What Is a Digital Signature?
A digital signature checks a message against a signing key and detects changes. Learn what it proves and what it does not.
Free video lesson · 1:13
What Is a Digital Signature?
Quantum Computing & Encryption for Beginners
Music: “Thinking Music” by Kevin MacLeod. Creative Commons Attribution 4.0.
A digital signature checks a message against a signing key and detects changes.
Read the video transcript
A digital signature helps check who signed a message and whether it changed. It is a mathematical check attached to the message, not a picture of someone writing their name.
Imagine your friend sends, meet at four. Their app uses a secret called a private key to sign those words. You use the matching public key to check the signature.
If somebody changes four to five, the old signature no longer passes the check. It belongs to the original message. Copying the signature onto different words does not make those words pass.
You must also know that the public key really belongs to your friend. A signature checks a connection to a key. By itself, it does not tell you the name of the person holding that key.
And a signature does not hide the message. People may still be able to read it. Its job here is checking the message and its signer, while encryption has the separate job of keeping contents private.
For the quantum risk, watch Can Quantum Computers Forge Digital Signatures? For payments, watch Can Quantum Computers Steal Bitcoin?
The short answer
A digital signature helps check who signed a message and whether it changed. It is a mathematical check attached to the message, not a picture of someone writing their name.
A simple example
Imagine your friend sends, meet at four. Their app uses a secret called a private key to sign those words. You use the matching public key to check the signature.
If somebody changes four to five, the old signature no longer passes the check. It belongs to the original message. Copying the signature onto different words does not make those words pass.
You must also know that the public key really belongs to your friend. A signature checks a connection to a key. By itself, it does not tell you the name of the person holding that key.
And a signature does not hide the message. People may still be able to read it. Its job here is checking the message and its signer, while encryption has the separate job of keeping contents private.
What the check actually proves
A valid signature connects a message to a signing key and helps detect changes. To connect that key to a person, you need another trusted check. A valid signature does not prove that the message is honest or that the signer made a good decision.
Check your understanding
If a signed message contains a lie, does its valid signature make the lie true?
Answer
No. It checks the signature and the message, not the truth of the words.
Keep learning
Follow the free video course one question at a time. For help applying these ideas at work, explore team training or a cryptography audit.