1. A method for securely transmitting digital data relating to a document between a scanner, a printer and a host computer, interconnected by a digital network, the method comprising the steps of: at a sending station: generating a document file through a data source, inputting a session-specific user-defined character string by an operator, automatically forming an encryption key based on the user-defined character string, encrypting the document file to produce an encrypted document file, using the encryption key, while automatically leaving one or more predetermined specifying header items in the document file as unencrypted, forming a transmission file including the encrypted document file, and transmitting the transmission file towards a receiving station; and at the receiving station: storing the received transmission file, inputting the same user-defined character string by an operator, automatically forming a decryption key based on the user-defined character string, and decrypting the encrypted document file to a decrypted document file, using the decryption key.