For example, we could write a program that hard-codes a secret key, and then signs bitcoin transactions with that secret key, but only if the transaction has va

For example, we could write a program that hard-codes a secret key, and then signs bitcoin transactions with that secret key, but only if the transaction has value less than 0.5 bitcoin. Then anyone who has the program can sign things of small value on our behalf; if the program is indistinguishably-obfuscated, they won't be able to get any additional information about the key itself, or use it in any other way. I guess just encrypting the key won't let you use it to sign things.