check for empty string

[suntorytimed]

Adding a check to see if keyFileContents is empty:

• this fixes a download error and an exception if the data content for encryption is empty
• An error occured: Bad signature - corrupted encrypted files #3958: for recovering encrypted files with a damaged signature this is necessary in addition to turning the signature check off

Signed-off-by: Stefan Weiberg sweiberg@suse.com

[ChristophWurst]

@schiessle 🏓

[MorrisJobke]

@schiessle 🏓

[schiessle]

Makes sense not to try to decrypt an empty string, so for the pr as such 👍

but...
@suntorytimed could you elaborate on what this exactly fixes and how to reproduce it? While I see the point that it doesn't make sense to decrypt an empty string I fail to see how this is related to an broken signature. Thanks!

[suntorytimed]

@schiessle this is pointed out in the linked ticket #3958

If for some reason the signature of an encrypted file is corrupted and I switch the signature check off to bypass this check the download of the file won't finish as the last data block of the file is empty and Nextcloud throws an exception due to an empty string that can't be decrypted.

#3958 (comment)

[schiessle]

Thanks for clarifying, @suntorytimed as soon as we have a second review this can be merged. I would also suggest to backport it. Can you prepare a second PR against the stable14 branch? Thanks!

[suntorytimed]

@schiessle done: #11783