When calculating the "strength" of an encryption algorithm, there are three things you need to look out for.
- The computational power required to brute-force the algorithm.
- The likelihood of a vulnerability, which reduces the computational power required. (e.g. Meet-in-the-Middle attacks)
- The likelihood of making the algorithm trivial to crack.
Let's look at each of these and how they affect your scenario.
1. Computational Requirements
These are very well understood. For example, AES-256 uses 256 bits of random input as key. As long as your random number generator produces sufficient entropy, a 256 bit random key will not be cracked. This is also the reason why there is no AES-512, AES-1024, etc. - it is simply not necessary.
But for algorithms such as RSA, small key sizes are quite practical to brute force. So if you use RSA with 1024 bit long primes, then it is certainly plausible that a sufficiently powerful adversary could recover the private key in reasonable time.
In short: To defend against the growing computational power available to adversaries, use algorithms, which are considered "unbreakable" by conventional hardware.
2. Attacks giving adversaries an advantage
There are certain cryptographic attacks, which don't outright break a scheme, but make it easier for an attacker to recover the key. For example, DES uses 56 bits of entropy, which is considered insecure and can be recovered on modern hardware within reasonable amount of time. "Double-DES" would be 112 bits, right? 56+56 = 112 after all.
Not exactly. There is an attack called "Meet-in-the-Middle" (YouTube explanation), which reduces the computational complexity from a theoretical 112 bits down to 57, which means it's 2^55 or 36,028,797,018,963,968 times more effective. This is a significant gain.
So how can one defend against such attacks? By using well-studied algorithms. In general, older algorithms have been studied extensively, thus making it rather unlikely for new vulnerabilities to be found. I'm not saying it's impossible, but AES has been published in 1998 and subsequently analyzed extensively both by the global cryptography community and nation state actors. So far, only side-channel attacks against AES have been discovered, but no attacks, which offer significant advantage compared to brute-force.
Does this mean no attacks against AES can ever be found? No. It just means it is very, very unlikely that after 25 years of research, someone has a "Eureka!" moment and shows how this one weird trick can be used to crack AES.
What about quantum computers?
That is a great hypothetical question. Quantum computers already exist, but aren't yet powerful enough to pose serious risks. There is a whole field dedicated to this, called Post-Quantum Cryptography. It may be worth looking into this, if the lifetime of your data is sufficiently long. Again, this is still a small risk overall.