The Mathematics FAQ implies that we shouldn't send many questions over that way:
And some kinds of questions are considered off-topic:
- Physics, engineering and financial questions — it is fine to ask the mathematical model and techniques, but concepts of the field would be off-topic.
- Typesetting equations — you can read our guideline for writing equations on this site, or visit TeX, LaTeX and Friends for questions about TeX
- Questions about the site itself — please ask on meta instead
The way I see it, we should only migrate purely mathematical questions that aren't exclusively related to cryptography to Math.SE, and they should be migrating purely (theoretical) cryptography questions to us.
As I am neither a cryptography expert nor a particularly talented mathematician, I can't give explicit examples of questions that would/wouldn't be migrated to Math.SE, but I expect they will be few and far between.
Also related is their list of questions that aren't off-topic but will receive better replies elsewhere:
There are certain subjects that, while still being on-topic here, you can get better response on our sister sites:
- Research-level mathematics — Math Overflow
- Research level Algorithm design, complexity theory, etc — Theoretical Computer Science - Stack Exchange
- Algorithm implementation/design, computer simulation and modelling, etc — Stack Overflow
- Statistical analysis — Statistical Analysis - Stack Exchange
Same thing here really, we shouldn't migrate research or hash/crypto design questions to Math.SE.
I also asked meta.math.SE what they have to say on this, but I expect it'll take some time before consensus is reached.