![alt text][1]
\documentclass[border=5pt]{standalone}
\usepackage{amsmath}
\usepackage{tikz}
\usetikzlibrary{positioning}
\begin{document}
\begin{tikzpicture}[%
x={(1cm,0cm)},
y={(0cm,1cm)},
z={({0.5*cos(45)},{0.5*sin(45)})},
]
\def\a{2}
\begin{tikzpicture}[%
x={(1cm,0cm)},
y={(0cm,1cm)},
z={({0.5*cos(45)},{0.5*sin(45)})},
]
\coordinate (A) at (0,0,0);
\coordinate (B) at (\a,0,0) ;
\coordinate (C) at (\a,\a,0);
\coordinate (D) at (0,\a,0);
\coordinate (E) at (0,0,\a);
\coordinate (F) at (\a,0,\a);
\coordinate (G) at (\a,\a,\a);
\coordinate (H) at (0,\a,\a);
%Ecken
\node[left= 1pt of A]{$A$};
\node[right= 1pt of B]{$B$};
\node[right= 1pt of C]{$C$};
\node[left= 1pt of D]{$D$};
\node[left= 1pt of E]{$E$};
\node[right= 1pt of F]{$F$};
\node[right= 1pt of G]{$G$};
\node[left= 1pt of H]{$H$};
%Kanten
\draw[] (A)
-- (B) node[midway, below]{\tiny$\a$cm}
-- (C) node[midway, right]{\tiny$\a$cm}
-- (D) node[midway, above]{\tiny$\a$cm}
-- (A) node[midway, left]{\tiny$\a$cm};
\draw[] (B) -- (F) -- (G) -- (C);
\draw[] (G) -- (H) -- (D);
\draw[densely dashed] (A) -- (E) -- (F);
\draw[densely dashed] (E) -- (H);
\end{tikzpicture}
\end{document}
[1]: http://texwelt.de/wissen/upfiles/Namenlos-11e4yyy_24.png
