Among the entities involved in software citation, software source code requires special attention, due to the role it plays in ensuring scientific reproducibility. To reference source code we need identifiers that are not only unique and persistent, but also support \emph{integrity} checking intrinsically. Suitable identifiers must guarantee that denoted objects will always stay the same, without relying on external third parties and administrative processes. We analyze the role of identifiers for digital objects (IDOs), whose properties are different from, and complementary to, those of the various digital identifiers of objects (DIOs) that are today popular building blocks of software and data citation toolchains. We argue that both kinds of identifiers are needed and detail the syntax, semantics, and practical implementation of the persistent identifiers (PIDs) adopted by the Software Heritage project to reference billions of software source code artifacts such as source code files, directories, and commits.