diff --git a/src/core/transaction.js b/src/core/transaction.js
index 275de7c..4c71ec6 100644
--- a/src/core/transaction.js
+++ b/src/core/transaction.js
@@ -16,25 +16,22 @@ class Transaction {
         this.gas            = gas;            // Gas that transaction consumed + tip for miner
         this.additionalData = additionalData; // Additional data that goes into the transaction
         this.timestamp      = timestamp;      // Creation timestamp (doesn't matter if true or not, just for randomness)
+        this.signature      = "";             // Transaction's signature, will be generated later
+    }
 
-        // Transaction's hash
-        this.hash = SHA256(
-            this.sender                         + 
-            this.recipient                      + 
-            this.amount.toString()              + 
-            this.gas.toString()                 + 
-            JSON.stringify(this.additionalData) + 
-            this.timestamp.toString()
-        );
-        
-        this.signature = ""; // Transaction's signature, will be generated later
+    static getHash(tx) {
+        return SHA256(
+            tx.sender                         +
+            tx.recipient                      +
+            tx.amount.toString()              +
+            tx.gas.toString()                 +
+            JSON.stringify(tx.additionalData) +
+            tx.timestamp.toString()
+        )
     }
 
     static sign(transaction, keyPair) {
-        if (keyPair.getPublic("hex") === transaction.sender) {
-            // The signature is generated by signing transaction's hash.
-            transaction.signature = keyPair.sign(transaction.hash, "base64").toDER("hex"); 
-        }
+        transaction.signature = keyPair.sign(Transaction.getHash(transaction), "base64").toDER("hex");
     }
 
     static async isValid(tx, stateDB) {
@@ -57,15 +54,7 @@ class Transaction {
         return ( 
             tx.amount >= 0 &&
             ((senderBalance >= tx.amount + tx.gas + (tx.additionalData.contractGas || 0) && tx.gas >= 1) || tx.sender === MINT_PUBLIC_ADDRESS) && 
-            tx.hash === SHA256(
-                tx.sender                         +
-                tx.recipient                      +
-                tx.amount.toString()              +
-                tx.gas.toString()                 +
-                JSON.stringify(tx.additionalData) +
-                tx.timestamp.toString()
-            ) &&
-            ec.keyFromPublic(tx.sender, "hex").verify(tx.hash, tx.signature) &&
+            ec.keyFromPublic(tx.sender, "hex").verify(Transaction.getHash(tx), tx.signature) &&
             !usedTimestamps.includes(tx.timestamp) &&
             tx.timestamp <= Date.now()
         )