Using two or more encryption algorithms together, how do we compute the strength of the final encryption?

If two or more encryption algorithms are used together, how do we compute the strength of the final encryption?

And how would the application perform against quantum computers?

The first two tables are accurate, correct?

In the third table where we combine two encryptions together, can we calculate the key length? And bits of encryption? Is it correct?

In the 3rd table, the bits are just added, correct?

Let's say:

encryption | Key length | bits of encryption
AES-128    | 128        | 128

Now, if we do add two of them together (combine using cascade encryption)

AES-128 + AES-128 = 256 key length ? | 256 bits of encryption ?

Is that correct?

Or what is the correct answer for:

AES-128 + AES-128 = ___ key length ? ___ bits of encryption ?

• First of all the same algorithm used twice does NOT double the key length. At best it doubles the difficulty which is one extra bit, so AES-128 twice at best becomes 129. Depending upon the particular algorithms used, using it twice may add nothing. A substitution cipher is still just a substitution cipher regardless of how many times you use it. This comment has been encoded with ROT-13 TWICE. – user10216038 Dec 5 '20 at 4:43
• Use AES-256 and be fine, for double encryption see an attack on DES double encryption that is why 3DES is suggested and used. The Question has fails to mention well-known methods and posted an image from an unknown resource. The source of the image must be included! Probably a HW question need to be closed! – kelalaka Dec 5 '20 at 10:25
• I'll note that RSA should never be used to encrypt most messages, due to the length limits. It's almost always used to encrypt a symmetric key (eg AES), which then encrypts the message. So breaking RSA means you get the AES key back. That's why the combined chart is using the weakest of the two: it's not cascading encryption of the message at all! For cascading encryption, meet-in-the-middle attacks and related-key attacks may become relevant. Assuming this is homework, that should give you enough to search for to find the answers you need. – SAI Peregrinus Dec 6 '20 at 17:29
• thank you so much good information – AED ER Dec 6 '20 at 19:08