digital forensics would refer to the investigation of a device, when a person is under interrogation for breaching information or data. Digital forensics would recover information through complicated tools to bring a person to justice for tampering or exploiting with private information. Computer forensic services would also do the same thing. However, the difference would be to work solely with computers and the routers or servers affiliated with a computer.
Cyber security deals with use of tactics or software for protecting the device or network from hackers. Such technology would use the information gathered by digital forensic services for the prevention of cyber attacks. Therefore, cyber security is dependent on digital forensics.