Service tax input credit setoff with another division liabil

Dear Experts,

I have one query that, I am running a software development firm for which i am raising invoice with service tax.

likewise, I am also selling digital signature certificates to the customers, while purchasing digital signature certificates we are paying service tax as signature certificates are transferred as soft copies but, in selling point we are charging VAT as it is sold in a hardware.

Please tell me that, can we set off the input service tax credit received for purchase of digital signatures with the tax liability of software development charges. 

Please provide the solution with the relevant section/clarification/ judgement

Thanks in advance