Update README

README.md

@@ -11,3 +11,29 @@ cryptpng --image <name of the image> --in <input file> --out <output file>
 # decrypt
 cryptpng --decrypt --image <crypt image> --out <decrypted output file>
 ```
+
+## Technical Information
+
+It should be possible to store data with a size up to ~ 4GB, but in reality most image viewers have
+problems with chunks that are bigger than several Megabytes.
+The data itself is stored in a [png chunk](http://www.libpng.org/pub/png/spec/1.2/PNG-Structure.html)
+and encrypted via aes. The encryption chunk is stored right before the `IDAT` chunk that contains the
+image data. The steps for encrypting are:
+
+### Encrypt
+
+1. Parse the png file and split it into chunks.
+2. Prompt for a password and use the sha512 32byte value.
+3. Create a base64 string out of the data.
+4. Encrypt the base64 string using aes and the provided hashed key.
+5. Store the data into the `crPt` chunk.
+6. Write the png header and chunks to the output file.
+
+### Decrypt
+
+1. Parse the png file and split it into chunks.
+2. Get the `crPt` chunk.
+3. Prompt for the password and create the sha512 32byte hash.
+4. Decrypt the data using aes and the provided hash key.
+5. Decode the base64 data.
+6. Write the data to the specified output file.

cryptpng.go

@@ -60,6 +60,7 @@ func main() {
 	}
 }
 
+// encrypts the data of fin inside the png (f) and writes it to fout
 func EncryptDataPng(f *os.File, fin *os.File, fout *os.File) {
 	png := PngData{}
 	err := png.Read(f)
@@ -99,6 +100,7 @@ func encryptData(data []byte) ([]byte, error) {
 	return encrypt(key, data)
 }
 
+// decrypts the data of a png chunk
 func decryptData(data []byte) ([]byte, error) {
 	key := readPassword()
 	return decrypt(key, data)