All Underhanded Crypto Entries

Honestly, 8/16b ints in C and C++ are just a minefield of problems.

Here's a snippet from an open source 8051 simulator that I was debugging today:

        if( ((RAM[ACC] & 0xF0) >> 4) > 9 || 
            GetBit(RAM[PSW], CY) == 0x01 ) {
            tempAdd = RAM[ACC] + 0x60;
            RAM[ACC] = tempAdd;
            if( ((unsigned char*)&tempAdd)[1] != 0 ) {
                SetBit(RAM[PSW], CY);
            }
        }

The code is supposed to set the carry flag when RAM[ACC] + 0x60 overflows into two bytes. Can you spot the bug? It involves my favorite C++ feature: implicit conversions, and my second favorite feature: signed chars.

The bug is that when RAM[ACC] is something like 0xFF, it gets cast to (int) -1 so the the upper byte and hence the carry flag never get set.

I predict there's lots more more evil that can milked from this fount.

This is one of those examples where I can see how it's confusing, but I just don't think you should have been doing it that way in the first place.

   bool foo = (rx & 0x0800) == 0x0800;

Not only does this not have the bug, it communicates your intent better, and modern compilers are quite capable of optimizing this.

Er, okay, anyone care to explain why they downvoted this?

I participated in this contest and was blown away by a lot of the ingenuity the other contestants demonstrated. The judges clearly did not have an easy time picking the winners, but I think they did a very good job at being fair about it.

Some of the files contains descriptive READMEs.

Try using this command on the files:

tar xf foo.tar.gz

No judge commentary (I'm a little disappointed, I was hoping someone would comment on the constant I used), but most of the entries are documented.

I put my own entry on github last week for easier reading:

https://github.com/ryancdotorg/undercrypto2014

The README was added later (wasn't in the actual entry) - it explains what the other files are.

As mentioned elsewhere in the comments, all the entries are in this repo:

https://github.com/simcop2387/underhanded-crypto

I'm curious why you say that. I found it very impressive from a technical standpoint.