RSA key pair of size 3072 or 4096

Max supported is 4096

https://developer.apple.com/documentation/security/seckeysizes

Isn't it a parameter you select when generating a key ?

That is for MacOS. I wanted to find out for IOS devices. As per the documentation in Security framework header:

Example sizes for RSAkeys are: 512, 768, 1024, 2048.

I wanted to find out for IOS devices.

Why don’t you just try it? Start with a key size that you’re confident will work (2048, say) and then increase the key size to test the cases you care about.

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