Abstract:
A key agreement protocol for use in a public key cryptographic scheme between a pair of correspondents each of which has a long term public key and an ephemeral public key. The protocol includes the steps of exchanging the ephemeral public keys between the correspondents for computing a shared secret at each correspondent and utilizing the shared secret to obtain a common key, wherein the validity of the ephemeral public keys is checked by the recipient thereof prior to use of the common key.

Description:
This application claims priority from U.S. Provisional Patent Application No. 60/690,156 filed on Jun. 14, 2005. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to data transmission systems and in particular systems for implementing key transportation and key agreement protocols within a public key infrastructure. 
     BACKGROUND OF THE INVENTION 
     Various protocols exist for establishing common keys between a pair of entities connected within a data communication system or for transporting keys between such entities. Many of theses protocols are based upon the fundamental Diffie-Hellman protocol in which a piece of information private to one of the correspondents is combined with public information from the other correspondent to arrive at a common key. The protocol known as the MQV protocol after the inventors Menezes, Qu and Vanstone and exemplified in the PCT application WO 98/18234, is recognized as one of the most efficient of known authenticated Diffie-Hellman protocols that use public key authentication. It is recognized as offering superior performance whilst inherently possessing excellent security properties. As a result, MQV has been widely standardized and has recently been chosen by the NSA as the key exchange mechanism underlying the next generation of cryptography to protect the United States government information. 
     Proposals have been made to modify the MQV protocol to implement a variation of the protocol. Whilst these proposals have been made to address what are perceived as potential flaws in the underlying MQV concept, further examination has shown that such flaws do not exist and that the proposed modifications, contrary to the assumptions made by the proponents, themselves introduce additional security risks. 
     It is therefore an object of the present invention to obviate or mitigate the above disadvantages. 
     In general terms, the present invention provides a key agreement protocol in which a signature component of one correspondent includes a hash of the public key of the one correspondent and the identity of the intended recipient. During the exchange of information, the validity of at least one of the public keys used in the exchange is determined. The resultant shared key may also be checked for its validity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An embodiment of the invention will now be described by way of example only with the reference to accompanying drawings in which: 
         FIG. 1  is a schematic representation of a data communication system. 
         FIG. 2  is a flow chart showing the exchange of information between correspondents in the data communication system. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring therefore to  FIG. 1  a pair of correspondents  10 ,  12  exchange information over a data communication link  14 . Each of the correspondents implement a cryptographic protocol in a cryptographic unit  16 ,  18  embedded within the respective correspondents. The cryptographic protocol is a public key protocol key implemented over a finite field. Such protocols use the intractability of the discrete log problem to secure a private key even where the corresponding public is known. A particularly useful protocol is that based on the properties of an elliptic curve defined over a finite field. 
     An elliptic curve E is a set of points that satisfy the equation y 2 +a 1 xy+a 3 y=x 3 +a 2 x 2 +a 4 x+a 6 . The elements of a finite field that satisfy the equation of an elliptic curve, together with the point at infinity, define an elliptic curve group G. The group G may have subgroups S and the group G, or each subgroup S, as the case may be, has a generator g that will generate each element of the group or subgroup. The number of points in the group or subgroup is the order q of the group or subgroup. Certain types of curves will have a cofactor h, as explained more fully at page 114 of Guide to Elliptic curve Cryptography published by Springer under ISBN 0-387-95273-X. The domain parameters including underlying field F, the curve E, the group G or subgroup S and the generator g are all parameters of the protocol that are publically known. 
     Each of the correspondents  10 ,  12  have respective long term private keys a, b and corresponding public keys g a , g b  respectively. Each of the cryptographic unites  16 ,  18 , of the correspondents  10 ,  12  is also capable of generating a random integer x, y respectively and computing a corresponding ephemeral or session public key g x , g y  respectively. The long term public keys g a , g b  are initially presented to a certifying authority. CA, who determines that they satisfy certain arithmetic properties to ensure that they may validly be used as public keys. In particular, the CA establishes that the long term public keys do not belong to a small group or subgroup of less than a predetermined order, referred to as a small subgroup check, and that the keys represent points on the curve E. By performing the substitution of an unsuitable key by an interloper to gain access to secret information is avoided. 
     As a first exchange in a session between the correspondents  10 ,  12 , correspondent  10  forwards a message consisting of the identity of correspondent  10 , A, and the ephemeral public key g x . Similarly, the correspondent  12  forwards the message including the identity of correspondent  12 , B and the ephemeral public key g y . 
     Upon receipt of the ephemeral public keys, each correspondent  10 ,  12  utilizes the cryptographic unit  16 ,  18 , to perform a public key validation within the correspondent to again check for the suitability of the received ephemeral public key. The validation again requires checking the point is on the curve E and that it is not part of a small subgroup. The small subgroup check may be performed for particular types of curve by exponentiating the key by the cofactor h and checking that the result does not correspond to the point at infinity. 
     Each of the correspondents then computes a signature component s A  and s B  of the form s A =x+aH (g x , B) and s B =y+bH (g y ,A), where H is a cryptographically secure hash function. 
     A common shared secret is then computed at each correspondent with the correspondent  10  computing
 
 k =[( g   y ( g   b ) H(g     x     ,A) )] S     A    
 
and the correspondent  12  computing
 
 k =[( g   x ( g   a ) H(g     y     ,B) )] S     B   .
 
     Finally, the secret key K is obtained by applying a suitable key derivation function F (for example a cryptographic hash function) to the shared secret k: K=F(k). The secret key K is computed by each of the correspondent and should be the same to permit exchange of further messages in a secure manner by utilisation of the common key K. 
     By performing public key validation on each of the public keys utilised, malicious attacks on the protocol may be thwarted and the integrity of the data exchange may be assured. 
     If an elliptic curve group is used for which the cofactor h is small, then public key validation of the exchanged public keys g x  and g y  can be sped up by omitting the expensive exponentiation associated with the small subgroup check that guarantees that the key K is in the main group of order q. Instead, the recipient simply checks that the received public key g x , g y  is a point on the curve (but not necessarily one in the group of order q). Then, the shared secret k is raised to the power of the (small) cofactor h, and the result is checked to ensure that it does not correspond to the point at infinity. This modified shared secret is then hashed to yield the secret key K. 
     In an alternative embodiment, validation of the shared secret key k is computed by an exponent of either s A  mod q or s B  mod q where q is the order of the group G. Thus correspondent  10  computes
 
 k =[( g   y ( g   b ) H(g     x     ,A) )] S     A     mod q  
 
and correspondent  12  computes
 
 k =[( g   x ( g   a ) H(g     y     ,B) )] S     B     mod q  
 
     By reducing the exponent mod q, the bit length of the exponent is reduced and accordingly the computational efficiency increased. The shared secret k may then be checked for conformance with the required mathematical properties, e.g. by checking k≠∞ and then used to compute the shared key k.