Abstract:
A distributed split-key cryptosystem and application in a public-key setting wherein each of a plurality of trustees independently selects his own secret-public key pair. The trustees combine their public encryption keys into a single public encryption key. Using this combined public key for an electronic auction and other secure transactions.

Description:
This application is based on United States provisional patent application number 60/008,555, filed Dec. 13, 1995. 
    
    
     TECHNICAL FIELD 
     The present invention relates to secure communications. 
     BACKGROUND OF THE INVENTION 
     Recently, systems have been developed by which a secret decryption key is in the hands of several trustees, so that each trustee holds a guaranteed piece of the key, while he cannot predict the full key. Examples of such systems include Micali&#39;s Fair Cryptosystems and the Clipper Chip. 
     We want to develop such a system in a public key setting so that the secret key that is guaranteed to be shared among some trustees is the secret decryption key corresponding to a given public encryption key. The inventive system does not work by having an initial entity who (1) computes a public-secret key pair, and (2) divides the secret key among various trustees in a proper way. Indeed, whether or not such entity blows itself up after doing so (e.g., as suggested by Desmet), the doubt exists that leakage of the secret decryption key may have occurred, either maliciously or by accident. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention works by having each trustee independently select his own secret-public key pair, and then having the trustees combine their public encryption keys into a single public encryption key. To this combined public key corresponds a combined secret key, such that the individual secret keys selected by the trustees are guaranteed to be pieces of this combined secret key. Thus, while no one knows said combined secret key (and indeed while, possibly, this secret key has never been computed, if so wanted), the trustees are guaranteed that they can reconstruct this key, or, alternatively, that they can decrypt any message encrypted with the combined public key without revealing its corresponding secret key. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The sole FIGURE illustrates a bid and auction process according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Let us exemplify one way to achieve our goals with the Diffie-Hellman public-key cryptosystem. The skilled in the art will have no difficulty in obtaining similar results for other public-key cryptosystems. 
     In the Diffie-Hellman system, there is a prime p and a generator g common to all users. A user X chooses his own secret key x at random between 1 and p-1, and sets his public key to be g x  mod p. Let now y and g y  mod p be, respectively, the secret and public keys of user Y. Then X and Y essentially share the secret key g xy  mod p. Indeed, each of X and Y can compute this &#34;common secret key&#34; by raising the other&#39;s public key to his own secret key mod p. On the other hand, without knowledge of x or y, no other user can, given the public keys g x  mod p and g y  mod p, based on any known method, feasibly compute the secret key g xy  mod p. Thus X or Y can use this key to secure communications between each other (e.g., by using it as the key of a symmetric cipher). 
     Let now T 1 , . . . , T n  be our trustees. Then, each T i  chooses a secret key x i  and a matching public key g xi  mod p. Then the combined public key is set to be the product of these public keys mod p, g z  mod p (i.e., g z  =g xl+ . . . +xn mod p). This key can be made public or otherwise known to a group of users. Alternatively, the individual trustees&#39; public keys can be made known, so that anyone can compute the combined public key from the individual trustees&#39; public keys, just as the trustees themselves did. 
     Notice that each trustee has a share of the corresponding secret key, z, of the combined secret key so computed. Indeed, this combined secret key would be z=x 1  +.sup.. . . x n  mod p-1. Thus each trustee is guaranteed to have a piece of this combined key. Indeed, if the trustees want, they could reveal their secret keys and thus easily compute z. Notice that a trustee cannot reveal a false piece of z without being caught. For instance, if trustee T 1  reveals a piece p 1  other than x 1 , anyone can detect that g p1  ≠g x1  T 1  &#39;s public key. 
     Assume now that user Alice wishes to encrypt a message m with the combined public key. Then she selects a (preferably) temporary secret key a and its corresponding public key g a  mod p; computes the secret key g az  mod p; encrypts m conventionally with said key g az . ; and sends this ciphertext to a proper recipient together with the temporary public-key g a  mod p. 
     At this point, the encryption of Alice&#39;s message is known, while the message can be revealed by either Alice herself (e.g., by releasing a), or by the collection of Trustees; for instance, by having them release their individual secret keys x i , and thus the combined secret key z. 
     This method can be used for applications other than law-enforcement ones. For instance, it can be used for simultaneous (sealed-bid) electronic auctions, which is illustrated in the sole Figure, which shows bidders B 1  . . . B m  submitting sealed bids b 1  . . . b m  in a system having n trustees, T 1  . . . T n . Indeed, users, like Alice, may encrypt their bids with a proper combined key (preferably indicating within the secret bid their own identities in order, among other things, to avoid that their bids can be &#34;copied&#34; by others). At the appropriate time, all bids will be revealed; for instance, by having the trustees release their individual x i , values so that the combined secret key z can be reconstructed, and thus all secret keys g az  mod p are also reconstructed and used for decrypting all bids. 
     Notice that no one can leak an individual bid, before the proper time, without the consent of the user who made it. In fact, for decrypting a bid the cooperation of all trustees would be needed, and it is extremely unlikely that such an improper collusion will ever occur if the Trustees are chosen to be trustworthy (or properly functioning machines). Thus, each individual bidder is protected, and so are the trustees (e.g., from some frivolous accusation of bid leakage). 
     It should be appreciated that such an auction mechanism is just one application of the inventive technology. The system can also be implemented so that certain prescribed sets of trustees (e.g., any majority of trustees rather than all trustees) suffice for reconstructing the combined secret key. Also, decryption of user messages may occur without revealing the combined secret key, if so wanted. 
     While the invention has been disclosed in connection with the preferred embodiments shown and described in detail, various modifications and improvements thereon will become readily apparent to those skilled in the art. Accordingly, the spirit and scope of the present invention is to be limited only by the following claims.