Stuart Haber and W. Scott Stornetta, researchers at Bellcore (Bell Communications Research) in New Jersey, publish in 1991 "How to Time-Stamp a Digital Document" (Journal of Cryptology), and in 1993, together with Dave Bayer, incorporate Merkle trees into the system, creating the cryptographically chained block structure that makes it possible to prove a document existed on a given date with no possibility of retroactive alteration. Each block contains the hash of the previous block, so any modification breaks the entire subsequent chain. Nakamoto cites Haber and Stornetta in three of the eight references in the Bitcoin whitepaper (2008) — the largest proportion of citations to a single work in blockchain's founding paper. Despite this, Haber and Stornetta remained practically unknown outside academic cryptography until Bitcoin's rise around 2017 attracted retrospective attention to their work.