Wrote a blog post about it back in the day, for those wondering how to break (super easy) encryption: https://lgms.nl/blog-1-plain
Could you, please, explain how?
Now imagine there's a programmer who wants to make the ultimate cube scrambler despite not knowing any of the above. Their brilliant idea is to take the aforementioned algorithms and chain them together. (Result: snafu.)
In essence, the moral of the story is that one shouldn't try stacking encryption algorithms without first acquiring a pretty good understanding of how they all work.
[0] https://www.popsci.com/science/article/2010-08/gods-number-r...
If we move the goalposts to where the combined algorithm receives a much larger key than any of the individual parts we're comparing to in terms of crackability, then the likely failure mode isn't ‘weaker’ any more, but ‘stronger, though maybe not as much stronger as was intended’.
The history of triple DES provides a nice practical example: ‘double DES’ isn't a thing because encrypting already-DES-encrypted data with DES again, with a completely separate key (thus effectively doubling the size of the key), does almost nothing to improve security.
A typical example is the crapto-1 Mifare Classic algorithm used to encrypt NFC cards. The way they read from the shift register and combine the bits was dumb and it's complexity weakened the algorithm.
Another I've seen is using two sequential keys XORd against one another to produce and "encryption" key. Turns out reading from low entropic systems very quickly yields a similar enough key that when XORd, partially removes the first one.
The claims of "weakened by combining" are often aired, but all examples I've found so far are basically summarized as "remaining within a group structure" (as in your rubik's cube example whose god number is 20) - which might not be stronger than each individual, but is unlikely to be weaker either -- and algorithm combinations usually DON'T remain within a group structure (e.g. DES is not a group, so 3DES is strictly stronger than DES, even if it might not be 3-times strong in bits)
Without key re-use, there is no way adding another algorithm can weaken an existing encryption.
Is the ceiling for “max encryption” that low, or is just that one algorithm combined with another has a local maximum?
> running a few more algorithms on something before trying to decrypt will make it easier rather than harder to decrypt?
No, it could make it easier, harder, or about the same. The ‘harder’ case is just unlikely when the algorithms one started with were already state-of-the-art and the programmer didn't know what they were doing. It might seem tempting to think that a cryptanalyst now has to do twice the work, but what they're really doing isn't cracking multiple encryptions — they're just attacking a different encryption.
It's of course not that trivial with better encryption algorithms. But before stacking encryption algorithms, try to first answer what you are trying to achieve (that application of a single algorithm does not).
Deezer is doing weird stuff in comparison. A friend told me that each song has every third block of 2048 bytes encrypted using blowfish. The key is derived from the hex md5 of the song ID, xor the same hex md5 but with a Ceasar cipher shift of 16, xor a hard-coded secret. The initialization vector is 0,1,2,3,4,5,6,7.