Short verion: Forget it.
Long version:
Yes you have two hypothetically chances if the sign(s) uses symmetric encryption (like AES). The first thing is disassembling the skp and make a debug setup. (hw+sw) You have to start the debug, and the start the update process. In this point you have to halt the processor and starting the fun. I mean step-by-step disassembly all instructions until you will find the signature password in the memory.
But there is a small problem. The asymettric encyption (like RSA) is unbreakable (without a sell out RSA keypair or quantum computer) and everybody is using this instead of AES.
The second is: If somebody from the original SKP dev team sell the sign keypair and somebody in china make a third party update server to fix this problem. For good money.
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