## Welcome to Cryptography Stack Exchange

Cryptography Stack Exchange is a question and answer site for software developers, mathematicians and others interested in cryptography. It's built and run by you as part of the Stack Exchange network of Q&A sites. With your help, we're working together to build a library of detailed answers to every question about cryptography.

We're a little bit different from other sites. Here's how:

This site is all about getting answers. It's not a discussion forum. There's no chit-chat.

Just questions...

up vote

Good answers are voted up and rise to the top.

The best answers show up first so that they are always easy to find.

accept

Accepting doesn't mean it's the best answer, it just means that it worked for the person who asked.

# why DES uses exactly 16 rounds?

Why DES uses 16 rounds why not more or less then 16. what is the significant of using 16.

From Schneier's description of DES in Chapter 12 of Applied Cryptography (12.3): “DES with any number of rounds fewer than 16 could be broken with a known-plaintext attack more efficiently than by a brute-force attack.” This explains the "Why not less than 16". As for the "why not more than 16", that is a tradeoff for speed of execution (more rounds = less speed).

Applied Cryptography mentioned this. With 17 or 18 rounds a differential attack is about as costly as brute-force. And 19 rounds or more makes differential attack impossible since it requires more than 2^64 chosen plaintexts, which is impossible since the DES block size is 64 bits.

## Get answers to practical, detailed questions

Focus on questions about an actual problem you have faced. Include details about what you have tried and exactly what you are trying to do.

• Asymmetric and symmetric cryptographic algorithms
• Cryptographic protocols and cryptanalysis techniques
• Hashing
• Entropy and information theory
• Random number generation

Not all questions work well in our format. Avoid questions that are primarily opinion-based, or that are likely to generate discussion rather than answers.

Questions that need improvement may be closed until someone fixes them.

• Anything not directly related to cryptography
• Questions that are primarily opinion-based
• Questions with too many possible answers or that would require an extremely long answer

## Tags make it easy to find interesting questions

All questions are tagged with their subject areas. Each can have up to 5 tags, since a question might be related to several subjects.

Click any tag to see a list of questions with that tag, or go to the tag list to browse for topics that interest you.

# why DES uses exactly 16 rounds?

Why DES uses 16 rounds why not more or less then 16. what is the significant of using 16.

## You earn reputation when people vote on your posts

+5 question voted up
+2 edit approved

As you earn reputation, you'll unlock new privileges like the ability to vote, comment, and even edit other people's posts.

Reputation Privilege
15 Vote up
125 Vote down (costs 1 rep on answers)

At the highest levels, you'll have access to special moderation tools. You'll be able to work alongside our community moderators to keep the site focused and helpful.

500 Vote to close, reopen, or migrate questions Edit other people's posts Access to moderation tools
see all privileges

## Improve posts by editing or commenting

Our goal is to have the best answers to every question, so if you see questions or answers that can be improved, you can edit them.

Use edits to fix mistakes, improve formatting, or clarify the meaning of a post.

You can always comment on your own questions and answers. Once you earn 50 reputation, you can comment on anybody's post.

Remember: we're all here to learn, so be friendly and helpful!

From Schneier's description of DES in Chapter 12 of Applied Cryptography (12.3): “DES with any number of rounds fewer than 16 could be broken with a known-plaintext attack more efficiently than by a brute-force attack.” This explains the "Why not less than 16". As for the "why not more than 16", that is a tradeoff for speed of execution (more rounds = less speed).

yes, but with 16 rounds you have to use brute force, since other type of attacks aren't faster than brute force at 16 rounds, while they are faster with less rounds. Regarding the "possibility to break 16 rounds", see wikipedia - DES Challenges; in 1992 it took 22 hours and 15 minutes to break DES. - tech Aug 14 '13 at 9:06

## Unlock badges for special achievements

Badges are special achievements you earn for participating on the site. They come in three levels: bronze, silver, and gold.