| Network Working Group | D. Kügler |
| Internet-Draft | BSI |
| Intended status: Standards Track | J. Schmidt |
| Expires: April 21, 2016 | secunet Security Networks |
| October 19, 2015 |
Using Password-Authenticated Key Agreement (PAKE) schemes in TLS
draft-schmidt-pake-tls-00
This document describes how to integrate Password-Authenticated Key Agreement (PAKE) schemes into TLS. These schemes enable two parties who share a potentially weak password to derive a common cryptographic key, allowing them to establish a secure channel. The current document defines a generic way to integrate PAKE schemes into TLS. In addition, it demonstrates how to use the well-known Password Authenticated Connection Establishment (PACE) scheme in TLS as an example for the generic construction.
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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].
Assume two or more parties want to communicate in a protected manner. Using a shared password might be a simple possibility, since it is easy to remember and to transfer, e.g. via phone. However, it does not deliver a strong security, in particular not the level that is provided by a cryptographic key. Password-Authenticated Key Agreement (PAKE) schemes provide a combination of both benefits. They allow the parties to derive a cryptographic key from a password to establish a secure communication. A PAKE scheme uses the password in a way that prevents an adversary from performing a brute-force attack without interaction with a legitimate communication partner. This way, an adversary trying to iterate through all possible passwords is detected and hence such attacks are mitigated. This memo shows how to integrate PAKE schemes into TLS.
As an example of how to use PAKE in TLS, it is shown how to integrate the Password Authenticated Connection Establishment (PACE), a realization of such a PAKE. It is balanced in the sense that both parties possess the same representation of the shared password. It comes with a security proof based on number-theoretic assumptions related to the Diffie-Hellman problem using random oracles and ideal ciphers.
This section gives a definitions and preliminaries required for PACE and describes the scheme itself.
The following notation is used in this memo:
Most PAKE schemes rely on finite groups. Here, we consider two different groups: (1) groups over finite fields and (2) groups over elliptic curves. They have in common that, using appropriate parameters, the discrete logarithm problem in both groups is hard to solve.
In the following, if the expressions are valid for both groups, we will use the multiplicative notation, i.e. a*b and a^c, which maps to a+b and c*a for the ECC case.
In order to describe PACE, this memo makes use of the following cryptographic primitives:
Note that the algorithms that realized these primitives are given together with the identifier of the used PAKE scheme in our construction.
In addition, a function Hash2Point that maps from a random bit string to a point, either in the finite field or on the elliptic curve, is chosen. How this function operates depends on the used parameters:
| Name | u | g(u) |
|---|---|---|
| secp256r1 | 1 | 0x5ac635d8aa3a93e7 b3ebbd55769886bc 651d06b0cc53b0f6 3bce3c3e27d26049 |
| secp384r1 | 1 | 0xb3312fa7e23ee7e4 988e056be3f82d19 181d9c6efe814112 0314088f5013875a c656398d8a2ed19d 2a85c8edd3ec2aed |
| secp521r1 | 3 | 0x51953eb9618e1c9a 1f929a21a0b68540 eea2da725b99b315 f3b8b489918ef109 e156193951ec7e93 7b1652c0bd3bb1bf 073573df883d2c34 f1ef451fd46b503f 12 |
| brainpoolP256r1 | 0 | 0x26dc5c6ce94a4b44 f330b5d9bbd77cbf 958416295cf7e1ce 6bccdc18ff8c07b6 |
| brainpoolP384r1 | 0 | 0x04a8c7dd22ce2826 8b39b55416f0447c 2fb77de107dcd2a6 2e880ea53eeb62d5 7cb4390295dbc994 3ab78696fa504c11 |
| brainpoolP512r1 | 0 | 0x3df91610a83441ca ea9863bc2ded5d5a a8253aa10a2ef1c9 8b9ac8b57f1117a7 2bf2c7b9e7c1ac4d 77fc94cadc083e67 984050b75ebae5dd 2809bd638016f723 |
PACE is a balanced PAKE scheme that is used for e.g. travel documents [BFK2009]. In a nutshell, a nonce is generated by the initiating party and send, encrypted with the (hash of) the shared password, to the second party. The possession of the correct password is proven by the ability to encrypt and decrypt the nonce. The parties generate jointly, based on the nonce, a group for a Diffie-Hellman key agreement. Finally, the encryption and authentication keys are derived from the Diffie-Hellman key.
The precondition for the execution of PACE is that both parties share a password. If the domain parameters are not agreed beforehand, the initializing party can send (a set of) supported parameter together with the nonce in the first message, the choice can be done by the second party by reflecting the parameters to use. Hence, we assume that Alice and Bob share a finite group, i.e. a finite field or domain parameters for an elliptic curve, its generator g with order q. All following computations are performed in this group.
After performing this protocol, both parties share a secret key that can be used for further communication.
Key agreement using PACE requires exchanging 2 messages from Alice to Bob and 2 responses. This makes PACE a 2-round protocol. The sizes of the messages are defined by the domain parameters: The first message contains the encrypted nonce and possibly domain parameters. Bobs response contains a group element. Alice and Bob require the selection of 2 random variables, each. Both perform 2 exponentiations and one evaluation of Hash2Point.
The passwords that are used in the PAKE schemes can be stored plain or protected either by hashing them or even using a salt with the hash. Furthermore, in case an augmented PAKE scheme is used, the representation of the password of the client and the server differs.
In order to integrate PAKE schemes into TLS, the Hello Messages and the KeyExchange Messages have to be adapted. The client sends an additional extension with the ClientHello message in order to initiate a key-exchange using a PAKE. Furthermore, the extension may contain details to the desired PAKE like the identity of the client, the desired group parameters, and potential nonces. We show how to integrate a PAKE scheme using the example of PACE.
The client notifies the Server with the ClientHallo messages that a PAKE scheme should be used for the key-agreement and sends a list of supported schemes. This is done by an extension of the Hello message. Moreover, the extension contains the information on the identity of the client and furthermore which set of schemes is supported.
enum{PAKE(TBD1)} ExtensionType;
struct{
opaque ClientName <1..2^8-1>
enum{PACE_AES128_SHA256(TBD2),
PACE_ECC_AES128_SHA256(TBD3),
} PAKEType;
} PAKEParams;
The parameter ClientName in the extension is used by the server to identify the client and determine which password to use for further messages. The ClientName SHALL be a UTF-8 encoded character string processed according to the [RFC4013] profile of [RFC3454]. The PAKEType parameter defines which PAKE is supported by the client.
In case all requested PAKEs are unknown to the server or the requested domain parameters do not fit the expected ones, the server shall respond with an unknown cipher error. In case the supplied ClientName is unknown, the server should keep running the protocol using random elements that fit the expected answers and reject the TLS clients finished by sending a bad_record_mac alert.
The PAKEType PACE_AES128_SHA256 / PACE_ECC_AES128_SHA256 defines PACE using a finite field / an elliptic curve. As block cipher, AES128 is used, as MAC function an HMAC with SHA256.
Note that the client indicates whether a finite field, an elliptic curve or both can be used for PACE. In case ECC is supported, the client can further specify the supported curves and the point format by the Supported Elliptic Curves and Supported Point Formats extensions as defined in [RFC4492]. The selection of the used parameter set is done by the server.
The ServerKeyExchange message contains the response of the server to the PAKE request in the ClientHello message. The response should reflect one of the PAKEs supported by the client and send an appropriate answer back to the client.
enum{PACE_AES128_SHA256(TBD2),
PACE_ECC_AES128_SHA256(TBD3)} KeyExchangeAlgorithm;
struct {
opaque p<1..2^16-1>
opaque g<1..2^16-1>
opaque q<1..2^16-1>
opaque t[16];
opaque Y_s<1..2^16-1>
}PACE_parameters
struct {
ECParameters elliptic_curve;
opaque t[16];
ECPoint Y_s;
}PACE_ECC_parameters
struct {
select (KeyExchangeAlgorithm)
case PACE_AES128_SHA256:
opaque PACE_parameters;
case PACE_ECC_AES128_SHA256:
opaque PACE_ECC_parameters;
} ServerKeyExchange;
Upon reception of the ClientHello Message with PAKEType PACE_AES128_SHA256, the server selects appropriate domain parameters, i.e. a prime p and a generator g with order q. The server looks up the password corresponding to the ClientName sent and uses it to decrypt the first 16 bytes of the ClientRandom variable to s. Note that the ClientRandom is interpreted an encrypted value even though it was randomly selected by the client. This spares the parties from exchanging an additional nonce. The server furthermore selects two random numbers t < 2^128 and y_s < q and computes g'= Hash2Point(s||t) and verifies g' > 1. Otherwise, the server repeats the selection of t. The server computes Y_s=(g')^y_s mod p and answers by reflecting PACE_AES128_SHA256 as KeyExchangeAlgorithm and sending the values p, g, t and Y_s to the client.
Upon reception of the ClientHello Message with PAKEType PACE_ECC_AES128_SHA256, the server selects appropriate domain parameters. If the client sent the Supported Elliptic Curves and/or Supported Point Formats extension, the server MUST select one of the supported parameters. The server looks up the password corresponding to the ClientName sent and uses it to decrypt the first 16 bytes of the ClientRandom variable to s. Note that the ClientRandom is interpreted an encrypted value even though it was randomly selected by the client. This spares the parties from exchanging an additional nonce. The server furthermore selects two random numbers t<2^128 and y_s<q, computes G'= Hash2Point(s||t) and verifies that G' is not in a small subgroup for curves with cofactor > 1. In case this check fails, the server repeats the selection of t. Afterwards, the server computes Y_s=y_s*G'. The server answers by reflecting PACE_ECC_AES128_SHA256 as KeyExchangeAlgorithm and sending the domain parameters together with t and Y_s, encoded a ECPoint as defined in [RFC4492] to the client.
The ClientKeyExchange message finalizes the key exchange.
struct {
select (PAKEType)
case PACE_AES128_SHA256: opaque Y_c<1..2^16-1>
case PACE_ECC_AES128_SHA256: ECPoint Y_c;
} ClientKeyPAKEParams;
Upon reception of the ServerKeyExchange Message with KeyExchangeAlgorithm PACE_AES128_SHA256, the client verifies the received parameters. In particular, it verifies that the domain parameters are valid and that Y_s is a not-null element of GF(p). After successful verification, the client uses the shared password to derive s from the first 16 Bytes of the ClientRandom value sent to the server in the ClientHello message. The client selects a random number y_c < q, computes g'= Hash2Point(s||t), verifies g' > 1 and computes Y_c = g'^y_c mod p as well as K = Y_s^y_c mod p. In case g' = 0 or g' = 1, the client SHALL NOT immediately report an error but use a random g' for all further computations and reject the server's Finished message. The client send Y_c to the server, which also derives K=Y_c^y_s mod p after validation of Y_c. Both parties now use the shared value K to derive an encryption key K_enc = H(K||1), a MAC key K_mac=H(K||2), as well as MAC key for the verification tag K_tag=H(K||3). The validation tag of the client is T_c=MAC(K_tag,"PACE_AES128_SHA256"||Y_c) and for the server T_s=MAC(K_tag,"PACE_AES128_SHA256"||Y_s).
Upon reception of the ServerKeyExchange Message with KeyExchangeAlgorithm PACE_ECC_AES128_SHA256, the client verifies the received parameters. In particular, it verifies the domain parameters and that Y_s is an element of the specified curve and is not equal to the point at infinity. After successful verification, the client uses the shared password to derive s from the first 16 Bytes of the ClientRandom value sent to the server in the ClientHello message. The client computes G'= Hash2Point(s||t) and verifies that G' is not in a small subgroup for curves with cofactor > 1. In case this check fails, the client SHALL NOT immediately report an error but use a random G' for all further computations and reject the server's Finished message. After successful verification, the client selects a random number y_c<q and and computes K=y_c*Y_s. The client sends Y_c, encoded a ECPoint as defined in [RFC4492], to the server, which also derives K=y_s*Y_c after validation of Y_c. Both parties now use the x-coordinate of the shared value K denoted as K_x to derive an encryption key K_enc = H(K_x||1), a MAC key K_mac=H(K_x||2), as well as MAC key for the verification tag K_tag=H(K_x||3). The validation tag of the client is T_c=MAC(K_tag,"PACE_ECC_AES128_SHA256"||Y_c) and for the server T_s=MAC(K_tag,"PACE_ECC_AES128_SHA256"||Y_s).
The Finished messages contain the verification tags to proof that both sides derived the same cryptographic keys. In particular, the tags T_c and T_s are exchanged. After verification of the received tags, both parties are ready to establish a secure channel for application data.
This document request IANA to assign a new value for TLS extension type from the TLS ExtensionType Registry defined in [RFC5246] with the name "PAKE" and the number TBD1.
In addition, this document defines a new registry to be maintained by IANA:
Overall, the PAKE schemes defined here preserves the structure of the TLS Handshake protocol and hence the security arguments of TLS after key-agreement carry over. The security of PAKE is discussed below.
This memo does not make any assumptions about the strength of the shared credential, i.e. the password; best practices like a sufficient password length and use of multiple character classes SHOULD be followed. Nevertheless, an adversary can perform a dictionary attack enumerating all possible passwords. In order to prevent such attacks, implementations MUST include protection mechanisms like restricting the allowed password attempts or even locking out identities after a specific number of failed tries. Please note that this holds for server and client implementations in equal measure.
For using PAKE schemes, storing the password in plain is not required. Both parties SHOULD use a hashed version of it. In case of an augmented PAKE, the server key SHOULD be derived from the hashed password.
In [BFK2009], Bender et al. show the security of the PACE scheme in the model of Bellare et al. [BPR2000]. Their proof makes use of ideal ciphers and random oracles, i.e., idealized hash functions. In practice, the security of this scheme relies on the quality of the used randomness. In particular, if the nonce is not chosen in an uniform way, an adversary might use this information to compromise the password. Hence, using a strong pseudo random function to the raw random material in order to ensure the statistical quality of the random data is RECOMMENDED.
| [BFK2009] | Bender, J., Fischlin, M. and D. Kügler, "Security Analysis of the PACE Key-Agreement Protocol", ISC 2009, LNCS 5735, 2009. |
| [BHKL2013] | Bernstein, D., Hamburg, M., Krasnova, A. and T. Lange, "Elligator: Elliptic-curve points indistinguishable from uniform random strings", ACM CCS 2013, 2013. |
| [BPR2000] | Bellare, M., Pointcheval, D. and P. Rogaway, "Authenticated Key Exchange Secure against Dictionary Attacks", Euorcrypt 2000, LNCS 1807, 2000. |
| [SW2006] | Shallue, A. and C. Woestijne, "Construction of Rational Point on Elliptic Curves over Finite Fields", ANTS 2006, LNCS 4076, 2006. |
| [U2007] | Ulas, M., "Rational Points on certain Hyperelliptic Curves over Finite Fields", arXiv 0706.1448, 2007. |