Beschrifteter Würfel (Kanten, Diagonale)

\documentclass[border=5pt]{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}[scale=2]
  % Sommets arrière
  \coordinate (E) at (0.8,0.5);
  \coordinate (F) at (2.8,0.5);
  \coordinate (G) at (2.8,2.5);
  \coordinate (H) at (0.8,2.5);
  % Sommets avant
  \coordinate (A) at (0,0);
  \coordinate (B) at (2,0);
  \coordinate (C) at (2,2);
  \coordinate (D) at (0,2);
  % Arêtes arrière (pointillés)
  \draw[dashed,gray] (E) -- (F);
  \draw[dashed,gray] (E) -- (H);
  \draw[dashed,gray] (E) -- (A);
  % Arêtes visibles
  \draw[thick] (A) -- (B) -- (C) -- (D) -- cycle;
  \draw[thick] (B) -- (F) -- (G) -- (C);
  \draw[thick] (D) -- (H) -- (G);
  % Diagonale du cube
  \draw[red,thick,dashed] (A) -- (G) node[midway,above right,font=\small] {$a\sqrt{3}$};
  % Labels
  \node[below left,font=\small] at (A) {$A$};
  \node[below right,font=\small] at (B) {$B$};
  \node[right,font=\small] at (C) {$C$};
  \node[above left,font=\small] at (D) {$D$};
  \node[left,font=\small] at (E) {$E$};
  \node[below right,font=\small] at (F) {$F$};
  \node[above right,font=\small] at (G) {$G$};
  \node[above,font=\small] at (H) {$H$};
  % Côté
  \draw[<->,blue] (0,-0.3) -- (2,-0.3) node[midway,below,font=\small] {$a$};
\end{tikzpicture}
\end{document}