A method and apparatus for obtaining an encryption key for an item of data transmitted from a client to a server. The method includes: determining a number R of registers available within the client for carrying out a plurality of calculations of encryption keys; determining a maximum number N of iterations necessary for obtaining at least one encryption key at the server; obtaining a structure of data representative of a key calculation state effected within the R available registers; calculating the at least one encryption key as a function: —of the number of available registers R, by performing at most N calls to a pseudo-random function F and —of the data structure; so that the at least one encryption key can be obtained from a combination of at most T=CR+NN−1 encryption keys based on a secret previously shared between the server and client.