I still have no idea where to find the part which says it should add ff.
RFC says you use ASN.1, which says:
10.4.6 A minimum octet 2's-complement-binary-integer encoding of the whole number has a field-width that is a
multiple of eight bits and also satisfies the condition that the leading nine bits of the field shall not all be zero and shall
not all be ones.
If you add FF to a negativ integer, the first nine bits are all ones. Which would violate this, so i guess you use another standard.
I really can't find the ff part
Are there more exceptions that i have to handle except the one with the FF for negativ integers?
I should really check for further deviations between my client and your agents de/encodings, before getting into the clients source code.