The comments in this question debate about the added security of multi-layered encryption. There seems to be some disagreement, and I thought a proper question would be helpful here.
So, to provide some common background, consider the following two scenarios:
I apply symmetric encryption to a given file, as follows:
gpg --symmetric --cipher-algo AES256 my_file.txt
to which I add the password "mydogisamazing"
I apply four layers of encryption to a given file, as follows:
gpg --symmetric --cipher-algo AES256 my_file.txt gpg --symmetric --cipher-algo AES256 my_file.txt.gpg gpg --symmetric --cipher-algo AES256 my_file.txt.gpg.gpg gpg --symmetric --cipher-algo AES256 my_file.txt.gpg.gpg.gpg
where the passwords supply to each are, respectively: "amazing" "is" "dog" "my" (so, when I decrypt all the layers, I have entered "my" "dog" "is" "amazing")
Is option 2 more secure than option 1? Knowing almost nothing about encryption security, it seems to me it is, because anyone wanting to break in would have to run some password algorithm four times, whereas in option 1 the algorithm needs to be run 1 time only. What if different
chiper-algo were used instead of the same?
All in all, it seems also obvious to me that the answer does depend on the nature of the passwords. For instance, if I have 15 layers of encryption and each layer's password is merely one letter, it seems "trivial" to break the code.
UPDATE: in response to a comment, I stress that the example above was trying to present an apparent "equivalent" case, i.e "shorter passwords + more layers" versus "longer passwords + less layers". It seems only obvious to me (maybe wrong) that merely adding more layers of identical complexity will only increase the security of the encryption (in the mere sense of taking longer to hack the passwords). Hence my stress on the varying length of passwords.