ClassifyIt video · 2 minutes 34 seconds
CryptIt - Public Certificates and Private Keys Explained
Understand public certificates and private keys through a simple envelope and padlock example, including delivery through an intermediary and encryption for multiple recipients.
2 minutes 34 seconds · English captions · Published 2026-10-23
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Video transcript
This video explains how digital certificates work. We will use a document, an envelope, a padlock and a matching key.
Bob has two mathematically connected parts: a public key and a private key. His public key is contained in a digital certificate, together with information identifying its owner. Bob can share his public certificate. His private key stays secret and protected. Sharing the certificate does not share the private key.
Think of Bob’s public key as an open padlock, and his private key as the matching key. Alice has a document and Bob’s open padlock. She places the document inside an envelope, closes the envelope, and snaps the padlock shut. Anyone can close this lock. Alice sends the locked envelope to Bob. Only Bob’s matching private key can open the padlock. He unlocks it, opens the envelope, and takes out the document.
Now imagine Charly carries the locked envelope from Alice to Bob. He can carry or copy the encrypted message, but without Bob’s private key, he cannot decrypt its contents. Bob’s software uses his private key to recover the original document.
The same works in the other direction. Alice has her own public certificate and private key. She shares her public certificate with Bob and other senders. They can now encrypt files for Alice. Her matching private key lets her open them. Charly can deliver these encrypted files too, without reading their contents.
You can also protect one message for several recipients. Imagine the envelope has a separate padlock for Bob, Alice and Dana. These are alternative ways in, not locks that must all be opened together. Each recipient uses their own matching private key. Bob can open the document with his key. Alice can open it with hers. Dana can use hers. Any one of these authorised recipients can open the same document.
Before using a certificate, check that it really belongs to the intended recipient through a trusted process. Keep your private key protected. Anyone who obtains it may be able to decrypt files intended for you. Remember: share your public certificate so others can send you protected information. Keep your private key to open it.