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Birthday Attack Collision

2^(n/2) inputs for 50% collision. Free online Birthday Attack Collision for technology — instant, accurate results, mobile-friendly, no signup needed.

Inputs
18,446,744,073,709,552,000

Derivation

  1. ├── 01Givenbits = 128
  2. ├── 02Formula(2)^(t / 2)
  3. └── 03Compute Inputs1.8447e+19
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§01What is

Understanding the Birthday Attack Collision

The Birthday Attack Collision computes Inputs from 1 input: hash bits. 2^(n/2) inputs for 50% collision.

Quick calculators for the math that shouldn’t need a notepad — instant, accurate, private to your browser. The Birthday Attack Collision sits in that toolkit — it 2^(n/2) inputs for 50% collision. Enter your numbers above and the result updates instantly; every step of the math is shown in the Derivation panel so you can see exactly how the answer was reached.

§02The Formula

How it’s calculated

(2)^(t / 2)

Where

bits
Hash bits
§03Practical Example

Step-by-step walkthrough

Scenario

Apply the formula to a realistic set of inputs: Hash bits = 128.

  1. 01Start by noting the input — Hash bits: 128.
  2. 02Substitute these values into the formula: (2)^(t / 2)
  3. 03Compute Inputs: the calculator returns 1.84467e+19.
  4. 04Cross-check the answer by opening the Derivation panel above — every line of math is shown so you can follow the computation end-to-end.
§04Variants

Common Birthday Attack Collision Problems

The formula gets rearranged depending on which variable you need. Here are the patterns you’ll run into in the real world — find the one that matches your problem and follow the worked steps.

01 · PATTERN

Hash bits halved

bits = 64 (from 128)

Keep every other input at its default and halve the hash bits. See how inputs responds.

  1. 01New Hash bits: 64
  2. 02Baseline Inputs: 1.84467e+19
  3. 03New Inputs: 4.29497e+9
  4. 04Inputs decreases by 100% → use this sensitivity to plan for real-world variation.
02 · PATTERN

Hash bits doubled

bits = 256 (from 128)

Keep every other input at its default and double the hash bits. See how inputs responds.

  1. 01New Hash bits: 256
  2. 02Baseline Inputs: 1.84467e+19
  3. 03New Inputs: 3.40282e+38
  4. 04Inputs increases by 1.8446744073709552e+21% → use this sensitivity to plan for real-world variation.
§05FAQ

Frequently asked questions

Yes. The calculator implements the standard formula as documented and returns exact floating-point results. No approximations are used unless noted in the formula.
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