topic 7 - steganography part 1

Question 1

what is steganography?

Answer 1

Steganography – the art/science of communicating by hiding secret messages in innocuous ‘cover’ objects (such as an image).

Question 2

what is an acrostic?

Answer 2

Acrostic – a text, usually a poem, in which particular letters, such as the first letters of each line, spell a word or phrase.

Question 3

whats an example of an acrostic?

Answer 3

“Accept your overture. Next Friday, about eleven, come away anywhere”

Take second letter of each word to decode hidden message.

Secret message = c o v e r b l o w n

Question 4

what is a cover object?

Answer 4

a message that can be transmitted without suspicion.

E.g. Image file, digital sound or written text.

Must contain sufficient redundant data so that the secret information can be embedded without detection.

Question 5

what is a stego object?

Answer 5

a cover object containing an embedded secret object.

The same cover should never be used twice as two versions of one cover could easily be detected and possibly decoded.

Question 6

what is a stego key?

Answer 6

used to encrypt the secret message – (not applicable to pure steganography).

Question 7

what is pure steganography?

Answer 7

does not require the exchange of prior information such as a stego-key.

Both sender and receiver must have access to the embedding and extraction algorithm but the algorithm should not be public.

Question 8

what is a secret key steganography?

Answer 8

Secret key steganography – a secret stego-key is exchanged prior to communicating using a stego-object.

Question 9

how do you embed a binary image?

Answer 9

Make a copy of the cover image, c, and call this the stego-object, s.

Copy the value of the ith element in the secret binary image, m, to the LSB of the ith element in stego-object (where i refers to a column-wise/top to bottom index)

Repeat for all l(m) – number of elements (pixels) in the secret image.