Zig 0.17.0-dev (Split by item)

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rejectNonCanonical

Reject a scalar whose encoding is not canonical.

scalar.rejectNonCanonical

pub fn rejectNonCanonical(s: CompressedScalar) NonCanonicalError!void

File

lib/std/crypto/25519/scalar.zig:23

Code

pub fn rejectNonCanonical(s: CompressedScalar) NonCanonicalError!void {
    var c: u8 = 0;
    var n: u8 = 1;
    var i: usize = 31;
    while (true) : (i -= 1) {
        const xs = @as(u16, s[i]);
        const xfield_order_s = @as(u16, field_order_s[i]);
        c |= @as(u8, @intCast(((xs -% xfield_order_s) >> 8) & n));
        n &= @as(u8, @intCast(((xs ^ xfield_order_s) -% 1) >> 8));
        if (i == 0) break;
    }
    if (c == 0) {
        return error.NonCanonical;
    }
}