documentclass{article}
usepackage{amsmath}
usepackage{tikz}
usetikzlibrary{matrix,decorations.pathreplacing,calc}

begin{document}
[
   begin{tikzpicture}[decoration={brace,amplitude=5pt},baseline=(current bounding box.west)]
     matrix (magic) [matrix of math nodes,left delimiter=(,right delimiter=)] {
      1 \
      & 1 \
      & & ddots \
      & & & 1 \
      & & & & -1 \
      & & & & & -1 \
      & & & & & & ddots \
      & & & & & & & -1\
     };
     draw[decorate] (magic-1-1.north) -- (magic-4-4.north east) node[above=5pt,midway,sloped] {$n$ copies of $1$};
     draw[decorate] (magic-5-5.north east) -- (magic-8-8.north east) node[above=5pt,midway,sloped] {$n$ copies of $-1$};
   end{tikzpicture}
]
end{document}

draw[decorate] ($(magic-1-1.north)+(.2,.1)$) -- ($(magic-4-4.north east)+(.15,.1)$) node[above=5pt,midway,sloped] {$n$ copies of $1$};