I am just concerned if they can modified received address.
You just created your transaction using Electrum UI without manually modifying anything and then signed it, right? In that case it is impossible to change anything in that signed transaction. Even changing a single byte in it would invalidate the signature and the tx becomes invalid (can not be mined).
The only scenarios where the receiving address (better said the outputs since that includes the amount as well) can change is if you manually changed the sighash type used for each input you were spending. Keep in mind that this is not something that can change in the UI or when you just click "spend".
There are multiple different types but the 3 that you should know of here are:
- SIGHASH_ALL (0x01) this is the default value that Electrum sets for everything. In this sighash type you are signing everything and as I explained nothing in the tx can change without making it invalid.
- SIGHASH_NONE (0x02) as the name suggests none of the outputs are signed which means the outputs can be changed without invalidating the transaction itself.
- SIGHASH_SINGLE (0x03) as the name suggests you are only signing a "single" output, the output at the same index as the input that has the signature with the said sighash type (eg. second input signs the second output). The rest of your outputs can be change without invalidating the transaction itself.
