Negotiated Discrete Log Diffie-Hellman Ephemeral Parameters for TLS
draft-gillmor-tls-negotiated-dl-dhe-00
The information below is for an old version of the document.
| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Author | Daniel Kahn Gillmor | ||
| Last updated | 2014-03-28 | ||
| Replaced by | draft-ietf-tls-negotiated-dl-dhe | ||
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draft-gillmor-tls-negotiated-dl-dhe-00
Internet Engineering Task Force D. Gillmor
Internet-Draft ACLU
Intended status: Informational March 28, 2014
Expires: September 29, 2014
Negotiated Discrete Log Diffie-Hellman Ephemeral Parameters for TLS
draft-gillmor-tls-negotiated-dl-dhe-00
Abstract
Traditional discrete logarithm-based Diffie-Hellman (DH) key exchange
during the TLS handshake suffers from a number of security,
interoperability, and efficiency shortcomings. These shortcomings
arise from lack of clarity about which DH group parameters TLS
servers should offer and clients should accept. This document offers
a solution to these shortcomings for compatible peers by establishing
a registry of DH parameters with known structure and a mechanism for
peers to indicate support for these groups.
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 September 29, 2014.
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
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Client Behavior . . . . . . . . . . . . . . . . . . . . . . . 3
3. Server Behavior . . . . . . . . . . . . . . . . . . . . . . . 4
4. Optimizations . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Checking the Peer's Public Key . . . . . . . . . . . . . 5
4.2. Short Exponents . . . . . . . . . . . . . . . . . . . . . 5
4.3. Table Acceleration . . . . . . . . . . . . . . . . . . . 6
5. Open Questions . . . . . . . . . . . . . . . . . . . . . . . 6
5.1. Server Indication of support . . . . . . . . . . . . . . 6
5.2. Normalizing Weak Groups . . . . . . . . . . . . . . . . . 7
5.3. Arbitrary Groups . . . . . . . . . . . . . . . . . . . . 7
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8.1. Negotiation resistance to active attacks . . . . . . . . 8
8.2. DHE only . . . . . . . . . . . . . . . . . . . . . . . . 8
8.3. Client fingerprinting . . . . . . . . . . . . . . . . . . 9
8.4. Deprecating weak groups . . . . . . . . . . . . . . . . . 9
8.5. Choice of groups . . . . . . . . . . . . . . . . . . . . 9
8.6. Timing attacks . . . . . . . . . . . . . . . . . . . . . 10
8.7. Replay attacks from non-negotiated DL DHE . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Normative References . . . . . . . . . . . . . . . . . . 10
9.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Named Group Registry . . . . . . . . . . . . . . . . 11
A.1. dldhe2432 . . . . . . . . . . . . . . . . . . . . . . . . 11
A.2. dldhe3072 . . . . . . . . . . . . . . . . . . . . . . . . 12
A.3. dldhe4096 . . . . . . . . . . . . . . . . . . . . . . . . 13
A.4. dldhe6144 . . . . . . . . . . . . . . . . . . . . . . . . 13
A.5. dldhe8192 . . . . . . . . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
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Traditional TLS [RFC5246] offers a Diffie-Hellman ephemeral (DHE) key
exchange mode which provides Perfect Forward Secrecy for the
connection. The client offers a ciphersuite in the ClientHello that
includes DHE, and the server offers the client group parameters g and
p. If the client does not consider the group strong enough (e.g. if
p is too small, or if p is not prime, or there are small subgroups),
or if it is unable to process it for other reasons, it has no
recourse but to terminate the connection.
Conversely, when a TLS server receives a suggestion for a DHE
ciphersuite from a client, it has no way of knowing what kinds of DH
groups the client is capable of handling, or what the client's
security requirements are for this key exchange session. Some
widely-distributed TLS clients are not capable of DH groups where p >
1024. Other TLS clients may by policy wish to use DHE only if the
server can offer a stronger group (and are willing to use a non-PFS
key-exchange mechanism otherwise). The server has no way of knowing
which type of client is connecting, but must select DHE parameters
with insufficient knowledge.
Additionally, the DH parameters chosen by the server may have a known
structure which renders them secure against small subgroup attack,
but a client receiving an arbitrary p has no efficient way to verify
that the structure of a new group is reasonable for use.
This extension solves these problems with a registry of groups of
known reasonable structure, an extension for clients to advertise
support for them and servers to select them, and guidance for
compliant peers to take advantage of the additional security,
availability, and efficiency offered.
The use of this extension by one compliant peer when interacting with
a non-compliant peer should have no detrimental effects.
1.1. Requirements Language
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].
2. Client Behavior
A TLS client that is capable of using strong discrete log Diffie-
Hellman groups can advertise its capabilities and its preferences for
stronger key exchange by using this mechanism.
The client SHOULD send an extension of type
"negotiated_dl_dhe_groups" in the ClientHello, indicating an list of
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known discrete log Diffie-Hellman groups, ordered from most preferred
to least preferred.
The "extension_data" field of this extension SHALL contain
"DiscreteLogDHEGroups" where:
enum {
dldhe2432(0), dldhe3072(1), dldhe4096(2),
dldhe6144(3), dldhe8192(4), (255)
} DiscreteLogDHEGroup;
struct {
DiscreteLogDHEGroup discrete_log_dhe_group_list<1..2^8-1>;
} DiscreteLogDHEGroups;
A client that offers this extension SHOULD include at least one DHE-
key-exchange ciphersuite in the Client Hello.
The known groups defined by the DiscreteLogDHEGroup registry are
listed in Appendix A. These are all safe primes, designed to be
sparse, and with the high and low 64 bits set to 1 for efficient
Montgomery or Barrett reduction.
A client who offers a group MUST be able and willing to perform a DH
key exchange using that group.
3. Server Behavior
A TLS server MUST NOT send the NegotiatedDHParams extension to a
client that does not offer it first.
A compatible TLS server that receives this extension from a client
SHOULD NOT select a DHE ciphersuite if it is unwilling to use one of
the DH groups named by the client. In this case, it SHOULD select an
acceptable non-DHE ciphersuite from the client's offered list. If
the extension is present, none of the client's offered groups are
acceptable by the server, and none of the client's proposed non-DHE
ciphersuites are acceptable to the server, the server SHOULD end the
connection with a fatal TLS alert of type insufficient_security.
A compatible TLS server that receives this extension from a client
and selects a DHE-key-exchange ciphersuite selects one of the offered
groups and indicates it to the client in the ServerHello by sending a
"negotiated_dl_dhe_groups" extension. The "extension_data" field of
this extension on the server side should be a single one-byte value
DiscreteLogDHEGroup.
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A TLS server MUST NOT select a named group that was not offered by
the client.
When the server sends the "negotiated_dl_dhe_groups" extension in the
ServerHello, the ServerDHParams member of the subsequent
ServerKeyExchange message should indicate a one-byte zero value (0)
in place of dh_g and the identifier of the named group in place of
dh_p, represented as a one-byte value. dh_Ys must be transmitted as
normal.
This re-purposing of dh_p and dh_g is unambiguous: there are no
groups with a generator of 0, and no implementation should accept a
modulus of size < 9 bits. This change serves two purposes:
The size of the handshake is is reduced (significantly, in the
case of a large prime modulus).
The signed struct should not be re-playable in a subsequent key
exchange that does not indicate named DH groups.
4. Optimizations
In a successfully negotiated discrete log DH group key exchange, both
peers know that the group in question uses a safe prime as a modulus,
and that the group in use is of size p-1 or (p-1)/2. This allows at
least two optimizations that can be used to improve performance.
4.1. Checking the Peer's Public Key
Peers should validate the each other's public key Y (dh_Ys offered by
the server or DH_Yc offered by the client) by ensuring that 1 < Y <
p-1. This simple check ensures that the remote peer is properly
behaved and isn't forcing the local system into a small subgroup.
To reach the same assurance with an unknown group, the client would
need to verify the primality of the modulus, learn its factors, and
test Y against each of its factors.
4.2. Short Exponents
Traditional Discrete Log Diffie-Hellman has each peer choose their
secret exponent from the range [2,p-2]. Using exponentiation by
squaring, this means each peer must do roughly 2*log_2(p)
multiplications, twice (once for the generator and once for the
peer's public key).
Peers concerned with performance may also prefer to choose their
secret exponent from a smaller range, doing fewer multiplications,
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while retaining the same level of overall security. Each named group
indicates its approximate security level, and provides a lower-bound
on the range of secret exponents that should preserve it. For
example, rather than doing 2*2*2048 multiplications for a dldhe2048
handshake, each peer can choose to do 2*2*224 multiplications by
choosing their secret exponent in the range [2,2^224] and still keep
the approximate 112-bit security level.
A similar short-exponent approach is used in SSH's Diffie-Hellman key
exchange (See section 6.2 of [RFC4419]).
4.3. Table Acceleration
Peers wishing to further accelerate DHE key exchange can also pre-
compute a table of powers of the generator of a known group. This is
a memory vs. time tradeoff, and it only accelerates the first
exponentiation of the ephemeral DH exchange (the exponentiation using
the peer's public exponent as a base still needs to be done as
normal).
5. Open Questions
[This section should be removed, and questions resolved, before any
formalization of this draft]
5.1. Server Indication of support
Some servers will support this extension, but for whatever reason
decide to not negotiate a ciphersuite with DHE key exchange at all.
Some possible reasons include:
The client indicated that a server-supported non-DHE ciphersuite
was preferred over all DHE ciphersuites, and the server honors
that preference.
The server prefers a client-supported non-DHE ciphersuite over all
DHE ciphersuites, and selects it unilaterally.
The server would have chosen a DHE ciphersuite, but none of the
client's offered groups are acceptable to the server,
Clients will not know that such a server supports the extension.
Should we offer a way for a server to indicate its support for this
extension to a compatible client in this case?
Should the server have a way to advertise that it supports this
extension even if the client does not offer it?
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5.2. Normalizing Weak Groups
Is there any reason to include a weak group in the list of groups?
Most DHE-capable peers can already handle 1024-bit DHE, and therefore
1024-bit DHE does not need to be negotiated. Properly-chosen
2432-bit DH groups should be roughly equivalent to 112-bit security.
And future implementations should use sizes of at least 3072 bits
according to [ENISA].
5.3. Arbitrary Groups
This spec currently doesn't indicate any support for groups other
than the named groups. Other DHE specifications have moved away from
staticly-named groups with the explicitly-stated rationale of
reducing the incentive for precomputation-driven attacks on any
specific group (e.g. section 1 of [RFC4419]). However, arbitrary
large groups are expensive to transmit over the network and it is
computationally infeasible for the client to verify their structure
during a key exchange. If we instead allow the server to propose
arbitrary groups, we could make it a MUST that the generated groups
use safe prime moduli, while still allowing clients to signal support
(and desire) for large groups. This leaves the client in the
position of relying on the server to choose a strong modulus, though.
Note that in at least one known attack against TLS
[SECURE-RESUMPTION], a malicious server uses a deliberately broken
discrete log DHE group to impersonate the client to a different
server.
6. Acknowledgements
Thanks to Tom Ritter and Nikos Mavrogiannopolous and Niels Moeller
and Kenny Paterson for their comments and suggestions on the idea for
this draft. Any mistakes here are not theirs.
7. IANA Considerations
This document defines a new TLS extension, "negotiated_dh_group",
assigned a value of XXX from the TLS ExtensionType registry defined
in section 12 of [RFC5246]. This value is used as the extension
number for the extensions in both the client hello message and the
server hello message.
This extension also defines a registry of TLS named Discrete Log DH
groups, derived initially from some of the IKE DH groups [RFC3526],
indicating the advised strength of each group and whether it is
recommended for use in TLS. These recommendations may be updated by
future revisions.
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8. Security Considerations
8.1. Negotiation resistance to active attacks
Because the contents of this extension is hashed in the finished
message, an active MITM that tries to filter or omit groups will
cause the handshake to fail, but possibly not before getting the peer
to do something they would not otherwise have done.
An attacker who impersonates the server can try to do the following:
Pretend that a non-compatible server is actually capable of this
extension, and select a group from the client's list, causing the
client to select a group it is willing to negotiate. It is
unclear how this would be an effective attack.
Pretend that a compatible server is actually non-compatible by
negotiating a non-DHE ciphersuite. This is no different than MITM
ciphersuite filtering.
Pretend that a compatible server is actually non-compatible by
negotiating a DHE ciphersuite and no extension, with an explicit
(perhaps weak) group chosen by the server. [XXX what are the
worst consequences in this case? What might the client leak
before it notices that the handshake fails? XXX]
An attacker who impersonates the client can try to do the following:
Pretend that a compatible client is not compliant (e.g. by not
offering this extension). This could cause the server to
negotiate a weaker DHE group during the handshake, but it would
fail to complete during the final check of the Finished message.
Pretend that a non-compatible client is compatible. It is not
clear how this could be an attack.
Change the list of groups offered by the client (e.g. by removing
the stronger of the set of groups offered). This could cause the
server to negotiate a weaker group than desired, but again should
be caught by the check in the Finished message.
8.2. DHE only
Note that this extension specifically targets only discrete log-based
Diffie-Hellman ephemeral key exchange mechanisms. It does not cover
the non-ephemeral DH key exchange mechanisms, nor does it cover
elliptic curve-based DHE key exchange, which has its own list of
named groups.
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8.3. Client fingerprinting
This extension provides a few additional bits of information to
distinguish between classes of TLS clients (see e.g.
[PANOPTICLICK]). To minimize this sort of fingerprinting, clients
SHOULD support all named groups at or above their minimum security
threshhold. New named groups SHOULD NOT be added to the registry
without consideration of the cost of browser fingerprinting.
8.4. Deprecating weak groups
Advances in hardware or in discrete log cryptanalysis may cause some
of the negotiated groups to not provide the desired security margins,
as indicated by number of years to protect the premaster secret (and
therefore the confidentiality and integrity of the TLS session)
against a powerful adversary.
Revisions of this extension or updates should mark known-weak groups
as explicitly deprecated, and implementations that require strong
confidentiality and integrity guarantees should avoid using any
deprecated groups.
8.5. Choice of groups
Other lists of named discrete log Diffie-Hellman groups
[STRONGSWAN-IKE] exist. This draft chooses to not reuse them for
several reasons:
Using the same groups in multiple protocols increases the value
for an attacker with the resources to crack any single group.
The IKE groups include weak groups like MODP768 which are
unacceptable for secure TLS traffic.
The IKE groups do not have sparse moduli, which makes modular
exponentiation less efficient.
Mixing group parameters across multiple implementations leaves
open the possibility of some sort of cross-protocol attack. This
shouldn't be relevant for ephemeral scenarios, and even with non-
ephemeral keying, services shouldn't reuse keys; however, using
different groups avoids these failure modes entirely.
The DL DHE groups are not collected in a single IANA registry, or
are mixed with non-DL DHE groups, which makes them inconvenient
for re-use in TLS.
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8.6. Timing attacks
Any implementation of discrete log Diffie-Hellman key exchange should
use constant-time modular-exponentiation implementations. This is
particularly true for those implementations that ever re-use DHE
parameters (so-called "semi-static" ephemeral keying).
8.7. Replay attacks from non-negotiated DL DHE
[SECURE-RESUMPTION] shows a malicious peer using a bad DL DHE group
to maneuver a client into selecting a pre-master secret of the peer's
choice, which can be replayed to another server using a non-DHE key
exchange, and can then be bootstrapped to replay client
authentication.
To prevent this attack (barring the fixes proposed in
[SESSION-HASH]), a client would need not only to implement this
draft, but also to reject non-negotiated DL DHE ciphersuites whose
group structure it cannot afford to verify. Such a client would need
to abort the initial handshake and reconnect to the server in
question without listing any DL DHE ciphersuites on the subsequent
connection.
This tradeoff may be too costly for most TLS clients today, but may
be a reasonable choice for clients performing client certificate
authentication, or who have other reason to be concerned about
server-controlled pre-master secrets.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
9.2. Informative References
[ENISA] European Union Agency for Network and Information Security
Agency, "Algorithms, Key Sizes and Parameters Report,
version 1.0", October 2013, <http://www.enisa.europa.eu/
activities/identity-and-trust/library/deliverables/
algorithms-key-sizes-and-parameters-report>.
[PANOPTICLICK]
Electronic Frontier Foundation, "Panopticlick: How Unique
- and Trackable - Is Your Browser?", 2010, <https://
panopticlick.eff.org/>.
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[RFC3526] Kivinen, T. and M. Kojo, "More Modular Exponential (MODP)
Diffie-Hellman groups for Internet Key Exchange (IKE)",
RFC 3526, May 2003.
[RFC4419] Friedl, M., Provos, N., and W. Simpson, "Diffie-Hellman
Group Exchange for the Secure Shell (SSH) Transport Layer
Protocol", RFC 4419, March 2006.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[SECURE-RESUMPTION]
Delignat-Lavaud, A., Bhargavan, K., and A. Pironti,
"Triple Handshakes Considered Harmful: Breaking and Fixing
Authentication over TLS", March 2014, <https://secure-
resumption.com/>.
[SESSION-HASH]
Bhargavan, K., Delignat-Lavaud, A., Pironti, A., Langley,
A., and M. Ray, "Triple Handshakes Considered Harmful:
Breaking and Fixing Authentication over TLS", March 2014,
<https://secure-resumption.com/draft-bhargavan-tls-
session-hash-00.txt>.
[STRONGSWAN-IKE]
Brunner, T. and A. Steffen, "Diffie Hellman Groups in
IKEv2 Cipher Suites", October 2013, <https://
wiki.strongswan.org/projects/strongswan/wiki/
IKEv2CipherSuites#Diffie-Hellman-Groups>.
Appendix A. Named Group Registry
A.1. dldhe2432
The 2432-bit group has registry value 0, and is calcluated from the
following formula:
The modulus is: p = 2^2432 - 2^2368 + 2332920 * 2^64 - 1
Its hexadecimal representation is:
FFFFFFFF FFFFFFFF 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
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00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 002398F7 FFFFFFFF FFFFFFFF
The generator is: g = 2
The group size is (p-1)/2
Peers using dldhe2432 that want to optimize their key exchange with a
short exponent (Section 4.2) should choose a secret key of at least
224 bits.
A.2. dldhe3072
The 3072-bit prime has registry value 1, and is calcluated from the
following formula:
p = 2^3072 - 2^3008 + 425754 * 2^64 -1
Its hexadecimal representation is:
FFFFFFFF FFFFFFFF 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00067F19 FFFFFFFF FFFFFFFF
The generator is: g = 2
The group size is: (p-1)/2
Peers using dldhe3072 that want to optimize their key exchange with a
short exponent (Section 4.2) should choose a secret key of at least
256 bits.
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A.3. dldhe4096
The 4096-bit group has registry value 2, and is calcluated from the
following formula:
The modulus is: p = 2^4096 - 2^4032 + 341664 * 2^64 - 1
Its hexadecimal representation is:
FFFFFFFF FFFFFFFF 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000 00000000 0005369F
FFFFFFFF FFFFFFFF
The base is: g = 2
The group size is: (p-1)/2
Peers using dldhe4096 that want to optimize their key exchange with a
short exponent (Section 4.2) should choose a secret key of at least
XXX bits.
A.4. dldhe6144
The 6144-bit group has registry value 3, and is calcluated from the
following formula:
The modulus is: p = 2^6144 - 2^6080 + XXX * 2^64 - 1
Its hexadecimal representation is:
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XXX ...still calculating... XXX
The generator is: 2
Peers using dldhe6144 that want to optimize their key exchange with a
short exponent (Section 4.2) should choose a secret key of at least
XXX bits.
A.5. dldhe8192
The 8192-bit group has registry value 4, and is calcluated from the
following formula:
The modulus is: p = 2^8192 - 2^8128 + XXX * 2^64 - 1
Its hexadecimal representation is:
XXX ...still calculating... XXX
The base is: g = 2
Peers using dldhe8192 that want to optimize their key exchange with a
short exponent (Section 4.2) should choose a secret key of at least
XXX bits.
Author's Address
Daniel Kahn Gillmor
ACLU
125 Broad Street, 18th Floor
New York, NY 10004
USA
Email: dkg@fifthhorseman.net
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