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Description  |
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TECHNICAL FIELD
The invention relates to the field of photography, and more particularly to
a digital camera equipped with apparatus for enabling an image file
produced by the camera to be authenticated.
BACKGROUND ART
Photographs have in general been regarded as mediums that "do not lie."
However, the inherent trustworthiness of photographs have been compromised
by the increasing sophistication and ease with which photographic images
can be manipulated. This problem is particularly severe in the case of
digital cameras which store a recorded image in digital memory, such as on
magnetic tape or laser disc instead of photographic film. There the
concept of an "original" image is no longer meaningful.
While a need for authenticating photographs exists today, the seriousness
of the problem is not yet widely acknowledged. Consider, for example, that
while a photograph can be readily altered to a limited extent, it can be
authenticated only to some degree by comparing a positive print with its
negative film recorded by a camera while inspecting the negative film for
evidence of physical alteration. Authentication of an image recorded by a
digital camera is more difficult because there is no physical "original"
to compare or negative to examine; furthermore, the digital image file
produced by these cameras (as well as other digitally recorded information
such as audio and video files) can be easily manipulated using
sophisticated computers which make such manipulation easy and, as
techniques improve, more difficult to detect.
Today, most pictures that appear in newspapers and magazines have been
altered to some degree, with the severity varying from the trivial (such
as deliberately removing distracting background while cleaning up "noise")
to the point of deliberate deception, such as substituting heads on
people's bodies. As the power, flexibility and ubiquity of image-altering
computers continues to increase, the notion that photographs "do not lie"
will become less widely held.
BACKGROUND ON DIGITAL SIGNATURES
The concept of a digital "signature" is based upon recent encryption
techniques called "public key encryption." [Whitfield Diffie and Martin E.
Hellman, "New Directions in Cryptography," IEEE Transactions on
Information Theory, Vol. IT-22, No. 6, pp. 644-654, November, 1976; Taher
Elgamal, "A Subexponential-Time Algorithm for Computing Discrete
Logarithms over GF(p.sup.2)," IEEE Transactions on Information Theory,
Vol. IT-31, No. 4, pp. 473-481, July 1985; and R. L. Rivest, A. Shamir and
L. Adleman, "A Method for Obtaining Digital Signatures and Public-Key
Cryptosystems," Communications of the ACM, Vol. 21, No. 2, pp. 120-126,
February, 1978]
Older encryption/decryption schemes require that both the Sender and
receiver possess the same secret "key": the sender uses the key to
transform the text message into ciphertext, and the receiver uses the same
key to perform an inverse transformation on the ciphertext, revealing the
original text message. If the correct key transforms the ciphertext into
unreadable garbage, it is reasonable to conclude that either the wrong key
is being used, the message has been altered, or the sender has been
impersonated by someone ignorant of the fact that a key is required to
encrypt so the receiver can decipher. The historic drawback to this secret
key encryption scheme has been in the secure distribution of keys;
disclosure of the key by the sender to the receiver must occur
out-of-band, either transmitted via an expensive alternate path or at some
prior time, such as when the sender and the receiver were in the same
space at the same time.
Public-key encryption techniques differ in that they enable the recipient
of a message to decrypt it using a public key that is different from the
one used by the sender to encrypt it, but mathematically related to it.
Public key encryption employs two different keys: a private key, which is
held by the security conscious party sending the message, and a
corresponding public key, which need not be kept secret. The public key is
generated based upon the private key, making the pair unique to each
other.
All public-key cryptography is based on the principle that it is easy to
multiply two large prime numbers together, but extremely difficult (taking
perhaps centuries using today's supercomputers) to work backwards and
uncover the factors (public and private keys) that could have been used to
generate the resulting product of the multiplication process.
The public-key scheme is illustrated in FIG. 1 and works as follows: to
send a secret message that only the recipient can read, the recipient's
public key would first be made known to the sender via any nonsecure
medium, such as a letter, a telephone conversation, or a newspaper notice.
Anyone wishing to send a secure message would encrypt the message using
this public key and transmit it to the recipient. The recipient, having
sole possession of the corresponding private key, is the only one able to
decrypt the message. The need to transmit a secret key that both parties
must possess beforehand is thus obviated. The tradeoff is that, although
only the recipient can decrypt the message, anyone who intercepts the
public key can send a message with anonymity.
The process described above can also be implemented "backwards" to great
advantage. In a second scenario, it is the sender who maintains possession
of the private key, and anyone who has the widely disseminated
corresponding public key is able to decrypt the message encrypted using
the private key. Although this procedure no longer performs the
traditional function of encryption (which is to provide confidential
communication between two parties), it does provide a way to insure that
messages are not forged: only the private key could have produced a
message that is decipherable by anyone having the corresponding public
key.
The foregoing provides the foundation for the concept of digital
signatures; encoding a smaller representation of the message to verify the
integrity and source of the message, as will now be described with
reference to FIG. 2. The sender of a message generates a unique digital
signature by hashing the message using a regular algorithm and encrypting
the message using a private key. There is a public key corresponding to
the private key to enable decryption of the hashed message. The digital
signature is transmitted along with the plain (unhashed and unencrypted)
text message so that a recipient may decrypt the digital signature and
thus authenticate the message by comparison of the decrypted hash and a
hash of the plain text message transmitted with the signature. Any
difference will readily show that the message has been altered or has been
transmitted by a person other than the one possessing the private key,
which is itself a digital code. Thus, as long as the private key remains
private, only the private key holder can produce messages decipherable by
anyone holding the public key. Furthermore, it is extremely difficult to
"reverse-engineer" the encrypted message using the public key to ascertain
the private key. Without knowledge of the private key, a digital
"signature" cannot be forged.
Digital signatures thus build upon the public-key cryptographic technique
and allow a recipient to not only authenticate the contents of a plain
text message but also the identity of the sender without obscuring the
original message. Digital signatures are produced by creating a hash of
the original plain text message, and then encrypting the hash using the
sender's private key, as shown in the top half of FIG. 2. The result is a
second, smaller digital file referred to as a signature which accompanies
the original plain text message as a separate block of data.
A "hash" is created by a mathematical function which maps values from a
large domain into a smaller range. For example, a checksum is a simple
kind of hash. A more complex example of a hash algorithm would involve
dividing a binary file into a collection of, say, 16 kilobit pieces and
performing a cumulative exclusive-OR function between successive pieces.
That produces a simple 16 kilobit hash which is smaller than the original
file yet is particularly unique to it. Many more complex and secure
transformations are also possible. An advantageous characteristic of a
hash is that changing a single bit in the original message input would
produce a very different hash output if subjected to the same mathematical
function. This advantage of a hash will become apparent in the use of a
hash in the present invention. Another advantage is that reverse
engineering a message so it will yield a given hash and also make sense to
the reader is virtually impossible. Thus, a key to the present invention
is that a unique encrypted digital signature can be created by encrypting
the output of the hashing function using a sender's private key.
In the process of hashing a message to produce a digital signature, the
original message is retained unaltered; only the message's hash is altered
by encryption with a private key. This way the original file can be read
by anyone, yet each recipient may authenticate the message by decrypting
the message's unique digital signature using the public key. If the
decrypted digital signature and hash of the message in question is created
by the same mathematical function that matches, both the integrity of the
message and the authenticity of the sender are assured.
DIGITAL CAMERAS
Standard digital cameras are filmless; they sense light and color via an
electronic device, such as a charge coupled device commonly known as a
CCD, and produce as an output a computer file which describes the image
using data bits (1's and 0's) arranged in a meaningful, predefined format.
Often this digital image file is first stored on a small mass-storage
medium inside the camera itself, such as a magnetic floppy disc, or a
magneto-optical disk commonly referred to as a compact disc (CD), for
later transference to a digital system for image processing and/or
display. Alternatively, the image file can be sent directly to a digital
system via a transmission medium. Once inside the digital system, it then
can be displayed and easily altered in any number of different ways.
There are several ways of lying with an image, some as old as photography
itself. The primary methods are: attaching false captions to the published
photograph, using false perspectives and distortions of a scene, and
manipulating the appearance of objects or persons in the scene, such as by
computer-assisted manipulation of pixels while the image is still stored
in digital form. The present invention addresses techniques for thwarting
attempts to lie with photographs using one or more of these ways of lying.
STATEMENT OF THE INVENTION
An object of the invention is to provide a solution to the problem of
authenticating digital image files, where the image file may be, for
example, a single still image from a digital camera, a sequence of images
from a digital video camera or even digital holograms. Authentication in
this sense means to establish as worthy of belief that the image file has
not been altered after its digital signature has been created. The term
"image file" is thus used herein to refer to the recorded file of a
digital still camera, a digital video camera or a digital holographic
camera.
In accordance with one aspect of the invention, a digital camera system is
equipped with not only the means for providing an image file (either
stored in an internal medium for later transmission or transmitted
directly to a digital image processing system) but also means for first
providing a hash of the image file or blocks of the file in the case of a
very long image file, such as from a video camera and means for then
encrypting the hash with a unique private key embedded in the digital
camera system. The private key is not known by anyone except perhaps the
manufacturer of the digital camera system, or the encrypting means
implemented, for example, as a programmed microprocessor embedded in the
digital camera system. All that the user need know is a public key unique
to the digital camera system. The encrypted hash is stored as a digital
signature along with the image file and public key for later
authentication.
For authentication of the image file, means for hashing the image file in
question produces a checking hash, and means for decrypting the digital
signature using the public key reveals the true hash produced by the
digital camera system from the true, plain text image file and means for
comparing the checking hash with the true hash. If the two hashes match,
it is certain that the image file is authentic, i.e., that the image file
has not been altered.
To further enhance the authenticatableness of an image from a digital
camera system, information is added in a border of the digital camera
image frame, such as the public key which is a unique serial number
identifying the camera, the date, time, light level, color temperature,
f/stop, shutter speed, latitude and longitude of the camera position, the
camera pointing direction, focusing distance of the camera lens system and
range (distance) from the camera of prominent objects in the image. All of
the added information that is recorded in the border of the image frame is
hashed and encrypted together with the image to become part of the
signature. That information cannot be altered in the original image file
containing this information without it being detected by later comparing
the original image file hash with secure image hash of the original image
file obtained from the digital signature by decrypting the digital
signature with a public key that is mathematically related to the private
key. Any alteration of the original image file transmitted with the
digital signature, including the information entered on the border of an
image file, will result in a mismatch of the two hashes compared. Only if
there is not a mismatch will the original image file be declared
authentic, and as an additional benefit the source of the image file will
be declared as "authentic" since only a source having the private key
mathematically related to the public key can produce and send a digital
signature that contains a matching hash of the image file hash.
The range information of prominent objects in the digital image scene
provides authenticatable data about the scene to an investigator which
helps in the identification of inconsistencies in the image as well as in
any image caption added to a published photograph or video scene. It also
helps prevent use of an altered image that is projected at lifesize
proportions and then photographed again with the original camera system at
a distance similar to that image's content would suggest for then all
objects in the photograph would be recorded as being at virtually the same
range since all points in the projected image would be virtually the same
as at the center of the projected image
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram illustrating public key encryption.
FIG. 2 is a block diagram illustrating the method of producing and using a
digital signature.
FIG. 3a illustrates a preferred application of the present invention to
digital cameras.
FIG. 3b is a block diagram of the present invention as applied to digital
cameras for producing a file image with a digital signature to allow
authentication of the file image by the recipient.
FIG. 3c is a block diagram of a system for authenticating the file image
from the digital camera of FIG. 3a.
FIG. 4 illustrates a colored border of an image from a digital camera.
DETAILED DESCRIPTION OF THE INVENTION
As applied to digital cameras, the object is to provide a digital signature
for an image file as it emerges in digital form from a digital camera
system 10 for later authentication as required. To accomplish this, the
digital camera 11 produces from a contained processor 12 two output files
for each captured image as shown in FIG. 3a: the first is an all digital,
industry standard format image file representing the captured image. The
second would be an encrypted digital signature of the image file produced
as shown in FIG. 3b by using the camera's unique private key (embedded
within the digital camera's secure microprocessor 12b) to encrypt a hash
of the captured image file (produced by hashing microprocessor 12a) for
creating an encrypted image hash, thus producing a digital signature
which, like the image file, is recorded in a recorder 12c of the digital
camera system 10, or transmitted directly to a digital processor's memory
for later authenticating and viewing the captured image. It is the
responsibility of the user to keep track of the image and digital
signature files once they leave the camera 10 since both are required to
authenticate the image file.
Once the digital image file and the digital signature are generated and
stored on a medium in the camera system and/or transferred into a digital
processor memory for processing, the image file's authenticity can be
checked at any time thereafter by a decrypting authentication system 20
shown in FIG. 3c using a public key taken from the camera name plate or
the image's border. The public key can be freely distributed to users for
authentication of image files using the authentication system 20. This
authentication system 20 has neither the public nor the private key
stored. It requires as inputs the digital image file in question, its
accompanying digital signature file, and the public key which is unique to
the camera believed to have originated the image and digital signature
files. It is perfectly reasonable to use as the public key the camera's
serial number appearing on the camera's name plate which is used by the
manufacturer to identify the camera for such purposes as warranty repair
or replacement. The public key is mathematically related to the private
key embedded in the secure microprocessor 12b of the digital camera 10 to
permit decrypting the digital signature with the public key in the
conventional way of encryption and decryption using public and private
keys. For image authentication, the image file is hashed for comparison,
with the secure image hash obtained by decrypting the digital signature
that was encrypted with a private key using the public key. If there are
no bit mismatches between the secure image hash and the image hash of the
image file, authenticity is confirmed, as will now be described in more
detail.
The authentication system 20 calculates its own image file hash using a
hash calculator 21 comprising a digital processing system programmed with
the same hashing algorithm used in the digital camera (which need not be
kept a secret) and a secure image hash using a decryptor 22 comprising a
digital processing system with the public key as a second input to decrypt
the digital signature. That then reveals the hash originally calculated by
the digital camera processor 12 at the time the image was taken. Note that
both the hash calculator 21 and the decryptor 22 may be implemented in the
same digital processing system 20. A comparator 23 receives the image hash
from the hash calculator 21 and the secure image hash from the decryptor
22. If these two hashes match, it is certain to any required degree that
the digital image in question is indeed identical to what the digital
camera system 10 originally produced. If, on the other hand, even one
single bit in the image being authenticated has been altered, the two
hashes will not even closely match and the image's authenticity will be
indicated as not being affirmed by an authenticity output signal A;
otherwise the comparator will indicate authenticity by an output signal A.
If the technique is to be effective (i.e., no false positives or false
negatives) and extended to larger data sets such as digital video, or even
digital holograms, reliance must be made upon the digital memory file
systems of the computer mass storage industry, which has already achieved
the ability to store and deliver extremely large binary data sets without
errors. On the other hand, analog techniques such as the NTSC encoding on
video tape formats are not sufficiently reliable for the present
invention. Noncorrected digital formats, such as the popular compact disc
(CD), are also not sufficiently reliable. In fact CD recording is so
unreliable that CD player manufacturers now utilize special techniques,
such as "oversampling" to combat the problem of missed bits. Such
techniques introduce a large number of errors upon playback which are
normally imperceptible and therefore unsuitable for the purposes of image
authentication in accordance with this invention. Consequently, the
present invention is directed to extremely reliable digital recording
systems of all kinds.
Measures of Protection
The invention as described above is resistant to forgery attempts since the
private key (which is not known to anybody except the manufacturer of the
camera) is embedded in a probe-proof microprocessor which itself is deeply
integrated into the camera's digital system. Even if some adept person
were to disassemble the camera and replace the microprocessor chip with
one containing a different key, the digital signature produced thereafter
would not be decryptable by any public key published by the manufacturer
since the private key must be mathematically related to the public key in
the conventional way of public key cryptography, and the only private key
that is properly related to the public key could not feasibly be derived
from the public key.
The advantages of freely distributing the verification algorithm and valid
public keys are great; with the algorithm freely available, verification
can become commonplace and routine. No special certification authority
need be involved in routine authenticity checks and no angst among the
parties involved need be created as a result of image file authenticity
being challenged. But the mass distribution of the verification algorithm
does carry one danger: it would be possible for someone to create a bogus
program which looks, behaves, and has the same file length as the genuine
verification algorithm with the only difference being it always proclaims
a "hash match" regardless of the authenticity of the image being verified.
With the algorithm freely and Widely available, this is not a large risk
as additional copies of the verification algorithm can be easily obtained
from multiple sources and a more reliable authenticity check made using a
best two-out-of-three scheme. Alternatively, as a precaution, a digital
signature and an image file known to have an altered image may be used to
authenticate the algorithm of the authenticator to be used to determine
whether or not it has been created to always proclaim a "hash match." When
the stakes are high and it is extremely important that the authentication
algorithm be known to be genuine, an independent certification authority
or the manufacturer of the digital camera could be called upon to provide
a copy of their own algorithm and the public key from their list of public
keys (camera serial numbers) at the time of authentication.
The algorithm and private key necessary for encrypting the digital
signature file from within the digital camera are to be embedded inside a
new class of secure microprocessors whose ROM contents cannot be read once
the recording system leaves the factory. Because the private key used for
encryption is hard-coded into the microprocessor chip by the manufacturer
(who must then ensure the private key remains secret), credibility of the
recording system's output becomes an extension of that of the
manufacturer; the digital signature from the digital camera can be
considered to be just as reliable and secure as if the signature had been
generated by the manufacturer.
Each digital camera should possess its own unique pair of private and
public keys, with the private key etched into the system's secure
microprocessor and the public key stored in three places: in a public key
list kept by the manufacturer, on the digital camera's name plate itself
(which can then also double as the camera's serial number), and in a
border as shown in FIG. 4 which contains more data about the captured
image as will be discussed in more detail below.
Assigning unique keys to each camera has the benefit of avoiding instant
obsolescence which would occur if only one private key were used for all
cameras, and that key were to be compromised. An even higher level of
security would occur if the manufacturer were to destroy all records of
each private key as the cameras are manufactured, since at that point the
private key is no longer needed by the manufacturer; only the record of
public keys are retained, which could be the same as the assigned serial
number as noted above. This would eliminate the possibility of private
keys being compromised by industrial espionage or theft.
Finally, regular and free distribution of all valid public keys is
desirable to defeat a counterfeiter who has learned of the encryption
algorithm employed and has written a program to produce digital signatures
based on fictitious private key. Decoding these digital signatures would
require the use of a public key not generated by the manufacturer and thus
expose them as forgeries at the outset. Freely distributing updated public
key lists would make it easy to identify and thwart such attempts. In
fact, it is recommended that the public key of the camera be recorded on
the border of the image file for use as the public key in the decryptor 22
since alteration of that public key in the image file to be authenticated
would result in the declaration by the comparator 23 of a mismatch between
the secure image hash from the decryptor 22 and the hash of the image file
from the hash calculator 21.
Uses of the Invention
The single most obvious use of the present invention in digital cameras
would be in situations where proof of image authenticity is necessary;
such as for legal evidence, insurance claims, or intelligence gathering.
The inevitable transition to digital cameras and
electronically-transmitted images will also make it more difficult for the
photographer to protect his image copyright, since electronic images tend
to proliferate faster and with less control from the author than the
traditional distribution method which places image control in the hands of
whoever holds the original negative or transparency. Just as it is common
practice today to obtain releases from photographed persons for any
published picture containing a recognizable image of the person, it is
reasonable that in the future no electronic image will be published
without first having authenticated the image using the digital signature
of the camera which is registered with the photographer in order to thwart
claims that photographs published have been altered and to improve the
trustworthiness of all publications and to uphold copyright claims.
This technique need not be limited to still digital images. Because digital
signatures can be used to verify any block of digital data in an image
file, it can also be implemented in digital video cameras as noted
hereinbefore. In all of these devices, a digital signature can be
generated and recorded each time the recording process stops or pauses, or
after each block of data; each video "take" is hashed, encoded and written
at the time it is recorded, or in the case of continuous recording by a
digital, video recorder at the time each block is completed by using the
technique of multiplexing between a set of three sets of buffers; as one
buffer is filled with a block of data, another block of data in a second
buffer is hashed and encrypted and a third block of data is recorded in a
third buffer at a faster clock rate so that its algorithms are complete in
the second buffer in time to transfer the digital signature in a third
buffer to a camera bulk storage medium before a multiplexer shifts
functions among the set of three buffers for the next block of data. Thus,
with a set of three buffers A, B and C, assume A is receiving an image
block, B is hashing and encrypting a previous block, and C is transferring
an encrypted block into the camera bulk storage medium. During the next
block interval, the functions are switched A to B, B to C and C to A, and
during the third block interval the functions are again switched B to C, C
to A, and A to B. The following block interval commences a new
multiplexing cycle. In that manner, real-time recording in the camera bulk
storage medium is delayed by only three block intervals.
A Special Border
In the case of the digital camera equipped with the present invention being
targeted towards legal authentication, a few additional features can be
implemented to better serve this use. A brightly colored border could
automatically be generated as part of each captured image file. Within the
border would appear (as shown in FIG. 4) textual information about the
image: the camera public key, the date and time it was taken, the ambient
light level seen by the camera at the time of exposure, the original color
temperature of the scene, the software version of the camera's firmware,
f/stop and shutter speed (or the CCD equivalent "sample time"), the
focusing distance of the lens at the time of exposure, the distance
(range) of prominent objects in the scene being imaged using a range
finder 13, such as an acoustic, infrared or laser range finder or other
range finders such as multiple-object optical range finding or contrast
detection range finding systems, a unique image sequence number and, when
the technology allows for a Global Positioning System (GPS) receiver to be
built into the camera, the geographical coordinates of the camera
indicating where in the world the picture was taken.
The present invention as it applies to authentication of camera photographs
as described with reference to FIGS. 3a, 3b, 3c and 4 completely mitigates
the new threats of computer-assisted alteration of digital image files and
eliminates the threat of deception by attaching or substituting false
captions or other information recorded in the border as shown in FIG. 4.
Such information helps an investigator identify and interpret information
by what is depicted in the photographs.
One piece of information captured in the border of the photographs is a
record of the distance at which the lens was focused. A second piece of
information captured in the border that may be useful in interpretation of
the photograph in a more sophisticated way is range information of
prominent objects in the scene at the time the picture was taken using the
range finder 13, such as an acoustic range finder, or an infrared or laser
range finder. Optical range finders could also be used. In an acoustic
range finder, the acoustic module (sometimes referred to as an "ultrasonic
transducer" operates at frequencies selected beyond the range of human
hearing, and often pulses of multiple frequencies may be used to guard
against the absorption of a single frequency by a particular object. Once
the timing of the return echo has been obtained, the distance can be
determined by employing the formula d=(1,100 t)2 where the distance of the
object d is in feet and the t is the number of seconds. For example, if
the echo returns in 9 milliseconds, the distance of an object would be
recorded as 5 feet. Listening for multiple echoes could allow for
recording the distance of several prominent objects in the field of view
(typically at least three). Less prominent objects could return echoes of
lesser energy, but prominent objects should be selected by adjustment of a
threshold selection amplifier. Range information recorded in the image
border not only makes digital manipulation of the photograph in a computer
more difficult without the manipulated photograph being declared not
authentic by the comparator 23, but also aids one viewing and analyzing
the scene with some perspective of the two-dimensional photograph of a
three-dimensional scene, such that the depths of all other objects may be
estimated by the relative positions of the prominent objects the range of
which are determined and recorded.
The range information also thwarts attempts to make a duplicate "original"
image file which has been manipulated by projecting at lifesize
proportions the manipulated image file and photographing it with the same
camera as was used to record the truly original image file at a distance
similar to what the image's content would suggest. Such a manipulated
duplicate "original" would be readily declared to be not authentic since
all ranges recorded for the objects in the image would be virtually the
same as the range recorded for objects at or near the center of the
projected image instead of the true original ranges recorded in the truly
original image file.
The lens' focused distance and f/stop are there to help detect potential
abuse of the camera, such as taking a close-up picture of a modified
photograph and trying to pass it off as an unaltered original. The ambient
light level and color temperature readings would be useful for getting a
feel for exactly what the scene was like at the time of exposure;
something a sensitive optical element might inadvertently hide via
automatic exposure and color correction.
Since all the textual data in the image file border are part of the
authenticated image file, their credibility are also upheld when
authenticated by the authentication process. The accuracy of the date and
time information would again be the responsibility of the secure
microprocessor; in addition to being able to keep its algorithm and
private key a secret, it also could have a lithium battery powering a
system clock set to Universal (Greenwich Mean) Time at the time of
manufacture. If the timer should ever fail or is tampered with, the system
would be programmed to fill the time and date fields with XXXX's,
eliminating the chance of an erroneous random time being indicated for the
actual time.
Higher Level of Security
Although the present invention offers a satisfactory level of security,
nevertheless there still exists a small possibility that a determined
person may be able to discover the camera's private key given an extended
amount of time. (No cryptographic scheme will protect data forever; given
sufficient time, advancements in code breaking or improved computer power
may some day be enough to render any existing level of cryptographic
protection obsolete.) If the discovered private key were to be available
to the custodian of the camera image files, it would allow the custodian
of the image files from that camera to generate authentic-looking digital
signatures on altered image files, essentially undermining the credibility
offered by the compromised camera. However, the security level of other
cameras in use, and of images taken with those other cameras will remain
uncompromised.
It would be wise for a manufacturer of such digital cameras to regularly
upgrade and enhance the sophistication of the encryption implementation as
newer camera models are introduced, typically those using longer
encryption/decryption key lengths and improved encryption/decryption and
hash algorithms. It is expected that evolving authentication algorithms
(the public domain component of this authentication invention which is to
be freely distributed) will be designed to be compatible with earlier
versions, i.e., to recognize, identify and authenticate image files made
with all previous digital camera models equipped with a microprocessor for
hash calculation and a secure microprocessor for encryption with a private
key.
Because the encryption details must necessarily be changed often (depending
on the technological capabilities of the day), no single image format, key
length encryption, or hashing algorithm is bein | | |
