Public Key vs Private Key

John Spacey, updated on January 13, 2023

A public key is used to encrypt messages. A public key is published so that anyone can send a particular receiver a secure message.

A private key can be used to decrypt messages encrypted with a matching public key. As the term suggests, private keys are intended to be secret.

Asymmetric Encryption

Asymmetric encryption is a technique that allows anyone to send encrypted messages to a receiver. It is based on cryptographic algorithms for generating matching pairs of public/private keys such that the private key can't be guessed from the public key.

The public key is then published using techniques such as digital certificates. The private key is configured in encryption enabled software and kept secret.

Symmetric Encryption

Symmetric encryption is based solely on private keys. For example, symmetric encryption may be used to encrypt data in storage. It is also used in communications by sharing private keys between endpoints.

In many cases, asymmetric encryption is used to initialize a session and exchange private keys. Then communication is encrypted with symmetric encryption. This is done because symmetric encryption is faster than asymmetric encryption.

Public Key vs Private Key
	Public Key	Private Key
Definition	A published key that can be used to send a secure message to a receiver.	A secret key that can be used to decrypt messages encrypted with the corresponding public or private key.
Applies to	Asymmetric Encryption	Asymmetric Encryption Symmetric Encryption

Authentication Schemes

In order to secure data or messages you encrypt with a public key and decrypt with a private key. The opposite is sometimes done for the reasons of signing a message to prove that you're the source. In other words, encryption with a private key can be used to prove that you hold the private key. This is confirmed by decrypting with the corresponding public key. This technique is used in authentication schemes such as digital signatures. In practice, different private-public key pairs are used for signing messages and securing message communication. The following chart illustrates the various patterns of encryption with public and private keys.

Scenario	Public Key	Private Key
Asymmetric Encryption	Encrypt	Decrypt
Authenticate the origin of a message	Decrypt	Encrypt
Symmetric Encryption	N/A	Encrypt Decrypt

In practice, all three of these scenarios may be performed on the same message using different keys. For example, data that is encrypted with a private key to prove the origin of a message is often sent inside a message encrypted with a public key.

Encryption

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References

Fujisaki, Eiichiro, and Tatsuaki Okamoto. "Secure integration of asymmetric and symmetric encryption schemes." Annual International Cryptology Conference. Springer, Berlin, Heidelberg, 1999.

Rivest, Ronald L., Adi Shamir, and Leonard Adleman. "A method for obtaining digital signatures and public-key cryptosystems." Communications of the ACM 21.2 (1978): 120-126.