Take the 2-minute tour ×
Information Security Stack Exchange is a question and answer site for Information security professionals. It's 100% free, no registration required.

This question concerns the session send and receive keys used in SSL/TLS protocol. my understanding is that this key uses symmetric encryption (DES, AES, BlowFish, etc.) I'm wondering, if public-private key pairs are superior to symmetric keys regarding key exchange security, why don't use asymmetric encryption for the session keys too?

this is an extension of an existing question: security of PKI, Certificates, certificate authorities, forward secrecy

share|improve this question

2 Answers 2

up vote 23 down vote accepted

2 reasons:

  1. Asymmetric encryption is slower, much slower, than symmetric encryption. Orders of magnitude slower.
  2. Given the same keylength, asymmetric is much weaker than symmetric, bit-for-bit. Therefore, you need a much larger key to provide equivalent protection. This also contributes to the slowness mentioned in 1.
  3. (As per @ThomasPornin's comment:) Asymmetric encryption carries with it an increase in size of output. For instance, if you use RSA, encrypted data is at least 10% larger than the cleartext. Symmetric encryption, on the other hand, has a fixed size overhead even when encrypting gigabytes of data.
share|improve this answer
3  
Another important point is that asymmetric encryption implies an increase in data length. For instance, if you use RSA, encrypted data is at least 10% larger than the cleartext. Symmetric encryption, on the other hand, has a fixed size overhead even when encrypting gigabytes of data. –  Thomas Pornin May 8 '11 at 12:06

Asymmetric encryption algorithms are far less efficient than symmetric algorithms. So essentially all use of encryption via asymmetric keys involves encrypting a symmetric session key, with which the actual message is encrypted.

Besides AviD's helpful notes about key length, note that if quantum computing attacks become feasible, they will render all mainstream public-key algorithms ineffective. On the other hand, AES-256 would remain strong even given a quantum computer attack. See Key size - Effect of Quantum Computing Attacks - Wikipedia.

share|improve this answer
1  
LOL, though this is true enough, Quantum computing always makes me laugh... Though of course I realize that it's probably just a question of time... –  AviD May 8 '11 at 7:20
1  
Shor's algorithm sounds quite remarkable! Best quote from the wikipedia article on it: another way to explain Shor's algorithm is by noting that it is just the quantum phase estimation algorithm in disguise... well, of course! :-) –  Rory Alsop May 10 '11 at 8:23
    
Reading this approximately 17 months latter, coincidentally i just read this morning that at University of New South Whales, Australia, a major break through has been made in production of quantum computers. It is said a key component has been successfully designed using the existing development methods used by semiconductor industry –  fayyazkl Sep 30 '12 at 19:51

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.