Add main file [WIP]

Add cryptpng.go file as the main entrance. The writing of the data is currently not working.

.gitignore

@@ -0,0 +1,2 @@
+*.png
+.idea

README.md

@@ -1 +1,3 @@
-cryptpng
+# cryptpng
+
+A proof of concept implementation of storing encrypted data inside of png metadata chunks.

cryptpng.go

@@ -0,0 +1,159 @@
+package main
+
+import (
+	"crypto/cipher"
+	"crypto/rand"
+	"encoding/base64"
+	"flag"
+	"fmt"
+	"io"
+	"os"
+	"log"
+	"crypto/aes"
+	"bufio"
+	"hash/crc32"
+	"encoding/binary"
+)
+
+type ChunkData struct {
+	length uint32
+	name string
+	data []byte
+	crc uint32
+}
+
+var filename string
+func main() {
+	flag.StringVar(&filename, "file", "image.png", "The path of the png file.")
+	f, err := os.Open(filename)
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer f.Close()
+	valid, header := validatePng(f)
+	if valid {
+		fout, err := os.Create("encrypted-" + filename)
+		if err != nil {
+			log.Fatal(err)
+		}
+		defer fout.Close()
+		_, _ = fout.Write(header)
+		chunk, err := readChunk(f)
+		if err != nil {
+			log.Fatal(err)
+		}
+		for chunk.name != "IDAT" {
+			fmt.Printf("l: %d, n: %s, c: %d\n", chunk.length,  chunk.name, chunk.crc)
+			_, _ = fout.Write(chunk.data)
+			chunk, err = readChunk(f)
+			if err != nil {
+				log.Fatal(err)
+			}
+		}
+		reader := bufio.NewReader(os.Stdin)
+		fmt.Print("Data to encrypt: ")
+		inputDataString, _ := reader.ReadString('\n')
+		inputData := []byte(inputDataString)
+		inputData, err = encryptData(inputData)
+		if err != nil {
+			panic(err)
+		}
+		cryptChunk := createChunk(inputData, "crPt")
+		_, _ = fout.Write(cryptChunk.data)
+		for {
+			_, _ = fout.Write(chunk.data)
+			chunk, err = readChunk(f)
+			if err != nil {
+				break
+			}
+		}
+	} else {
+		log.Fatal("Invalid png.")
+	}
+}
+
+// validates the png by reading the header of the file
+func validatePng(f *os.File) (bool, []byte) {
+	headerBytes := make([]byte, 8)
+	_, err := f.Read(headerBytes)
+	if err != nil {
+		log.Fatal(err)
+	}
+	firstByteMatch := headerBytes[0] == 0x89
+	pngAsciiMatch := string(headerBytes[1:4]) == "PNG"
+	dosCRLF := headerBytes[4] == 0x0d && headerBytes[5] == 0x0a
+	dosEof := headerBytes[6] == 0x1a
+	unixLF := headerBytes[7] == 0x0a
+	return firstByteMatch && pngAsciiMatch && dosCRLF && dosEof && unixLF, headerBytes
+}
+
+// reads the data of one chunk
+// it is assumed that the file reader is at the beginning of the chunk when reading
+func readChunk(f *os.File) (ChunkData, error) {
+	lengthRaw := make([]byte, 4)
+	_, err := f.Read(lengthRaw)
+	length := binary.BigEndian.Uint32(lengthRaw)
+	crcRaw := make([]byte, 4)
+	nameRaw := make([]byte, 4)
+	_, _ = f.Read(nameRaw)
+	name := string(nameRaw)
+	data := make([]byte, length)
+	_, err = f.Read(data)
+	_, err = f.Read(crcRaw)
+	crc := binary.BigEndian.Uint32(crcRaw)
+	return ChunkData{
+		length: length,
+		name:   name,
+		data:   data,
+		crc:    crc,
+	}, err
+}
+
+// creates a chunk with the given data and name
+func createChunk(data []byte, name string) ChunkData {
+	rawLength := make([]byte, 4)
+	binary.BigEndian.PutUint32(rawLength, uint32(len(data)))
+	rawName := []byte(name)
+	dataAndName := make([]byte, 0) // len(rawName) + len(data)
+	dataAndName = append(dataAndName, rawName...)
+	dataAndName = append(dataAndName, data...)
+	crc := crc32.ChecksumIEEE(dataAndName)
+	rawCrc := make([]byte, 4)
+	binary.BigEndian.PutUint32(rawCrc, crc)
+	fullData := make([]byte, 0) // len(rawLength) + len(dataAndName) + 4
+	fullData = append(fullData, rawLength...)
+	fullData = append(fullData, dataAndName...)
+	fullData = append(fullData, rawCrc...)
+	return ChunkData{
+		length: uint32(len(data)),
+		name:   name,
+		data:   fullData,
+		crc:    crc,
+	}
+}
+
+// creates an encrypted png chunk
+func encryptData(data []byte) ([]byte, error) {
+	reader := bufio.NewReader(os.Stdin)
+	fmt.Print("Password: ")
+	pw, _ := reader.ReadString('\n')
+	key := make([]byte, 32 - len(pw))
+	key = append(key, []byte(pw)...)
+	return encrypt(key, data)
+}
+
+func encrypt(key, text []byte) ([]byte, error) {
+	block, err := aes.NewCipher(key)
+	if err != nil {
+		return nil, err
+	}
+	b := base64.StdEncoding.EncodeToString(text)
+	cipherText := make([]byte, aes.BlockSize+len(b))
+	iv := cipherText[:aes.BlockSize]
+	if _, err := io.ReadFull(rand.Reader, iv); err != nil {
+		return nil, err
+	}
+	cfb := cipher.NewCFBEncrypter(block, iv)
+	cfb.XORKeyStream(cipherText[aes.BlockSize:], []byte(b))
+	return cipherText, nil
+}