What Are Digital Signatures?

A digital signature is a cryptographic technique used to prove that a message or document was genuinely created by a specific individual or entity and that it hasn’t been tampered with.

It’s the digital equivalent of signing a document with a pen but far more secure, tamper-resistant, and verifiable through math rather than trust.

Digital signatures are used across the internet, from signing transactions on blockchains to verifying software downloads, to ensuring that the websites we visit are really who they say they are.

And as we’ve seen in the last few articles, especially when understanding how Bitcoin secures its data, this idea becomes central.

Just like cryptographic hashes and public-private keys, digital signatures are one of the building blocks of trust in a decentralized world.

The Basics: Public Key Cryptography at Work

To understand digital signatures, we start with public key cryptography—a system where every user has -

• A private key (kept secret)
• A public key (shared freely)

The two are mathematically linked as what is encrypted with one can be verified or decrypted by the other.

In the case of 'Digital Signatures' the private key is used to sign a message and the public key is used to verify it.

This lets others confirm that a message truly came from the person who holds the corresponding private key.

Signing a Message

Looking at the diagram above we can see how encryption works while signing a message.

1. You write a message (or produce any data you want to send).
2. You use your private key to sign it—effectively encrypting a fingerprint of the data.
3. You send both the original message and the signature to the recipient.

The recipient then -

1. Uses your public key to verify the signature.
2. Confirms that the signature matches the message and could only have come from your private key.

The Problem With Large Messages

Encrypting an entire message with a private key, especially large documents, it is inefficient and risky. It can expose security vulnerabilities (especially with very small messages). And it opens up possible attack vectors if done improperly.

So we don’t sign the whole document. Instead, we use something we already covered...

Where Hash Functions Come In

Rather than signing the entire message, we -

1. Use a cryptographic hash function (like SHA-256) to generate a fixed-length digest of the message.
2. Then sign that digest with the private key.

This keeps the signature short, efficient, and secure—no matter how large (or small) the original message is.

The verifier:

• Hashes the message themselves.
• Decrypts the signature using the sender’s public key.
• Compares the two hashes.

If they match, the message is verified. If they don’t, something’s wrong.

A simpler example would be - How It Works in TLS and the Web

This exact process happens every time you connect to a secure website (HTTPS).

• The server sends you its certificate.
• That certificate contains a public key and a digital signature issued by a Certificate Authority (CA).

No manual key exchange. No guessing. Just verification, backed by cryptographic signatures.

Digital signatures are invisible but essential. They prove authorship, preserve integrity, and allow trust in systems where the parties don’t know or trust each other.

Whether it's securing financial transactions, verifying software, or loading a website, you’re relying on digital signatures far more often than you think.

Next up | We will take a look at the the infrastructure that makes all this work—Public Key Infrastructure (PKI) and how the web of trusted authorities lets your browser decide which signatures to believe.