Skip to content

PNGStego

stegobox.codec.PNGStego

Bases: BaseCodec

PNG steganography - simple LSB steganography in PNG images

  • To encode: The input image and message are both converted into binary, and the message is embedded into the least significant bits (LSB) of the input image. Once the message is encoded, a trailing delimiter is written. The resulting image is saved as a PNG file.
  • To decode: The least significant bits of the input image are read off until the delimiter is hit. After which the secret message would be recovered.

Source: nzimm/png-stego

Source code in stegobox/codec/png_stego.py 
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
class PNGStego(BaseCodec):
    """PNG steganography - simple LSB steganography in PNG images

    * To encode: The input image and message are both converted into binary, and the
      message is embedded into the least significant bits (LSB) of the input image. Once
      the message is encoded, a trailing delimiter is written. The resulting image is
      saved as a PNG file.
    * To decode: The least significant bits of the input image are read off until the
      delimiter is hit. After which the secret message would be recovered.

    Source: [nzimm/png-stego](https://github.com/nzimm/png-stego)
    """

    def __init__(self) -> None:
        super().__init__()

    def encode(self, carrier: Image.Image, payload: str) -> Image.Image:
        """Encoder requires the carrier image to be PNG and the payload to be a string.

        Args:
            carrier: Carrier image in format PNG. Read with `stegobox.io.image.read()`.
            payload: Payload (secret message) to be encoded.

        Returns:
            The encoded PNG image object with the payload embedded.
        """
        self._check_png(carrier)
        self._check_empty_payload(payload)

        # Maximum number of bytes that can be encoded
        max_payload_length = carrier.size[0] * carrier.size[1] * 3

        # Convert payload to binary (removes leading '0b')
        payloadb = to_binary(payload)[2:]
        if len(payloadb) > max_payload_length:
            raise Exception(f"Payload must be shorter than {max_payload_length} bits.")

        bits_encoded = 0
        delimiter_bits = 0
        image_data = []

        # Iterate over all pixels in the image
        for y in range(carrier.size[1]):
            for x in range(carrier.size[0]):
                for color in carrier.getpixel((x, y)):
                    if bits_encoded < len(payloadb):
                        # Set LSB of color to one bit of the payload. Mask color with
                        # 254 to guarantee LSB is 0, then set LSB to the payload bit
                        image_data.append((color & 254) | int(payloadb[bits_encoded]))
                        bits_encoded += 1

                    elif delimiter_bits < 8:
                        # Zero out 8 LSB as delimiter for the payload
                        image_data.append(color & 254)
                        delimiter_bits += 1

                    else:
                        # Finally, append the rest of the image data
                        image_data.append(color)

        # Returned encoded image - must be saved as PNG
        return Image.frombytes("RGB", carrier.size, bytes(image_data))

    def decode(self, carrier: Image.Image) -> str:
        """Decode the secret payload from the carrier image.

        Args:
            carrier: Carrier image in format PNG. Read with `stegobox.io.image.read()`.

        Returns:
            The decoded payload (secret message).
        """
        self._check_png(carrier)

        # Buffer for the payload
        payload = ""

        # Return message after finding the first \x00 byte
        bit_count = 1

        # Byte list to convert back to ascii string
        byte_list = []
        byte_str = "0"

        for y in range(carrier.size[1]):
            for x in range(carrier.size[0]):
                for color in carrier.getpixel((x, y)):
                    byte_str += bin(color)[-1]
                    bit_count += 1

                    if bit_count == 8:
                        if byte_str == "00000000":
                            break

                        byte_list.append(byte_str)
                        bit_count = 0
                        byte_str = ""

        # Convert byte list to ascii string
        try:
            for byte in byte_list:
                payload += to_string(byte)
        except TypeError:
            return ""

        return payload

    def _check_png(self, image: Image.Image) -> None:
        if image.format != "PNG":
            raise Exception("Carrier image must be PNG in PNGStego.")

    def _check_empty_payload(self, payload: str) -> None:
        if not payload:
            raise Exception("Payload must not be empty.")

encode(carrier, payload)

Encoder requires the carrier image to be PNG and the payload to be a string.

Parameters:

Name Type Description Default
carrier Image

Carrier image in format PNG. Read with stegobox.io.image.read().

 required
payload str

Payload (secret message) to be encoded.

 required

Returns:

Type Description
Image

The encoded PNG image object with the payload embedded.
Source code in stegobox/codec/png_stego.py 
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
def encode(self, carrier: Image.Image, payload: str) -> Image.Image:
    """Encoder requires the carrier image to be PNG and the payload to be a string.

    Args:
        carrier: Carrier image in format PNG. Read with `stegobox.io.image.read()`.
        payload: Payload (secret message) to be encoded.

    Returns:
        The encoded PNG image object with the payload embedded.
    """
    self._check_png(carrier)
    self._check_empty_payload(payload)

    # Maximum number of bytes that can be encoded
    max_payload_length = carrier.size[0] * carrier.size[1] * 3

    # Convert payload to binary (removes leading '0b')
    payloadb = to_binary(payload)[2:]
    if len(payloadb) > max_payload_length:
        raise Exception(f"Payload must be shorter than {max_payload_length} bits.")

    bits_encoded = 0
    delimiter_bits = 0
    image_data = []

    # Iterate over all pixels in the image
    for y in range(carrier.size[1]):
        for x in range(carrier.size[0]):
            for color in carrier.getpixel((x, y)):
                if bits_encoded < len(payloadb):
                    # Set LSB of color to one bit of the payload. Mask color with
                    # 254 to guarantee LSB is 0, then set LSB to the payload bit
                    image_data.append((color & 254) | int(payloadb[bits_encoded]))
                    bits_encoded += 1

                elif delimiter_bits < 8:
                    # Zero out 8 LSB as delimiter for the payload
                    image_data.append(color & 254)
                    delimiter_bits += 1

                else:
                    # Finally, append the rest of the image data
                    image_data.append(color)

    # Returned encoded image - must be saved as PNG
    return Image.frombytes("RGB", carrier.size, bytes(image_data))

decode(carrier)

Decode the secret payload from the carrier image.

Parameters:

Name Type Description Default
carrier Image

Carrier image in format PNG. Read with stegobox.io.image.read().

 required

Returns:

Type Description
str

The decoded payload (secret message).
Source code in stegobox/codec/png_stego.py 
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
def decode(self, carrier: Image.Image) -> str:
    """Decode the secret payload from the carrier image.

    Args:
        carrier: Carrier image in format PNG. Read with `stegobox.io.image.read()`.

    Returns:
        The decoded payload (secret message).
    """
    self._check_png(carrier)

    # Buffer for the payload
    payload = ""

    # Return message after finding the first \x00 byte
    bit_count = 1

    # Byte list to convert back to ascii string
    byte_list = []
    byte_str = "0"

    for y in range(carrier.size[1]):
        for x in range(carrier.size[0]):
            for color in carrier.getpixel((x, y)):
                byte_str += bin(color)[-1]
                bit_count += 1

                if bit_count == 8:
                    if byte_str == "00000000":
                        break

                    byte_list.append(byte_str)
                    bit_count = 0
                    byte_str = ""

    # Convert byte list to ascii string
    try:
        for byte in byte_list:
            payload += to_string(byte)
    except TypeError:
        return ""

    return payload