How Do Digital Signatures Work?

How Do Digital Signatures Work in Bitcoin?

Now that you’ve learned the foundational concepts—like hash functions, private/public keys, and asymmetric encryption—you’re ready to understand how digital signatures actually work in Bitcoin and other cryptocurrencies.

🔍 If you’re unfamiliar with any of these concepts, make sure to check out the earlier lessons—they explain everything in beginner-friendly terms.

🔐 How a Digital Signature Is Created

When you send bitcoin, your wallet doesn’t just send coins—it constructs a transaction message. This message includes:

• The amount of BTC to send
• The recipient’s Bitcoin address
• Other technical details (like transaction fees)

Here’s the step-by-step process of how your wallet creates a digital signature:

1. Hashing the Message
   The transaction message is passed through a hash function to generate a unique output called a message hash (or simply, a hash).
2. Signing with Your Private Key
   Your wallet then uses your private key to encrypt the hash.
3. Creating the Digital Signature
   The encrypted hash becomes the digital signature.

At this point, your wallet now holds two key items:

• The original transaction message
• The digital signature (the encrypted version of the message hash)

🚫 This signature is tied specifically to this transaction. It cannot be reused for another transaction because the signature depends on the unique data from this message.

✅ How a Digital Signature Is Verified

To send bitcoin, you must prove that you’re authorized to spend it—that is, you must prove ownership of the private key associated with the address holding the funds.

But here’s the challenge:
You must prove ownership without revealing your private key.

This is exactly what a digital signature lets you do.

When your transaction is broadcast to the Bitcoin network, your wallet includes three things:

1. The original transaction message
2. Your digital signature
3. Your public key (corresponding to your Bitcoin address)

Here’s how the Bitcoin network verifies your signature:

1. A Bitcoin node receives the three items.
2. It runs the transaction message through the same hash function to generate its own version of the message hash.
3. It uses your public key to decrypt the digital signature (which contains the original hash you signed).
4. It compares the two hashes:
   • 🔁 If the hashes match:
     ✔️ The transaction is valid. It proves you’re the true owner and the message hasn’t been altered.
   • ❌ If they don’t match:
     ❌ The transaction is rejected.

🔎 Why This Works

Two important things are being proven here:

1. Ownership (Authenticity)

• If your public key can decrypt the digital signature, it proves that only your private key could have created it.
• That means the transaction could only have come from you—establishing your identity as the sender.

2. Integrity (Tamper-Proof)

• The network compares the decrypted hash from the signature with the newly hashed version of the message.
• If they don’t match, it means the message was changed in transit.
• Even a small change in the message (like altering the recipient’s address) would produce a completely different hash.

This ensures that:

• The message is authentic (came from you)
• The message is untampered (hasn’t been changed)

📌 Final Analogy

Just like a handwritten signature proves a person signed a specific document, a digital signature cryptographically proves:

• Who created the message
• That the message is unchanged

🧠 Summary

Now that you’ve learned how digital signatures are created and verified in Bitcoin, here’s the big picture:

• A digital signature is created by encrypting a hashed transaction with your private key.
• It proves your ownership of the bitcoins being sent.
• It ensures the transaction data is authentic and untampered.
• Anyone on the network can verify the signature using your public key—without ever needing access to your private key.

This is what it truly means to “sign” a transaction in the Bitcoin network.