> with a key that repeats
There's a _lot_ hiding in that, all of the interesting stuff for security and potential breaks. So...yeah it is based on complicated math, it's just in that bit instead of the xor.
Even the xor is a bit of a fudge, but probably close enough.
There's a lot of basic computer science hiding in it that's been remarkably stable for generations of computer scientists, which is not something you can say about asymmetric cryptography.
Is there? Like...kind of, but on the face of it I'd say about the same amount in both. If you look back at DES there's a _lot_ in common with modern ciphers, but like, RSA is still in use and that's old as shit.
I think you're right if your point is that we're more likely to see big breaks in asymmetric crypto, but it's kind of based on vibes to me, it's not really clear that it's provable in any way with anything like our current understanding.
AES doesn't reduce to a fundamental mathematical problem we're uncertain about, in the same way as discrete logs, factoring, the elliptic curve discrete log, or shortest vectors. It's a simpler idea, mathematically: rigorously understood linear operations to propagate key-driven changes quickly, disrupted by nonlinear operations to keep the cipher from being solvable with algebra, driven by a key schedule, and iterated enough times to destroy the signal that differential cryptography (and its analogs) would use to mount attacks.
It's just radically different levels of exposure to mathematical theory.
I'm fond of pointing out that JP Aumasson, who is (unlike me) an academic cryptographer of some repute, believes SHA2 will never be broken.
For any kind of symmetric encryption method, if you are in the context of a "known-plaintext attack", i.e. where besides the cipher that was used you know both the non-encrypted text and its corresponding encrypted text, if you know a segment of text that is long enough, i.e. longer than the number of bits corresponding to the secret key and of some ancillary information that might be needed, like an initialization vector, the problem of breaking the cipher is known to have a unique solution, because it is equivalent to the solution of a system of equations that has enough equations to determine the unknowns.
However, for a well-designed cipher that system of equations is non-linear and exceedingly complex and it cannot be formulated in any mathematically simple form, therefore no methods of solution are known.
The more rounds are performed inside the encryption algorithm, the more complex that system of equations becomes, and the more unlikely is to find any method to solve it.
Even for the much simpler systems of non-linear equations that occur in physics, when they cannot be linearized even in the neighborhood of each point, then there typically are no better methods of solution than searching randomly through the solution space, which in cryptography corresponds to brute-force search, which is infeasible for big enough secret keys.
>if you are in the context of a "known-plaintext attack", i.e. where besides the cipher that was used you know both the non-encrypted text and its corresponding encrypted text
I know with some pre-modern ciphers like Caesar that was an issue, and it famously came up due to implementation/operator issues with systems like Enigma. But is that actually still a relevant attack context at all with modern symmetric encryption? Outside of academic interest I'm struggling to think of any time where you would ever be in that situation. I thought input got scrambled pretty well so you can't really trivially predict things like text word count (putting aside that these days the vast super majority of bits aren't text). When used to protect your own data (ie, storage encryption etc) then the "end points" are you and future-you and by definition fully trusted, including operation of the encryption system bits. If someone hostile roots it and has access the device computation they'll also have access to device data. When symmetric encryption is used between end points controlled by two different parties keys forward secrecy should be pretty standard these days right? So attackers still would have to root one trusted side or the other and at that point it's game over regardless.
On the face of it seems like it'd have to be a pretty strange situation nowadays where the attacker somehow gets to reuse the same encryption key as the trusted parties without knowing it or being able to access the secrets they're trying to keep without touching the encryption at all? DRM sorts of use-cases maybe, one of the few situations where hostile attacker and trusted user are supposed to be the same person which has always been a tension with it.