
USA
Email: peter@andyet.com
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