The Venti archival storage system paper might be an interesting read for you[1]. The researchers deemed Sha1 (160-bit) acceptable to hash 8Kb data blocks for an

The Venti archival storage system paper might be an interesting read for you[1]. The researchers deemed Sha1 (160-bit) acceptable to hash 8Kb data blocks for an exabyte of data (10^18 bytes). The risk of collision at that level is less than 10^-20. There are now 512-bit sha hashes, so that takes it to another level. So, I think Dropbox is safe from hash collision risks at this point. [1] http://doc.cat-v.org/plan_9/4th_edition/papers/venti/ http://doc.cat-v.org/plan_9/4th_edition/papers/venti/