First data to be sent by a first party to a second party is encrypted using an encryption key string formed using at least a hash value generated using second data and a secret, shared with a trusted party, that serves as identification of the first party. The second data comprises, for example, one or more conditions that serve as identifiers of the second party, and a hash-value element generated by hashing the first data. The encrypted first data and the encryption key string is made available to the second party which forwards the encryption key string to the trusted party with a request for the corresponding decryption key. The trusted party carries out at least one check on the basis of data contained in the encryption key string and, if this at least one check is satisfactory, provides a decryption key to the second party.