ethclient, mobile: add TransactionSender (#15127)

* core/types: make Signer derive address instead of public key

There are two reasons to do this now: The upcoming ethclient signer
doesn't know the public key, just the address. EIP 208 will introduce a
new signer which derives the 'entry point' address for transactions with
zero signature. The entry point has no public key.

Other changes to the interface ease the path make to moving signature
crypto out of core/types later.

* ethclient, mobile: add TransactionSender

The new method can get the right signer without any crypto, and without
knowledge of the signature scheme that was used when the transaction was
included.

1 files changed, 7 insertions, 0 deletions

diff --git a/mobile/ethclient.go b/mobile/ethclient.go
index 4e8328501..7f31a8998 100644
--- a/mobile/ethclient.go
+++ b/mobile/ethclient.go
@@ -77,6 +77,13 @@ func (ec *EthereumClient) GetTransactionByHash(ctx *Context, hash *Hash) (tx *Tr
 	return &Transaction{rawTx}, err
 }
 
+// GetTransactionSender returns the sender address of a transaction. The transaction must
+// be included in blockchain at the given block and index.
+func (ec *EthereumClient) GetTransactionSender(ctx *Context, tx *Transaction, blockhash *Hash, index int) (sender *Address, _ error) {
+	addr, err := ec.client.TransactionSender(ctx.context, tx.tx, blockhash.hash, uint(index))
+	return &Address{addr}, err
+}
+
 // GetTransactionCount returns the total number of transactions in the given block.
 func (ec *EthereumClient) GetTransactionCount(ctx *Context, hash *Hash) (count int, _ error) {
 	rawCount, err := ec.client.TransactionCount(ctx.context, hash.hash)