FWIW: tarsnap is also rsync for cloud storage and Colin (guy who founded and runs tarsnap) also has won a putnam award for his work in mathematics and crypto.
Colin won the Putnam as an undergraduate student. The Putnam award is not a mathematics research award like the Fields Medal or the Abel Prize. It's a mathematics competition. As such, Colin didn't win the award for any particular work.
It's still quite impressive though. I would say Colin's work developing scrypt has more applicability to cryptography than his Putnam award.
One of the best comebacks in HN history.
A glorious exchange.
That's like saying the Google Drive client or the Dropbox client are "rsync for cloud storage" just because they incrementally add changes to a cloud service.
As in it encrypts your files, dedups and puts them on S3?
If you don't CBC, wherever is on the cloud side will have two files with one block off. Let's assume the file is a txt file. For smaller cipher block sizes, it is becoming very easy to guess your encryption key.
Tldr, you want any attacker to lose any diff ability on your encrypted data.
Good practice for file encryption is to pad some random bytes to the start of a file. This makes known cyphertext attacks much harder.
So encrypting identical files should result in different files and also slightly different length files.
FWIW, the client is open source: https://github.com/Tarsnap/tarsnap