ネット通販・フリマアプリ・家計管理の安全ガイド：
偽ショッピングサイトの見分け方と「家庭用Wi-Fi・子どもの課金」対策

#![forbid(unsafe_code)]

/// Telemetry extracted from an E-Commerce storefront before entering payment credentials.
#[derive(Debug, Clone, Copy)]
pub struct StorefrontTelemetry<'a> {
    pub sld_label: &'a [u8],           // Second-level domain ASCII bytes (e.g., b"x9k2-sale-jp")
    pub ct_log_first_seen_days: u32,   // Days since first RFC 6962 SCT issuance
    pub advertised_discount_pct: u8,   // Advertised discount percentage (0..=99)
    pub personal_bank_transfer_only: bool,
    pub redirects_to_external_chat_pay: bool,
    pub supports_emv_3ds2: bool,
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum StorefrontVerdict {
    TrustedMerchant { risk_score: u8 },
    CautionVerifyAddress { risk_score: u8, reason: &'static str },
    BlockFakeStorefront { risk_score: u8, reason: &'static str },
}

/// Computes Shannon entropy H(X) in bits per byte for an ASCII domain label.
pub fn compute_label_entropy(label: &[u8]) -> f64 {
    if label.is_empty() {
        return 0.0;
    }
    let mut counts = [0u32; 256];
    for &b in label {
        counts[b as usize] += 1;
    }
    let len = label.len() as f64;
    let mut entropy = 0.0;
    for &c in &counts {
        if c > 0 {
            let p = (c as f64) / len;
            entropy -= p * p.log2();
        }
    }
    entropy
}

/// Evaluates whether an online shop exhibits disposable fake-EC characteristics.
pub fn inspect_storefront_safety(t: &StorefrontTelemetry<'_>) -> StorefrontVerdict {
    if t.personal_bank_transfer_only || t.redirects_to_external_chat_pay {
        return StorefrontVerdict::BlockFakeStorefront {
            risk_score: 100,
            reason: "Personal bank account transfer or off-platform chat payment demand detected.",
        };
    }

    let mut risk: u8 = 0;

    // Newly registered domain (NRD) check via Certificate Transparency log age
    if t.ct_log_first_seen_days < 14 {
        risk = risk.saturating_add(45);
    } else if t.ct_log_first_seen_days < 45 {
        risk = risk.saturating_add(25);
    }

    // High discount bait check
    if t.advertised_discount_pct >= 70 {
        risk = risk.saturating_add(35);
    }

    // DGA-like random domain label entropy check
    let entropy = compute_label_entropy(t.sld_label);
    if entropy > 3.55 && t.sld_label.len() >= 10 {
        risk = risk.saturating_add(25);
    }

    if !t.supports_emv_3ds2 {
        risk = risk.saturating_add(15);
    }

    if risk >= 75 {
        StorefrontVerdict::BlockFakeStorefront {
            risk_score: risk,
            reason: "Newly registered high-entropy domain with extreme discount bait.",
        }
    } else if risk >= 40 {
        StorefrontVerdict::CautionVerifyAddress {
            risk_score: risk,
            reason: "Verify corporate registration and avoid entering primary card details.",
        }
    } else {
        StorefrontVerdict::TrustedMerchant { risk_score: risk }
    }
}