Steganography vs cryptography

Cryptography and steganography pursue the same goal — protecting a communication — but by two opposite routes. Confusing them is a classic mistake.

Two opposing strategies

Cryptography makes a message unreadable. It is plain to see that a communication exists, but its content is scrambled: without the key, all you read is a string of unintelligible characters.

Steganography does something else: it hides the very existence of the message. No one suspects that a communication is taking place, because the secret is concealed within an innocuous-looking carrier — a holiday photo, an audio recording, a mundane piece of text.

In Greek, steganos means "covered, hidden" and graphein "to write": literally, hidden writing.

The table to remember

	Cryptography	Steganography
What is hidden	the CONTENT of the message	the EXISTENCE of the message
The observer...	sees an encrypted message	sees nothing unusual
Typical medium	openly encrypted text	image, sound, innocuous text
If intercepted	it is known there is a secret	it goes unsuspected

A few historical examples

The idea is very old:

• invisible ink (lemon juice, revealed by heat): the message only appears if you know it is there;
• the photographic microdots of the 20th century: an entire text reduced to the size of a punctuation mark;
• Greek antiquity: Herodotus recounts that Histiaeus had a message tattooed on the shaved scalp of a slave, then waited for the hair to grow back before sending him.

In all these cases, an observer sees nothing: no ciphertext, no sign of communication.

The two are complementary

The opposition should not mislead: in practice, you do not choose one against the other, you combine them.

The right method is to encrypt first, hide afterwards:

1. you encrypt the message (it becomes unreadable);
2. you hide this encrypted message in an innocuous carrier (it becomes invisible).

Thus, if the concealment is discovered, the attacker still comes across a ciphertext they cannot read. The two layers reinforce each other: steganography protects the existence, cryptography protects the content.

In summary

• Cryptography makes the message unreadable; you can see there is a secret.
• Steganography hides the existence of the message within an innocuous carrier.
• The Greek etymology means "hidden writing".
• Historical examples: invisible ink, microdots, scalp tattoo (Histiaeus).
• The two are complementary: you encrypt, then you hide.