A method, comprising: acquiring candidate data in association with a request for accessing a resource, the candidate data comprising first data and second data; processing the first data with a first key in an attempt to effect decryption of the first data, thereby to obtain first processed data; processing the second data with a second key in an attempt to effect decryption of the second data, thereby to obtain second processed data; and granting the request if a pre-determined portion of the first processed data is derivable from the second processed data. The method may further comprise extracting from the first processed data a group identifier and the pre-determined portion of the first processed data, and effecting a comparison of the group identifier to a reference group identifier in order to conclude whether the first data has been successfully decrypted based on an outcome of the comparison.