happy-cracking 0.3.0

A fast, comprehensive CTF toolkit for cryptographic encoding/decoding, classic ciphers, hash operations, and analysis tools
Documentation 
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use anyhow::Result;
use clap::Subcommand;

#[derive(Subcommand)]
pub enum EntropyAction {
    #[command(about = "Calculate Shannon entropy of input")]
    Analyze {
        #[arg(help = "Input text")]
        input: String,
    },
}

pub fn run(action: EntropyAction) -> Result<()> {
    match action {
        EntropyAction::Analyze { input } => {
            let entropy = calculate(&input);
            let classification = classify(entropy);
            println!("Entropy: {:.4} bits/byte", entropy);
            println!("Classification: {}", classification);
        }
    }
    Ok(())
}

// Calculate Shannon entropy in bits per byte.
// H = -Σ p(x) * log2(p(x)), maximum is 8.0 bits/byte.
pub fn calculate(input: &str) -> f64 {
    if input.is_empty() {
        return 0.0;
    }

    let bytes = input.as_bytes();
    let len = bytes.len() as f64;

    // Optimization: Use array instead of HashMap for pure byte counting
    // Avoids hashing overhead and memory allocation
    let mut counts = [0usize; 256];
    for &b in bytes {
        counts[b as usize] += 1;
    }

    let mut entropy = 0.0;
    for &count in counts.iter() {
        if count > 0 {
            let p = count as f64 / len;
            entropy -= p * p.log2();
        }
    }

    entropy
}

// Classify the entropy value into a human-readable category.
pub fn classify(entropy: f64) -> &'static str {
    if entropy >= 7.5 {
        "High (likely encrypted/compressed)"
    } else if entropy >= 4.0 {
        "Medium (natural language/code)"
    } else {
        "Low (low entropy data)"
    }
}