Goals Identify image tampering methods. Assess methods available for protecting images. Assess image authentication techniques. Identify directions for future work.
Categories of Image Tampering There are three main categories of image tampering: Enhancing Compositing Copy/Move
Enhancing Changing the color of objects Changing the weather conditions Blurring out objects
Compositing Combining two or more images to create a new image
Copy-Move Copying regions of the original image and pasting into other areas. The yellow area has been copied and moved to conceal the truck.
What can be done to protect digital images? Watermarking Fragile watermarks Semi-fragile watermarks Self-embedding watermarks Digital cameras with watermarking capabilities Digital Fingerprinting/Signatures Digital cameras with fingerprinting capabilities
Digital Watermarking The basic concept of digital watermarking an image is that a low level signal is placed directly into the image data. Any manipulation of the image will impact the watermark and subsequent retrieval of the watermark and examination of its condition will indicate if tampering has occurred.
Fragile Watermarks Fragile watermarks are designed to detect every possible change in pixel values. Variety of Techniques but in most cases, the watermark is embedded in the least significant bit (LSB) of the image. Advantages: Pick up all image manipulations – malicious and non- malicious Disadvantages: Too sensitive
Semi-Fragile Watermarks They are robust, to a certain extent, and are less sensitive to pixel modifications. Techniques: Divide image into blocks and utilize bits from each block to calculate a spread spectrum noise like signal which is combined with DCT coefficients and inserted as a watermark. Divide image into blocks, construct watermark in DCT domain from pseudo-random zero-mean unit variance Gaussian numbers, take the inverse DCT and insert into the image. Advantage: less sensitive than fragile watermarks
Self-Embedding Tampered images result in lost information. The previous techniques will only detect and localize areas of interest when authentication is carried out. Self-embedding allows tamper detection and recovery of missing information. General concept is that the image is embedded in itself in an encrypted form. Advantage: Potential for original data to be retrieved. Disadvantage: Tampering with the image can remove blocks of the original image making retrieval of content impossible
Digital Cameras with Watermarking Capabilities Watermarking based on secret key, block ID and content. The image is divided into blocks and each block watermarked using a frequency based spread spectrum technique incorporating the secret key, block ID and block content. Image of photographers iris is combined with the camera ID, the hash of the original image and other details specific to the camera.
Digital Fingerprints/Signatures Based on the concept of public key encryption. Hashed version of image is encrypted using a private key. Encrypted file provides a unique signature/fingerprint of the image which can be used to authenticate by decryption with public key. Mainly used in transmission of images.
Digital Cameras with Fingerprinting Capabilities Epson Image Authentication System (IAS) The IAS software in the camera instantly seals the captured images with an invisible digital fingerprint. Verification of image is achieved by any PC with Image Authentication System software installed
Authentication Techniques Active Authentication Rely on the presence of a watermark or fingerprint. Require knowledge original image Algorithm/key used to embed the watermark or fingerprint. Passive Authentication No requirement of knowledge of original image. Does not rely of presence of watermark or fingerprint.
Copy-Move Detection Original ImageTampered Image Exact MatchRobust Match
Copy-Move Detection Original Image Tampered Image PCA Detection
Re-sampling Detection Original ImageTampered Image Periodic pattern in Fourier Transform of altered region Fourier Transform of unaltered region
Inconsistencies in Lighting Genuine Image Tampered Image
Future Research Development of a better self embedding technique. Development of an all inclusive passive authentication technique.
Conclusions Digital image forgeries can be used to deceive the public and the authorities. They are here to stay. Until non destructible/ non removal digital watermarks are perfected, passive authentication will remain necessary. Currently no single passive authentication technique can detect all types of digital forgeries.
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