Encryption on the cloud with versioning Can file-versioning, which is available for instance in Dropdox, weaken the security of encrypted files if someone is able to see two or more snapshots of the ciphertext at different times?

To be specific, the files are encrypted by PGP on the file level (each file, making extension: important.doc.pgp ) with asymmetric keys.
I'm assuming that you mean any time the file changes, it is re-encrypted with PGP. Here is a description of how PGP encryption works:


  


Whenever you change the file and re-encrypt with PGP, a new, independent session key is chosen. So what you end up with is a (potentially only slightly) modified file, being encrypted with a brand new session key.

All modern block ciphers, when used correctly, will protect against cryptanalysis, even when the analyst has access to multiple encryptions of the same (or slightly modified) plaintexts. Since PGP is generating a new, fresh, independent key, this would fall under the category of when used correctly. So, no, an attacker having access to all these encrypted versions gains no advantage in breaking the plaintext or the keys.