How to change bibliography item labels to boldface?

Question

I would like to change the labels for my bibliography items to boldface (only in the reference list, not for citations in the text). The complete label should be printed in bold, i.e., including the square brackets.

I had a look at these questions: How to change the appeareance of bibliography item labels in amsrefs? and BibLaTeX bibliography without square brackets. However, I was unable to transfer them to my case. I even tried to change the bst file, but couldn't get what I wanted to achieve.

What is the correct approach to change only the appearance of the labels in the bibliography?

Here's my example:

\documentclass{article}

\bibliographystyle{alpha}

\begin{document}

I cite \cite{A1} and \cite{B2} in my text.

\bibliography{bibliography}

\end{document}

And this is the content of bibliography.bib:

@BOOK{A1,
   author = {Author One},
   title = {Alpha},
   publisher = {Publisher A},
   year = {2000},
   }

@BOOK{B2,
   author = {Author Two},
   title = {Beta},
   publisher = {Publisher B},
   year = {2001},
   }

This is what I want the output to look like:

Answer 1

Without any packages, you just have to redefine the way \@biblabel works:

\documentclass{article}
\usepackage{filecontents}
\begin{filecontents*}{\jobname.bib}
@BOOK{A1,
   author = {Author One},
   title = {Alpha},
   publisher = {Publisher A},
   year = {2000},
   }

@BOOK{B2,
   author = {Author Two},
   title = {Beta},
   publisher = {Publisher B},
   year = {2001},
   }
\end{filecontents*}
\makeatletter
\renewcommand{\@biblabel}[1]{\textbf{[#1]}}
\makeatother
\bibliographystyle{alpha}

\begin{document}

I cite \cite{A1} and \cite{B2} in my text.

\bibliography{\jobname}

\end{document}