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Symmetric-key ciphers aren't used because they "save a round trip". They're used because they're much faster. One could use a public key encryption system to
by YZF 13y ago
Symmetric-key ciphers aren't used because they "save a round trip". They're used because they're much faster. One could use a public key encryption system to encrypt the entire session between two parties (e.g. RSA) but AES is much faster than RSA.
- oakenshield 13y agoSpeed is one concern but far from the main reason. You couldn't use a typical-length RSA key to encrypt a large chunk of data because there's a max input size for any given RSA key size. Standard block/stream cipher modes of operation are defined only for symmetric key ciphers.
- dspillett 13y agoYou can certainly use RSA as a block cypher, you are just limited in the size of the blocks.
- YZF 13y agoThe max input size is for one message. It's effectively the "block size" for RSA. You can send multiple messages using a single RSA key (and in fact pretty much everyone does that, otherwise you'd need a new public key for every message). RSA is a complete crypto system, you don't need any additional ciphers. You do need randomiziation (e.g. random padding).
- YZF 13y agoFor the sake of completeness, one such construction where RSA is used as a cipher is OAEP (Optimal Asymmetric Encryption Padding). Beyond being slow it has some other inefficiencies but that's another story.