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> It is generally harder to build a crypto system than to break one

It was harder to design Symmetric Encryption using e.g. AES-256 than to break it? As far as I know it's design is pretty straight-forward but there is no known way to break it even with all compute of the planet at your disposal (excluding trivial ways like exposing the key).

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Yes, it was. An attack only needs to work one way, but an encryption standard must resist all attacks.

There are many examples of would be crypto algorithms that died on the vine because an attack was found. It is often recommended for beginners to study breaking cryptographic algorithms long before they attempt to create them.

It didn't take 25 years to come up with Symmetric Encryption and AES but even though people try extensively to break it for the last 25 years no-one has come even close to anything that is not brute force (which is practically impossible even assuming very generous computing resources). So at least in that case coming up with the encryption seems to be a very solvable problem but breaking it seems close to impossible.

It’s worth considering AES is the result of many decades of development of prior block cipher algorithms. It is no so hard to make a variant of AES that looks similar but is not secure.

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