Method for secure and economical sharing of files between a plurality of entities (UEA, UEB) using at least one virtual folder, and implemented by a server web application connected to a cloud storage provider and adapted to use a published application programming interface, API, of the cloud storage provider, the method comprising the steps of: - generating a master key (Ks) for each virtual folder dedicated to the encryption of files to be shared with the group in said virtual folder, wherein the virtual folder is associated with an authorization control list specifying one or more entities that have access to the files stored in the virtual folder; - in response to receipt from a first entity of a request to store a clear-text file in the virtual folder, verifying in the authorization control list whether the first entity is authorized to access the virtual folder, and if the first entity is authorized, encrypting the clear-text file using the master key (Ks) and storing the encrypted file in the cloud storage provider in a storage folder of the first entity; - in response to receipt from a second entity of a request to retrieve a clear-text file in the virtual folder, verifying in the authorization control list whether the second entity is authorized to access the virtual folder and if the second entity is authorized, retrieving the encrypted file from the cloud storage provider in the storage folder of the first entity, decrypting the clear-text file using the master key (Ks) and sending the clear-text file to the second entity; - in response to receipt from a third entity of a request to remove a fourth entity from the authorization control list of a virtual folder, verifying in the authorization control list whether the third entity is authorized to remove the fourth entity, and if the third entity is authorized, removing the fourth entity from the authorization control list, identifying all files in the virtual folder that were contributed by the fourth entity, and moving the encrypted files corresponding to the identified files to a storage folder of another entity in the authorization control list. - generating a master key (Ks) for each virtual folder dedicated to the encryption of files to be shared with the group in said virtual folder, wherein the virtual folder is associated with an authorization control list specifying one or more entities that have access to the files stored in the virtual folder; - in response to receipt from a first entity of a request to store a clear-text file in the virtual folder, verifying in the authorization control list whether the first entity is authorized to access the virtual folder, and if the first entity is authorized, encrypting the clear-text file using the master key (Ks) and storing the encrypted file in the cloud storage provider in a storage folder of the first entity; - in response to receipt from a second entity of a request to retrieve a clear-text file in the virtual folder, verifying in the authorization control list whether the second entity is authorized to access the virtual folder and if the second entity is authorized, retrieving the encrypted file from the cloud storage provider in the storage folder of the first entity, decrypting the clear-text file using the master key (Ks) and sending the clear-text file to the second entity; - in response to receipt from a third entity of a request to remove a fourth entity from the authorization control list of a virtual folder, verifying in the authorization control list whether the third entity is authorized to remove the fourth entity, and if the third entity is authorized, removing the fourth entity from the authorization control list, identifying all files in the virtual folder that were contributed by the fourth entity, and moving the encrypted files corresponding to the identified files to a storage folder of another entity in the authorization control list.