I'm currently doing a project on RSA, and have just come across this website.


On here, it says that

Since encrypted data transmission takes too much time in case of asymmetric encryption, this kind of encryption is used for a secure symmetric key exchange that is used for actual transmitted data encryption and decryption.

Now I was not aware that this was the case, so my question is, does this mean that asymmetric encryption is relatively slow compared to symmetric encryption, or is it impractically slow to be implemented in things like SSL?

Also, out of curiosity, exactly how much slower is it in terms of a time value?


Another question, why do the primes have to be far apart? I guess since the numbers used are so big they are far apart anyway, but is this a prerequisite for RSA, or just a fact that holds due to the size of the numbers?


  • $\begingroup$ It is not a good idea to ask two unrelated questions in the same CSE question. Your second question was covered there in detail. In a nutshell: RSA primes should be far apart in order to prevent Fermat factoring of the public modulus, but that happens naturally, with overwhelming odds, when choosing the primes at random in a suitably large interval. $\endgroup$
    – fgrieu
    Nov 23 '15 at 15:36
  • $\begingroup$ Related question over at SecSE: 2014-05-08: Asymmetric vs symmetric encryption benchmarks $\endgroup$ Nov 25 '15 at 14:37

Yes, asymmetric encryption is slow compared to symmetric encryption.

With symmetric ciphers, encryption and decryption speed can be several gigabytes per seconds on a common PC core; see these benchmarks.

With RSA encryption, on comparable hardware, we are talking tens of thousands encryptions per second, and only few hundreds of decryption per seconds, for common key sizes, and small messages (like 1 bit to 250 bytes, way enough for a session keys and authenticators); see these benchmarks.

Pure asymmetric encryption would often be like 3 to 5 decimal orders of magnitude slower than symmetric encryption is. That's the main reason why in practice, when enciphering messages that could be over what fits the capacity of a single asymmetric encryption, we use hybrid encryption.

  • 1
    $\begingroup$ Ah okay that clears it up, thanks! Can I ask, why is it that decryption is so much slower than encryption? The process is essentially the same isnt it? $\endgroup$
    – Ali
    Nov 23 '15 at 17:04
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    $\begingroup$ @Ali: in RSA, the public exponent (used for encryption) can be (and typically is) small, e.g. 1 to 5 decimal digits, when the private exponent (used for decryption) must be large, like hundreds of decimal digits. All things being equal, time is roughly proportional to the size of the exponent. There are other differences between RSA encryption vs decryption, but the bulk of the performance difference comes from that. $\endgroup$
    – fgrieu
    Nov 23 '15 at 17:21
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    $\begingroup$ @Ali: also, that speed difference between encryption and decryption is specific to RSA (and related schemes, such as Rabin); other methods have different tradeoffs. $\endgroup$
    – poncho
    Nov 23 '15 at 18:06
  • $\begingroup$ @fgrieu Yeah though it might be something like that, thanks for clearing it up. Also do you have anything to offer in regards to my second question? Why do the primes have to be far apart? $\endgroup$
    – Ali
    Nov 23 '15 at 18:24
  • $\begingroup$ Another data point: on my Intel Core i7-4400U laptop using OpenSSL 1.0.2d, AES-128 encryption is 366 times faster than RSA-2048 encryption when operating on 2048-bit blocks of data. $\endgroup$
    – rmalayter
    Nov 23 '15 at 18:47

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