Steganography: Understanding the Basics

Steganography represents the practice of representing information within another message or a physical object. The motive of steganography is to make information invisible to human inspection. It is a combination of two Greek words, including steganós (concealed) and graphia (writing).

In this article, we will be exploring the details of Steganography. We will explore the types, use and areas of applications.

Table of Contents

• What is Steganography?
• How is it used?
• Types of Steganography
  • Physical Steganography
  • Digital Steganography 
  • Network Steganography
  • Audio steganography 
  • Image Steganography
• How does Steganography Work?
• Difference Between Steganography and Cryptography

What is Steganography?

It is the practice of hiding secret messages within any unsuspicious objects. One very basic example of this is the use of invisible ink on private letters. Some other examples include hiding text within images, videos, shopping lists or documents. While it has been used for centuries, it has been digitized in the recent year. The aim is to disguise sensitive information which can later be extracted at the destination.

How is Steganography Used?

Here are some examples of how steganography is used:

1. Data Security and Confidentiality: It secures and protects sensitive data from unauthorized access. By hiding information within seemingly innocent files or communication channels, it adds an extra layer of security, making it difficult for adversaries to detect or intercept the hidden data.
2. Digital Watermarking: Steganography techniques are utilized in digital watermarking to embed copyright information, ownership details, or metadata within multimedia files. This allows content creators to protect their intellectual property and track unauthorized use or distribution.
3. Covert Communication: Steganography includes covert communication and clandestine messaging. In certain situations, where privacy or secrecy is crucial, steganography can be used to hide messages within seemingly harmless files or media, making it challenging for eavesdroppers or surveillance systems to detect or decipher the hidden content.
4. Digital Forensics and Anti-Fraud Measures: Steganography is utilized in digital forensics to detect hidden information or evidence within digital files. It helps investigators uncover hidden messages, identify tampered images, or fraud detection, thus aiding in criminal investigations and preventing cybercrimes.
5. Information Concealment in Social Media: Steganography can be used to hide sensitive or private information within social media posts, images, or comments. This allows individuals to share confidential data or communicate discreetly within public platforms without drawing attention or arousing suspicion.
6. Protecting Sensitive Documents: Steganography techniques can be employed to secure confidential documents, such as legal contracts, financial records, or classified information. By hiding sensitive data within innocuous files or images, it adds an extra layer of protection against unauthorized access or data breaches.

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Types of Steganography

The following types of steganography are used at present:

1. Physical Steganography

It is a method of hiding secret information within an ordinary, non-secret message to avoid detection. Steganography is a term derived from Greek words “steganos” (hidden or covered) and “graph” (to write). Unlike cryptography, which converts data into an unreadable format that can only be understood when decrypted with a key, steganography hides information in plain sight. To an observer without prior knowledge, the message appears completely normal.

At present, digital steganography has become more prevalent. It is known as cryptography’s “dark cousin” because, unlike encryption, which results in detectable gibberish that can sometimes be broken depending on computing power, steganography hides the secret message within a legitimate message that appears normal to the naked eye. This makes detection difficult, especially when the hidden messages are encrypted, making them even more untraceable.

2. Digital Steganography 

Digital steganography is a technique used to hide secret information within digital media such as images, audio files, or video files. The goal is to hide the information in such a way that it does not attract attention to itself. The hidden information can be a text message, another image, or digital data. In digital images, various methods are used for steganography:

• Least Significant Bits (LSB): In this method, the least significant bits of the color values of pixels in an image are changed to encode the secret message. For example, if a pixel has the color defined by RGB values (124, 5, 78) and it is changed to (123, 6, 79) to hide some message, the difference in colors will be minimal and almost imperceptible to the human eye.
• Masking: This technique is similar to watermarking. It involves modifying the luminance of parts of the image. Luminance determines how bright a particular object will appear. Without an available comparison to the original, these changes will hardly be noticeable. Masking and filtering techniques are mostly used on 24-bit and grayscale images.
• Palette-Based Technique: In this method, messages are embedded in palette-based images such as PNG or GIF. There is a color lookup table that holds every color used in the image. Every pixel is represented as single byte, and pixel data is an index to color palette. In pallete based images, there are two approaches for hiding messages: embedding messages into the palette and embedding into the image data.

3. Network Steganography

Network steganography is a method of hiding secret information within network communication protocols. It can work with any network protocol, from the physical layer (layer 1) to the application layer (layer 7) of the OSI model. 

In network steganography, instead of modifying the payload (the actual data being transmitted), parts of the network protocol that are not recognized as payload are modified. This can be done in various ways, such as:

• Permutation of wave-shape data: The shape of the signal wave used in the communication can be subtly altered to encode secret information.
• Permutation of fragments data: The order or arrangement of data fragments can be changed to hide information.
• Permutation of optional tag data: Optional tags in network protocols can be manipulated to carry hidden data.
• Permutation of header data: The header of a packet, which usually contains metadata about the packet, can be altered to include secret information.
• Permutation of invalid/malformed frames and packets: Invalid or malformed frames and packets, which would typically be ignored or discarded by network devices, can be used to carry hidden information.

4. Audio Steganography 

It is a technique used to hide information within audio signals without altering the perceptible quality of the audio. It involves embedding secret data into the audio files in a way that is imperceptible to the human ear. Here is a detailed explanation of audio steganography:

• Least Significant Bit (LSB) Substitution: It is a common method used in audio steganography. In this technique, the least significant bits of the audio samples are replaced with the secret data bits. Since the human ear is less sensitive to changes in the least significant bits, the modifications are typically undetectable.
• Spread Spectrum Technique: The spread spectrum technique spreads the secret data across the entire spectrum of the audio signal. This can be achieved by modifying the amplitude or phase of the audio samples at specific intervals to represent the hidden data. The changes are carefully controlled to avoid audible distortion.
• Echo Hiding: It involves adding inaudible echoes to the audio signal to represent the hidden data. These echoes are created by delaying certain portions of the audio and blending them back into the original signal. The delay values and blending ratios are carefully calculated to embed the secret information.
• Phase Coding: Such techniques exploit the phase information of the audio signal to hide the secret data. By modifying the phase of specific audio samples, the hidden information can be encoded. Since the human ear is more sensitive to changes in the phase, sophisticated algorithms are used to ensure imperceptibility.
• Spread Spectrum Modulation: It is a technique where the secret data is modulated onto a carrier signal in the audio spectrum. The carrier signal is carefully chosen to be perceptually similar to the audio content, making it difficult to detect the presence of the hidden data.
• Frequency Domain Techniques: Frequency domain techniques involve transforming the audio signal into the frequency domain. For this purpose it uses techniques like the Discrete Fourier Transform (DFT) or Discrete Cosine Transform (DCT). The secret data is then embedded in the transformed coefficients of specific frequency components.
• Statistical Methods: Statistical methods exploit the statistical properties of the audio signal to hide the secret data. These techniques aim to modify the statistical distribution or correlations in the audio samples to encode the hidden information.

5. Image Steganography

Image steganography is a technique used to hide secret information within digital images without visibly altering the image’s appearance. It involves embedding data within the image pixels in a way that is imperceptible to the human eye. Here is a detailed explanation of image steganography:

• Transform Domain Techniques: These techniques involve applying mathematical transforms, including Discrete Fourier Transform (DFT) or Discrete Cosine Transform (DCT), to the image. The secret data is then embedded in the transformed coefficients of specific frequency components. These changes are usually invisible to the human eye.
• Palette-based Techniques: Such techniques are used for steganography in images with limited color palettes, such as indexed color images or GIF files. These techniques modify the color indexes or color tables to embed the hidden data without significantly affecting the visual quality of the image.
• Noise-based Techniques: Noise-based techniques introduce imperceptible noise into the image to encode the secret data. The noise can be added uniformly across the image or selectively in specific regions. By analyzing the noise pattern, the hidden information can be extracted.
• Spatial Domain Techniques: Spatial domain techniques directly modify the pixel values or arrangement in the image to embed the secret data. This can involve rearranging the pixel order, altering pixel positions, or modifying color values within certain tolerances.
• Visual Cryptography: This technique splits the image into multiple shares or layers, where each share individually appears as random noise. When the shares are combined or overlaid, the hidden information becomes visible. This method provides a secure and visually impressive way to hide data within images.

How does Steganography Work?

The following points provide a step-by-step explanation of how steganography works:

1. Choose a Cover Medium: The first step in steganography is to choose a suitable cover medium. This is the file or data that will hide the secret message. It could be an image, audio file, video file, or even a network protocol.
2. Prepare the Secret Message: Next, you need to have the secret message that you want to hide. This could be a text message, another image, or any form of digital data.
3. Embed the Secret Message: Now, you need to embed the secret message into the cover medium. This is done in such a way that it does not significantly alter the appearance or structure of the cover medium. For example, if you’re using an image as your cover medium, you might change the least significant bits of the color values of certain pixels to encode the secret message. These changes are usually so small that they’re not noticeable to the human eye.
4. Create the Stego-Medium: The result of the embedding process is what’s known as the “stego-medium”. This is the cover medium that now contains the hidden message. To an observer, it should look just like an ordinary file or piece of data.
5. Transmit the Stego-Medium: The stego-medium is then sent to the intended recipient. This could be done over the internet, on a physical storage device, or in any other way that data can be transferred.
6. Receive and Extract the Secret Message: The recipient, who knows that there’s a hidden message in the stego-medium, can then extract it. This usually requires some knowledge of how the message was embedded. For example, if the message was hidden in the least significant bits of an image, the recipient would need to know to look there.
7. Decrypt the Secret Message (if necessary): If the secret message was encrypted before it was embedded in the cover medium, the final step is for the recipient to decrypt it. This will require the correct decryption key.

Difference Between Steganography and Cryptography

The following is the tabular format comparing steganography and cryptography based on various parameters:

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