Embodiments of the present invention relate to systems, methods, and computer-storage media for maintaining privacy while delivering advertisements based on encrypted user profile identifiers. In embodiments, a Public key Encryption with Keyword Search (PEKS) is used to generate a public key and a private key. In embodiments, a public key and a private key are used to encrypt user profile identifiers and generate trapdoors associated with defined profile identifiers, respectively. A portion of the encrypted user profile identifiers are compared to a portion of the trapdoors. If a match is present between at least one encrypted user profile identifier and an associated trapdoor, a delivery engine is provided with an identification of content to be delivered to the user. The provided description is then used to determine an advertisement to present to a user. The advertisement is then presented to the user.