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I have confusion on Feistel word in key size. Means, "what is the meaning of key size with different feistel word ?"

What does m=2,3,4 represent? Where m represent different size of feistel word or we can say 'm' is keywords defined in paper on SIMON and Speck titled "THE SIMON AND SPECK FAMILIES OF LIGHTWEIGHT BLOCK CIPHERS" by NSA, USA.

example: SIMON block cipher with an n-bit word (and hence a 2n-bit block) is denoted SIMON 2n, where n is required to be 16, 24, 32, 48, or 64. SIMON 2n with m-word (mn-bit) key will be referred to as SIMON 2n/mn.

So, I think 'm' is cleared now.

Nikhil Singh
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In the case of SIMON and SPECK families, they are both balanced Feistel ciphers (or similar in the case of SPECK), meaning the block is size is composed of 2 equal sized halves. SIMON and SPECK have the same combinations for block and key size.

The variables refer to the cipher internals, with $n$ being the size of the "word", which is the size of the half-block and also the size of the primitive operations such as addition modulo $2^n$, and $m$ is the quantity of said words that make up the input key.

The block size is $2n$, which means the 96-bit block uses a word size of 48-bits. The 2 different key sizes for this block size are 96 and 144-bits, which are equal to 2 or 3 48-bit words.

Richie Frame
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