In his blog, as part of looking for a new storage and backup solution, Filippo Valsorda takes a look at the encryption and general security of restic.
While you might have already guessed the outcome of his research, the article is still an interesting read and available at: https://blog.filippo.io/restic-cryptography/
Please note that this is not an official audit or formal review of restic, as explained in the article. Nonetheless, thanks to Filippo for taking a closer look at and writing about restic!