Asymmetric encryption

Asymmetric encryption is public-key encryption: a way to protect data using a mathematically related public–private key pair. The public key is used to encrypt data, while the matching private key is used to decrypt it. In a secure scheme, deriving the private key from the public key should be computationally infeasible. It is one function within public-key cryptography, which also includes digital signatures and key-establishment techniques.

How Data Is Encrypted and Decrypted

The recipient generates a key pair, keeps the private key secret, and distributes the public key. A sender obtains that public key, checks that it belongs to the intended recipient, and uses it to encrypt data into ciphertext. The recipient then uses the matching private key to recover the plaintext. The parties do not need to share a secret encryption key in advance.

Asymmetric encryption therefore helps address secret-key distribution, but distributing a public key is not enough by itself: the sender must trust its ownership. An observer may still intercept or copy ciphertext; encryption protects its contents rather than preventing interception. It also does not, on its own, establish who sent the message or guarantee that alterations will be detected.

Asymmetric vs. Symmetric Encryption

• Key Use: Asymmetric encryption uses a public key for encryption and a private key for decryption. Symmetric encryption uses a shared secret key that both parties must protect.
• Computational Cost: Symmetric encryption is generally more efficient for bulk data. Public-key operations are typically more computationally expensive, and neither approach is universally safer.
• Hybrid Systems: Hybrid systems use public-key techniques to establish or protect a symmetric key, then use symmetric encryption for the payload. This avoids applying costly public-key encryption to all the data.

Related Public-Key Functions

Digital signatures serve a different purpose from asymmetric encryption. A signer uses a signing private key to create a signature, and a verifier uses the corresponding public key to check the message's integrity and origin. The signature does not hide the message, and attributing it to a particular person or organisation requires a trustworthy link between that signer and the key.

Key agreement lets parties establish a shared secret for subsequent cryptographic use. It need not involve encrypting a pre-existing secret for delivery. Signing and key agreement are related public-key functions, not stages required to decrypt a message; individual schemes do not necessarily support all three functions.

Trust and Key Management

If an attacker substitutes their own public key for the recipient's, the sender may encrypt data that the attacker can decrypt. The attacker may then relay it to the recipient, enabling a man-in-the-middle attack. Certificates validated through a trusted system or independently verified key fingerprints can help establish the correct key-to-identity link.

The protection offered by asymmetric encryption depends on a sound scheme, correct implementation, and private-key secrecy. Key size alone does not establish security. Depending on the key's use and the surrounding protocol, a compromised private key can expose data protected by that key or allow impersonation; it does not automatically compromise every system or all past traffic.

Public distribution does not remove key-management responsibilities. Private keys still need access controls, and compromised keys need replacement and, where applicable, revocation. If the only usable decryption key is lost, the protected data may become unrecoverable.