Note that we know this well enough that entire industries depend upon it, and they are considered safe. RSA Public Key Cryptography is an international standard that backs SSL and other internet security. We do security audits of institutions dependent upon that technology and find them secure.
We may know it better than we know how to build safe bridges. So, do we not know how to build safe bridges?
The components of this definition seem categorical, but they are all open to degrees of interpretation.
Examples like this do not present a challenge to the definition itself, so much as they raise the 'sorites' problem implicit in everything with a degree. How certain is certain? How justified is justified? If I add one strong hint toward reliability at some point does that change conjecture into knowledge?
In math, clearly not. In industry, probably so, though that point is not consistent or clear. You know subjectively when you are past it, but you cannot observe yourself passing it.