### try explaining better the computation of n'

parent 12e23f72
 ... ... @@ -11,13 +11,14 @@ The core of the X25519 key-exchange protocol is a scalar\hyp{}multiplication function, which we will also refer to as X25519. This function receives as input two arrays of $32$ bytes each. One of them is interpreted as the little-endian encoding of a non-negative integer $n$ (see \ref{subsec:integer-bytes}). non-negative 256-bit integer $n$ (see \ref{subsec:integer-bytes}). The other is interpreted as the little-endian encoding of the \xcoord $x_P \in \F{p}$ of a point in $E(\F{p^2})$, using the standard mapping of integers modulo $p$ to elements in $\F{p}$. The X25519 function first computes a scalar $n'$ by setting bit 255 of $n$ to \texttt{0}, setting bit 254 to \texttt{1}, and setting the lower 3 bits to \texttt{0}. The X25519 function first computes a scalar $n'$ from $n$ by setting bits at position 0, 1, 2 and 255 to \texttt{0}; and to \texttt{1} bit at position 254. This operation is often called clamping'' of the scalar $n$. Note that $n' \in 2^{254} + 8\{0,1,\ldots,2^{251}-1\}$. X25519 then computes the \xcoord of $n'\cdot P$. ... ...
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