Traceable method and system for broadcasting digital data
A method and system for a user to obtain a derived value K r of m bits, in which, given two pseudo-random functions g 0 and g 1 from m bits to m bits, said user obtains, on the basis of an input parameter consisting of a word r of n bits, a derived value K r =g r n ∘ . . . g r 2 ∘g r 1 (S), where, for i= 1 , . . . , n, g r i =g 0 if r i =0 , and g r i =g 1 if r i =1 , and where S is a master value of m bits which is not disclosed to said user. This method comprises the following steps: a search is conducted, from among a set of words of m bits U j , where j= 1 , . . . , p, recorded in a table T U and equal to U j =g v l(j) (j) ∘ . . . g v 2 (j) ∘g v 1 (j) (S), where, for i= 1 , . . . , l(j), the indices v i (j) are predetermined bits, for a word U σ equal to U σ =g r l(σ) ∘ . . . g r 2 ∘g r 1 (S); said derived value K r is thereafter obtained by calculating K r =g r n ∘ . . . g r l(σ)+1 (U σ ). Application to the encryption/decryption of pay-per-use digital contents, and to authentication.
1. A method, carried out on a computerized platform, for decryption, the method comprising:
receiving a cryptogram C r (M), where M is a plaintext content and C r is a predetermined encryption algorithm;
receiving an input parameter comprising a word r of n bits related to C r (M);
given two pseudo-random functions g 0 and g 1 from m bits to m bits obtaining, on the basis of r, a derived value K r of m bits, wherein
K r =g r n ∘ . . . g r 2 ∘g r 1 ( S ),
where, for i=1, . . . , n, g r i =g 0 if r i =0, and g r i =g 1 if r i =1, and where S is a master value of m bits, wherein S is not disclosed to said user;
conducting a search from among a set of words of m bits U j , where j=1, . . . , p recorded in a table T U and equal to
U j =g v l(j) (j) ∘ . . . g v 2 (j) ∘g v 1 (j) ( S ),
where, for i=1, . . . , l(j), the indices v i (j) are predetermined bits,
for a word U σ equal to
U σ =g r l(σ) ∘ . . . g r 2 ∘g r 1 ( S ),
obtaining K r by calculating
K r =g r n ∘ . . . g r l(σ)+1 ( U σ ); and
decrypting C r (M) using, as a decryption key, a value obtained on the basis of at least one derived value K r , so as to obtain M.
2. The method of claim 1 , wherein said computerized platform uses as secret key for authentication with a verificator entity a value that may be obtained on the basis of the at least one derived value K r , the latter being calculated by the obtaining and conducting steps, after reception of r.
3. A device for decryption, the device comprising:
a memory that stores a computer program;
a central processing unit that executes the computer program to perform steps comprising:
receiving an input parameter comprising a word r of n bits related to C r (M);
producing two pseudo-random functions g 0 and g 1 from m bits to m bits, and making it possible to obtain, on the basis of r, a derived value K r of m bits, wherein
K r =g r n ∘ . . . g r 2 ∘g r 1 ( S ),
where, for i=1, . . . , n, g r i =g 0 if r i =0, and g r i =g 1 if r i =1, and where S is a master value of m bits, wherein said master value S is not disclosed to said user, said device being configured for:
receiving a cryptogram C r (M), where M is a plaintext content and C r is a predetermined encryption algorithm;
searching, from among a set of words of m bits U j , where j=1, . . . , p, recorded in a table T U and equal to
U j =g v l(j) (j) ∘ . . . g v 2 (j) ∘g v 1 (j) ( S ),
where, for i=1, . . . , l(j), the indices v i (j) are predetermined bits,
for a word U σ equal to
U σ =g r l(σ) ∘ . . . g r 2 ∘g r 1 ( S ),
obtaining said derived value K r by calculating
K r =g r n ∘ . . . g r l(σ)+1 ( U σ ); and
decrypting C r (M) using, as a decryption key, a value obtained on the basis of at least one derived value K r , so as to obtain M.
4. An electronic circuit, comprising the device as claimed in claim 3 .
5. A decoder, comprising an electronic circuit as claimed in claim 4 .
6. A non-transitory computer-readable medium having stored thereon a computer program that is downloadable from a communication network, the computer program comprising instructions for the execution of the method as claimed in claim 1 .
7. A non-transitory computer-readable medium having stored thereon a computer program that is downloadable from a communication network, the computer program comprising instructions for the execution of the method as claimed in claim 2 .