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Rolling your own crypto: Everything you need to build AES from scratch

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Re: Rolling your own crypto: Everything you need to build AES from scratch

#61

Post by Retr0id »

nullc wrote: post #2

It's much more interesting to implement AES without the table lookups-- as doing so requires constructing a boolean circuit that computes the same result as the tables, more useful too since the table lookups result in security killing side channels. :) The author might be surprised at how often someone's random "learn 2 crypto" ends up in use in production. Kudos for the warnings, though I doubt that actually accomp…

Further, you can compute the whole block transform as a giant boolean circuit (aka bitslicing). This has various advantages, sidechannel resistence being one, but surprisingly it outperforms a more traditional implementation in many circumstances (e.g. in pure-python implementations: https://github.com/DavidBuchanan314/python-bitsliced-aes )

Re: Rolling your own crypto: Everything you need to build AES from scratch

#62

Post by sireat »

fstokesman wrote: post #58
zgs wrote: post #34

Earlier quoted context omitted.

Most cryptographic operations are easy to implement. The best part is when you make a mistake, you'll most likely get random nonsense out the end. Floating point code is much more difficult, often you'll get an answer that's almost correct or worse one that's correct most places...

Heard about a great floating point implementation bug just today. When nintendo released Super Mario 64 for the Wii virtual console, they essentially just made their own emulator that plays a single ROM. The emulator is pretty accurate, though they made a mistake when implementing floating point operation rounding: All operations end up rounding upwards to the next floating point number (as opposed to alternating, de…

Shades of the floating point truncation bug in Vancouver Stock Exchange Index in the early 1980s:

https://en.wikipedia.org/wiki/Vancouver_Stock_Exchange#Round...

The bug caused the index to creep up to twice the real index.

Re: Rolling your own crypto: Everything you need to build AES from scratch

#63

Post by dingleberry420 »

traceroute66 wrote: post #18
stingraycharles wrote: post #14

Earlier quoted context omitted.

It’s absolutely worth it to roll your own crypto if you want to learn from it. I feel like there is almost a stigma against articles like these because we’ve all been conditioned so much to avoid even thinking about it lol. I did some deep dive into AES and RSA at some point in a distant past, it was a learning experience that to this day allows me to make much better decisions when choosing algorithms.

> It’s absolutely worth it to roll your own crypto if you want to learn from it No. If you want to "learn from it", the first thing you should do is buy a copy of Bruce Schneier's Applied Cryptography . Just reading (and fully understanding !) that book will alone put you in a position where you already know more about cryptography than 90% of other people. If after that you still want to play around with rolling you…

No. If you want to "learn from it", you should do whatever you want that works for you. You're not our dad.

Re: Rolling your own crypto: Everything you need to build AES from scratch

#64

Post by IncRnd »

chmod600 wrote: post #47
Mordisquitos wrote: post #6

Earlier quoted context omitted.

> you shouldn't use your (or their) self-made cryptography in production Absolutely, and that was the very first thing I thought when I saw the title. For some reason I've got the "don't roll your own crypto" commandment heavily ingrained into my brain (even though I've never been in any context where I might have tried), together with "don't let the frying pan handle stick out over the edge of the cooker in case a k…

"don't roll your own crypto" is good advice, but it's also against the hacker ethos. It's another way of saying "you'll never really understand this so don't try, just rely on a mysterious cabal of experts". May be true, but not exactly a great message.

>It's another way of saying "you'll never really understand this so don't try, just rely on a mysterious cabal of experts".

It's more along the lines of, "you absolutely can understand this if you try, buy trying will take years of your life to learn and apply all attack vectors and their mitigations."

The admonition not to roll your own crypto isn't so that people don't look at crypto. Take a look, fiddle, and hack all you want - just not in production. The saying comes from crypto people noticing avoidable security issues that get created many times people have touched or used crypto code - because even 100% correct crypto code is often also broken in practice.

Re: Rolling your own crypto: Everything you need to build AES from scratch

#65

Post by stu2b50 »

traceroute66 wrote: post #18
stingraycharles wrote: post #14

Earlier quoted context omitted.

It’s absolutely worth it to roll your own crypto if you want to learn from it. I feel like there is almost a stigma against articles like these because we’ve all been conditioned so much to avoid even thinking about it lol. I did some deep dive into AES and RSA at some point in a distant past, it was a learning experience that to this day allows me to make much better decisions when choosing algorithms.

> It’s absolutely worth it to roll your own crypto if you want to learn from it No. If you want to "learn from it", the first thing you should do is buy a copy of Bruce Schneier's Applied Cryptography . Just reading (and fully understanding !) that book will alone put you in a position where you already know more about cryptography than 90% of other people. If after that you still want to play around with rolling you…

> If after that you still want to play around with rolling your own crypto, then fine, go for it. But be aware you are very much making your own bed and should be prepared to lie in the inevitable mucky consequences.

I'm very confused. OP was suggesting to roll your own crypto as a learning exercise. What possible consequences could there be? Let alone "murky" ones?

Will your RSA implementation summon an eldritch monster or something?

Re: Rolling your own crypto: Everything you need to build AES from scratch

#66

Post by tptacek »

tialaramex wrote: post #37
stingraycharles wrote: post #14

Earlier quoted context omitted.

It’s absolutely worth it to roll your own crypto if you want to learn from it. I feel like there is almost a stigma against articles like these because we’ve all been conditioned so much to avoid even thinking about it lol. I did some deep dive into AES and RSA at some point in a distant past, it was a learning experience that to this day allows me to make much better decisions when choosing algorithms.

> It’s absolutely worth it to roll your own crypto if you want to learn from it. It's essentially a waste of your time. Because of Schneier's Law: "Any person can invent a security system so clever that she or he can't think of how to break it". The thing that you might learn from, if you put the work in, would be breaking other people's stuff. Ideally you would find something that's actually in use and vulnerable en…

How dare you.

Re: Rolling your own crypto: Everything you need to build AES from scratch

#68

Post by unethical_ban »

traceroute66 wrote: post #18
stingraycharles wrote: post #14

Earlier quoted context omitted.

It’s absolutely worth it to roll your own crypto if you want to learn from it. I feel like there is almost a stigma against articles like these because we’ve all been conditioned so much to avoid even thinking about it lol. I did some deep dive into AES and RSA at some point in a distant past, it was a learning experience that to this day allows me to make much better decisions when choosing algorithms.

> It’s absolutely worth it to roll your own crypto if you want to learn from it No. If you want to "learn from it", the first thing you should do is buy a copy of Bruce Schneier's Applied Cryptography . Just reading (and fully understanding !) that book will alone put you in a position where you already know more about cryptography than 90% of other people. If after that you still want to play around with rolling you…

It is shocking to me how many people in the computer community explicitly tell curious minds not to even try building a crypto protocol.

It isn't enough to say "you'll never get it right for production, please use a framework or library for production". You simply don't want them to even experiment on their own.

I agree with saying "the most effective way to learn cryptography is through books and tools X, Y, Z". But one would imagine that writing a poor hashing algo would open up a door to alternate dimensions by how superstitious some are about it on this site.

Re: Rolling your own crypto: Everything you need to build AES from scratch

#69

Post by watwut »

traceroute66 wrote: post #24
watwut wrote: post #22

Earlier quoted context omitted.

Just no. This idea that people are not allowed to learn by doing things and must read huge books first is absurd. The guidelines for learning you gave here are impractical for majority of people who are interested. It is actually good when people are trying to learn about security. Just about worst thing these absurd rules achieve is that effectively only rule breakers are allowed in.

> Just no. This idea that people are not allowed to learn by doing things and must read huge books first is absurd. No, its not absurd. Sure, I agree, for many things in life you can "learn by doing". But this is cryptography. There is no escaping that cryptography IS mathematics and an algorithm built on top of that mathematics. Unfortunately the only way to learn the theory is by reading and understanding books or…

First, majority of exploits are not even in math, they are in implementation. Second, as much as math is fun, the claim that you need to start with it is plain absurd.

First thing this mantra does is that we collectively know less a out crypto and security. Second thing it does is that it selects stick-in-ass rule followers away which is exactly contraproductive. And third, it makes us stuck with crappy convoluted code crypto libs had twenty years ago, cause supply of people capable to improve it is not build up.

Re: Rolling your own crypto: Everything you need to build AES from scratch

#70

Post by jedisct1 »

MarekKnapek wrote: post #25

Implementing AES from scratch is easy and fun, just read the spec and implement it 1:1, until ... you encounter GCM mode and its Galois Field "weird" math. Very. Not. Fun.

AES is useful beyond AES-GCM.

Constructions such as AES-CMAC, but also recent ones such as AEGIS only require the AES round function.

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