A re-encryption scheme of public key cryptography is proposed for secure data sharing in public cloud. The re-encryption scheme offers owner to securely store his data in cloud by encrypting the data over an asymmetric data encryption and symmetric data encryption is further encrypted by the public key of data owner which is also stored in the cloud and is made available to all the legitimate recipients in accordance with the access control. The cloud maintained here is a public cloud and is a semi-trust as because it does not allow the owner. Data owner’s stores encrypted data in the cloud to ensure security for his data in the cloud computing environment and issues decryption key to only authorized user to access the data from cloud. When user is revoked, data owner as to reencrypt the data so that revoked user cannot access the data again .To perform this operation he will issue re-encryption command to cloud so that data in cloud gets re-encrypted. Once re-encryption is done there is a need for generation of new decryption keys to valid user, so that they can continue to access the data.