Gallager's codes use so-called parity bits — extra bits that contain information about message bits.
In Gallager's codes, by contrast, the correlation between parity bits and message bits was random: the first parity bit might describe, say, the sum of message bits 4, 27, and 83; the next might do the same for message bits 19, 42, and 65.
One happens when some message bits are used to encrypt others, the other exists because the key bits are not truly random.
One parity bit might indicate, say, whether the sum of message bits 1, 2, and 4 is even or odd; the next parity bit might do the same for message bits 3, 4, and 6; and so on, through successive triplets of bits.
But earlier codes generated the parity bits in a systematic fashion: the first parity bit might indicate whether the sum of message bits one through three was even; the next parity bit might do the same for message bits two through four, the third for bits three through five, and so on.
In order to embed three secret message bits, the algorithm requires four bits of the cover image, but due to the coding mechanism, no more than two of the four bits will be changed when producing the stego image.
The purpose of the modification is to eliminate involved distribution of message bits among the cipher rounds in order to reduce occupation of in-array resources and also to improve performance metrics.
Message bits are obtained.
Because, and hold the same message bits.
(3) Shuffle message bits according to.
The source can encode k 1 = 11 message bits, and the relay can encode only k 2 = 5 message bits out of k 1 = 11 message bits (recovered at the relay).
