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Conditions

Overview​

Writing smart contracts with Fully Homomorphic Encryption (FHE) changes how we handle conditionals. Since all data is encrypted, we can’t use traditional if...else statements—there’s no way to view the values being compared.

Moreover, conditionals in FHE must evaluate both branches simultaneously. This is similar to constant-time cryptographic programming, where branching can leak information through timing attacks—for example, if one path takes longer to execute, an observer could infer which condition was true.

Basic Usage​

To handle encrypted conditionals, Fhenix uses a concept called a selector—a function that takes an encrypted condition and two possible values, returning one based on the encrypted result.

In practice, this is done with the select function. It behaves like a ternary operator (condition ? a : b) but works entirely on encrypted data.

For example, FHE.select takes the encrypted ebool returned by gt. If isHigher represents encrypted true, it returns a; otherwise, it returns b—all without revealing which path was taken.

Quick Start​

euint32 a = FHE.asEuint32(10);
euint32 b = FHE.asEuint32(20);
euint32 max;

// Instead of this (won't work) :
if (a.gt(b)) { // gt returns encrypted boolean (ebool), traditional if..else won't work as expected
   max = a;
} else {
   max = b;
}

// Do this:
ebool isHigher = a.gt(b);
max = FHE.select(isHigher, a, b);

Key Points to Remember​

  • All operations take place on encrypted data, so the actual values and comparison results stay concealed
  • Using traditional if...else on encrypted data might result in unexpected behavior and leak information
  • The select function is the only way to handle conditional execution in FHE without leaking information

Common Use Cases​

Here are some common scenarios where you'll use select:

  1. Maximum/Minimum Operations
euint32 max = FHE.select(a.gt(b), a, b);
  1. Conditional Updates
euint32 newValue = FHE.select(shouldUpdate, newValue, currentValue);
  1. Threshold Checks
ebool isAboveThreshold = value.gt(threshold);
euint32 result = FHE.select(isAboveThreshold, value, threshold);

Best Practices​

  1. Always use select instead of trying to implement branching logic
  2. Keep your conditional logic simple and linear
  3. Remember that all operations must be performed on encrypted data
  4. Consider the performance implications of complex conditional chains