RSA is not a symmetric encryption algorithm. It is a public-key, or asymmetric, cryptographic system. The key distinction is that symmetric cryptography uses a shared secret key, while public-key cryptography uses separate, related keys.
How to tell symmetric and asymmetric cryptography apart
Look at the relationship between the keys. In symmetric cryptography, the same secret key is used for the operation and, when applicable, to reverse its effects. The National Institute of Standards and Technology (NIST) gives AES as an example of a symmetric algorithm: NIST’s definition of symmetric cryptography.
Public-key cryptography, also called asymmetric cryptography, uses separate but related keys. Depending on the scheme, one key can be used to encrypt or digitally sign, while the other is used to decrypt or verify a signature. See NIST’s definition of public-key cryptography.
Why RSA is not symmetric
RSA is a public-key system, so it belongs to the asymmetric category rather than the symmetric one. In the familiar public-key encryption example, a sender encrypts a message with the recipient’s public key, and the recipient decrypts it with the matching private key. NIST describes RSA as an early public-key system in its introductory cryptography chapter.
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RSA and AES illustrate different key arrangements; they are not interchangeable names for the same kind of operation. AES is a supported example of symmetric cryptography, whereas RSA is a public-key system.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Public-key cryptography is not always encryption
“Cryptography” covers more than encrypting messages. Public-key schemes can support digital signatures as well: one key signs, and the related key verifies. That does not mean every asymmetric algorithm encrypts arbitrary messages. Whether a system is used for encryption or signatures depends on the scheme.
For a brief historical overview of public-key cryptography, NIST’s publication record for “Public-Key Cryptography” is available.
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