Authenticated data structures are a technique for outsourcing data storage and maintenance to an untrusted server. The server is required to produce an efficiently checkable and cryptographically secure proof that it carried out precisely the requested computation. Miller et al. introduced λ• (pronounced lambda auth)—a functional programming language with a built-in primitive authentication construct, which supports a wide range of user-specified authenticated data structures while guaranteeing certain correctness and security properties for all well-typed programs. We formalize λ• and prove its correctness and security properties. With Isabelle's help, we uncover and repair several mistakes in the informal proofs and lemma statements. Our findings are summarized in an ITP'19 paper.