Short answer : You can't !

MD5 stands for Message Digest 5
It's a one way fonction which computes a text's digest.
You can't decrypt a MD5 digest.
You usually use it to check that the digest of the value just entered is equal to the digest stored.
(usually passwd management works this way)

Use other usual encryption schemes if you want to decrypt text.
(RIJNDAEL, RSA, ELGAMAL...)

UPDATE :
I warmly recommend btrott's impressive work (Crypt::OpenPGP)
which handles several schemes
(IDEA, DES3, Blowfish, Rijndael, Rijndael192, Rijndael256, Twofish,DSA, RSA, and ElGamal)

"Only Bad Coders Code Badly In Perl" (OBC2BIP)

In reply to Re: big important text encryption with MD5 or something else? by arhuman
in thread big important text encryption with MD5 or something else? by Alex the Serb

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":



  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.