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First, you have to edit your blogger template. Go to Layout >>>>>Edit HTML and find [/head]. If you can't find it, mark the box "Expand Widget Templates". Then paste this code BEFORE the [/head].
Next, you need to encrypt your post and put it in your post. Go to this page, insert your desired key (which will act as the password) and also the text that you want to keep hidden. Press the [Encrypt] button and copy the encrypted text in the red/pink box.
Choose a unique_name and paste the text you copied from the pink/red box into PASTE_ENCRYPED_TEXT_HERE. Make sure the unique_name is same for both lines of the code below.
[a href="javascript:Decrypt_text('unique_name',%20'PASTE_ENCRYPED_TEXT_HERE');"]Show encrypted text[/a]
[div id="unique_name"]
[/div]
**change [] to <>**
source: http://forum.lowyat.net/topic/690621
