Using the PSK shared secret to identify the balance makes it into a long-lived token, which is confusing because in other places, PSK receiver tokens, and therefore also PSK shared secrets are short-lived. So if somebody would use an off-the-shelf PSK implementation, people's prepaid balances would be wiped out by it all the time.
Using the PSK shared secret to identify the balance makes it into a long-lived token, which is confusing because in other places, PSK receiver tokens, and therefore also PSK shared secrets are short-lived. So if somebody would use an off-the-shelf PSK implementation, people's prepaid balances would be wiped out by it all the time.