We don't ever know, in the information theory sense, that a crypto algorithm wont fail suddenly. If we ever knew that, we'd quit using it. However, it has been shown that crypto algorithm failing has a strong tendency to fail according to a two step process:

• Most crypto algorithms fail quickly in the initial analysis phase, as we apply a pile of known crypto-breaking tools to the problem, along with gobs of expert opinion.
• The remaining algorithms have been shown to generally fall in phases. Attacks usually weaken the algorithm before finally breaking it. This is often measured in terms of bits. If a brute force attack would take 2^64 operations, and someone found a technique that takes 2^58 operations, then we say it has been weakened by 6 bits.

The latter is a perfect case for quoting "past performance does not predict future results." There is no guarantee that any future algorithm wont break at any unknown point in time. As a hedge against this, most modern crypto algorithms are designed in multiple rounds using one of the well known structures like a feistel network. We generally understand how to make feistel networks which fall gracefully, and those which do not have well-understood mechanics are often regarded with suspicion for many many years. As a result, there is a tendency (not a guarantee) for measuring weakness in bits to be a reasonable indicator.

Perhaps a better question would be "how bad is it if your current crypto algorithm fails overnight?" If you can put a dollar figure to that, you can at least try to apply Bayesian statistics to put in place rational safeguards against this possibility.

We don't ever know, in the information theory sense, that a crypto algorithm won't fail suddenly. If we ever knew that, we'd quit using it. However, it has been shown that when a crypto algorithm fails, it has a strong tendency to fail according to a two-step process:

• Most crypto algorithms fail quickly in the initial analysis phase, as we apply a pile of known crypto-breaking tools to the problem, along with gobs of expert opinion.
• The remaining algorithms have been shown to generally fail in phases. Attacks usually weaken the algorithm before finally breaking it. This is often measured in terms of bits. If a brute force attack would take 2^64 operations, and someone found a technique that takes 2^58 operations, then we say it has been weakened by 6 bits.

The latter is a perfect case for quoting "past performance does not predict future results." There is no guarantee that any future algorithm won't break at any unknown point in time. As a hedge against this, most modern crypto algorithms are designed in multiple rounds using one of the well known structures like a Feistel network. We generally understand how to make Feistel networks which fail gracefully, and those which do not have well-understood mechanics are often regarded with suspicion for many, many years. As a result, there is a tendency (not a guarantee) for measuring weakness in bits to be a reasonable indicator.

Perhaps a better question would be "how bad is it if your current crypto algorithm fails overnight?" If you can put a dollar figure to that, you can at least try to apply Bayesian statistics to put in place rational safeguards against this possibility.