collisions-club

Microservice for generating strings which all produce the same Java hashCode. Note that the generated output is almost always outside the ASCII range.

Why?

¯\_(ツ)_/¯

The formula

The generator abuses the fact that in Java, String::hashCode essentially sums every character's value, multiplying by 31 at each iteration.

Knowing this, we can trivially generate colliding strings. For example, for four characters, the formula would be:

arr[0] = 2114 - a
arr[1] = a * 31 - b
arr[2] = b * 31 - c
arr[3] = c * 31

where

0 <= c <= b <= a <= 2114

The magic number 2114 is derived from 0xFFFF / 31 where 0xFFFF is the maximum value of an UTF-16 code point (Java uses UTF-16 internally!) and 31 is the multiplier used by the Java hashCode algorithm.

Try it

You can also execute this Java fiddle to see that all of the strings returned indeed have the same hashCode:

https://www.tutorialspoint.com/viewproject.php?URL=compile_java8_online.php&PID=0Bw_CjBb95KQMSWpDWVdGZHBzX3M

API Endpoints

GET /generate

Generates specified amount of strings which all produce the same hashCode in Java.

Parameter	Description	Type
count	Amount of strings to generate. Must be inside range 1...1024 (inclusive).	int32
string_length	Length of strings to generate (amount of UTF-16 code units, no guarantees about the byte count of the UTF-8 output). Currently only value supported is 4.	int32
seed	(Optional) The seed to use for the random number generator.	int64

Example

Request

GET /generate?string_length=4&count=5&seed=1245125551

Response

GET /generate?string_length=4&count=5&seed=1245125551

{
  "data": [
    "ů퀍櫶⾶",
    "U䝙䋘",
    "˅꛴徠式",
    "Ł틢뮡뉾",
    "ج㻞㝐జ"
  ]
}

GET /hashes

Gets the hashes produced by the specified string lengths.

Example

Request

GET /hashes

Response

{
  "data": [
    {
      "string_length": 4,
      "hash": 62978174
    }
  ]
}

Download

See Releases.

Running

1. Build or download the jar.
2. Run the jar: java -jar collisions-club-<version>.jar.

Building

1. Download or install Apache Maven.
2. Clone or download this repository.
3. In the root folder, execute mvn clean package. This produces collisions-club-<version>.jar in the target folder.

License

collisions-club is licensed under the MIT License. See LICENSE.

collisions-club uses third party libraries that are distributed under their own tems. See LICENSE-3RD-PARTY.