For transmitting an information string from a source entity to a destination entity the information is encrypted through a time-sequence of encrypting keys that are each valid only in a predetermined associated time interval within the string. The destination entity is presented with control information regarding a changeover instant between two particular encryption keys. In the information string, a data block is singled out at a predetermined localization in the string relative to a changeover that is known to the destination entity, and the data block is encrypted with an actually valid encryption key. A fingerprint is formed of the encrypted data block and transmitted at the predetermined localization instead of the encrypted data block. The encrypted data block is transmitted in an out-of-band control message which indicates that an update occurred.