Encrypted In Numbers!
brainfuck, 66 bytes

,[&gt;&gt;&gt;+&lt;[-&lt;+&gt;&gt;-&lt;]&lt;[-&gt;+&lt;],]&gt;[&lt;&lt;,&gt;&gt;[-&lt;+&gt;]&lt;-]&lt;&lt;&lt;[&gt;[-&lt;-&gt;&gt;+&lt;]&lt;&lt;]&gt;.&gt;&gt;[.&gt;]


Input is the encrypted string. Assumes infinite sized cells and 0 on EOF.

How it Works:

,[&gt;&gt;&gt;+&lt;[-&lt;+&gt;&gt;-&lt;]&lt;[-&gt;+&lt;],] Gets input and the number of characters divided by 2
&gt;[&lt;&lt;,&gt;&gt;[-&lt;+&gt;]&lt;-]&lt;&lt;&lt; Remove the second half of the string (the multiplication part)
[&gt;[-&lt;-&gt;&gt;+&lt;]&lt;&lt;] Subtract each character from the previous one, while keeping a copy of the previous one.
&gt;.&gt;&gt;[.&gt;] Print the characters