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TEXTBOOK

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Digital data (text, images, audio, etc.) can easily be stored and transmitted, but they are also easy to copy, tamper with, or steal, leading to copyright issues.
Steganography is the art and science of hiding secret information inside digital data so that the existence of the hidden data is not noticeable.
It differs from cryptography:
Cryptography hides the content by encryption.
Steganography hides the existence of the information by embedding it in a cover file.

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Steganography is useful even when cryptography is available, because encrypted files attract attention, while stego files look normal and do not raise suspicion.

Secret data should be embedded in redundant regions (holes) of the cover so that the size and appearance of the file does not noticeably change.

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Watermarking, a sub-area of steganography, is used to prove ownership of digital content and prevent unauthorized use.

The chapter further explores applications, watermarking, fingerprinting, and text steganography in later sections

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Basic Features

The main features of a data hiding algorithm.

Embedding Capacity:
Refers to how much secret data can be hidden inside a cover file (image, text, audio, etc.).
Measured as bits of hidden data per bit of cover data (bpb).
Higher capacity is good, but sometimes only a small message needs to be hidden.

Invisibility (Perceptual Transparency):
The secret data should not visibly change the cover.
If humans cannot notice any difference before and after hiding, the method has high invisibility.

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Undetectability:

Even if humans cannot see changes, a computer should also not detect any unusual patterns.
The hidden data should not alter the statistical features of the file.

Robustness:

The hidden data should remain even if the cover is edited, compressed, resized, or processed.
This is especially important for watermarking (proof of ownership).

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Tamper Resistance:
Protects the hidden data from being altered or replaced by attackers.
Important for copyright information that must remain intact for many years.

Tradeoff Between Features:
Increasing robustness or tamper resistance usually reduces embedding capacity (amount of data that can be hidden).
Not all desirable qualities can be maximized at the same time.

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Data Should Be Hidden in the Body of the Cover:
Hidden data should not be stored in file headers or footers, because these parts may change when converting file formats.

No Method is Fully Unbreakable:
Skilled attackers may still break or remove hidden data despite strong protection techniques.

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Signal-to-Noise Ratio (SNR):
In steganography, a low SNR is preferred because it means the hidden data (signal) is lost inside the cover (noise), making it less detectable.

Cover Escrow vs. Blind Cover:
Cover Escrow: Original cover is needed to extract hidden data.
Blind Cover: Hidden data can be extracted without needing the original cover.

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Conflicting Requirements:

A single steganographic method cannot simultaneously provide high capacity, high robustness, and high undetectability.
Each method prioritizes only one or two of these features.

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Steganography has many other “digital” features, a few of which are listed here.

Stego-Key:
Like encryption keys, a secret stego-key is used to hide or extract the message.
Security depends on keeping the key secret, not the algorithm.

Algorithm Complexity:
Some methods may take more time to hide or retrieve data.
This is acceptable if they provide better invisibility and robustness.

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Asymmetric Hiding:
Encoding (hiding data) can be slow, as long as decoding (retrieving data) is fast and easy.

Error-Correcting Codes:
These help to detect and correct hidden data if the cover file gets slightly damaged or changed.

Parallel Processing:
Steganographic algorithms can be designed to work in parts, allowing multiple processors to embed or extract data at the same time.
Useful for both embedding and for steganalysts trying to break the system.

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Applications of Data Hiding

Hiding Private Messages:
Sensitive data can be hidden inside images, text, or audio files.
The sender can post the file publicly, and the receiver extracts the secret message using a stego-key.

Watermarking (Ownership Marking):
A small hidden mark is added to prove ownership or authorship.
The watermark must be resistant to removal to prevent copyright theft.

Fingerprinting:
Each distributed copy gets a unique hidden mark.
Helps to trace the source of illegal copies (e.g., music CDs).

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Tamper-Proofing:
Hidden data is used to detect whether a file has been modified.
Useful for images and audio to check for editing or removal of content.

Sealing a File:
A hidden checksum is stored inside the file.
If the seal is broken, it means the file was altered.

Alternative to Encryption:
When strong encryption is restricted by law, steganography can be used to protect privacy without attracting attention

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Audio-Based Applications:
Caller ID or commercial tracking can be hidden in audio broadcasts to monitor usage.

Feature Tagging:
Extra details like captions, names, or dates can be hidden inside files.
Requires high capacity, but does not need high robustness.

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Watermarking

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Watermarking is a technique used to embed ownership or identification information into digital data (images, audio, video, text).
The purpose of watermarking is to prove authorship or ownership and to identify illegal copying, theft, or alteration of digital content

The watermark should:

Not significantly increase the size of the original data.

Be robust so that it survives operations like compression, filtering, or cropping.

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Watermarking may use steganographic methods, where a string (e.g., owner's name) is hidden in the data and extracted later using a secret key.

Difference between Steganography and Watermarking:
In Steganography, the hidden data is important, and the cover data is not. The data may be fragile.
In Watermarking, the cover data is valuable, and the hidden data is not. The hidden data must be robust.

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Intuitive Methods

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Historical Methods of Data Hiding (Steganography)

Wax Tablet Method

Write the secret message on a wooden tablet.
Cover it with wax and write an ordinary message on the wax surface.
Used in ancient Greece to hide real messages beneath fake ones.

Tattoo on Messenger’s Scalp

Shave a messenger’s head and tattoo the secret message on the skin.
After the hair grows back, send the messenger to the destination.
Receiver shaves the head again to reveal the message.
Used by Histiaeus (Greek history).

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Invisible ink (like milk, vinegar, fruit juice) can be used to hide messages on paper. The secret text becomes visible only when the paper is heated.

Another historical method hides the secret message by using specific letters from a “normal-looking” cover text.

A variation of this method writes the secret message vertically as the first word of each line. The writer then adds more words to make the text look natural, so the secret message blends into poetry or sentences.

ACCEPTED YOU OVER NEIL BROWN

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6 . One old method hides data by placing tiny dots under specific letters in a normal text. When those marked letters are read together, they reveal the secret message.

A variation of this method hides data by slightly shifting or altering the position of certain letters instead of using dots.

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7. Check digits help check if numbers like ISBN or bank numbers are correct.

• An ISBN has 10 digits: first part for country, second for publisher, third for book, and the last digit is the check digit.

• To find the check digit, each digit is multiplied by a number from 10 down to 2 and added together.

• The check digit is chosen so the total becomes divisible by 11.

• If the check digit turns out to be 10, it is written as X.

What is the check digit for the 9-digit ISBN 0-387-98682?

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Digits of the ISBN (before the check digit):�0, 3, 8, 7, 9, 8, 6, 8, 2

We multiply each digit by weights from 10 down to 2:

0×10 = 0�3×9 = 27�8×8 = 64�7×7 = 49�9×6 = 54�8×5 = 40�6×4 = 24�8×3 = 24�2×2 = 4

Now add them all:

0 + 27 + 64 + 49 + 54 + 40 + 24 + 24 + 4 = 286

We need a check digit c such that:�(286 + c) is a multiple of 11.

286 is already divisible by 11 (because 286 ÷ 11 = 26).

So c = 0.

Final ISBN:�0-387-98682-0

Answer: The check digit is 0.

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8. Every letter in the alphabet is given a 3-digit code (e.g., A = 111, D = 121, G = 131, etc.).

• To hide a letter, you convert each digit of its code into a word in the cover text.

• If the digit is 1, choose a word with 1, 4, or 7 syllables.

• If the digit is 2, choose a word with 2 or 5 syllables.

• If the digit is 3, choose a word with 3 or 6 syllables.

• Each letter becomes three words, so a message can be hidden smoothly inside normal sentences.

• The table simply lists each letter and its assigned 3-digit code

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Each letter of the alphabet can be given a 5-bit binary code (since 5 bits allow 32 combinations, more than enough for 26 letters).

• So any message can be converted into a binary string (0s and 1s).

• To hide this binary string, each bit is hidden in the letters of a normal-looking text.

• For example, you can show 0 by using lowercase and 1 by using uppercase (or italics vs. normal font).

• Example: The binary sequence 011000011 is hidden in the phrase "to be or not" by changing some letters to uppercase → tO Be or nOT.

• This method of hiding data in letter forms is known as Bacon’s biliteral cipher.

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10. Choose a keyword (example: blah).�• Convert each letter to its alphabet position:�b = 2, l = 12, a = 1, h = 8.�• Make a grid with 12 columns (because the highest number is 12).�• In each row, place the secret message letters into columns 2, 12, 1, and 8 (in that order).�• Fill the rest of the empty grid spaces with other letters to make the text look natural and meaningful

11. Each of the 26 letters of the alphabet is matched with a musical note.�• The secret message is then written as a sequence of musical notes instead of letters.�• The music appears like a normal composition, but it actually hides the message.�• This method was introduced by Gaspari Schotti in his work Schola Steganographica.

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12. Write or print the secret message normally on paper.�• Then photograph it and reduce it to a tiny microdot (as small as a period mark).�• Paste this microdot into a normal letter, usually at the end of a sentence.�• The receiver can find it by holding the paper up to the light and spotting the small, shiny dot.

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Simple Digital Methods

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Modern steganography uses computers and binary data to hide information.

It has three types:
Naive steganography: No key used.
Secret steganography: Uses a secret key.
Public-key steganography: Uses public and private keys.

Hidden data replaces natural randomness in communication without changing how it looks or works.
File hiding: A data file can be disguised as a system file (e.g., renamed to MsProLibMod.DLL) so it blends with other files.
Camouflage method: Scrambles and attaches a hidden file to another file; looks normal but may seem suspicious if too large.
Disk hiding method: Stores hidden data in “free space” on a disk using special software, but it’s risky—new data can overwrite it.

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Data Hiding in Text

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Fixed-space word processors allow data hiding by adjusting spaces.
One space at the end of a sentence = hides bit 0
Two spaces at the end of a sentence = hides bit 1
Appending spaces at line ends is a simple method but can be detected by programs that remove extra spaces

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Right-margin justification can also be used for hiding bits by varying spaces between words.
Improved algorithm (Rules for bit interpretation):
Single space followed by a double space → bit 0
Double space followed by a single space → bit 1
Single space before and after a word → no bit
Double spaces before and after a word → no bit

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Syntactic Methods:

These methods hide data using ambiguous punctuation or by modifying sentence structure while preserving the original meaning.
Types:
Ambiguous punctuation: Changes in punctuation marks (e.g., commas, periods) to represent bits.
Meaning-preserving modification: Rearranging words or phrases without altering meaning.

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Example 1 – Punctuation:
“A common, boring, but responsible task.”
“A common, boring but responsible task.”�→ The presence or absence of a comma can represent binary 0 or 1.
Example 2 – Abbreviations:
“See e.g., page 100” vs. “See e.g. page 100”�→ Slight punctuation variation hides a bit.
Example 3 – Word Order:
“When you finish this, you can go.”
“You can go when you finish this.”�→ Both mean the same but encode different bits.

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Advantages:

Hard to detect automatically; requires human reading to decode.

Disadvantages:

Low embedding capacity (only one bit per phrase).
Manual effort required to encode and decode.
Risky if punctuation inconsistencies appear unnatural.

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Semantic Methods

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Hiding Data Using Synonyms

Data is hidden by choosing specific synonyms from a thesaurus.
Sender and receiver both use the same online thesaurus.
Each word in the cover text is checked in the thesaurus.
Example:

The word child appears as the 3rd word (index 2) in the synonym list: bud, chick, child, kid, minor.
So it hides the 2-bit value 2 (01 in binary).

If the required bits don’t match, the encoder replaces the word with another suitable synonym.
If no synonym works, a human may need to rewrite the sentence.

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Innocuous Text

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Steganography can hide data by generating normal-looking sentences using dictionary words arranged in valid syntax.
Binary data is converted into words by using bits as pointers to positions in dictionary lists (Article, Noun, Verb, etc.).
The decoder recovers data by finding each word’s position in the dictionary; it doesn’t need grammar rules.
Dictionaries must be carefully designed to avoid ambiguity when a word appears in multiple types.
This method increases file size a lot, so data is usually compressed, encrypted, encoded, then compressed again.

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Mimic Functions

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Mimic functions hide data by generating artificial text that looks like natural language so computers cannot detect the hidden message.

Requirements for generated text:

Letter frequencies must look like real English.
Most words must appear in a dictionary.
Sentences must be grammatically correct.

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How data is hidden:

At each step of generating a sentence, the encoder must choose between different grammar options.
The choice (option 1, 2, 3, …) is controlled by the next data bit(s).

Uses Context-Free Grammar (CFG):

A CFG contains rules for generating sentences.
Each rule has multiple choices (e.g., noun → Alice | Bob).
Each choice corresponds to a binary string (00, 01, 10, 11…).

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CFG is a set of rules used to generate valid sentences or strings in a language.
It uses terminal symbols (actual words/characters) and nonterminal symbols (placeholders like noun, verb, expression).
Each rule (production) has the form Nonterminal → combination of terminals/nonterminals.
CFG starts with a start symbol, and by repeatedly replacing nonterminals using the rules, it generates sentences.
Productions can be recursive, allowing generation of infinite sentences (e.g., nested expressions).
CFG defines the structure of languages, especially programming languages and natural languages.
It is called context-free because each rule can be applied independently, without depending on surrounding symbols

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THANK YOU

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Sri Krishna Institute of Technology

(Approved by AICTE, Accredited by NAAC, Affiliated to VTU, Karnataka)

11/18/2025