The Substitution Technique

(Ema_0, Unmasking/Decryption) – The replacement technique does exactly the opposite of the substitution technique. The substitution algorithm reads the contents of the key sequentially, performs XOR (^) with the contents of the block and overwrites it. This restores the values of the original data source.

If both techniques were performed in the masking stage and the replacing technique was performed first, the substitution must be performed last in the unmasking stage.

Below is the substitution technique (unmasking) applied to the previous replacement (masking) example.

KEY & DATA INDEX	KEY VALUES	DATA BEFORE SUBSTITUTION	SUBSTITUTION (MASKING)																DATA RESULT
000	033	@	a																a
001	048	R		b															b
002	043	H			c														c
003	045	I				d													d
004	044	I					e												e
005	037	C						f											f
006	046	I							g										g
007	040	@								h									h
008	041	@									i								i
009	042	@										j							j
010	036	O											k						k
011	034	N												l					l
012	038	K													m				m
013	039	I														n			n
014	035	L															o		o
015	047	_																p	p
			000	001	002	003	004	005	006	007	008	009	010	011	012	013	014	015

Example of substitution at the unmasking/decryption stage

ASCII decimal unsigned values:
@=64, R=82, H=72, I=73, I=73, C=67, I=73, @=64, @=64, @=64, O=79, N=78, K=75, I=73, L=76, _=95

The four first replacement:
1. @=64; key(0)=33; 64^33=97, a → data(0)
2. R=82; key(1)=48; 82^48=98, b → data(1)
3. H=72; key(2)=43; 72^43=99, c → data(2)
4. I=73; key(3)=45; 73^45=100, d → data(3)
and so on.

With the substitution technique we get the source string [a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p] as output.