Impossible
Good quality encryption is already in the public domain (eg OpenPGP ) and any attempt to insert a backdoor is very unlikely to succeed when the source is publicly available.
For people who REALLY want to keep information secret - it is possible to use multiple encryption programs in series (eg use 7-Zip to create a password protected Zip file then use ccrypt to encrypt the Zip file then use OpenPGP to encrypt the output from ccrypt.). Done properly there is no way of recovering the original message without knowing the keys even if one of the programs has a backdoor.
Biting the hand that feeds IT
