From the OpenSSL documentation on the subject:


  The challenge string permits a primitive form of proof of possession of private key. By checking the SPKAC signature and a random challenge string some guarantee is given that the user knows the private key corresponding to the public key being certified. This is important in some applications. Without this it is possible for a previous SPKAC to be used in a "replay attack".


spkac: what is the purpose of the challenge string?
Some people refer to tires that are more resistant to punctures as "puncture proof" tires.
For example: Specialized All Condition Armadillo Tyre

Even though these tires are not really puncture proof, they do reduce a lot the chances of puncturing. So the advantage is obvious.

Apparently, they're not too expensive compared to regular tires. So, I was wondering why people would choose not to use these more resistant tires. What are their disadvantages?
