Dual Watermarking for image tamper detection and recovery

From Anil jaiswal:AN-02

Ankur kulhari: AN-04Mahesh Gour: AN-14

Piyush Shrivastava: AN-20Pranshu jaiswal: AN-21

Digital watermarking???

• Digital watermarking is a technique to embed a digital signature(data string) into an image for the purpose of image integrity and authentication.

It is of two types:• Percetible watermark• Imperceptible watermark

Cont…• Perceptible Watermark: It is an message or

institute logo which can be recognized together with the host image.

• Imperceptible watermark: Information hidden in the image and this information or may logo or message which denotes ownership of image, parts of host image or even full copy of image itself which can later extracted.

Cont…

Imperceptible watermark can be classified based on functionality as:• Fragile :sensitive to all kind of modifications,

so used for image integrity• Semi-fragile: It can locate modified regions in

an image and extract remaining watermark from undamaged regions so capable of image authentication even after manipulation.

Cont…

• Robust : It is designed to resist attacks that attempt to destroy the watermark in an image. Its main purpose for ownership authentication or copyright protection.

Method for watermark generation

Cont…

Earlier approach of tamper detection

Lin’s method :It uses 3 level hierarchical structure.• It is storage effective.• It only requires secret key and a public algorithm to

recover a tampered image.

Drawbacks: • Too sensitive to error pixels.• No Second chance for block recovery.• Blocky effect

Cont…In this algorithm there are two copies of watermark for each non overlapping block in the image.Algorithm contains three steps:• Watermark embedding: It hides watermark of each

pair of partner block to another two mapping block. • Tampered block detection : It is done through 3

level hierarchical error block checking.• Tampered block recovery: It is done through 2

stage error block recovery scheme.

Watermark Generation

Watermark Generation

• Block mapping sequence is needed for one to one mapping sequence

X’=[f(X)=(k*X)mod N]+1It is known as 1-D transformationWhere X,X’([ 0,N-1]) are block number, k (a prime number ∈

Z-{N’s factors} is a secret key and N ( Z-{0})is the total ∈ ∈number of blocks in the image.

• A lookup table is used to record the mapping addresses of each block.

Cont…

Cont…

• Block watermark generation:– Calculate average of each of the partner block– Now calculate representative information

Cont…

Block Watermark embedding

Block Watermark embedding• Using 12-bit watermark of each pair of partner

block, the three LSB of each pixel of the block are used for watermark embedding.

Cont…

• Before physically changing the value of a pixel in the mapping block, a smoothing function is applied to minimize the difference between original and watermarked pixel.

Detection of tempered blocks

Detection of tempered blocks

Cont…

A 3-level hierarchical tempered Algorithm is as

Level-1 Detection: For non- overlapping block B of size 2×2 pixel in image

1. Retrieve the 12-bit watermark information of block B.2. Get the parity check bit p and v from the watermark.3. Calculate the exclusive- Or value of the first 10- bit of

watermark , denoted as p`.4. If p= p` and p!= v, mark block B valid, otherwise mark

it invalid.

Cont…Level 2 Detection : For each block B which mark valid after level-

1 detection , check following four triples neighboring block situations (N NE E), (E SE S), (S SW W), (W NW N) . The 3×3 blocks-neighborhood of central block B can be denote by compass directions;

NW N NE W B E SW S SEIf all blocks in any of the four triples are marked invalid, Mark

block B invalid.

Cont…

Level -3 Detection : For each valid block after level-2 detection, mark this block invalid if there are five or more than neighboring blocks in its 3×3 block-neighbourhood that are invalid.

This technique introduce much less false alarms because we use small block size (2×2).

Recovery of tampered blocks

Recovery of tampered blocks

• After the tamper detection process, all blocks are marked either valid or invalid.

• Only those invalid blocks need to be recovered.

Method of recovery of tampered blocks

• A two-stage recovery scheme is applied for tamper recovery as follow:

• Stage 1:• For each non-overlapping block B of size 2x2

pixels which is marked invalid.1. Find the mapping block of B from the look-up

table, denote by B’.2. If B’ is valid then B’ is the candidate block, go

to 5.

Cont…3. Find the mapping block of B’s partner block, denote by B”.4. If B” is valid then B” is the candidate block: otherwise stop, leave

block B alone.5. Retrieve the 12-bit watermark information from the candidate

block.6. If block B locates in the upper half of the image, the 5-bit

representative information of block B starts from the 1st bit (the left most bit) of the 12-bit watermark: otherwise, it starts from the sixth bit.

7. Pad three 0’s to the end of the 5th representative information to form a new 8-bit intensity.

8. Replace each pixel in block B with this new 8-bit intensity and mark block B as valid.

Cont…

• Stage 2: • The darker pixels surrounding the block(which

is being processed) are used for stage2 recovery process.

Thank You