Is it possible to write in bootloader addreess?

[AvaTar]

What bootloader addresses ?

For the system BL, no. And that's a good thing.

[Tesla DeLorean]

User should expect it to act like ROM, expect it will Hard Fault if you attempt.

[Arman Ilmak]

Which part is writable?

[Arman Ilmak]

I want to change the start address of the main code and no one else can figure out the start address by reading the flash.How can I do such a thing?

[Tesla DeLorean]

The boot loader doesn't determine that. Assume your actual adversaries understand the platform/technologies better than you, they have more time and more resources. Figure you are matching wits with Einstein or Tesla, or both at once.

If the micro-controller can figure it out how hard can it be for anyone else?

Are you talking about your own loader, or the system loader (BOOT0=High)?

The memory at 0x1FFxxxxx other than the options and OTP is generally not writable.

On the H7 I think you can specify a start block for the vector table, but not sure how useful that is.