1. A device for generating the convolution function of two series of 4p discrete values {a.sub.n } and {b.sub.n }, where p is a prime number, said device including: a. first means for introducing sequentially and in series by bit, terms {a.sub.n } into a device computing by successive accumulations, the following terms: ##EQU25## wherein k takes all the integer values comprised between 0 and 4p-1, the double parentheses ((--)) indicate that the enclosed quantity is to be taken modulo the Mersenne number M.sub.p ' 2.sup.p -1 and the brakcets &lt;--&gt; indicate that the enclosed quantity is to be taken modulo p; b. a second means connected to receive and to retain the terms A.sub.i,k computed by said first means and for providing a cyclic access to said terms; c. a third means connected to said second means for adding and subtracting alternatively said terms A.sub.i,k by pairs so as to provide the following terms: EQU A.sub.k ' = ((A.sub.1,k + (-1).sup.k A.sub.3,k)) and EQU A.sub.k " = ((A.sub.2,k + (-1).sup.k A.sub.4,k)), wherein k takes all the integer values comprised between 0 and 4p-1; d. a fourth means coupled to said third means for computing terms {A.sub.k } = A.sub.k ' + j.sup.k A.sub.k "; where j equals the square root of minus one; e. a fifth means comprising a second set of said first to said fourth means, said fifth means acting to process the terms of said series {b.sub.n } so as to provide terms {B.sub.k } = B.sub.k ' + j.sup.k B.sub.k "; f. a sixth means responsive to the {B.sub.k } terms from said fifth means for multiplying each term B.sub.k by a given coefficient q; g. a seventh means connected to said fourth means and to said sixth means for multiplying said series A.sub.k and qB.sub.k together term by term to provide a sequence qC.sub.k = ((q A.sub.k B.sub.k)); h. an eighth means including switching means for selecting sequentially and in series by bit, said terms {qC.sub.k } generated by said seventh means and a device connected to said switching means and computing by successive accumulations, the following terms: ##EQU26## where m takes all integer values comprised between 0 and p-1; i. a ninth means cyclically connected to said eighth means and provided with cyclic access to the terms c.sub.i,m provided by said eighth means; j. a tenth means receiving the terms cyclically accessed by said ninth means and alternately adding and subtracting pairs of terms c.sub.i,m ; and k. an eleventh means receiving the output of said tenth means and providing the circular convolution terms {c.sub.n } by adding and subtracting said terms provided by said tenth means.