However, it's a slippery slope to say that encryption can't be illegal because in is math, and math can't be illegal.
It's equivalent to say that making bombs is chemistry, and chemistry can't be illegal. It's an oversimplification.
Presenting the practical consequences of a backdoor produces a far more sound argument.
I think the banning of encryption or forcing encryption products to have government backdoors would be horrifically bad policies, I don't think there is anything in our current legal framework that prevents it.
Even when an encryption algorithm is implemented on a computer, it's still not a physical object, it's just a mathematical algorithm.
A computer simulation of explosive chemical reaction is not going to get you arrested, why should an encryption algorithm?
Math can not harm anybody because math isn't real, it's speech (yes, maybe except for psychological harm). Chemistry is quite real.
Anyway, I don't see merit on the author's argument. It's true, but useless. The discussion is about protected speech and evidence gathering, nobody is claiming cryptography directly harms people.