Performing searches over encrypted data is a very current and active area. Several efficient solutions have been provided for the single-writer scenario in which all sensitive data originates with one party (the Data Owner) that encrypts it and uploads it to a public repository. Subsequently the Data Owner (or authorized clients, the Query Sources) accesses the encrypted data through a Query Processor which has direct access to the public encrypted repository. Motivated by the recent trend in pervasive data collection, we depart from this model and consider a multi-writer scenario in which data originates with several and mutually untrusted parties. In this new scenario the Data Owner provides public parameters so that each item of the generated data stream can be put into an encrypted stream; moreover, the Data Owner keeps some related secret information needed to generate tokens so that different subscribers can access different subsets of the encrypted stream in clear, as specified by corresponding access policies. We propose a new public-key scheme, Secure Selective Stream (SSS), built upon an Amortized Encryption Scheme (AOE), that can be used to encrypt each item in the stream so that the ciphertexts have size proportional to the un-encrypted data; moreover, encryption and decryption take time linear in the data item size. We provide constructions for SSS and AOE. We provide a game-based and an indistinguishability-based security notions for SSS, we prove that the SSS scheme is game-base secure given that the AOE scheme is game-based secure as well. We prove that AOE is secure under hardness assumptions in the bilinear setting. We provide an implementation in C++ all the basic operations in our multi-writer scenario using one round of communication.