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I noticed that several hash algorithms, in this case MD5, defines some constants.

MD5 defines 64 constant values (the shift count per round) which also gets expand (via sine) to a summand for a specific round. It is like a lookup table.

But I cannot see anything (no documentation, no implementation,...) that calls this table an substitution box (S-Box). But I saw some implementations that use a variable called "S" for this table. A hint?

So my question is: Is it valid to call this table an S-Box? Or is it only common to call this kind of table an S-Box when it's used into a real encryption/decryption?

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Is it valid to call this table an S-Box?

Not really; at least, not with the meaning we usually give to "S-box".

The "S" in "S-box" stands for Substitution; we take the data, and replace it with a value from the S-box (using the data as an index into the S-box). The classical (if not the original) example is the S-boxes within AES; at certain points within the cipher, we take each 8 bit value $V$ from the block, and replace it with $SBox[V]$.

We don't do anything like that in MD5; instead, the "S" constants stand for "Shift". They don't replace anything; instead, they modify how much shifting (rotating in more common parlance) we do per round.

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And what is with the expanded values? I mean the "K" in this: en.wikipedia.org/wiki/MD5#Pseudocode It replaces the round index (data) with some other data. –  Bastian Born Apr 22 at 21:20
    
@BastianBorn: no, that data is added to the current data; it's no more an "sbox" than the "S" constants are. –  poncho Apr 22 at 21:23
    
Technically the rotation could be viewed as a linear substitution, although in cryptography an s-box is generally a nonlinear substitution. –  Richie Frame Apr 23 at 3:57
    
More original than AES was the use of S-boxes in DES. But the term predates even that, I think. –  Henno Brandsma Apr 23 at 9:11
    
@HennoBrandsma: Yes, I know; I did say that the sboxes in AES were not the original. However, it is easier to explain the sboxes there than the sboxes within DES, hence I used that as the example. –  poncho Apr 23 at 20:32

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