Internet DRAFT - draft-ietf-tls-pwd
draft-ietf-tls-pwd
Transport Layer Security D. Harkins, Ed.
Internet-Draft HP Enterprise
Intended status: Standards Track D. Halasz, Ed.
Expires: August 28, 2016 Halasz Ventures
February 25, 2016
Secure Password Ciphersuites for Transport Layer Security (TLS)
draft-ietf-tls-pwd-07
Abstract
This memo defines several new ciphersuites for the Transport Layer
Security (TLS) protocol to support certificate-less, secure
authentication using only a simple, low-entropy, password. The
ciphersuites are all based on an authentication and key exchange
protocol that is resistant to off-line dictionary attack.
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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material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 28, 2016.
Copyright Notice
Copyright (c) 2016 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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. The Case for Certificate-less Authentication . . . . . . 3
1.2. Resistance to Dictionary Attack . . . . . . . . . . . . . 3
2. Keyword Definitions . . . . . . . . . . . . . . . . . . . . . 4
3. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Notation . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Discrete Logarithm Cryptography . . . . . . . . . . . . . 5
3.2.1. Elliptic Curve Cryptography . . . . . . . . . . . . . 5
3.2.2. Finite Field Cryptography . . . . . . . . . . . . . . 7
3.3. Instantiating the Random Function . . . . . . . . . . . . 8
3.4. Passwords . . . . . . . . . . . . . . . . . . . . . . . . 8
3.5. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 8
4. Specification of the TLS-PWD Handshake . . . . . . . . . . . 9
4.1. Protecting the Username . . . . . . . . . . . . . . . . . 10
4.1.1. Construction of a Protected Username . . . . . . . . 11
4.1.2. Recovery of a Protected Username . . . . . . . . . . 12
4.2. Fixing the Password Element . . . . . . . . . . . . . . . 13
4.2.1. Computing an ECC Password Element . . . . . . . . . . 14
4.2.2. Computing an FFC Password Element . . . . . . . . . . 16
4.3. Changes to Handshake Message Contents . . . . . . . . . . 17
4.3.1. Client Hello Changes . . . . . . . . . . . . . . . . 17
4.3.2. Server Key Exchange Changes . . . . . . . . . . . . . 18
4.3.2.1. Generation of ServerKeyExchange . . . . . . . . . 19
4.3.2.2. Processing of ServerKeyExchange . . . . . . . . . 20
4.3.3. Client Key Exchange Changes . . . . . . . . . . . . . 21
4.3.3.1. Generation of Client Key Exchange . . . . . . . . 21
4.3.3.2. Processing of Client Key Exchange . . . . . . . . 22
4.4. Computing the Premaster Secret . . . . . . . . . . . . . 22
5. Ciphersuite Definition . . . . . . . . . . . . . . . . . . . 23
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 23
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
8. Security Considerations . . . . . . . . . . . . . . . . . . . 25
9. Human Rights Considerations . . . . . . . . . . . . . . . . . 28
10. Implementation Considerations . . . . . . . . . . . . . . . . 28
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 29
11.1. Normative References . . . . . . . . . . . . . . . . . . 29
11.2. Informative References . . . . . . . . . . . . . . . . . 30
Appendix A. Example Exchange . . . . . . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 35
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1. Background
1.1. The Case for Certificate-less Authentication
TLS usually uses public key certificates for authentication
[RFC5246]. This is problematic in some cases:
o Frequently, TLS [RFC5246] is used in devices owned, operated, and
provisioned by people who lack competency to properly use
certificates and merely want to establish a secure connection
using a more natural credential like a simple password. The
proliferation of deployments that use a self-signed server
certificate in TLS [RFC5246] followed by a PAP-style exchange over
the unauthenticated channel underscores this case.
o The alternatives to TLS-PWD for employing certificate-less TLS
authentication-- using pre-shared keys in an exchange that is
susceptible to dictionary attack, or using an SRP exchange that
requires users to, a priori, be fixed to a specific finite field
cryptorgraphy group for all subsequent connections-- are not
acceptable for modern applications that require both security and
cryptographic agility.
o A password is a more natural credential than a certificate (from
early childhood people learn the semantics of a shared secret), so
a password-based TLS ciphersuite can be used to protect an HTTP-
based certificate enrollment scheme like EST [RFC7030] to parlay a
simple password into a certificate for subsequent use with any
certificate-based authentication protocol. This addresses a
significant "chicken-and-egg" dilemma found with certificate-only
use of [RFC5246].
o Some PIN-code readers will transfer the entered PIN to a smart
card in clear text. Assuming a hostile environment, this is a bad
practice. A password-based TLS ciphersuite can enable the
establishment of an authenticated connection between reader and
card based on the PIN.
1.2. Resistance to Dictionary Attack
It is a common misconception that a protocol that authenticates with
a shared and secret credential is resistent to dictionary attack if
the credential is assumed to be an N-bit uniformly random secret,
where N is sufficiently large. The concept of resistence to
dictionary attack really has nothing to do with whether that secret
can be found in a standard collection of a language's defined words
(i.e. a dictionary). It has to do with how an adversary gains an
advantage in attacking the protocol.
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For a protocol to be resistant to dictionary attack any advantage an
adversary can gain must be a function of the amount of interactions
she makes with an honest protocol participant and not a function of
the amount of computation she uses. The adversary will not be able
to obtain any information about the password except whether a single
guess from a single protocol run which she took part in is correct or
incorrect.
It is assumed that the attacker has access to a pool of data from
which the secret was drawn-- it could be all numbers between 1 and
2^N, it could be all defined words in a dictionary. The key is that
the attacker cannot do a an attack and then enumerate through the
pool trying potential secrets (computation) to see if one is correct.
She must do an active attack for each secret she wishes to try
(interaction) and the only information she can glean from that attack
is whether the secret used with that particular attack is correct or
not.
2. Keyword Definitions
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. Introduction
3.1. Notation
The following notation is used in this memo:
password
a secret, and potentially low-entropy word, phrase, code or key
used as a credential for authentication. The password is shared
between the TLS client and TLS server.
y = H(x)
a binary string of arbitrary length, x, is given to a function H
which produces a fixed-length output, y.
a | b
denotes concatenation of string a with string b.
[a]b
indicates a string consisting of the single bit "a" repeated "b"
times.
x mod y
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indicates the remainder of division of x by y. The result will
be between 0 and y.
len(x)
indicates the length in bits of the string x.
lgr(a,b)
takes "a" and a prime, b and returns the legendre symbol (a/b).
LSB(x)
returns the least-significant bit of the bitstring "x".
G.x
indicates the x-coordinate of a point, G, on an elliptic curve.
3.2. Discrete Logarithm Cryptography
The ciphersuites defined in this memo use discrete logarithm
cryptography (see [SP800-56A]) to produce an authenticated and shared
secret value that is an element in a group defined by a set of domain
parameters. The domain parameters can be based on either Finite
Field Cryptography (FFC) or Elliptic Curve Cryptography (EEC).
TLS [RFC5246] allows for both FFC and ECC domain parameter sets to be
conveyed verbosely by the server. This opens up the possibility of a
malicious server offering a weak group, or one with a trapdoor, that
would lead to a leaking of information during a run of the protocol.
Therefore, if explicit domain parameter sets are used with TLS-PWD,
they MUST be agreed-upon a priori in an out-of-band fashion. Clients
MUST NOT accept explicit domain parameter sets from a server that it
has not previously agreed to accept.
Elements in a group, either an FFC or EEC group, are indicated using
upper-case while scalar values are indicated using lower-case.
3.2.1. Elliptic Curve Cryptography
The authenticated key exchange defined in this memo uses fundamental
algorithms of elliptic curves defined over GF(p) as described in
[RFC6090].
Domain parameters for the ECC groups used by this memo are:
o A prime, p, determining a prime field GF(p). The cryptographic
group will be a subgroup of the full elliptic curve group which
consists points on an elliptic curve-- elements from GF(p) that
satisfy the curve's equation-- together with the "point at
infinity" that serves as the identity element.
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o Elements a and b from GF(p) that define the curve's equation. The
point (x,y) in GF(p) x GF(p) is on the elliptic curve if and only
if (y^2 - x^3 - a*x - b) mod p equals zero (0).
o A point, G, on the elliptic curve, which serves as a generator for
the ECC group. G is chosen such that its order, with respect to
elliptic curve addition, is a sufficiently large prime.
o A prime, q, which is the order of G, and thus is also the size of
the cryptographic subgroup that is generated by G.
o A co-factor, f, defined by the requirement that the size of the
full elliptic curve group (including the "point at infinity") is
the product of f and q.
This memo uses the following ECC Functions:
o Z = elem-op(X,Y) = X + Y: two points on the curve X and Y, are
sumed to produce another point on the curve, Z. This is the group
operation for ECC groups.
o Z = scalar-op(x,Y) = x * Y: an integer scalar, x, acts on a point
on the curve, Y, via repetitive addition (Y is added to itself x
times), to produce another EEC element, Z.
o Y = inverse(X): a point on the curve, X, has an inverse, Y, which
is also a point on the curve, when their sum is the "point at
infinity" (the identity for elliptic curve addition). In other
words, R + inverse(R) = "0".
o z = F(X): the x-coordinate of a point (x, y) on the curve is
returned. This is a mapping function to convert a group element
into an integer.
Only ECC groups over GF(p) can be used with TLS-PWD. ECC groups over
GF(2^m) SHALL NOT be used by TLS-PWD. In addition, ECC groups with a
co-factor greater than one (1) SHALL NOT be used by TLS-PWD.
A composite (x, y) pair can be validated as a point on the elliptic
curve by checking whether: 1) both coordinates x and y are greater
than zero (0) and less than the prime defining the underlying field;
2) the x- and y- coordinates satisfy the equation of the curve; and
3) they do not represent the point-at-infinity "0". If any of those
conditions are not true the (x, y) pair is not a valid point on the
curve.
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3.2.2. Finite Field Cryptography
Domain parameters for the FFC groups used by this memo are:
o A prime, p, determining a prime field GF(p), the integers modulo
p. The FFC group will be a subgroup of GF(p)*, the multiplicative
group of non-zero elements in GF(p).
o An element, G, in GF(p)* which serves as a generator for the FFC
group. G is chosen such that its multiplicative order is a
sufficiently large prime divisor of ((p-1)/2).
o A prime, q, which is the multiplicative order of G, and thus also
the size of the cryptographic subgroup of GF(p)* that is generated
by G.
This memo uses the following FFC Functions:
o Z = elem-op(X,Y) = (X * Y) mod p: two FFC elements, X and Y, are
multiplied modulo the prime, p, to produce another FFC element, Z.
This is the group operation for FFC groups.
o Z = scalar-op(x,Y) = Y^x mod p: an integer scalar, x, acts on an
FFC group element, Y, via exponentiation modulo the prime, p, to
produce another FFC element, Z.
o Y = inverse(X): a group element, X, has an inverse, Y, when the
product of the element and its inverse modulo the prime equals one
(1). In other words, (X * inverse(X)) mod p = 1.
o z = F(X): is the identity function since an element in an FFC
group is already an integer. It is included here for consistency
in the specification.
Many FFC groups used in IETF protocols are based on safe primes and
do not define an order (q). For these groups, the order (q) used in
this memo shall be the prime of the group minus one divided by two--
(p-1)/2.
An integer can be validated as being an element in an FFC group by
checking whether: 1) it is between one (1) and the prime, p,
exclusive; and 2) if modular exponentiation of the integer by the
group order, q, equals one (1). If either of these conditions are
not true the integer is not an element in the group.
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3.3. Instantiating the Random Function
The protocol described in this memo uses a random function, H, which
is modeled as a "random oracle". At first glance, one may view this
as a hash function. As noted in [RANDOR], though, hash functions are
too structured to be used directly as a random oracle. But they can
be used to instantiate the random oracle.
The random function, H, in this memo is instantiated by using the
hash algorithm defined by the particular TLS-PWD ciphersuite in HMAC
mode with a key whose length is equal to block size of the hash
algorithm and whose value is zero. For example, if the ciphersuite
is TLS_ECCPWD_WITH_AES_128_GCM_SHA256 then H will be instantiated
with SHA256 as:
H(x) = HMAC-SHA256([0]32, x)
3.4. Passwords
The authenticated key exchange used in TLS-PWD requires each side to
have a common view of a shared credential. To protect the server's
database of stored passwords, though, the password SHALL be salted
and the result, called the base, SHALL be used as the authentication
credential.
The salting function is defined as:
base = HMAC-SHA256(salt, username | password)
The password used for generation of the base SHALL be represented as
a UTF-8 encoded character string processed according to the rules of
the [RFC4013] profile of [RFC3454] and the salt SHALL be a 32 octet
random number. The server SHALL store a triplet of the form:
{ username, base, salt }
And the client SHALL generate the base upon receiving the salt from
the server.
3.5. Assumptions
The security properties of the authenticated key exchange defined in
this memo are based on a number of assumptions:
1. The random function, H, is a "random oracle" as defined in
[RANDOR].
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2. The discrete logarithm problem for the chosen group is hard.
That is, given g, p, and y = g^x mod p, it is computationally
infeasible to determine x. Similarly, for an ECC group given the
curve definition, a generator G, and Y = x * G, it is
computationally infeasible to determine x.
3. Quality random numbers with sufficient entropy can be created.
This may entail the use of specialized hardware. If such
hardware is unavailable a cryptographic mixing function (like a
strong hash function) to distill enropy from multiple,
uncorrelated sources of information and events may be needed. A
very good discussion of this can be found in [RFC4086].
If the server supports username protection (see Section 4.1), it is
assumed that the server has chosen a domain parameter set and
generated a username-protection keypair. The chosen domain parameter
set and public key are assumed to be conveyed to the client at the
time the client's username and password were provisioned.
4. Specification of the TLS-PWD Handshake
The key exchange underlying TLS-PWD is the "dragonfly" PAKE as
defined in [RFC7664].
The authenticated key exchange is accomplished by each side deriving
a password-based element, PE, in the chosen group, making a
"commitment" to a single guess of the password using PE, and
generating the Premaster Secret. The ability of each side to produce
a valid finished message authenticates itself to the other side.
The authenticated key exchange is dropped into the standard TLS
message handshake by modifying some of the messages.
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Client Server
-------- --------
Client Hello (name) -------->
Server Hello
Server Key Exchange (commit)
<-------- Server Hello Done
Client Key Exchange (commit)
[Change cipher spec]
Finished -------->
[Change cipher spec]
<-------- Finished
Application Data <-------> Application Data
Figure 1
4.1. Protecting the Username
The client is required to identify herself to the server before the
server can look up the appropriate client credential with which to
perform the authenticated key exchange. This has negative privacy
implicaitons and opens up the client to tracking and increased
monitoring. It is therefore useful for the client to be able to
protect her username from passive monitors of the exchange and
against active attack by a malicious server. TLS-PWD provides such a
mechsnism. Support for protected usernames is RECOMMENDED.
To enable username protection a server choses a domain parameter set,
chooses a random private key, s, such that 1 < s < (q-1), where q is
the order of the chosen group, uses scalar-op() with the selected
group's generator to generate a public key, S:
S = scalar-op(s, G)
This keypair SHALL only be used for username protection. For
efficiency, the domain parameter set used for userame protection MUST
be based on elliptic curve cryptography. Any ECC group that is
approprate for TLS-PWD (see Section 3.2.1) is suitable for this
purpose but for interoperability, brainpoolP256r1 MUST be supported.
The domain parameter set used for username protection does not
restrict the choice of domain parameter set used for the underlying
key exchange in any way.
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When the client's username and password are provisioned on the
server, the server conveys the chosen group and its public key to the
client. This is stored on the client along with the server-specific
state (e.g. the hostname) it uses to initiate a TLS-PWD exchange.
The server uses the same group and public key with all clients.
To protect a username, the client and server perform a static-
ephemeral Diffie-Hellman exchange, using compact representation (and
therefore compact output, see [RFC6090]). The result of the Diffie-
Hellman exchange is passed to HKDF [RFC5869] to create a key-
encrypting key suitable for AES-SIV [RFC5297]. The length of the
key-encrypting key, l, and the hash function to use with HKDF depends
on the length of the prime, p, of the group used to provide username
protection:
o SHA-256, SIV-128, l=256 bits: when len(p) <= 256
o SHA-384, SIV-192, l=384 bits: when 256 < len(p) <= 384
o SHA-512, SIV-256, l=512 bits: when len(p) > 384
4.1.1. Construction of a Protected Username
Prior to initiating a TLS-PWD exchange, the client chooses a random
secret, c, such that 1 < c < (q-1), where q is the order of the group
from which the server's public key was generated, and uses scalar-
op() with the group's generator to create a public key, C. It uses
scalar-op() with the server's public key and c to create a shared
secret and derives a key-encrypting key, k, using the "salt-less"
mode of HKDF [RFC5869].
C = scalar-op(c, G)
Z = scalar-op(c, S)
k = HKDF-expand(HKDF-extract(NULL, Z.x), "", l)
Where NULL indicates the salt-free invocation and "" indicates an
empty string (i.e. there is no "context" passed to HKDF).
The key, k, and the client's username is then passed to SIV-encrypt
with no AAD and no nonce to produce an encrypted username, u:
u = SIV-encrypt(k, username)
Note: the format of the ciphertext output from SIV includes the
authenticating synthetic initialization vector.
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The protected username SHALL be the concatenation of the x-coordinate
of the client's public key, C, and the encrypted username, u. The
length of the x-coordinate of C MUST be equal to the length of the
group's prime, p, pre-pended with zeros, if necessary. The protected
username is inserted into the PWD_name extension and the
ExtensionType MUST be PWD_protect (see Section 4.3.1).
The length of the ciphertext output from SIV, minus the synthetic
initialization vector, will be equal to the length of the input
plaintext, in this case the username. To further foil traffic
analysis, it is RECOMMENDED that clients append a series of NULL
bytes to their usernames prior to passing them to SIV-encrypt() and
to vary the number of bytes added with each distinct run of TLS-PWD.
4.1.2. Recovery of a Protected Username
A server that receives a protected username needs to recover the
client's username prior to performing the key exchange. To do so,
the server computes the client's public key, completes the static-
ephemeral Diffie-Hellman exchange, derives the key encrypting key, k,
and decrypts the username.
The length of the x-coordinate of the client's public key is known
(it is the length of the prime from the domain parameter set used to
protect usernames) and can easily be separated from the ciphertext in
the PWD_name extension in the Client Hello-- the first len(p) bits
are the x-coordinate of the client's public key and the remaining
bits are the ciphertext.
Since compressed representation is used by the client, the server
MUST compute the y-coordinate of the client's public key by using the
equation of the curve:
y^2 = x^3 + ax + b
and solving for y. There are two solutions for y but since
compressed output is also being used, the selection is irrelevant.
The server reconstructs the client's public value, C, from (x, y).
If there is no solution for y, or if (x, y) is not a valid point on
the elliptic curve (see Section 3.2.1), the server MUST treat the
Client Hello as if it did not have a password for a given username
(see Section 4.3.1).
The server then uses scalar-op() with the reconstructed point C and
the private key it uses for protected passwords, s, to generate a
shared secret, and derives a key-encrypting key, k, in the same
manner as in Section 4.1.1.
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Z = scalar-op(s, C)
k = HKDF-expand(HKDF-extract(NULL, Z.x), "", l)
The key, k, and the ciphertext portion of the PWD_name extension, u,
are passed to SIV-decrypt with no AAD and no nonce to produce the
username:
username = SIV-decrypt(k, u)
If SIV-decrypt returns the symbol FAIL indicating unsuccessful
decryption and verification the server MUST treat the ClientHello as
if it did not have a password for a given username (see
Section 4.3.1). If successful, the server has obtained the client's
username and can process it as needed. Any NULL octets added by the
client prior to encryption can be easily stripped off of the string
that represents the username.
4.2. Fixing the Password Element
Prior to making a "commitment" both sides must generate a secret
element, PE, in the chosen group using the common password-derived
base. The server generates PE after it receives the Client Hello and
chooses the particular group to use, and the client generates PE upon
receipt of the Server Key Exchange.
Fixing the password element involves an iterative "hunting and
pecking" technique using the prime from the negotiated group's domain
parameter set and an ECC- or FFC-specific operation depending on the
negotiated group.
To thwart side channel attacks which attempt to determine the number
of iterations of the "hunting-and-pecking" loop are used to find PE
for a given password, a security parameter, m, is used to ensure that
at least m iterations are always performed.
First, an 8-bit counter is set to the value one (1). Then, H is used
to generate a password seed from the a counter, the prime of the
selected group, and the base (which is derived from the username,
password, and salt):
pwd-seed = H(base | counter | p)
Then, using the technique from section B.5.1 of [FIPS186-3], the pwd-
seed is expanded using the PRF to the length of the prime from the
negotiated group's domain parameter set plus a constant sixty-four
(64) to produce an intermediate pwd-tmp which is modularly reduced to
create pwd-value:
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n = len(p) + 64
pwd-tmp = PRF(pwd-seed, "TLS-PWD Hunting And Pecking",
ClientHello.random | ServerHello.random) [0..n];
pwd-value = (pwd-tmp mod (p-1)) + 1
The pwd-value is then passed to the group-specific operation which
either returns the selected password element or fails. If the group-
specific operation fails, the counter is incremented, a new pwd-seed
is generated, and the hunting-and-pecking continues. This process
continues until the group-specific operation returns the password
element. After the password element has been chosen, the base is
changed to a random number, the counter is incremented and the
hunting-and-pecking continues until the counter is greater than the
security parameter, m.
The probability that one requires more than n iterations of the
"hunting and pecking" loop to find an ECC PE is roughly (q/2p)^n and
to find an FFC PE is roughly (q/p)^n, both of which rapidly approach
zero (0) as n increases. The security parameter, m, SHOULD be set
sufficiently large such that the probability that finding PE would
take more than m iterations is sufficiently small (see Section 8).
When PE has been discovered, pwd-seed, pwd-tmp, and pwd-value SHALL
be irretrievably destroyed.
4.2.1. Computing an ECC Password Element
The group-specific operation for ECC groups uses pwd-value, pwd-seed,
and the equation for the curve to produce PE. First, pwd-value is
used directly as the x-coordinate, x, with the equation for the
elliptic curve, with parameters a and b from the domain parameter set
of the curve, to solve for a y-coordinate, y. If there is no
solution to the quadratic equation, this operation fails and the
hunting-and-pecking process continues. If a solution is found, then
an ambiguity exists as there are technically two solutions to the
equation and pwd-seed is used to unambiguously select one of them.
If the low-order bit of pwd-seed is equal to the low-order bit of y,
then a candidate PE is defined as the point (x, y); if the low-order
bit of pwd-seed differs from the low-order bit of y, then a candidate
PE is defined as the point (x, p - y), where p is the prime over
which the curve is defined. The candidate PE becomes PE, a random
number is used instead of the base, and the hunting and pecking
continues until it has looped through m iterations.
Algorithmically, the process looks like this:
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found = 0
counter = 0
base = H(username | password | salt)
n = len(p) + 64
do {
counter = counter + 1
seed = H(base | counter | p)
tmp = PRF(seed, "TLS-PWD Hunting And Pecking",
ClientHello.random | ServerHello.random) [0..n]
val = (tmp mod (p-1)) + 1
if ( (val^3 + a*val + b) mod p is a quadratic residue)
then
if (found == 0)
then
x = val
save = seed
found = 1
base = random()
fi
fi
} while ((found == 0) || (counter <= m))
y = sqrt(x^3 + a*x + b) mod p
if ( lsb(y) == lsb(save))
then
PE = (x, y)
else
PE = (x, p-y)
fi
Figure 2: Fixing PE for ECC Groups
Checking whether a value is a quadradic residue modulo a prime can
leak information about that value in a side-channel attack.
Therefore, it is RECOMMENDED that the technique used to determine if
the value is a quadratic residue modulo p blind the value with a
random number so that the blinded value can take on all numbers
between 1 and p-1 with equal probability. Determining the quadratic
residue in a fashion that resists leakage of information is handled
by flipping a coin and multiplying the blinded value by either a
random quadratic residue or a random quadratic nonresidue and
checking whether the multiplied value is a quadradic residue or a
quadradic nonresidue modulo p, respectively. The random residue and
nonresidue can be calculated prior to hunting-and-pecking by
calculating the legendre symbol on random values until they are
found:
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do {
qr = random()
} while ( lgr(qr, p) != 1)
do {
qnr = random()
} while ( lgr(qnr, p) != -1)
Algorithmically, the masking technique to find out whether a value is
a quadratic residue modulo a prime or not looks like this:
is_quadratic_residue (val, p) {
r = (random() mod (p - 1)) + 1
num = (val * r * r) mod p
if ( lsb(r) == 1 )
num = (num * qr) mod p
if ( lgr(num, p) == 1)
then
return TRUE
fi
else
num = (num * qnr) mod p
if ( lgr(num, p) == -1)
then
return TRUE
fi
fi
return FALSE
}
The random quadratic residue and quadratic non-residue (qr and qnr
above) can be used for all the hunting-and-pecking loops but the
blinding value, r, MUST be chosen randomly for each loop.
4.2.2. Computing an FFC Password Element
The group-specific operation for FFC groups takes pwd-value, and the
prime, p, and order, q, from the group's domain parameter set (see
Section 3.2.2 when the order is not part of the defined domain
parameter set) to directly produce a candidate password element, by
exponentiating the pwd-value to the value ((p-1)/q) modulo the prime.
If the result is greater than one (1), the candidate password element
becomes PE, and the hunting and pecking terminates successfully.
Algorithmically, the process looks like this:
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found = 0
counter = 0
base = H(username | password | salt)
n = len(p) + 64
do {
counter = counter + 1
pwd-seed = H(base | counter | p)
pwd-tmp = PRF(pwd-seed, "TLS-PWD Hunting And Pecking",
ClientHello.random | ServerHello.random) [0..n]
pwd-value = (pwd-tmp mod (p-1)) + 1
PE = pwd-value ^ ((p-1)/q) mod p
if (PE > 1)
then
found = 1
base = random()
fi
} while ((found == 0) || (counter <= m))
Figure 3: Fixing PE for FFC Groups
4.3. Changes to Handshake Message Contents
4.3.1. Client Hello Changes
The client is required to identify herself to the server by adding a
either a PWD_protect or PWD_clear extension to the Client Hello
message depending on whether the client wishes to protect its
username (see Section 4.1) or not, respectively. The PWD_protect and
PWD_clear extensions use the standard mechanism defined in [RFC5246].
The "extension data" field of the PWD extension SHALL contain a
PWD_name which is used to identify the password shared between the
client and server. If username protection is performed, and the
ExtensionType is PWD_protect, the contents of the PWD_name SHALL be
constructed according to Section 4.1.1).
enum { PWD_clear(TBD1), PWD_protect(TBD2) } ExtensionType;
opaque PWD_name<1..2^8-1>;
An unprotected PWD_name SHALL be UTF-8 encoded character string
processed according to the rules of the [RFC4013] profile of
[RFC3454] and a protected PWD_name SHALL be a string of bits.
A client offering a PWD ciphersuite MUST include one of the PWD_name
extensions in her Client Hello.
If a server does not have a password for a client identified by the
username either extracted from the PWD_name, if unprotected, or
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recovered using the technique in Section 4.1.2, if protected, or if
recovery of a protected username fails, the server SHOULD hide that
fact by simulating the protocol-- putting random data in the PWD-
specific components of the Server Key Exchange-- and then rejecting
the client's finished message with a "bad_record_mac" alert. To
properly effect a simulated TLS-PWD exchange, an appropriate delay
SHOULD be inserted between receipt of the Client Hello and response
of the Server Hello. Alternately, a server MAY choose to terminate
the exchange if a password is not found.
The server decides on a group to use with the named user (see
Section 10 and generates the password element, PE, according to
Section 4.2.2.
4.3.2. Server Key Exchange Changes
The domain parameter set for the selected group MUST be specified in
the ServerKeyExchange, either explicitly or, in the case of some
elliptic curve groups, by name. In addition to the group
specification, the ServerKeyExchange also contains the server's
"commitment" in the form of a scalar and element, and the salt which
was used to store the user's password.
Two new values have been added to the enumerated KeyExchangeAlgorithm
to indicate TLS-PWD using finite field cryptography, ff_pwd, and TLS-
PWD using elliptic curve cryptography, ec_pwd.
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enum { ff_pwd, ec_pwd } KeyExchangeAlgorithms;
struct {
opaque salt<1..2^8-1>;
opaque pwd_p<1..2^16-1>;
opaque pwd_g<1..2^16-1>;
opaque pwd_q<1..2^16-1>;
opaque ff_selement<1..2^16-1>;
opaque ff_sscalar<1..2^16-1>;
} ServerFFPWDParams;
struct
opaque salt<1..2^8-1>;
ECParameters curve_params;
ECPoint ec_selement;
opaque ec_sscalar<1..2^8-1>;
} ServerECPWDParams;
struct {
select (KeyExchangeAlgorithm) {
case ec_pwd:
ServerECPWDParams params;
case ff_pwd:
ServerFFPWDParams params;
};
} ServerKeyExchange;
4.3.2.1. Generation of ServerKeyExchange
The scalar and Element that comprise the server's "commitment" are
generated as follows.
First two random numbers, called private and mask, between zero and
the order of the group (exclusive) are generated. If their sum
modulo the order of the group, q, equals zero the numbers must be
thrown away and new random numbers generated. If their sum modulo
the order of the group, q, is greater than zero the sum becomes the
scalar.
scalar = (private + mask) mod q
The Element is then calculated as the inverse of the group's scalar
operation (see the group specific operations in Section 3.2) with the
mask and PE.
Element = inverse(scalar-op(mask, PE))
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After calculation of the scalar and Element the mask SHALL be
irretrievably destroyed.
4.3.2.1.1. ECC Server Key Exchange
EEC domain parameters are specified, either explicitly or named, in
the ECParameters component of the EEC-specific ServerKeyExchange as
defined in [RFC4492]. The scalar SHALL become the ec_sscalar
component and the Element SHALL become the ec_selement of the
ServerKeyExchange. If the client requested a specific point format
(compressed or uncompressed) with the Support Point Formats Extension
(see [RFC4492]) in its Client Hello, the Element MUST be formatted in
the ec_selement to conform to that request. If the client offered
(an) elliptic curve(s) in its ClientHello using the Supported
Elliptic Curves Extension, the server MUST include (one of the) named
curve(s) in the ECParameters field in the ServerKeyExchange and the
key exchange operations specified in Section 4.3.2.1 MUST use that
group.
As mentioned in Section 3.2.1, elliptic curves over GF(2^m), so
called characteristic-2 curves, and curves with a co-factor greater
than one (1) SHALL NOT be used with TLS-PWD.
4.3.2.1.2. FFC Server Key Exchange
FFC domain parameters sent in the ServerKeyExchange are for the
group's prime, generator (which is only used for verification of the
group specification), and the order of the group's generator. The
scalar SHALL become the ff_sscalar component and the Element SHALL
become the ff_selement in the FFC-specific ServerKeyExchange.
As mentioned in Section 3.2.2 if the prime is a safe prime and no
order is included in the domain parameter set, the order added to the
ServerKeyExchange SHALL be the prime minus one divided by two--
(p-1)/2.
4.3.2.2. Processing of ServerKeyExchange
Upon receipt of the ServerKeyExchange, the client decides whether to
support the indicated group or not. If the client used the Supported
Elliptic Curves Extension to offer (a) named curve(s) in her
ClientHello, the named curve in the ServerKeyExchange MUST be one
offered. If the server is explicitly specifying a group, either an
FFC or ECC group, the client and server MUST have agreed upon groups
prior to beginning the exchange (see Section 3.2) and the client MUST
compare each field of the explicit offer to the agreed-upon group(s).
Any discrepency SHALL result in the exchange being aborted.
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If the client decides to support the indicated group the server's
"commitment" MUST be validated by ensuring that: 1) the server's
scalar value is greater than zero (0) and less than the order of the
group, q; and 2) that the Element is valid for the chosen group (see
Section 3.2.2 and Section 3.2.1 for how to determine whether an
Element is valid for the particular group. Note that if the Element
is a compressed point on an elliptic curve it MUST be uncompressed
before checking its validity).
If the group is acceptable and the server's "commitment" has been
successfully validated, the client extracts the salt from the
ServerKeyExchange and generates the password element, PE, according
to Section 3.4 and Section 4.2.2. If the group is not acceptable or
the server's "commitment" failed validation, the eexchange MUST be
aborted.
4.3.3. Client Key Exchange Changes
When the value of KeyExchangeAlgorithm is either ff_pwd or ec_pwd,
the ClientKeyExchange is used to convey the client's "commitment" to
the server. It, therefore, contains a scalar and an Element.
struct {
opaque ff_celement<1..2^16-1>;
opaque ff_cscalar<1..2^16-1>;
} ClientFFPWDParams;
struct
ECPoint ec_celement;
opaque ec_cscalar<1..2^8-1>;
} ClientECPWDParams;
struct {
select (KeyExchangeAlgorithm) {
case ff_pwd: ClientFFPWDParams;
case ec_pwd: ClientECPWDParams;
} exchange_keys;
} ClientKeyExchange;
4.3.3.1. Generation of Client Key Exchange
The client's scalar and Element are generated in the manner described
in Section 4.3.2.1.
For an FFC group, the scalar SHALL become the ff_cscalar component
and the Element SHALL become the ff_celement in the FFC-specific
ClientKeyExchange.
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For an ECC group, the scalar SHALL become the ec_cscalar component
and the ELement SHALL become the ec_celement in the ECC-specific
ClientKeyExchange. If the client requested a specific point format
(compressed or uncompressed) with the Support Point Formats Extension
in its ClientHello, then the Element MUST be formatted in the
ec_celement to conform to its initial request.
4.3.3.2. Processing of Client Key Exchange
Upon receipt of the ClientKeyExchange, the server must validate the
client's "commitment" by ensuring that: 1) the client's scalar and
element differ from the server's scalar and element; 2) the client's
scalar value is greater than zero (0) and less than the order of the
group, q; and 3) that the Element is valid for the chosen group (see
Section 3.2.2 and Section 3.2.1 for how to determin whether an
Element is valid for a particular group. Note that if the Element is
a compressed point on an elliptic curve it MUST be uncompressed
before checking its validity. If any of these three conditions are
not met the server MUST abort the exchange.
4.4. Computing the Premaster Secret
The client uses the server's scalar and Element, denoted here as
ServerKeyExchange.scalar and ServerKeyExchange.Element, and the
random private value, denoted here as client.private, she created as
part of the generation of her "commit" to compute an intermediate
value, z, as indicated:
z = F(scalar-op(client.private,
element-op(ServerKeyExchange.Element,
scalar-op(ServerKeyExchange.scalar, PE))))
With the same notation as above, the server the client's scalar and
Element, and his random private value, denoted here as
server.private, he created as part of the generation of his "commit"
to compute the premaster secret as follows:
z = F(scalar-op(server.private,
element-op(ClientKeyExchange.Element,
scalar-op(ClientKeyExchange.scalar, PE))))
The intermediate value, z, is then used as the premaster secret after
any leading bytes of z that contain all zero bits have been stripped
off.
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5. Ciphersuite Definition
This memo adds the following ciphersuites:
CipherSuite TLS_FFCPWD_WITH_3DES_EDE_CBC_SHA = ( TBD, TBD );
CipherSuite TLS_FFCPWD_WITH_AES_128_CBC_SHA = (TBD, TBD );
CipherSuite TLS_ECCPWD_WITH_AES_128_CBC_SHA = (TBD, TBD );
CipherSuite TLS_ECCPWD_WITH_AES_128_GCM_SHA256 = (TBD, TBD );
CipherSuite TLS_ECCPWD_WITH_AES_256_GCM_SHA384 = (TBD, TBD );
CipherSuite TLS_FFCPWD_WITH_AES_128_CCM_SHA = (TBD, TBD );
CipherSuite TLS_ECCPWD_WITH_AES_128_CCM_SHA = (TBD, TBD );
CipherSuite TLS_ECCPWD_WITH_AES_128_CCM_SHA256 = (TBD, TBD );
CipherSuite TLS_ECCPWD_WITH_AES_256_CCM_SHA384 = (TBD, TBD );
Implementations conforming to this specification MUST support the
TLS_ECCPWD_WITH_AES_128_CBC_SHA ciphersuite; they SHOULD support
TLS_ECCPWD_WITH_AES_128_CCM_SHA, TLS_FFCPWD_WITH_AES_128_CCM_SHA,
TLS_ECCPWD_WITH_AES_128_GCM_SHA256,
TLS_ECCPWD_WITH_AES_256_GCM_SHA384; and MAY support the remaining
ciphersuites.
When negotiated with a version of TLS prior to 1.2, the Pseudo-Random
Function (PRF) from that version is used; otherwise, the PRF is the
TLS PRF [RFC5246] using the hash function indicated by the
ciphersuite. Regardless of the TLS version, the TLS-PWD random
function, H, is always instantiated with the hash algorithm indicated
by the ciphersuite.
For those ciphersuites that use Cipher Block Chaining (CBC)
[SP800-38A] mode, the MAC is HMAC [RFC2104] with the hash function
indicated by the ciphersuite.
6. Acknowledgements
The authenticated key exchange defined here has also been defined for
use in 802.11 networks, as an EAP method, and as an authentication
method for IKE. Each of these specifications has elicited very
helpful comments from a wide collection of people that have allowed
the definition of the authenticated key exchange to be refined and
improved.
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The authors would like to thank Scott Fluhrer for discovering the
"password as exponent" attack that was possible in an early version
of this key exchange and for his very helpful suggestions on the
techniques for fixing the PE to prevent it. The authors would also
like to thank Hideyuki Suzuki for his insight in discovering an
attack against a previous version of the underlying key exchange
protocol. Special thanks to Lily Chen for helpful discussions on
hashing into an elliptic curve. Rich Davis suggested the defensive
checks that are part of the processing of the ServerKeyExchange and
ClientKeyExchange messages, and his various comments have greatly
improved the quality of this memo and the underlying key exchange on
which it is based.
Martin Rex, Peter Gutmann, Marsh Ray, and Rene Struik, discussed the
possibility of a side-channel attack against the hunting-and-pecking
loop on the TLS mailing list. That discussion prompted the addition
of the security parameter, m, to the hunting-and-pecking loop. Scott
Flurer suggested the blinding technique to test whether a value is a
quadratic residue modulo a prime in a manner that does not leak
information about the value being tested.
7. IANA Considerations
IANA SHALL assign two values for a new TLS extention type from the
TLS ExtensionType Registry defined in [RFC5246] with the name
"pwd_protect" and "pwd_clear". The RFC editor SHALL replace TBD1 and
TBD2 in Section 4.3.1 with the IANA-assigned value for these
extensions.
IANA SHALL assign nine new ciphersuites from the TLS Ciphersuite
Registry defined in [RFC5246] for the following ciphersuites:
CipherSuite TLS_FFCPWD_WITH_3DES_EDE_CBC_SHA = ( TBD, TBD );
CipherSuite TLS_FFCPWD_WITH_AES_128_CBC_SHA = (TBD, TBD );
CipherSuite TLS_ECCPWD_WITH_AES_128_CBC_SHA = (TBD, TBD );
CipherSuite TLS_ECCPWD_WITH_AES_128_GCM_SHA256 = (TBD, TBD );
CipherSuite TLS_ECCPWD_WITH_AES_256_GCM_SHA384 = (TBD, TBD );
CipherSuite TLS_FFCPWD_WITH_AES_128_CCM_SHA = (TBD, TBD );
CipherSuite TLS_ECCPWD_WITH_AES_128_CCM_SHA = (TBD, TBD );
CipherSuite TLS_ECCPWD_WITH_AES_128_CCM_SHA256 = (TBD, TBD );
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CipherSuite TLS_ECCPWD_WITH_AES_256_CCM_SHA384 = (TBD, TBD );
The RFC editor SHALL replace (TBD, TBD) in all the ciphersuites
defined in Section 5 with the appropriate IANA-assigned values. The
"DTLS-OK" column in the ciphersuite registry SHALL be set to "Y" for
all ciphersuites defined in this memo.
8. Security Considerations
A security proof of this key exchange in the random oracle model is
found in [lanskro].
A passive attacker against this protocol will see the
ServerKeyExchange and the ClientKeyExchange containing the server's
scalar and Element, and the client's scalar and Element,
respectively. The client and server effectively hide their secret
private value by masking it modulo the order of the selected group.
If the order is "q", then there are approximately "q" distinct pairs
of numbers that will sum to the scalar values observed. It is
possible for an attacker to iterate through all such values but for a
large value of "q", this exhaustive search technique is
computationally infeasible. The attacker would have a better chance
in solving the discrete logarithm problem, which we have already
assumed (see Section 3.5) to be an intractable problem.
A passive attacker can take the Element from either the
ServerKeyExchange or the ClientKeyExchange and try to determine the
random "mask" value used in its construction and then recover the
other party's "private" value from the scalar in the same message.
But this requires the attacker to solve the discrete logarithm
problem which we assumed was intractable.
Both the client and the server obtain a shared secret, the premaster
secret, based on a secret group element and the private information
they contributed to the exchange. The secret group element is based
on the password. If they do not share the same password they will be
unable to derive the same secret group element and if they don't
generate the same secret group element they will be unable to
generate the same premaster secret. Seeing a finished message along
with the ServerKeyExchange and ClientKeyExchange will not provide any
additional advantage of attack since it is generated with the
unknowable premaster secret.
An active attacker impersonating the client can induce a server to
send a ServerKeyExchange containing the server's scalar and Element.
It can attempt to generate a ClientKeyExchange and send to the server
but the attacker is required to send a finished message first so the
only information she can obtain in this attack is less than the
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information she can obtain from a passive attack, so this particular
active attack is not very fruitful.
An active attacker can impersonate the server and send a forged
ServerKeyExchange after receiving the ClientHello. The attacker then
waits until it receives the ClientKeyExchange and finished message
from the client. Now the attacker can attempt to run through all
possible values of the password, computing PE (see Section 4.2),
computing candidate premaster secrets (see Section 4.4), and
attempting to recreate the client's finished message.
But the attacker committed to a single guess of the password with her
forged ServerKeyExchange. That value was used by the client in her
computation of the premaster secret which was used to produce the
finished message. Any guess of the password which differs from the
one used in the forged ServerKeyExchange would result in each side
using a different PE in the computation of the premaster secret and
therefore the finished message cannot be verified as correct, even if
a subsequent guess, while running through all possible values, was
correct. The attacker gets one guess, and one guess only, per active
attack.
Instead of attempting to guess at the password, an attacker can
attempt to determine PE and then launch an attack. But PE is
determined by the output of the random function, H, which is
indistinguishable from a random source since H is assumed to be a
"random oracle" (Section 3.5). Therefore, each element of the finite
cyclic group will have an equal probability of being the PE. The
probability of guessing PE will be 1/q, where q is the order of the
group. For a large value of "q" this will be computationally
infeasible.
The implications of resistance to dictionary attack are significant.
An implementation can provision a password in a practical and
realistic manner-- i.e. it MAY be a character string and it MAY be
relatively short-- and still maintain security. The nature of the
pool of potential passwords determines the size of the pool, D, and
countermeasures can prevent an attacker from determining the password
in the only possible way: repeated, active, guessing attacks. For
example, a simple four character string using lower-case English
characters, and assuming random selection of those characters, will
result in D of over four hundred thousand. An attacker would need to
mount over one hundred thousand active, guessing attacks (which will
easily be detected) before gaining any significant advantage in
determining the pre-shared key.
Countermeasures to deal with successive active, guessing attacks are
only possible by noticing a certain username is failing repeatedly
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over a certain period of time. Attacks which attempt to find a
password for a random user are more difficult to detect. For
instance, if a device uses a serial number as a username and the pool
of potential passwords is sufficiently small, a more effective attack
would be to select a password and try all potential "users" to
disperse the attack and confound countermeasures. It is therefore
RECOMMENDED that implementations of TLS-PWD keep track of the total
number of failed authentications regardless of username in an effort
to detect and thwart this type of attack.
The benefits of resistance to dictionary attack can be lessened by a
client using the same passwords with multiple servers. An attacker
could re-direct a session from one server to the other if the
attacker knew that the intended server stored the same password for
the client as another server.
An adversary that has access to, and a considerable amount of control
over, a client or server could attempt to mount a side-channel attack
to determine the number of times it took for a certain password (plus
client random and server random) to select a password element. Each
such attack could result in a successive paring-down of the size of
the pool of potential passwords, resulting in a manageably small set
from which to launch a series of active attacks to determine the
password. A security parameter, m, is used to normalize the amount
of work necessary to determine the password element (see
Section 4.2). The probability that a password will require more than
m iterations is roughly (q/2p)^m for ECC groups and (q/p)^m for FFC
groups, so it is possible to mitigate side channel attack at the
expense of a constant cost per connection attempt. But if a
particular password requires more than k iterations it will leak k
bits of information to the side-channel attacker, which for some
dictionaries will uniquely identify the password. Therefore, the
security parameter, m, needs to be set with great care. It is
RECOMMENDED that an implementation set the security parameter, m, to
a value of at least forty (40) which will put the probability that
more than forty iterations are needed in the order of one in one
trillion (1:1,000,000,000,000).
The server uses a database of salted passwords. While this will
prevent an adversary who gains access to the database from learning
the client's password, it does not prevent such an adversary from
impersonating the client back to the server. Each side uses the
salted password, called the base, as the authenticaiton credential so
the database of salted passwords MUST be afforded the security of a
database of plaintext passwords.
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Authentication is performed by proving knowledge of the password.
Any third party that knows the password shared by the client and
server can impersonate one to the other.
The static-ephemeral Diffie-Hellman exchange used to protect
usernames requires the server to reuse its Diffie-Hellman public key.
To prevent an invalid curve attack, an entity that reuses its Diffie-
Hellman public key needs to check whether the received ephemeral
public key is actually a point on the curve. This is done explicitly
as part of the server's reconstruction of the client's public key out
of only its x-coordinate ("compact representation").
9. Human Rights Considerations
At the time of publication, there was a growing interest in
considering the human rights impact of IETF (and IRTF) work. As
such, the Human Rights Considerations of TLS-PWD are presented.
The key exchange underlying TLS-PWD uses public key cryptography to
perform authentication and authenticated key exchange. The keys it
produces can be used to establish secure connections between two
people to protect their communication. Implementations of TLS-PWD,
like implementations of other TLS ciphersuites that perform
authentication and authenticted key establishment, are considered
'armaments' or 'munitions' by many governments around the world.
The most fundamental of Human Rights is the right to protect oneself.
The right to keep and bear arms is an example of this right.
Implementations of TLS-PWD can be used as arms, kept and borne, to
defend oneself against all manner of attackers-- criminals,
governments, laywers, etc. TLS-PWD is a powerful tool in the
promotion and defense of Universal Human Rights.
10. Implementation Considerations
The selection of the ciphersuite and selection of the particular
finite cyclic group to use with the ciphersuite are divorced in this
memo but they remain intimately close.
It is RECOMMENDED that implementations take note of the strength
estimates of particular groups and to select a ciphersuite providing
commensurate security with its hash and encryption algorithms. A
ciphersuite whose encryption algorithm has a keylength less than the
strength estimate, or whose hash algorithm has a blocksize that is
less than twice the strength estimate SHOULD NOT be used.
For example, the elliptic curve named brainpoolP256r1 (whose IANA-
assigned number is 26) provides an estimated 128 bits of strength and
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would be compatible with an encryption algorithm supporting a key of
that length, and a hash algorithm that has at least a 256-bit
blocksize. Therefore, a suitable ciphersuite to use with
brainpoolP256r1 could be TLS_ECCPWD_WITH_AES_128_GCM_SHA256 (see
Appendix A for an example of such an exchange).
Resistance to dictionary attack means that the attacker must launch
an active attack to make a single guess at the password. If the size
of the pool from which the password was extracted was D, and each
password in the pool has an equal probability of being chosen, then
the probability of success after a single guess is 1/D. After X
guesses, and removal of failed guesses from the pool of possible
passwords, the probability becomes 1/(D-X). As X grows so does the
probability of success. Therefore it is possible for an attacker to
determine the password through repeated brute-force, active, guessing
attacks. Implementations SHOULD take note of this fact and choose an
appropriate pool of potential passwords-- i.e. make D big.
Implementations SHOULD also take countermeasures, for instance
refusing authentication attempts by a particular username for a
certain amount of time, after the number of failed authentication
attempts reaches a certain threshold. No such threshold or amount of
time is recommended in this memo.
11. References
11.1. Normative References
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, DOI
10.17487/RFC2104, February 1997,
<http://www.rfc-editor.org/info/rfc2104>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3454] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454, DOI
10.17487/RFC3454, December 2002,
<http://www.rfc-editor.org/info/rfc3454>.
[RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User Names
and Passwords", RFC 4013, DOI 10.17487/RFC4013, February
2005, <http://www.rfc-editor.org/info/rfc4013>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/
RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
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[RFC5297] Harkins, D., "Synthetic Initialization Vector (SIV)
Authenticated Encryption Using the Advanced Encryption
Standard (AES)", RFC 5297, DOI 10.17487/RFC5297, October
2008, <http://www.rfc-editor.org/info/rfc5297>.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869, DOI 10.17487/
RFC5869, May 2010,
<http://www.rfc-editor.org/info/rfc5869>.
11.2. Informative References
[FIPS186-3]
National Institute of Standards and Technology, "Digital
Signature Standard (DSS)", Federal Information Processing
Standards Publication 186-3, .
[RANDOR] Bellare, M. and P. Rogaway, "Random Oracles are Practical:
A Paradigm for Designing Efficient Protocols", Proceedings
of the 1st ACM Conference on Computer and Communication
Security, ACM Press, 1993.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<http://www.rfc-editor.org/info/rfc4086>.
[RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
for Transport Layer Security (TLS)", RFC 4492, DOI
10.17487/RFC4492, May 2006,
<http://www.rfc-editor.org/info/rfc4492>.
[RFC6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
Curve Cryptography Algorithms", RFC 6090, February 2011.
[RFC7030] Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed.,
"Enrollment over Secure Transport", RFC 7030, DOI
10.17487/RFC7030, October 2013,
<http://www.rfc-editor.org/info/rfc7030>.
[RFC7664] Harkins, D., Ed., "Dragonfly Key Exchange", RFC 7664, DOI
10.17487/RFC7664, November 2015,
<http://www.rfc-editor.org/info/rfc7664>.
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[SP800-38A]
National Institute of Standards and Technology,
"Recommendation for Block Cipher Modes of Operation--
Methods and Techniques", NIST Special Publication 800-38A,
December 2001.
[SP800-56A]
Barker, E., Johnson, D., and M. Smid, "Recommendations for
Pair-Wise Key Establishment Schemes Using Discrete
Logarithm Cryptography", NIST Special Publication 800-56A,
March 2007.
[lanskro] Lancrenon, J. and M. Skrobot, "On the Provable Security of
the Dragonfly Protocol", Proceedings of 18th International
Information Security Conference (ISC 2015), pp 244-261,
DOI 10.1007/978-3-319-23318-5_14, September 2015.
Appendix A. Example Exchange
(Note: at the time of publication of this memo ciphersuites have
not yet been assigned by IANA and the exchange that follows uses
the private numberspace).
username: fred
password: barney
---- prior to running TLS-PWD ----
server generates salt:
96 3c 77 cd c1 3a 2a 8d 75 cd dd d1 e0 44 99 29
84 37 11 c2 1d 47 ce 6e 63 83 cd da 37 e4 7d a3
and a base:
6e 7c 79 82 1b 9f 8e 80 21 e9 e7 e8 26 e9 ed 28
c4 a1 8a ef c8 75 0c 72 6f 74 c7 09 61 d7 00 75
---- state derived during the TLS-PWD exchange ----
client and server agree to use brainpoolP256r1
client and server generate PE:
PE.x:
29 b2 38 55 81 9f 9c 3f c3 71 ba e2 84 f0 93 a3
a4 fd 34 72 d4 bd 2e 9d f7 15 2d 22 ab 37 aa e6
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server private and mask:
private:
21 d9 9d 34 1c 97 97 b3 ae 72 df d2 89 97 1f 1b
74 ce 9d e6 8a d4 b9 ab f5 48 88 d8 f6 c5 04 3c
mask:
0d 96 ab 62 4d 08 2c 71 25 5b e3 64 8d cd 30 3f
6a b0 ca 61 a9 50 34 a5 53 e3 30 8d 1d 37 44 e5
client private and mask:
private:
17 1d e8 ca a5 35 2d 36 ee 96 a3 99 79 b5 b7 2f
a1 89 ae 7a 6a 09 c7 7f 7b 43 8a f1 6d f4 a8 8b
mask:
4f 74 5b df c2 95 d3 b3 84 29 f7 eb 30 25 a4 88
83 72 8b 07 d8 86 05 c0 ee 20 23 16 a0 72 d1 bd
both parties generate pre-master secret and master secret
pre-master secret:
01 f7 a7 bd 37 9d 71 61 79 eb 80 c5 49 83 45 11
af 58 cb b6 dc 87 e0 18 1c 83 e7 01 e9 26 92 a4
master secret:
65 ce 15 50 ee ff 3d aa 2b f4 78 cb 84 29 88 a1
60 26 a4 be f2 2b 3f ab 23 96 e9 8a 7e 05 a1 0f
3d 8c ac 51 4d da 42 8d 94 be a9 23 89 18 4c ad
---- ssldump output of exchange ----
New TCP connection #1: Charlene Client <-> Sammy Server
1 1 0.0018 (0.0018) C>SV3.3(173) Handshake
ClientHello
Version 3.3
random[32]=
52 8f bf 52 17 5d e2 c8 69 84 5f db fa 83 44 f7
d7 32 71 2e bf a6 79 d8 64 3c d3 1a 88 0e 04 3d
cipher suites
TLS_ECCPWD_WITH_AES_128_GCM_SHA256_PRIV
TLS_ECCPWD_WITH_AES_256_GCM_SHA384_PRIV
Unknown value 0xff
compression methods
NULL
extensions
TLS-PWD unprotected name[5]=
04 66 72 65 64
elliptic curve point format[4]=
03 00 01 02
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elliptic curve list[58]=
00 38 00 0e 00 0d 00 1c 00 19 00 0b 00 0c 00 1b
00 18 00 09 00 0a 00 1a 00 16 00 17 00 08 00 06
00 07 00 14 00 15 00 04 00 05 00 12 00 13 00 01
00 02 00 03 00 0f 00 10 00 11
Packet data[178]=
16 03 03 00 ad 01 00 00 a9 03 03 52 8f bf 52 17
5d e2 c8 69 84 5f db fa 83 44 f7 d7 32 71 2e bf
a6 79 d8 64 3c d3 1a 88 0e 04 3d 00 00 06 ff b3
ff b4 00 ff 01 00 00 7a b8 aa 00 05 04 66 72 65
64 00 0b 00 04 03 00 01 02 00 0a 00 3a 00 38 00
0e 00 0d 00 1c 00 19 00 0b 00 0c 00 1b 00 18 00
09 00 0a 00 1a 00 16 00 17 00 08 00 06 00 07 00
14 00 15 00 04 00 05 00 12 00 13 00 01 00 02 00
03 00 0f 00 10 00 11 00 0d 00 22 00 20 06 01 06
02 06 03 05 01 05 02 05 03 04 01 04 02 04 03 03
01 03 02 03 03 02 01 02 02 02 03 01 01 00 0f 00
01 01
1 2 0.0043 (0.0024) S>CV3.3(94) Handshake
ServerHello
Version 3.3
random[32]=
52 8f bf 52 43 78 a1 b1 3b 8d 2c bd 24 70 90 72
13 69 f8 bf a3 ce eb 3c fc d8 5c bf cd d5 8e aa
session_id[32]=
ef ee 38 08 22 09 f2 c1 18 38 e2 30 33 61 e3 d6
e6 00 6d 18 0e 09 f0 73 d5 21 20 cf 9f bf 62 88
cipherSuite TLS_ECCPWD_WITH_AES_128_GCM_SHA256_PRIV
compressionMethod NULL
extensions
renegotiate[1]=
00
elliptic curve point format[4]=
03 00 01 02
heartbeat[1]=
01
Packet data[99]=
16 03 03 00 5e 02 00 00 5a 03 03 52 8f bf 52 43
78 a1 b1 3b 8d 2c bd 24 70 90 72 13 69 f8 bf a3
ce eb 3c fc d8 5c bf cd d5 8e aa 20 ef ee 38 08
22 09 f2 c1 18 38 e2 30 33 61 e3 d6 e6 00 6d 18
0e 09 f0 73 d5 21 20 cf 9f bf 62 88 ff b3 00 00
12 ff 01 00 01 00 00 0b 00 04 03 00 01 02 00 0f
00 01 01
1 3 0.0043 (0.0000) S>CV3.3(141) Handshake
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ServerKeyExchange
params
salt[32]=
96 3c 77 cd c1 3a 2a 8d 75 cd dd d1 e0 44 99 29
84 37 11 c2 1d 47 ce 6e 63 83 cd da 37 e4 7d a3
EC parameters = 3
curve id = 26
element[65]=
04 22 bb d5 6b 48 1d 7f a9 0c 35 e8 d4 2f cd 06
61 8a 07 78 de 50 6b 1b c3 88 82 ab c7 31 32 ee
f3 7f 02 e1 3b d5 44 ac c1 45 bd d8 06 45 0d 43
be 34 b9 28 83 48 d0 3d 6c d9 83 24 87 b1 29 db
e1
scalar[32]=
2f 70 48 96 69 9f c4 24 d3 ce c3 37 17 64 4f 5a
df 7f 68 48 34 24 ee 51 49 2b b9 66 13 fc 49 21
Packet data[146]=
16 03 03 00 8d 0c 00 00 89 00 20 96 3c 77 cd c1
3a 2a 8d 75 cd dd d1 e0 44 99 29 84 37 11 c2 1d
47 ce 6e 63 83 cd da 37 e4 7d a3 03 00 1a 41 04
22 bb d5 6b 48 1d 7f a9 0c 35 e8 d4 2f cd 06 61
8a 07 78 de 50 6b 1b c3 88 82 ab c7 31 32 ee f3
7f 02 e1 3b d5 44 ac c1 45 bd d8 06 45 0d 43 be
34 b9 28 83 48 d0 3d 6c d9 83 24 87 b1 29 db e1
00 20 2f 70 48 96 69 9f c4 24 d3 ce c3 37 17 64
4f 5a df 7f 68 48 34 24 ee 51 49 2b b9 66 13 fc
49 21
1 4 0.0043 (0.0000) S>CV3.3(4) Handshake
ServerHelloDone
Packet data[9]=
16 03 03 00 04 0e 00 00 00
1 5 0.0086 (0.0043) C>SV3.3(104) Handshake
ClientKeyExchange
element[65]=
04 a0 c6 9b 45 0b 85 ae e3 9f 64 6b 6e 64 d3 c1
08 39 5f 4b a1 19 2d bf eb f0 de c5 b1 89 13 1f
59 5d d4 ba cd bd d6 83 8d 92 19 fd 54 29 91 b2
c0 b0 e4 c4 46 bf e5 8f 3c 03 39 f7 56 e8 9e fd
a0
scalar[32]=
66 92 44 aa 67 cb 00 ea 72 c0 9b 84 a9 db 5b b8
24 fc 39 82 42 8f cd 40 69 63 ae 08 0e 67 7a 48
Packet data[109]=
16 03 03 00 68 10 00 00 64 41 04 a0 c6 9b 45 0b
85 ae e3 9f 64 6b 6e 64 d3 c1 08 39 5f 4b a1 19
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2d bf eb f0 de c5 b1 89 13 1f 59 5d d4 ba cd bd
d6 83 8d 92 19 fd 54 29 91 b2 c0 b0 e4 c4 46 bf
e5 8f 3c 03 39 f7 56 e8 9e fd a0 00 20 66 92 44
aa 67 cb 00 ea 72 c0 9b 84 a9 db 5b b8 24 fc 39
82 42 8f cd 40 69 63 ae 08 0e 67 7a 48
1 6 0.0086 (0.0000) C>SV3.3(1) ChangeCipherSpec
Packet data[6]=
14 03 03 00 01 01
1 7 0.0086 (0.0000) C>SV3.3(40) Handshake
Packet data[45]=
16 03 03 00 28 44 cd 3f 26 ed 64 9a 1b bb 07 c7
0c 6d 3e 28 af e6 32 b1 17 29 49 a1 14 8e cb 7a
0b 4b 70 f5 1f 39 c2 9c 7b 6c cc 57 20
1 8 0.0105 (0.0018) S>CV3.3(1) ChangeCipherSpec
Packet data[6]=
14 03 03 00 01 01
1 9 0.0105 (0.0000) S>CV3.3(40) Handshake
Packet data[45]=
16 03 03 00 28 fd da 3c 9e 48 0a e7 99 ba 41 8c
9f fd 47 c8 41 2c fd 22 10 77 3f 0f 78 54 5e 41
a2 21 94 90 12 72 23 18 24 21 c3 60 a4
1 10 0.0107 (0.0002) C>SV3.3(100) application_data
Packet data....
Authors' Addresses
Dan Harkins (editor)
HP Enterprise
1322 Crossman Avenue
Sunnyvale, CA 94089-1113
United States of America
Email: dharkins@arubanetworks.com
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Dave Halasz (editor)
Halasz Ventures
8401 Chagrin Road, Suite 10A
Chagrin Falls, OH 44023
United States of America
Email: david.e.halasz@gmail.com
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