Need to set hidden field using javascript function

I am trying to set the value of a hidden field using the return value of a
javascript function.

For example:

<input type="hidden" name="us">
	<xsl:attribute name="value">rot13(Get_Cookie("us")</xsl:attribute> 
</input>

Why doesn't this work?  Would it work to set a plain variable with
javascript instead?  Any suggestions would be much appreciated.

Thanks,

Rose	

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