Network Working Group P. Wouters
Internet-Draft Red Hat
Intended status: Standards Track October 21, 2013
Expires: April 24, 2014
Using DANE to Associate OTR public keys with email addresses
draft-wouters-dane-otrfp-01
Abstract
The Off-The-Record Messaging protocol (OTR) exchanges public keys in-
band. This document describes how to use DANE to securely associate
an Instant Message user identified by their email address with an OTR
public key. This association helps to authenticate users and protect
against MITM attacks.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on April 24, 2014.
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document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. The OTRFP Resource Record . . . . . . . . . . . . . . . . . . 3
2.1. Location of the OTRFP record . . . . . . . . . . . . . . 3
2.2. The OTRFP RDATA Format . . . . . . . . . . . . . . . . . 4
3. Building the fingerprint data . . . . . . . . . . . . . . . . 5
3.1. Multiprecision Integers (MPI) . . . . . . . . . . . . . . 5
3.2. OTR public key representation . . . . . . . . . . . . . . 6
3.3. Calculating a DSA fingerprint . . . . . . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
4.1. OTRFP RRtype . . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
5.1. Email address information leak . . . . . . . . . . . . . 7
5.2. UI presentation . . . . . . . . . . . . . . . . . . . . . 7
5.3. DNSSEC required . . . . . . . . . . . . . . . . . . . . . 7
6. Reference example . . . . . . . . . . . . . . . . . . . . . . 7
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
Off-the-Record [OTRSPEC] is an encryption and authentication method
for two parties exchanging messages that works independantly of the
transport layer. There are OTR implementations for Instant Message
networks such as [XMPP], IRC [RFC2812], commercial IM networks, the
GSM SMS network and others.
OTR offers encryption, authentication, repudiation and perfect
forward secrecy. To authenticate the other party after an
unauthenticated Diffie-Hellman key exchange, a "long term" identity
keypair is used. It is up to both users to mutually verify each
other's OTR public key. One can make an out-of-band phone call, and
read out each other's public key fingerprint, assuming both parties
can recognise and trust each other's voice. Another option is to use
the shared secret. A third option is for both parties to ask each
other a question to which only the other party knows the answer.
None of the above listed methods allow a person to pre-publish their
OTR public key or finger print, or allow for a trusted third party or
PKI to vouch for OTR public keys. As a result, most users feel it is
too cumbersome to authenticate each other. As such, these users are
not protected against MITM attacks.
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This document describes a mechanism to associate a user's OTR public
key with their email address, using a new DNS RRtype. This is
similar to the SSHFP [RFC4255] RRType, except that this method
associates keys with users, not hosts. Client implementations that
support this document will be able to protect their users against
MITM attacks, without requiring all users to manually verify each
other's identity.
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 RFC 2119 [RFC2119].
This document also makes use of standard DNSSEC and DANE terminology.
See DNSSEC [RFC4033], [RFC4034], [RFC4035], and DANE [RFC6698] for
these terms.
2. The OTRFP Resource Record
The OTRFP DNS resource record (RR) is used to associate an end entity
OTR public key with an email address, thus forming a "OTRFP public
key association".
The type value allocated for the OTRFP RR type is [TBD]. The OTRFP
RR is class independent. The OTRFP RR has no special TTL
requirements.
2.1. Location of the OTRFP record
Domain names are prepared for requests in the following manner.
1. The user name (the "left-hand side" of the email address, called
the "local-part" in the mail message format definition [RFC2822]
and the "local part" in the specification for internationalized
email [RFC6530]), is encoded with Base32 [RFC4648], to become the
left-most label in the prepared domain name. This does not
include the "@" character that separates the left and right sides
of the email address.
2. The string "_otrfp" becomes the second left-most label in the
prepared domain name.
3. The domain name (the "right-hand side" of the email address,
called the "domain" in RFC 2822) is appended to the result of
step 2 to complete the prepared domain name.
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For example, to request an OTRFP resource record for a user whose
address is "hugh@example.com", you would use
"nb2wo2a=._otrfp.example.com" in the request. The corresponding RR
in the example.com zone might look like:
nb2wo2a=._otrfp.example.com. IN OTRFP (
3 0 1 5fdb8166f9089e253b90f95a91f48a8d2d2359ce )
Design note: Encoding the user name with Base32 allows local parts
that have characters that would prevent their use in domain names.
For example, a period (".") is a valid character in a local part, but
would wreak havoc in a domain name. Similarly, RFC 6530 allows non-
ASCII characters in local parts, and encoding a local part with non-
ASCII characters with Base32 renders the name usable in the DNS.
2.2. The OTRFP RDATA Format
The RDATA for an OTRFP RR consists of an OTR protocol version, key
type, hash type and the fingerprint of the user's OTR public key.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+-------------------------------+---------------+
! protocol ! Key Type ! Hash Type |
+---------------+-------------------------------+---------------+
! !
~ Fingerprint data ~
! !
+---------------------------------------------------------------+
The fields have the following meaning and encoding:
Protocol:
One octet specifying the OTR protocol version. The following
values are assigned:
Value Protocol
----- --------------
0 reserved
1 reserved
2 version 2 (obsoleted)
3 version 3
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Key Type:
Two octets specifying the OTR Key Type as defined in
[draft-individual-otr]. The following values are assigned:
Value Key Type
----- --------------
0 DSA
Hash Type:
One octet value specifying the hash algorithm used to compute
the fingerprint data. The following values are assigned:
Value Hash Type
----- --------------
0 reserved
1 SHA-1
Fingerprint data:
The actual fingerprint data in hexadecimal (see below)
3. Building the fingerprint data
OTR represents keys using multiprecision integers (also called MPIs)
which are unsigned integers used to hold large integer values such as
the ones used in cryptographic calculations. The OTR Public Key
representation depends on the Key Type and the fingerprint is a human
readable representation of the message-digest (hash) of the OTR
Public Key.
3.1. Multiprecision Integers (MPI)
An MPI consists of two pieces: a two-octet scalar that is the length
of the MPI in bits followed by a string of octets that contain the
actual integer.
These octets form a big-endian number; a big-endian number can be
made into an MPI by prefixing it with the appropriate length.
Examples (all numbers are in hexadecimal):
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The string of octets [00 01 01] forms an MPI with the value 1. The
string [00 09 01 FF] forms an MPI with the value of 511.
Additional rules:
The size of an MPI is ((MPI.length + 7) / 8) + 2 octets.
The length field of an MPI describes the length starting from its
most significant non-zero bit. Thus, the MPI [00 02 01] is not
formed correctly. It should be [00 01 01].
Unused bits of an MPI MUST be zero.
Also note that when an MPI is encrypted, the length refers to the
plaintext MPI. It may be ill-formed in its ciphertext. OTR does not
use encrypted MPIs.
3.2. OTR public key representation
An OTR Public Key is representated using a concatenation of its two
octet Key Type, followed by the MPI typed components that make up a
key. The number of components is dependant on the Key Type used.
A fingerprint is a hexadecimal representation of the message-digest
(hash) of an OTR Public Key. Currently, the only supported hash is
SHA-1.
There is an exception for backwards compatibility: if the OTR Key
Type is 0x0000 (DSA), then the two leading 0x00 octets are omitted
from the data to be hashed.
This encoding assures that, assuming the hash function itself has no
useful collisions, and DSA keys have a length of less than 524281
bits (500 times larger than most DSA keys), no two public keys will
have the same fingerprint.
3.3. Calculating a DSA fingerprint
For OTR Key Type 0x0000 (DSA), the public key is represented by its
Key Type (0x0000) concatenated with p(MPI) q(MPI) g(MPI) y(MPI)
This representation is hashed by the desired hashing function and
represented in hexadecimal to create the fingerprint data to be
included in the OTRFP RDATA section.
4. IANA Considerations
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4.1. OTRFP RRtype
This document uses a new DNS RR type, OTRFP, whose value [TBD] has
been allocated by IANA from the Resource Record (RR) TYPEs
subregistry of the Domain Name System (DNS) Parameters registry.
5. Security Considerations
5.1. Email address information leak
DNS zones that are signed with DNSSEC using NSEC for denial of
existence are susceptible to zone-walking, a mechanism that allow
someone to enumerate all the names in the zone. Someone who wanted
to collect email addresses from a zone that uses OTRFP might use such
a mechanism. DNSSEC-signed zones using NSEC3 for denial of existence
are significantly less susceptible to zone-walking. Someone could
still attempt a dictionary attack on the zone to find OTRFP records,
just as they can use dictionary attacks on an SMTP server to see
which addresses are valid.
5.2. UI presentation
Client treatment of any information included in the OTRFP record is a
matter of local policy. Clients are strongly encouraged to not
visually equate a DANE verified fingerprint with a human-verified
fingerprint. Padlock icons are strongly discouraged as the
verification state of a public key is not a binary selection.
5.3. DNSSEC required
If an OTRFP resource record is received without DNSSEC protection, it
MUST be ignored. If the DNS request returned an "indeterminate" or
"bogus" answer, the user MUST be warned that they might be under
attack. Bogus data MUST NOT be used.
6. Reference example
Given a textual representation of a DSA private key for
hugh@example.com of:
(dsa
(p #0085CAB74C71F78ACBAF92E3959E772050E8332D1262CF7989D1ACDFBB545E
16E757DBAAD3047E306E250C69CA4347CC7347F68070DE46B8EC4C4B41579F
1D57F00E76EBFDD7EF5494D6E726428BE17D7E4BEFE0ECE6BA661E7BE799B6
E5BF5EF8BB02CD1466A06114EF517CCBC21D68CC769F5A15D4FC473BA27482
AC4F42C667#)
(q #0086CBA0573319CFA3D3EBD8225651E58B316B22F5#)
(g #2CE973832A3B23A9E8E5BC324E16A9445C0EBB73C03ACBBE5EEC6504F640DB
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A49C97A42282271BA6127F848EC70F1A4AB784BE0A712081DF721452C87EE6
9B5C4FA963E933C2E592AF9631C3FDF94A85593A1BF6170889DBB8DD55A0A2
BB19874EBDA2D96929A541576D7800BFEB467D6F991E437883F8BC7D6A3654
5C04BDFC#)
(y #30CCBADF74E0313E1E6274DB8CC08FA6DC5EB6FA4729BB573034D75EB564CB
1F3DBECA70DF6A7337BD2A9EFA63986C2F64B4D4B32CFDB9F3BF6719DC4A98
43E60319236D8EB936FFE845C5A6B856557F0E9A4CA769041FBA92CA2CCE88
E2E3441650437FF5FB315F4AFF5CA43BFF3539B520297EC1C5BAF3E437F829
3F9E2DA8#)
(x #4EB9993416934FAE476E4655B5A520373F1321CE#)
)
The fingerprint is calculated (leaving out the 0x0000 Key Type for
DSA) by hashing:
SHA-1( p(MPI) | q(MPI) | g(MPI) | y(MPI) )
which yields (in hexadecimal):
35b3c7c02cf9e74bd53f33a0bb815ccd39e60a8d
Resulting in the following OTRFP record:
nb2wo2a=._otrfp.example.com. IN OTRFP (
3 0 1 35b3c7c02cf9e74bd53f33a0bb815ccd39e60a8d )
7. Acknowledgements
This document is based on RFC-4255 and draft-ietf-dane-smime whose
authors are Paul Hoffman, Jacob Schlyter and W. Griffin. The MPI
section has been taken verbatim from RFC-4880.
8. References
8.1. Normative References
[OTRSPEC] , "Off-the-Record Messaging Protocol version 3", September
2012,
<http://www.cypherpunks.ca/otr/Protocol-v3-4.0.0.html>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", RFC
4033, March 2005.
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[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, March 2005.
[RFC4255] Schlyter, J. and W. Griffin, "Using DNS to Securely
Publish Secure Shell (SSH) Key Fingerprints", RFC 4255,
January 2006.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
[XMPP] , "XMPP Standards Foundation RFCs", February 2003,
<http://xmpp.org/xmpp-protocols/rfcs/>.
8.2. Informative References
[RFC2812] Kalt, C., "Internet Relay Chat: Client Protocol", RFC
2812, April 2000.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822, April
2001.
[RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for
Internationalized Email", RFC 6530, February 2012.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, August 2012.
Author's Address
Paul Wouters
Red Hat
Email: pwouters@redhat.com
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