TLS Working Group M. Marlinspike
Internet-Draft T. Perrin, Ed.
Intended status: Standards Track January 7, 2013
Expires: July 11, 2013
Trust Assertions for Certificate Keys
draft-perrin-tls-tack-02.txt
Abstract
This document defines a TLS Extension that enables a TLS server to
support "pinning" to a self-chosen signing key. A client contacting
a pinned host will require the server to present a signature from the
signing key over the TLS server's public key.
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
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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 July 11, 2013.
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document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements notation . . . . . . . . . . . . . . . . . . 3
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Tack life cycle . . . . . . . . . . . . . . . . . . . . . 4
2.2. Pin life cycle . . . . . . . . . . . . . . . . . . . . . . 5
3. TACK Extension . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Definition of TackExtension . . . . . . . . . . . . . . . 6
3.2. Explanation of TackExtension fields . . . . . . . . . . . 7
3.2.1. Tack fields . . . . . . . . . . . . . . . . . . . . . 7
3.2.2. TackExtension fields . . . . . . . . . . . . . . . . . 7
4. Client processing . . . . . . . . . . . . . . . . . . . . . . 8
4.1. TACK pins . . . . . . . . . . . . . . . . . . . . . . . . 8
4.2. High-level client processing . . . . . . . . . . . . . . . 8
4.3. Client processing details . . . . . . . . . . . . . . . . 9
4.3.1. Check whether the TLS handshake is valid . . . . . . . 9
4.3.2. Check tack generations and update min_generations . . 9
4.3.3. Determine the store's status . . . . . . . . . . . . . 10
4.3.4. Pin activation (optional) . . . . . . . . . . . . . . 10
5. Application protocols and TACK . . . . . . . . . . . . . . . . 12
5.1. Pin scope . . . . . . . . . . . . . . . . . . . . . . . . 12
5.2. TLS negotiation . . . . . . . . . . . . . . . . . . . . . 12
5.3. Certificate verification . . . . . . . . . . . . . . . . . 12
6. Fingerprints . . . . . . . . . . . . . . . . . . . . . . . . . 13
7. Advice . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.1. For server operators . . . . . . . . . . . . . . . . . . . 14
7.2. For client implementers . . . . . . . . . . . . . . . . . 15
8. Security considerations . . . . . . . . . . . . . . . . . . . 16
8.1. For server operators . . . . . . . . . . . . . . . . . . . 16
8.2. For client implementers . . . . . . . . . . . . . . . . . 16
8.3. Note on algorithm agility . . . . . . . . . . . . . . . . 17
9. IANA considerations . . . . . . . . . . . . . . . . . . . . . 18
9.1. New entry for the TLS ExtensionType Registry . . . . . . . 18
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
11. Normative references . . . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21
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1. Introduction
Traditionally, a TLS client verifies a TLS server's public key using
a certificate chain issued by some public CA. "Pinning" is a way for
clients to obtain increased certainty in server public keys. Clients
that employ pinning check for some constant "pinned" element of the
TLS connection when contacting a particular TLS host.
TACK allows clients to pin to a server-chosen signing key, known as a
"TACK signing key" or "TSK", which signs the server's TLS keys. This
enables pinning without limiting a site's flexibility to deploy
different certificates and TLS keys on different servers or at
different times. Since pins are based on TSKs instead of CA keys,
trust in CAs is not required. Additionally, a TSK may be used to
revoke compromised TLS private keys, and a pair of "overlapping" TSKs
may be used to quickly introduce a new TSK if an older one has become
compromised or suspect.
If requested, a compliant server will send a TLS Extension containing
its "tack". Inside the tack is a TSK public key and signature. Once
a client has seen the same (hostname, TSK) pair multiple times, the
client will "activate" a pin between the hostname and TSK for a
period equal to the length of time the pair has been observed for.
This "pin activation" algorithm limits the impact of bad pins
resulting from transient network attacks or operator error.
TACK pins are easily shared between clients. For example, a TACK
client may scan the internet to discover TACK pins, then publish
these pins through some 3rd-party trust infrastructure for other
clients to rely upon.
1.1. Requirements notation
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].
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2. Overview
2.1. Tack life cycle
A server operator using TACK may perform several processes:
Selection of a TACK signing key (TSK): The server operator first
chooses the ECDSA signing key to use for a set of hostnames. It
is safest to use a different TSK for each hostname, though a TSK
may be reused for closely-related hostnames (such as aliases for
the same host, or hosts sharing the same TLS key).
Creating initial tacks under a TSK: The TSK private key is then used
to sign the TLS public keys for all servers associated with those
hostnames. The TSK public key and signature are combined with
some metadata into each server's "tack".
Deploying initial tacks: For each hostname, tacks are deployed to
TLS servers in a two-stage process. First, each TLS server
associated with the hostname is given a tack. Once this is
completed, the tacks are activated by setting the "activation
flag" on each server.
Creating new tacks under a TSK: A tack needs to be replaced whenever
a server changes its TLS public key, or when the tack expires.
Tacks may also need to be replaced with later-generation tacks if
the TSK's "min_generation" is updated (see next).
Revoking old tacks: If a TLS private key is compromised, the tacks
signing this key can be revoked by publishing a new tack
containing a higher "min_generation".
Deactivating tacks: If a server operator wishes to stop deploying
tacks, all tacks for a hostname can be deactivated via the
activation flag, allowing the server to remove the tacks within 30
days (at most).
Overlapping tacks: If a server operator wishes to change the TSK a
hostname is pinned to, the server can publish a new tack alongside
the old one. This lets clients activate pins for the new TSK
prior to the server deactivating the older pins.
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2.2. Pin life cycle
A TACK pin associates a hostname and a TSK. Pins are grouped into
"pin stores". A client may populate its pin stores by either
performing "pin activation" directly, or by querying some other
party. For example, a client application may have a store for pin
activation as well as a store whose contents are periodically fetched
from a server.
Whenever a client performing "pin activation" sees a hostname and TSK
combination not represented in the "pin activation" pin store, an
inactive pin is created. Every subsequent time the client sees the
same pin, the pin is "activated" for a period equal to the timespan
between the first time the pin was seen and the most recent time, up
to a maximum period of 30 days.
A pin store may contain up to two pins per hostname. This allows for
overlapping pins when a server is securely transitioning from one pin
to another. If both pins are simultaneously active, then the server
must satisfy both of them by presenting a pair of tacks.
In addition to creating and activating pins, a TLS connection can
alter client pin stores by publishing new "min_generation" values in
a tack. Each pin stores the highest "min_generation" value it has
seen from the pinned TSK, and rejects tacks from earlier generations.
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3. TACK Extension
3.1. Definition of TackExtension
A new TLS ExtensionType ("tack") is defined and MAY be included by a
TLS client in the ClientHello message defined in [RFC5246].
enum {tack(TBD), (65535)} ExtensionType;
The "extension_data" field of this ClientHello extension SHALL be
empty. A TLS server which is not resuming a TLS session MAY respond
with an extension of type "tack" in the ServerHello. The
"extension_data" field of this ServerHello extension SHALL contain a
"TackExtension", as defined below using the TLS presentation language
from [RFC5246].
struct {
opaque public_key[64];
uint8 min_generation;
uint8 generation;
uint32 expiration;
opaque target_hash[32];
opaque signature[64];
} Tack; /* 166 bytes */
struct {
Tack tacks<166...332> /* 1 or 2 tacks */
uint8 activation_flags;
} TackExtension; /* 169 or 335 bytes */
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3.2. Explanation of TackExtension fields
3.2.1. Tack fields
public_key: Specifies the public key of the TSK that has signed this
tack. The field contains a pair of integers (x, y) representing a
point on the elliptic curve P-256 defined in [FIPS186-3]. Each
integer is encoded as a 32-byte octet string using the Integer-to-
Octet-String algorithm from [RFC6090], and these strings are
concatenated with the x value first. (NOTE: This is equivalent to
an uncompressed subjectPublicKey from [RFC5480], except that the
initial 0x04 byte is omitted).
min_generation: Publishes a min_generation for the tack's TSK.
generation: Assigns each tack a generation. Generations less than
the highest published min_generation for the tack's TSK are
considered revoked.
expiration: Specifies a time after which the tack is considered
expired. The time is encoded as the number of minutes, excluding
leap seconds, after midnight UTC, January 1 1970.
target_hash: A hash of the TLS server's SubjectPublicKeyInfo
[RFC5280] using the SHA256 algorithm from [FIPS180-2]. The
SubjectPublicKeyInfo is typically conveyed as part of the server's
X.509 end-entity certificate.
signature: An ECDSA signature by the tack's TSK over the 8 byte
ASCII string "tack_sig" followed by the contents of the tack prior
to the "signature" field (i.e. the preceding 102 bytes). The
field contains a pair of integers (r, s) representing an ECDSA
signature as defined in [FIPS186-3], using curve P-256 and SHA256.
Each integer is encoded as a 32-byte octet string using the
Integer-to-Octet-String algorithm from [RFC6090], and these
strings are concatenated with the r value first.
3.2.2. TackExtension fields
tacks: This field provides the server's tack(s). It SHALL contain 1
or 2 tacks.
activation_flags: This field contains "activation flags" for the
extension's tacks. If the low order bit is set, the first tack is
considered active. If the next lowest bit is set, the second tack
is considered active. All other bits are reserved for future use
and MUST be ignored by clients.
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4. Client processing
4.1. TACK pins
A client SHALL have a local store of pins, and MAY have multiple
stores. Each pin store consists of a map associating fully qualified
DNS hostnames with either one or two sets of the following values:
Initial time: A timestamp noting when this pin was created.
End time: A timestamp determining the pin's "active period". If set
to zero or a time in the past, the pin is "inactive". If set to a
future time, the pin is "active" until that time.
TSK public key (or hash): A public key or a cryptographically-
secure, second preimage-resistant hash of a public key.
Min_generation: A single byte used to detect revoked tacks. All
pins within a pin store sharing the same TSK SHALL have the same
min_generation.
A hostname along with the above values comprises a "TACK pin". Thus,
each store can hold up to two pins for a hostname (however, those two
pins MUST reference different public keys). A pin "matches" a tack
if they reference the same public key. A pin is "relevant" if its
hostname equals the TLS server's hostname.
4.2. High-level client processing
A TACK client SHALL send the "tack" extension defined previously, and
SHALL send the "server_name" extension from [RFC6066]. If not
resuming a session, the server MAY respond with a TackExtension.
Regardless of whether a TackExtension is returned, the client SHALL
perform the following steps prior to using the connection:
1. Check whether the TLS handshake is "valid".
2. For each pin store, do:
A. Check tack generations and update min_generations.
B. Determine the store's status.
C. Perform pin activation (optional).
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These steps SHALL be performed in order. If there is any error, the
client SHALL send a fatal error alert and close the connection,
skipping the remaining steps (see Section 4.3 for details).
Based on step 2B, each store will report one of three statuses for
the connection: "confirmed", "contradicted", or "unpinned". A
contradicted connection might indicate a network attack. How the
client responds to confirmed or contradicted connections is left to
other specifications and client policy (see Section 5.3 for an
example). If a client closes a connection due to a contradicting
pin, the client SHALL send an "access_denied" alert.
4.3. Client processing details
4.3.1. Check whether the TLS handshake is valid
A TLS handshake is "valid" if the following are true. Unless
otherwise specified, if any of the following are false a
"bad_certificate" fatal error alert SHALL be sent.
1. The handshake protocol negotiates a cryptographically secure
ciphersuite and finishes succesfully.
2. If a TackExtension is present then all length fields are correct
and the tacks are "valid" (see below).
3. If there are two tacks, they have different "public_key" fields.
A tack is "valid" if:
1. "generation" is >= "min_generation".
2. "expiration" specifies a time in the future, otherwise the client
SHALL send a fatal "certificate_expired" error alert.
3. "target_hash" is a correct hash of the SubjectPublicKeyInfo.
4. "signature" is a correct ECDSA signature.
4.3.2. Check tack generations and update min_generations
If a tack has matching pins in the pin store and a generation less
than the stored min_generation, then that tack is revoked and the
client SHALL send a fatal "certificate_revoked" error alert. If a
tack has matching pins and a min_generation greater than the stored
min_generation, the stored value SHALL be set to the tack's value.
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4.3.3. Determine the store's status
If there is a relevant active pin without a matching tack, then the
connection is "contradicted". If the connection is not contradicted
and there is a relevant active pin with a matching tack, then the
connection is "confirmed". Otherwise, the connection is "unpinned".
4.3.4. Pin activation (optional)
The TLS connection MAY be used to create, delete, and activate pins.
This "pin activation algorithm" is optional; a client MAY rely on an
external source of pins. If the connection was "contradicted" by the
previous processing step, then pin activation is skipped.
The first step is to evaluate each of the (0, 1, or 2) relevant pins:
1. If a pin has no matching tack, its handling will depend on
whether the pin is active. If active, the connection will have
been contradicted, skipping pin activation. If inactive, the pin
SHALL be deleted.
2. If a pin has a matching tack, its handling will depend on whether
the tack is active. If inactive, the pin is left unchanged. If
active, the pin SHALL have its "end time" set based on the
current, initial, and end times:
end = current + MIN(30 days, current - initial)
In sum: (1) deletes unmatched inactive pins, and (2) activates
matched pins with active tacks.
The remaining step is to add new inactive pins for any unmatched
active tacks. Each new pin uses the server's hostname, the tack's
public key and min_generation (unless the store has a higher
min_generation for the public key), an "initial time" set to the
current time, and an "end time" of zero. (Note that there are always
sufficient empty "slots" in the pin store for adding new pins without
exceeding two pins per hostname.)
The following tables summarize this behavior from the perspective of
a pin. You can follow the lifecycle of a single pin from "New
inactive pin" to "Delete pin".
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Relevant pin is active:
+--------------------+----------------+---------------------------+
| Pin matches a tack | Tack is active | Result |
+--------------------+----------------+---------------------------+
| Yes | Yes | Extend activation period |
| | | |
| Yes | No | - |
| | | |
| No | - | (Connection contradicted) |
+--------------------+----------------+---------------------------+
Relevant pin is inactive:
+--------------------+----------------+--------------+
| Pin matches a tack | Tack is active | Result |
+--------------------+----------------+--------------+
| Yes | Yes | Activate pin |
| | | |
| Yes | No | - |
| | | |
| No | - | Delete pin |
+--------------------+----------------+--------------+
Tack doesn't match any relevant pin:
+--------------------------+------------------+
| Unmatched tack is active | Result |
+--------------------------+------------------+
| Yes | New inactive pin |
| | |
| No | - |
+--------------------------+------------------+
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5. Application protocols and TACK
5.1. Pin scope
TACK pins are specific to a particular application protocol. In
other words, a pin for HTTPS at "example.com" implies nothing about
POP3 or SMTP at "example.com".
5.2. TLS negotiation
Some application protocols negotiate TLS as an optional feature (e.g.
SMTP using STARTTLS [RFC3207]). If such a server does not negotiate
TLS and there are relevant active pins, then the connection is
contradicted by the pin. If a client is performing pin activation
for a pin store and the server does not negotiate TLS, then any
relevant, inactive pins SHALL be deleted. Note that these steps are
taken despite the absence of a TLS connection.
5.3. Certificate verification
A TACK client MAY choose to perform some form of certificate
verification in addition to TACK processing. When combining
certificate verification and TACK processing, the TACK processing
described in Section 4 SHALL be followed, with the exception that
TACK processing MAY be terminated early (or skipped) if some fatal
certificate error is discovered.
If TACK processing and certificate verification both complete without
a fatal error, then client behavior is left to other specifications
and client policy. An example client policy would be to allow the
connection to proceed only if it passes certificate verification and
is not contradicted by a pin.
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6. Fingerprints
A "key fingerprint" may be used to represent a TSK public key to
users in a form that is easy to compare and transcribe. A key
fingerprint consists of the first 25 characters from the base32
encoding of SHA256(public_key), split into 5 groups of 5 characters
separated by periods. Base32 encoding is as specified in [RFC4648],
except lowercase is used. Examples:
g5p5x.ov4vi.dgsjv.wxctt.c5iul
quxiz.kpldu.uuedc.j5znm.7mqst
e25zs.cth7k.tscmp.5hxdp.wf47j
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7. Advice
7.1. For server operators
Key reuse: All servers that are pinned to a single TSK are able to
impersonate each other's hostnames, since clients will perceive
their tacks as equivalent. Thus, TSKs SHOULD NOT be reused with
different hostnames unless these hostnames are closely related.
Examples where it would be safe to reuse a TSK are hostnames
aliased to the same host, hosts sharing the same TLS key, or
hostnames for a group of near-identical servers.
Aliases: A TLS server might be referenced by multiple hostnames.
Clients might pin any of these hostnames. Server operators should
be careful when using DNS aliases that hostnames are not pinned
inadvertently.
Generations: To revoke older generations of tacks, the server
operator SHOULD first provide all servers with a new generation of
tacks, and only then provide servers with new tacks containing the
new min_generation. Otherwise, a client might receive a
min_generation update from one server but then try to contact an
older-generation server which has not yet been updated.
Tack expiration: When TACK is used in conjunction with certificates
it is recommended to set the tack expiration equal to the end-
entity certificate's expiration (or a later date), so that the
tack and certificate can both be replaced at the same time.
Alternatively, short-lived tacks MAY be used so that a compromised
TLS private key has limited value to an attacker.
Nonrevokable tacks A tack with generation 255 cannot be revoked.
Such tacks MAY be used to minimize the risk that a compromised TSK
private key could be used to affect site availability.
Tack/pin activation: Tacks should only be activated once all TLS
servers sharing the same hostname have a tack. Otherwise, a
client may activate a pin by contacting one server, then contact a
different server at the same hostname that does not yet have a
tack.
Tack/pin deactivation: If all servers at a hostname deactivate their
tacks (by clearing the activation flags), all existing pins for
the hostname will eventually become inactive. The tacks can be
removed after a time interval equal to the maximum active period
of any affected pins (30 days at most).
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Tack/pin overlap: When publishing overlapping tacks, the old and new
tacks SHOULD be active simultaneously for at least 60 days. This
ensures that any pin activation client who is contacting the
server at intervals of 30 days or less will not have its
activation periods interrupted. Example process: activate new
tacks; wait 60 days; deactivate old tacks; wait 30 days; remove
old tacks.
7.2. For client implementers
Sharing pin information: It is possible for a client to maintain a
pin store based entirely on its own TLS connections. However,
such a client runs the risk of creating incorrect pins, failing to
keep its pins active, or failing to receive min_generation
updates. Clients are advised to make use of 3rd-party trust
infrastructure so that pin data can be aggregated and shared.
This will require additional protocols outside the scope of this
document.
Clock synchronization: A client SHOULD take measures to prevent
tacks from being erroneously rejected as expired due to an
inaccurate client clock. Such methods MAY include using time
synchronization protocols such as NTP [RFC5905], or accepting
seemingly-expired tacks as valid if they expired less than T
minutes ago, where T is a "tolerance bound" set to the client's
maximum expected clock error.
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8. Security considerations
8.1. For server operators
All servers pinned to the same TSK can impersonate each other (see
Section 7.1). Think carefully about this risk if using the same TSK
for multiple hostnames.
Make backup copies of the TSK private key and keep all copies in
secure locations where they can't be compromised.
A TSK private key MUST NOT be used to perform any non-TACK
cryptographic operations. For example, using a TSK for email
encryption, code-signing, or any other purpose MUST NOT be done.
HTTP cookies [RFC6265] set by a pinned host can be stolen by a
network attacker who can forge web and DNS responses so as to cause a
client to send the cookies to a phony subdomain of the pinned host.
To prevent this, TACK HTTPS servers SHOULD set the "secure" attribute
and omit the "domain" attribute on all security-sensitive cookies,
such as session cookies. These settings tell the browser that the
cookie should only be presented back to the originating host (not its
subdomains), and should only be sent over HTTPS (not HTTP) [RFC6265].
8.2. For client implementers
A TACK pin store may contain private details of the client's
connection history. An attacker may be able to access this
information by hacking or stealing the client. Some information
about the client's connection history could also be gleaned by
observing whether the client accepts or rejects connections to phony
TLS servers without correct tacks. To mitigate these risks, a TACK
client SHOULD allow the user to edit or clear the pin store.
Aside from possibly rejecting TLS connections, clients SHOULD NOT
take any actions which would reveal to a network observer the state
of the client's pin store, as this would allow an attacker to know in
advance whether a "man-in-the-middle" attack on a particular TLS
connection will succeed or be detected.
An attacker may attempt to flood a client with spurious tacks for
different hostnames, causing the client to delete old pins to make
space for new ones. To defend against this, clients SHOULD NOT
delete active pins to make space for new pins. Clients instead
SHOULD delete inactive pins. If there are no inactive pins to
delete, then the pin store is full and there is no space for new
pins. To select an inactive pin for deletion, the client SHOULD
delete the pin with the oldest "end time".
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8.3. Note on algorithm agility
If the need arises for tacks using different cryptographic algorithms
(e.g., if SHA256 or ECDSA are shown to be weak), a "v2" version of
tacks could be defined, requiring assignment of a new TLS Extension
number. Tacks as defined in this document would then be known as
"v1" tacks.
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9. IANA considerations
9.1. New entry for the TLS ExtensionType Registry
IANA is requested to add an entry to the existing TLS ExtensionType
registry, defined in [RFC5246], for "tack"(TBD) as defined in this
document.
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10. Acknowledgements
Valuable feedback has been provided by Adam Langley, Chris Palmer,
Nate Lawson, and Joseph Bonneau.
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11. Normative references
[FIPS180-2]
National Institute of Standards and Technology, "Secure
Hash Standard", FIPS PUB 180-2, August 2002, <http://
csrc.nist.gov/publications/fips/fips180-2/fips180-2.pdf>.
[FIPS186-3]
National Institute of Standards and Technology, "Digital
Signature Standard", FIPS PUB 186-3, June 2009, <http://
csrc.nist.gov/publications/fips/fips186-3/fips_186-3.pdf>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, February 2002.
[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.
[RFC5480] Turner, S., Brown, D., Yiu, K., Housley, R., and T. Polk,
"Elliptic Curve Cryptography Subject Public Key
Information", RFC 5480, March 2009.
[RFC5905] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, June 2010.
[RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions:
Extension Definitions", RFC 6066, January 2011.
[RFC6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
Curve Cryptography Algorithms", RFC 6090, February 2011.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
April 2011.
Marlinspike & Perrin Expires July 11, 2013 [Page 20]
Internet-Draft Trust Assertions for Certificate Keys January 2013
Authors' Addresses
Moxie Marlinspike
Trevor Perrin (editor)
Email: tack@trevp.net
Marlinspike & Perrin Expires July 11, 2013 [Page 21]