Patent ID: 8681983

Claim:
A method for providing attribute-based encryption of a message using conjunctive normal form (CNF) expressions, the said CNF expression comprising at least one clause over a set of n attributes, the said method using a key generation engine, an encryption engine and a decryption engine, and comprising the steps of: a. generating by the key generation engine a random gεG, where G is a prime order group of order p where p is a primer number, four random values α,γ,β,rε R Z/pZ, and for i=1,2, . . . , n,n+2, . . . ,2n where n is the number of attributes, computing by the key generation engine at least 2n−1 values g i =g α i εG and at least one value v=g γ εG; b. generating by the encryption engine an encryption key PK=(g r ,g 1 r , . . . ,g n r ,g n+2 r , . . . ,g 2n r ,v r ,g n β ,g n ) comprising at least 2n+2 group elements; c. for an abovementioned CNF expression over a set of attributes, CNF expression comprising N clauses, generating by the encryption engine N random values t 1 ,t 2 , . . . ,t N ε R Z/pZ, computing by the encryption engine the value t = ∑ i = 1 N ⁢ ⁢ t i ⁢ mod ⁢ ⁢ p , generating by the encryption engine a cryptogram hdr=(g n t ,hdr 1 , . . . ,hdr N ) consisting of at least 2N+1 group elements and generating a session key SK, wherein the said session key or parts thereof is used to derive a symmetric key which is used to encrypt the message, or to encrypt the message with the said session key; d. generating by the key generation engine a plurality of private decryption keys dk u =(g 1 r(β+s u ) ,g 1 s u , . . . ,g n s u ,g n+2 s u , . . . ,g 2n s u ,d i 1 , . . . ,d i N ,d j 1 , . . . ,d j R ) each of the said decryption keys explicitly associated with at least one positive attribute by the mean of the group element d i j =g i j γ·s u and explicitly associated with at least one negative attribute by the means of the group element d j k =g j k γ·s u wherein the value s u is a random group element and is unique for every decryption key dk u .