Network Working Group S. Josefsson
Internet-Draft SJD AB
Updates: 4492, 5246 (if approved) M. Pegourie-Gonnard
Intended status: Informational Independent / PolarSSL
Expires: October 15, 2014 April 13, 2014
Curve25519 for ephemeral key exchange in Transport Layer Security (TLS)
draft-josefsson-tls-curve25519-05
Abstract
This document specifies the use of Curve25519 for ephemeral key
exchange in the Transport Layer Security (TLS) protocol, as well as
its DTLS variant. It updates RFC 5246 (TLS 1.2) and RFC 4492
(Elliptic Curve Cryptography for TLS).
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 October 15, 2014.
Copyright Notice
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document authors. All rights reserved.
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1. Introduction
In [Curve25519], a new elliptic curve function for use in
cryptographic applications was specified. Curve25519 is a Diffie-
Hellman function designed with performance and security in mind.
[RFC4492] defines the usage of elliptic curves for authentication and
key agreement in TLS 1.0 and TLS 1.1, and these mechanisms are also
applicable to TLS 1.2 [RFC5246]. The use of ECC curves for key
exchange requires the definition and assignment of additional
NamedCurve IDs. This document specify that value for Curve25519, as
well as the minor changes in key selection and representation that
are required to accommodate for Curve25519's slightly different
nature.
This document only describes usage of Curve25519 for ephemeral key
exchange (ECDHE). It does not define its use for signature, since
the primitive considered here is a Diffie-Hellman function; the
related signature scheme, Ed25519, is outside the scope of this
document. The use of Curve25519 with long-term keys embedded in
X.509 certificates is also out of scope here.
1.1. Requirements 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 [RFC2119].
2. Data Structures and Computations
2.1. Cryptographic computations
All cryptographic computations are done using the Curve25519 function
defined in [Curve25519]. In this memo, this function is considered
as a black box that takes as input a (secret key, public key) pair
and outputs a public key. Public keys are defined as strings of 32
bytes. Secret keys are defined as 255 bits numbers such as the high-
order bit (bit 254) is set, and the three lowest-order bits are
unset. In addition, a common public key, denoted by G, is shared by
all users.
An ECDHE key exchange using Curve25519 goes as follows. Each party
picks a secret key d uniformly at random and computes the
corresponding public key x = Curve25519(d, G). Parties exchange
their public keys (see Section 2.3) and compute a shared secret as
x_S = Curve25519(d, x_peer). This shared secret is used directly as
the premaster secret, which is always exactly 32 bytes when ECDHE
with Curve25519 is used.
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A complete description of the Curve25519 function, as well as a few
implementation notes, are provided in Appendix A.
2.2. Curve negotiation and new NamedCurve value
Curve negotiation uses the mechanisms introduced by [RFC4492],
without modification except the following restriction: in the
ECParameters structure, only the named_curve case can be used with
Curve25519. Accordingly, arbitrary_explicit_prime_curves in the
Supported Curves extension does not imply support for Curve25519,
even though the Curve25519 function happens to be defined using an
elliptic curve over a prime field.
The reason for this restriction is that explicit_prime is only suited
to the so-called Short Weierstrass representation of elliptic curves,
while Curve25519 uses a different representation for performance and
security reasons.
This document adds a new NamedCurve value for Curve25519 as follows.
enum {
Curve25519(TBD1),
} NamedCurve;
Curve25519 is suitable for use with DTLS [RFC6347].
Since Curve25519 is not designed to be used in signatures, clients
who offer ECDHE_ECDSA ciphersuites and advertise support for
Curve25519 in the elliptic_curves ClientHello extension SHOULD also
advertise support for at least one curve suitable for ECDSA. Servers
MUST NOT select an ECDHE_ECDSA ciphersuite if there are no common
curves suitable for ECDSA.
2.3. Public Key representation and new ECPointFormat value
This section defines a new point format suitable to encode Curve25519
public keys, as well as an identifier to negotiate this new format in
TLS, and includes guidance on their use.
The curves defined in [RFC4492] define a public key as a point on the
curve. In order to exchange public keys, the points are serialized
as a string of bytes using one of the formats defined in [SEC1].
These encodings begin with a leading byte identifying the format,
followed by a string of bytes, whose length is uniquely determined by
the leading byte and curve used.
Since Curve25519 public keys already are string of bytes, no
serialization is needed. However, a leading byte with value 0x41 is
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prepended to the public key to identify the format. The goal,
besides consistency with the SEC1 formats, is to allow using other
formats with Curve25519 in the future if needed.
In order to negotiate this format in TLS, a new ECPointFormat is
defined as follows.
enum {
montgomery_x_le(TBD2),
} ECPointFormat;
This format is currently the only format defined for use with
Curve25519. Clients offering Curve25519 in the Supported Elliptic
Curves extension MUST also offer montgomery_x_le in the Supported
Point Format extension. Servers selecting Curve25519 for key
exchange MUST include montgomery_x_le in their Supported Point Format
extension. Servers willing to use Curve25519 MUST NOT assume that
the client supports the montgomery_x_le format if the client did not
advertise it explicitly.
When included in a ServerKeyExchange or ClientKeyExchange message,
the public key is wrapped in an ECPoint structure as defined in
[RFC4492], whose payload is as described above. For example, a
public key with value 2A ... 2A appears on the wire as follows
(including the length byte of ECPoint.point).
21 41 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A
2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A
2.4. Public key validation
With the curves defined by [RFC4492], each party must validate the
public key sent by its peer before performing cryptographic
computations with it. Failing to do so allows attackers to gain
information about the private key, to the point that they may recover
the entire private key in a few requests, if that key is not really
ephemeral.
Curve25519 was designed in a way that the result of Curve25519(x, d)
will never reveal information about d, provided is was chosen as
prescribed, for any value of x.
Let's define legitimate values of x as the values that can be
obtained as x = Curve25519(G, d') for some d, and call the other
values illegitimate. The definition of the Curve25519 function shows
that legitimate values all share the following property: the high-
order bit of the last byte is not set.
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Since there are some implementation of the Curve25519 function that
impose this restriction on their input and others that don't,
implementations of Curve25519 in TLS SHOULD reject public keys when
the high-order bit of the last byte is set (in other words, when the
value of the leftmost byte is greater than 0x7F) in order to prevent
implementation fingerprinting.
Other than this recommended check, implementations do not need to
ensure that the public keys they receive are legitimate: this is not
necessary for security with Curve25519.
3. IANA Considerations
IANA is requested to assign numbers for Curve25519 listed in
Section 2.2 to the Transport Layer Security (TLS) Parameters registry
EC Named Curve [IANA-TLS] as follows.
+-------+-------------+---------+-----------+
| Value | Description | DTLS-OK | Reference |
+-------+-------------+---------+-----------+
| TBD1 | Curve25519 | Y | This doc |
+-------+-------------+---------+-----------+
IANA is also requested to assign numbers for Curve25519 listed in
Section 2.3 to the Transport Layer Security (TLS) Parameters registry
EC Point Format [IANA-TLS] as follows.
+-------+-----------------+---------+-----------+
| Value | Description | DTLS-OK | Reference |
+-------+-----------------+---------+-----------+
| TBD2 | montgomery_x_le | Y | This doc |
+-------+-----------------+---------+-----------+
4. Security Considerations
The security considerations of [RFC5246] and most of the security
considerations of [RFC4492] apply accordingly.
Curve25519 is designed to facilitate the production of high-
performance constant-time implementations of the Curve25519 function.
Implementors are encouraged to use a constant-time implementation of
the Curve25519 function. This point is of crucial importance if the
implementation chooses to reuse its supposedly ephemeral key pair for
many key exchanges, which some implementations do in order to improve
performance.
Curve25519 is believed to be at least as secure as the secp256r1
curve defined in [RFC4492], also know as NIST P-256. While the NIST
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curves are advertised as being chosen verifiably at random, there is
no explanation for the seeds used to generate them. In contrast, the
process used to pick Curve25519 is fully documented and rigid enough
so that independent verification has been done. This is widely seen
as a security advantage for Curve25519, since it prevents the
generating party from maliciously manipulating the parameters.
Another family of curves available in TLS, generated in a fully
verifiable way, is the Brainpool curves [RFC7027]. Specifically,
brainpoolP256 is expected to provide a level of security comparable
to Curve25519 and NIST P-256. However, due to the use of pseudo-
random prime, it is significantly slower than NIST P-256, which is
itself slower than Curve25519.
See [SafeCurves] for more comparisons between curves.
5. Acknowledgements
Several people provided comments and suggestions that helped improve
this document: Kurt Roeckx, Andrey Jivsov, Robert Ransom, Rich Salz,
David McGrew.
6. References
6.1. Normative References
[Curve25519]
Bernstein, J., "Curve25519: New Diffie-Hellman Speed
Records", LNCS 3958, pp. 207-228, February 2006,
.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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, May 2006.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012.
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6.2. Informative References
[IANA-TLS]
Internet Assigned Numbers Authority, "Transport Layer
Security (TLS) Parameters", .
[SafeCurves]
Bernstein, D. and T. Lange, "SafeCurves: choosing safe
curves for elliptic-curve cryptography.", January 2014,
.
[RFC7027] Merkle, J. and M. Lochter, "Elliptic Curve Cryptography
(ECC) Brainpool Curves for Transport Layer Security
(TLS)", RFC 7027, October 2013.
[SEC1] Certicom Research, , "Standards for Efficient Cryptography
(SEC) 1", September 2000.
[EFD] Bernstein, D. and T. Lange, "Explicit-Formulas Database:
XZ coordinates for Montgomery curves", January 2014,
.
[NaCl] Bernstein, D., "Cryptography in NaCL", March 2013,
.
Appendix A. The curve25519 function
A.1. Formulas
This section completes Section 2.1 by defining the Curve25519
function and the common public key G. It is meant as an alternative,
self-contained specification for the Curve25519 function, possibly
easier to follow than the original paper for most implementors.
A.1.1. Field Arithmetic
Throughout this section, P denotes the integer 2^255-19 =
0x7FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFED.
The letters X and Z, and their numbered variants such as x1, z2, etc.
denote integers modulo P, that is integers between 0 and P-1 and
every operation between them is implictly done modulo P. For
addition, subtraction and multiplication this means doing the
operation in the usual way and then replacing the result with the
remainder of its division by P. For division, "X / Z" means
mutliplying (mod P) X by the modular inverse of Z mod P.
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A convenient way to define the modular inverse of Z mod P is as
Z^(P-2) mod P, that is Z to the power of 2^255-21 mod P. It is also a
practical way of computing it, using a square-and-multiply method.
The four operations +, -, *, / modulo P are known as the field
operations. Techniques for efficient implementation of the field
operations are outside the scope of this document.
A.1.2. Conversion to and from internal format
For the purpose of this section, we will define a Curve25519 point as
a pair (X, Z) were X and Z are integers mod P (as defined above).
Though public keys were defined to be strings of 32 bytes, internally
they are represented as curve points. This subsection describes the
conversion process as two functions: PubkeyToPoint and PointToPubkey.
PubkeyToPoint:
Input: a public key b_0, ..., b_31
Output: a Curve25519 point (X, Z)
1. Set X = b_0 + 256 * b_1 + ... + 256^31 * b_31 mod P
2. Set Z = 1
3. Output (X, Z)
PointToPubkey:
Input: a Curve25519 point (X, Z)
Output: a public key b_0, ..., b_31
1. Set x1 = X / Z mod P
2. Set b_0, ... b_31 such that
x1 = b_0 + 256 * b_1 + ... + 256^31 * b_31 mod P
3. Output b_0, ..., b_31
A.1.3. Scalar Multiplication
We first introduce the DoubleAndAdd function, defined as follows
(formulas taken from [EFD]).
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DoubleAndAdd:
Input: two points (X2, Z2), (X3, Z3), and an integer mod P: X1
Output: two points (X4, Z4), (X5, Z5)
Constant: the integer mod P: a24 = 121666 = 0x01DB42
Variables: A, AA, B, BB, E, C, D, DA, CB are integers mod P
1. Do the following computations mod P:
A = X2 + Z2
AA = A2
B = X2 - Z2
BB = B2
E = AA - BB
C = X3 + Z3
D = X3 - Z3
DA = D * A
CB = C * B
X5 = (DA + CB)^2
Z5 = X1 * (DA - CB)^2
X4 = AA * BB
Z4 = E * (BB + a24 * E)
2. Output (X4, Z4) and (X5, Z5)
This may be taken as the abstract definition of an arbitrary-looking
function. However, let's mention "the true meaning" of this
function, without justification, in order to help the reader make
more sense of it. It is possible to define operations "+" and "-"
between Curve25519 points. Then, assuming (X2, Z2) - (X3, Z3) = (X1,
1), the DoubleAndAdd function returns points such that (X4, Z4) =
(X2, Z2) + (X2, Z2) and (X5, Z5) = (X2, Z2) + (X3, Z3).
Taking the "+" operation as granted, we can define multiplication of
a Curve25519 point by a positive integer as N * (X, Z) = (X, Z) + ...
+ (X, Z), with N point additions. It is possible to compute this
operation, known as scalar multiplication, using an algorithm called
the Montgomery ladder, as follows.
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ScalarMult:
Input: a Curve25519 point: (X, 1) and a 255-bits integer: N
Output: a point (X1, Z1)
Variable: a point (X2, Z2)
0. View N as a sequence of bits b_254, ..., b_0,
with b_254 the most significant bit
and b_0 the least significant bit.
1. Set X1 = 1 and Z1 = 0
2. Set X2 = X and Z2 = 1
3. For i from 254 downwards to 0, do:
If b_i == 0, then:
Set (X2, Z2) and (X1, Z1) to the output of
DoubleAndAdd((X2, Z2), (X1, Z1), X)
else:
Set (X1, Z1) and (X2, Z2) to the output of
DoubleAndAdd((X1, Z1), (X2, Z2), X)
4. Output (X1, Z1)
A.1.4. Conclusion
We are now ready to define the Curve25519 function itself.
Curve25519:
Input: a public key P and a secret key S
Output: a public key Q
Variables: two Curve25519 points (X, Z) and (X1, Z1)
1. Set (X, Z) = PubkeyToPoint(P)
2. Set (X1, Z1) = ScalarMult((X, Z), S)
3. Set Q = PointToPubkey((X1, Z1))
4. Output Q
The common public key G mentioned in the first paragraph of
Section 2.1 is defined as G = PointToPubkey((9, 1).
A.2. Test vectors
The following test vectors are taken from [NaCl]. Compared to this
reference, the private key strings have been applied the ClampC
function of the reference and converted to integers in order to fit
the description given in [Curve25519] and the present memo.
The secret key of party A is denoted by S_a, it public key by P_a,
and similarly for party B. The shared secret is SS.
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S_a = 0x6A2CB91DA5FB77B12A99C0EB872F4CDF
4566B25172C1163C7DA518730A6D0770
P_a = 85 20 F0 09 89 30 A7 54 74 8B 7D DC B4 3E F7 5A
0D BF 3A 0D 26 38 1A F4 EB A4 A9 8E AA 9B 4E 6A
S_b = 0x6BE088FF278B2F1CFDB6182629B13B6F
E60E80838B7FE1794B8A4A627E08AB58
P_b = DE 9E DB 7D 7B 7D C1 B4 D3 5B 61 C2 EC E4 35 37
3F 83 43 C8 5B 78 67 4D AD FC 7E 14 6F 88 2B 4F
SS = 4A 5D 9D 5B A4 CE 2D E1 72 8E 3B F4 80 35 0F 25
E0 7E 21 C9 47 D1 9E 33 76 F0 9B 3C 1E 16 17 42
A.3. Side-channel considerations
Curve25519 was specifically designed so that correct, fast, constant-
time implementations are easier to produce. In particular, using a
Montgomery ladder as described in the previous section ensures that,
for any valid value of the secret key, the same sequence of field
operations are performed, which eliminates a major source of side-
channel leakage.
However, merely using Curve25519 with a Montgomery ladder does not
prevent all side-channels by itself, and some point are the
responsibility of implementors:
1. In step 3 of SclarMult, avoid branches depending on b_i, as well
as memory access patterns depending on b_i, for example by using
safe conditional swaps on the inputs and outputs of DoubleAndAdd.
2. Avoid data-dependant branches and memory access patterns in the
implementation of field operations.
Techniques for implementing the field operations in constant time and
with high performance are out of scope of this document. Let's
mention however that, provided constant-time multiplication is
available, division can be computed in constant time using
exponentiation as described in Appendix A.1.1.
If using constant-time implementations of the field operations is not
convenient, an option to reduce the information leaked this way is to
replace step 2 of the SclarMult function with:
2a. Pick Z uniformly randomly between 1 and P-1 included
2b. Set X2 = X * Z and Z2 = Z
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This method is known as randomizing projective coordinates. However,
it is no guaranteed to avoid all side-channel leaks related to field
operations.
Side-channel attacks are an active reseach domain that still sees new
significant results, so implementors of the Curve25519 function are
advised to follow recent security research closely.
Authors' Addresses
Simon Josefsson
SJD AB
Email: simon@josefsson.org
Manuel Pegourie-Gonnard
Independent / PolarSSL
Email: mpg@elzevir.fr
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