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# Cryptpng
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A way to store encrypted data inside a png without altering the image itself.
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## Usage
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```shell script
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# encrypt
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cryptpng encrypt --image <name of the image> --in <input file> --out <output file>
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# decrypt
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cryptpng decrypt --image <crypt image> --out <decrypted output file>
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```
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## Technical Information
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It should be possible to store data with a size up to ~ 4GB, but in reality most image viewers have
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problems with chunks that are bigger than several Megabytes.
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The data itself is stored in a [png chunk](http://www.libpng.org/pub/png/spec/1.2/PNG-Structure.html)
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and encrypted via aes. The encryption chunk is stored right before the `IDAT` chunk that contains the
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image data. The steps for encrypting are:
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### Encrypt
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1. Parse the png file and split it into chunks.
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2. Prompt for a password and use the scrypt 32byte value with a generated salt.
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3. Store the salt in the `saLt` chunk.
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4. Encrypt the data using aes and the provided hashed key.
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5. Split the data into parts of 1 MiB of size.
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6. Store every data part into a separate `crPt` chunk.
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7. Write the png header and chunks to the output file.
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### Decrypt
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1. Parse the png file and split it into chunks.
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2. Get the `saLt` chunk.
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3. Get the `crPt` chunks and and concat the data.
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4. Prompt for the password and create the scrypt 32byte hash with the salt.
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5. Decrypt the data using aes and the provided hash key.
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6. Write the data to the specified output file.
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