I am using 3-DES to encrypt some data in Java. I decided to run some tests and p

Question

I am using 3-DES to encrypt some data in Java. I decided to run some tests and plot some graphics to decide how to optimally pack data for its encryption (see how encryption/decryption time behave depending on the length of the plain text), so I measured the time to encrypt one thousand Strings of incremental size (first one of length 1, second one of length 2, etc.) and then did the same the other way around (decrypt the first results).

I find that the decryption time increases with the size of the resulting plain text, although the time/character decreases as a rule. However, for encryption all the tests take approximately the same amount of time (it takes as long to encrypt a 2-character String than a 1000-character String, ~0.003 ms).

Is this something inherent to 3-DES, to how Java implements it or maybe I measured the time wrongly?

Explanation / Answer

This is both a DES (or any block cipher) and a programming concern. It is not a problem, just something to be aware of.

The reason time/character decreases with larger plaintexts is that the initial overhead and memory management takes up the bulk of time with small inputs. With large inputs (megabytes+) the time/char will stabilize to a fairly consistent number. Memory management techniques and encryption mode of operation also effect overhead. Making a copy of the plaintext instead of reading its memory space directly into the block cipher state for example.

DES processes blocks of 64-bits, which is 8 bytes at a time. So your time per character between 0 and 8 will be the same, as it is processing a single block.

There is also the accuracy of the measurements, which in Java is probably not using a high precision event timer, and will therefore be extremely inaccurate in measuring events that take small fractions of a second to complete. Try using a hand stopwatch to measure something that takes 0.02s to complete for example, you will never get a consistent or even accurate measurement.

Because block ciphers are fairly quick in processing block inputs, you will need to process several megabytes of data in order to get a more accurate time/block. Try 8 million bytes to start. You will need to increment the data size to get to around 10s of processing time before you can get an accurate time/block.

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