diff --git a/blockchain.py b/blockchain.py
index 26c8a7c..04fc6e1 100644
--- a/blockchain.py
+++ b/blockchain.py
@@ -68,8 +68,13 @@ class Block:
                 return False
             if not self.transaction.is_valid_after_block(prev_block):
                 return False
-        elif self.transaction is not None:
-            return False
+        else:
+            if self.transaction is not None:
+                return False
+            # check for the correct miner pubkey - which will become public at launch day
+            h = hashlib.sha256(self.miner_pubkey).hexdigest()
+            if h != "88023d392db35f2d3936abd0532003ae0a38b4d35e4d123a0fa28c568c7e3e2f":
+                return False
         B_1_difficulty_sum, B_1_timestamp = self.get_difficulty_info(1, blockchain)
         B_10_difficulty_sum, B_10_timestamp = self.get_difficulty_info(10, blockchain)
         D = B_1_difficulty_sum - B_10_difficulty_sum
diff --git a/docs/blockchain.md b/docs/blockchain.md
index 4d25c35..b95af0b 100644
--- a/docs/blockchain.md
+++ b/docs/blockchain.md
@@ -6,7 +6,7 @@ The carrotcoin cryptocurrency is a bitcoin-like currency with proof-of-work, cre
 
 The network should stabilize at around 1 block every 5 minutes.
 
-Blocks are hashed with SHA256 and need to start with a certain number of zeros (or, more precise, be below a given value depending on the difficulty)
+Blocks are hashed with SHA256 and this hash needs to start with a certain number of zeros (or, more precise, be below a given value depending on the difficulty)
 
 ed25519 keys are used to validate transactions.
 
@@ -100,6 +100,8 @@ A transaction that is stored in a block must fulfill the following criteria (in
 A block is valid if all of the following criteria are fulfilled:
 
 - The "previous hash" field contains a SHA256 hash value of a previous valid block. (Or is completely filled with nullbytes, in case of the first block.)
+- The first block has a public key with the following SHA256 hash value:  
+  88023d392db35f2d3936abd0532003ae0a38b4d35e4d123a0fa28c568c7e3e2f
 - The "timestamp" field is greater than the timestamp of the previous block (if there is any). Equal timestamps are not allowed.
 - The "timestamp" field is less than or equal to the current time. This needs to be decided by each node based on the local clock.
 - The "difficulty sum" must be precisely calculated, as specified in `calculating difficulty`