Of course, if it's is just signed-frame video, prior art doesn't begin to describe
Johns Hopkins University
Johns Hopkins APL Creates System To Detect Digital Video Tampering
The Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., has
opened the door to using reliable digital video as evidence in court by developing a
system that identifies an attempt to alter digital video evidence.
It's not too hard to make changes to digital video, says Tom Duerr, APL's project
manager. But our system quickly and conclusively detects any alterations made to the
original tape. For the past two years, Duerr has led development of the project for
the United States Postal Inspection Service.
We're satisfied that our system can accurately detect tampering and now we're
building a working prototype that can be attached to a camcorder, says Nick Beser,
lead engineer for the project. Our authenticator provides proof of tampering when the
human eye can't detect it. You might theorize that a change has been made, but this
system takes the theory out of that determination.
The U.S. Postal Inspection Service, the federal law enforcement agency that safeguards
the U.S. Postal Service, its employees and assets, and ensures the integrity of the
mail, uses video surveillance and cutting edge technology as investigative tools in
many of its cases. We are looking forward to field testing the prototype developed by
APL, says Dennis Jones, assistant postal inspector in charge of the agency's Forensic
Technical Services Division. Being able to present a certifiable digital recording
in court in support of our investigative efforts will minimize court challenges over
the admissibility of such evidence. This system could reinforce the public's
confidence in the work of law enforcement professionals.
Securing the System
The authentication system computes secure computer-generated digital signatures for
information recorded by a standard off-the-shelf digital video camcorder. While
recording, compressed digital video is simultaneously written to digital tape in the
camcorder and broadcast from the camera into the Digital Video Authenticator
(currently a laptop PC). There the video is separated into individual frames and three
digital signatures are generated per frame -- one each for video, audio, and
camcorder/DVA control data -- at the camcorder frame rate.
Public-key cryptography is used to create unique signatures for each frame. The keys
are actually parameters from mathematical algorithms embedded in the system. Duerr
says, The keys, signature, and original data are mathematically related in such a way
that if any one of the three is modified, the fact that a change took place will be
revealed in the verification process.
One key, called a private key, is used to generate the signatures and is destroyed
when the recording is complete. The second, a public key, is used for verification.
To provide additional accountability, a second set of keys is generated that
identifies the postal inspector who made the recording. This set of keys is embedded
in a secure physical token that the inspector inserts into the system to activate the
taping session. The token also signs the Digital Video Authenticator's public key,
ensuring that the public key released with the video signatures was created by the
inspector and can be trusted.
The signatures that are generated for the recording make it easy to recognize
tampering. If a frame has been added it won't have a signature and will be instantly
detected. If an original frame is altered, the signature won't match the new data and
the frame will fail verification. The method is so perceptive that tampering with even
a single bit (an eighth of a byte) of a 120,000-byte video frame is enough to trigger
an alert. After an event is recorded, the signatures and the signed public key are
transferred to a removable storage device and secured along with the original tape in
case the authenticity of a tape is challenged.
When finished, the Digital Video Authenticator is expected to be within the size and
cost range of consumer-grade digital camcorders. It will be attached to, rather than
embedded in, a video camera, which allows it to be transferred to different cameras
when current ones become obsolete. Comparison of signatures with recorded video and
analysis of the results will be accomplished in separate software that will run on a
Prototype development will include peer review by other researchers and potential
users and is expected to be completed by 2005. In addition to Postal Inspection
Service use, the system could serve state and local law enforcement needs and possibly
corporate and other business venues.
The Applied Physics Laboratory, a division