Systems and methods are provided for tracking electronic files in computer networks using electronic signatures. A signature program installed on a network node inserts an electronic signature into certain encoded media files when they are transferred to other network users. Each network user is issued a unique electronic signature based on public key infrastructure. A signature repository supplies the recipient signature to the signature program prior to transferring a file. The sender and recipient signatures are appended to a portion of the media file, preferably the lower order bits to minimize perceptible file degradation. A transaction record is thereby written into the file and a copy of the transfer information is stored centrally at the repository, thereby creating a traceable record of a file's movement.