Find a Minecraft SMP server seed based on known locations

It is totally possible to find a Minecraft seed in a reasonable amount of time on CPU. Even though there are about 2^64 possible seeds, there is a lot of bias in how structure and decorator features are placed. By approaching the problem in a smart way, you can get rid of major brute-forcing overhead.

When it comes to structures(villages, temples, monuments, etc) and decorators(dungeons, emerald ore, end gateways, etc), the game creates a java.util.Random object which is seeded with the world seed before sending it through a layer of hashing. Looking closely, one realizes that the java Random class is a 48-bit linear congruential generator. What does that mean? Well, the upper 16 bits of the seed get trimmed off when passed on to the LCG. In fact, ALL features in the game do this mistake: structures, decorators, heightmap, slime chunks, etc. This means that you only need to brute-force 2^48 seeds which is much more reasonable on a GPU(a few hours worth). Those lower 48 bits are referred to as "structure seed".

The odd one out is biomes. Those use the full 64 bit of the seed. Once you have fully reversed the structure seed, you can then proceed to brute-force across the upper 2^16 with a biome search. This was the way to do it prior to 1.9.

In 1.9, Mojang changed the end pillars so they would generate in a circle with random heights. First, the game generates a list of indices, 0 through 9 for the 10 pillars. Then, it shuffles the list through Collections.shuffle() by passing on a custom Random object.

Random rand = new Random(worldSeed); long pillarSeed = rand.nextLong() & 65535L; List pillars = IntStream.range(0, 10).boxed().collect(Collectors.toList()); Collections.shuffle(pillars, new Random(pillarSeed));

For the sake of this post not being massive, you'll have to take my word for it. Given the 10 pillar heights, We can reverse 16 direct bits from the world seed in a matter of milliseconds. Those 16 bits all overlap with the structure seed, tuning down the brute-forcing work from 2^48 to 2^32.

To conclude, YES, it is possible to reverse-engineer a Minecraft seed. The most advanced seed crackers nowadays can reverse a seed in about 2 minutes of CPU once all the data is provided. I first learned about this technique through Neil's channel:

The seed usually runs through Java's Random class, which only uses 48 bits of the seed. There are some complicating factors, but I'm finishing off a tool to do this. At the moment it narrows the options for the lower 48 bits to a few tens of thousands - will need to chain with some other technique to reduce further. With one technique, I'm extracting about 32 bits and have 32 to go ("only" 4 billion possibilities).

Actually there is a way to find a seed using a program. All you need is this program linked below https://github.com/KaptainWutax/SeedCracker

Personally, I don't really think that working backwards to a map's seed from structure coordinates will help, due to Minecraft's incredibly randomized map generation procedure. However, I have heard of a small mod/plugin called World Downloader: http://www.minecraftforum.net/topic/1444862-145-world-downloader-mod/

It's been updated to 1.4.5, and what it does is basically download a multiplayer map from the server side. If you're willing to use a small-scale mod, you can use this to get the world file in your hands.

Once you've done that, you can add the file to your saves file in Minecraft (assuming you run your game on a Mac as I am not sure about Windows). Next, open up your game and go to the save. If cheats have been enabled, you can simply type in /seed as Yamikuronue suggested. However, if cheats are presumably still in their default (set to false from the server side), you can press esc, open up the world to LAN, and when the menu pops up, simply enable cheats. Your LAN world will still be the same as the SMP world, just with cheats enabled. Finally, you can enter /seed as mentioned previously.

The best way to find the seed if you have the server on your computer is to upload the level.dat file from the world folder into mineatlas.com. It will tell you the seed there and even show you a map of the world!