What is cryptography?

What is Cryptography?

Cryptography is the science of securing information so that only intended parties can access and understand it. The term comes from Greek:

• Kryptos → hidden/secret
• Graphien → writing

In essence, cryptography ensures that data cannot be easily read, changed, or faked by unauthorized users. It is the reason we can:

• Safely use online banking
• Securely message friends on WhatsApp
• Trust blockchain transactions

Goals of Cryptography

Cryptography in computer science (and blockchain) serves four primary purposes:

1. Confidentiality → Only authorized users can read the message.
   Example: Messages on Signal are encrypted end-to-end.
2. Integrity → Data cannot be modified without detection.
   Example: Hashes ensure a block in the blockchain is not altered.
3. Authentication → Verify the identity of the sender.
   Example: Digital signatures prove that a transaction came from a wallet owner.
4. Non-repudiation → Sender cannot deny having sent the message/transaction.
   Example: Once Alice signs a Bitcoin transaction, she cannot deny it later.