What is cryptography?

Cryptography uses math to protect information. It also detects changes and helps verify who created or sent it. Encryption turns readable data into ciphertext that someone needs the right key to read. Other methods detect changes or verify digital signatures.

Encryption keeps a message private from people who do not have the key. A digital signature can help verify who signed it and whether it changed. Both depend on the method, its implementation, and how keys are handled.

What cryptography protects

Cryptography supports several kinds of protection. Confidentiality keeps information private. Integrity helps detect unauthorized changes. Authentication helps verify the identity of a person, device, or service. Some methods also provide evidence that a specific key signed data. Security standards call this non-repudiation. That evidence does not, by itself, settle questions of intent or legal responsibility.

A method can provide one kind of protection without providing the others. For example, encryption can hide a message without proving who sent it. A digital signature can help verify its origin and detect changes without hiding the message.

How encryption works

Encryption turns readable information, called plaintext, into ciphertext that is difficult to understand without the right key. Decryption uses a key to restore the ciphertext to readable information.

The Advanced Encryption Standard, or AES, is a widely used example. Its specification processes data in 128-bit blocks and supports keys that are 128, 192, or 256 bits long. Those numbers describe AES keys. They do not show how secure other cryptographic methods are.

Encryption protects information only while the relevant keys remain under control. If someone steals a decryption key, that person can read data protected by it. Encryption also does not prevent someone from deleting or blocking access to encrypted information.

Symmetric and public-key cryptography

Symmetric-key cryptography uses the same secret key to encrypt and decrypt data. This method suits people or devices that already have a secure way to share the key. They must keep the key secret from anyone who should not read the information.

Public-key cryptography, also called asymmetric cryptography, uses a related pair of keys: a public key and a private key. A public key can be shared, but its matching private key must be protected. Depending on the method, the pair can support encryption and decryption or digital signatures. For a signature, the private key signs data and the public key checks the signature. This approach helps people communicate without first sharing a secret key.

Hashes and signatures

A cryptographic hash turns data of any length into a fixed-length value, called a hash. Changing the input changes the result, and approved hash functions are designed to make it impractical to find different data with the same hash. A software provider, for example, can publish a hash so a user can check whether a downloaded file matches the expected version.

A hash does not hide data or prove who created it. If someone changes both a file and an unauthenticated hash, the check alone will not reveal the change. A digital signature addresses this limitation: it uses a private key to sign data and a public key to verify the signature. It can help detect tampering and authenticate the signing key, but it does not encrypt the signed content.

Where cryptography is used

Cryptography protects information in everyday digital services. A secure website connection uses cryptographic methods to help protect data moving between a browser and a server. Phones and other devices use cryptography to protect stored information, and digital signatures help verify software and other files.

The protection you see depends on the service or device. A padlock in a browser indicates that the connection uses encryption, but it does not guarantee that a website is trustworthy or that the information a user sends will remain private after the site receives it. Cryptography protects particular parts of a digital exchange, not every step around it.

Keys and implementation matter

Even a strong algorithm cannot make up for poor key handling. Security depends on how keys are generated, stored, used, and eventually destroyed. A lost key can lock its owner out of encrypted data; a stolen key can expose that data. Weak implementation or software vulnerabilities can also undermine protection.

Choose established products and services that explain what they encrypt and how they protect account recovery and keys. Keep devices and software updated, and use strong account authentication. These steps support cryptographic protections, but no single setting makes information impossible to access.

Cryptography after quantum computers

Some future quantum computers could threaten widely used public-key methods. Post-quantum cryptography refers to algorithms designed to resist attacks from quantum computers; quantum cryptography uses principles of quantum mechanics and is a different field. It remains uncertain when a quantum computer will be able to break current methods.

The National Institute of Standards and Technology has published post-quantum standards for implementation. These standards help prepare cryptographic systems for future changes while protecting information now.