Digital Signature has been made mandatory for filing written of income for AY2011-12 for the following persons:
1. Firms
2. Individual and HUF who require to get their books audited U/s44AB
Refer this link
https://law.incometaxindia.gov.in/DIT/File_opener.aspx?page=NOTF&schT=&csId=43232e56-0f10-423e-b222-79e363ea6e58&NtN=&yr=ALL&sec=&sch=&title=Taxmann%20-%20Direct%20Tax%20Laws
