Network Working Group I. Miers
Internet-Draft M. Green
Intended status: Standards Track Johns Hopkins University
Expires: August 18, 2014 E. Rescorla
Mozilla
February 14, 2014
Short Authentication Strings for TLS
draft-miers-tls-sas-00
Abstract
TLS and DTLS connections generally rely on a PKI, a shared secret, or
endpoint fingerprints for endpoint authentication. This document
describes an authentication mechanism which instead generates a
"short authentication string" (SAS) as an emergent property of the
connection. The SAS can then be verified via an external channel in
order to authenticate the connection.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Background: Coin-Flipping Protocols . . . . . . . . . . . . . . 3
4. Protocol Definition . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Coin Flipping . . . . . . . . . . . . . . . . . . . . . . . 4
4.2. Computing the raw SAS bits . . . . . . . . . . . . . . . . 5
4.3. Computing the SAS String . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
6. Normative References . . . . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 6
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1. Introduction
TLS [TLS12] and DTLS connections generally rely on a PKI, a shared
secret, or endpoint fingerprints for endpoint authentication. This
document describes an authentication mechanism which instead
generates a "short authentication string" (SAS) as an emergent
property of the connection. The SAS can then be verified via an
external channel in order to authenticate the connection.
While a fingerprint can be used for authentication (and is used in
SSH), it is too long to be conveniently read and compared by two
users. If a predictable subset of the fingerprint is compared (e.g.,
the first or last bits) an attacker can create a fingerprint which
just matches that subset. The mechanism described by this document
is based on fingerprints but compares a small number of bits derived
from the fingerprint and randomness generated by both endpoints, thus
requiring an attacker to match the entire fingerprint (which is too
long to be feasible) in order to produce a low probability of
detection. In order to compute the SAS, the endpoints run a "coin
flip" protocol to generate a short shared bitstring which is not
under the control of either endpoint. The bitstring is then used,
along with the TLS fingerprint, to derive a set of bits that are
mapped to a SAS.
2. 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].
3. Background: Coin-Flipping Protocols
The general pattern of a coin-flipping protocol is shown below:
Alice Bob
H(R_a) ------------->
<---------------- R_b
R_a ---------------->
Alice and Bob each generate a large random number R_a and R_b. Alice
(who speaks first) computes a commitment to R_a by hashing R_a and
sends it to Bob. Bob then sends R_b to Alice and finally then Alice
reveals R_a to Bob. Bob then verifies that R_a matches the hash Alice
sent in the first message and then each side computes the shared
value S = R_a XOR R_b.
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4. Protocol Definition
4.1. Coin Flipping
We map the coin flipping messages to TLS using a TLS extension and a
new handshake message, as shown below.
Client Server
ClientHello + SASXtn -------->
ServerHello + SASXtn
Certificate*
ServerKeyExchange*
CertificateRequest*
<-------- ServerHelloDone
Certificate*
ClientKeyExchange
CertificateVerify*
SASShare
[ChangeCipherSpec]
Finished --------> SASShare
[ChangeCipherSpec]
<-------- Finished
Application Data <-------> Application Data
Each side generates a 512-bit cryptographically random value R_client
and R_server.
The SASXtn is defined as follows:
struct {
opaque digest<255>;
} SASExtension;
The client's SASXtn is a zero-length value indicating the client's
desire to do the SAS handshake. The server's SASXtn is a digest of
R_server using the Hash defined for the Finished message in Section
7.4.9 of [TLS12].
The SASShare structure is defined as follows:
struct {
opaque share[64];
} SASShare;
"share" is the raw byte value of R_client or R_server, as
appropriate.
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4.2. Computing the raw SAS bits
The SAS bits are computed as follows:
1. If you are the client, verify that the server's R_server matches
their SASExtension value. If not, abort the handshake with error
handshake_failure
2. Compute R_shared = R_client ^ R_server
3. If both endpoints have certificate fingerprints compute
fingerprints=fingerprint_server | fingerprint_client. If only
the server has a fingerprint, compute
fingerprints=fingerprint_server.
4. Compute SAS_bits as HASH(fingerprints||R_shared) using the Hash
defined for the Finished message in Section 7.4.9 of [TLS12]
4.3. Computing the SAS String
The application should map the first 15<n<len(fingerprints) bits of
SAS_bits to some set of words or symbols defined for the application.
One option is the PGP word list. For a spoken language agnostic
solution, symbols could be use.
5. Security Considerations
Implementations MUST use fresh, random R_client and R_server values
for each TLS handshake.
Implementations MUST ensure their share of the coin flip remains
secret until after the TLS session key is established.
Applications SHOULD abort if the SAS strings do not match.
Applications SHOULD abort after multiple failed TLS handshakes and
notify the user. Failure to do so will allow an attacker multiple
attempts to guess a SAS. They will succeed after a few thousand
attempts.
6. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[TLS12] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
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Authors' Addresses
Ian Miers
Johns Hopkins University
Email: imiers@cs.jhu.edu
Matthew Green
Johns Hopkins University
Email: mgreen@cs.jhu.edu
Eric Rescorla
Mozilla
2064 Edgewood Drive
Palo Alto, CA 94303
USA
Phone: +1 650 678 2350
Email: ekr@rtfm.com
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