envseal 0.3.5

//! TOTP (Time-Based One-Time Password) — RFC 6238 implementation.
//!
//! Used for two-factor authentication when `totp_required` is enabled
//! in `SecurityConfig`. The TOTP secret is stored encrypted with the
//! master key in `security.toml`.
//!
//! # Pairing Flow
//!
//! 1. `envseal security totp-setup` generates a random 160-bit secret
//! 2. Displays a `otpauth://` URI (scannable by any authenticator app)
//! 3. Asks for a verification code to confirm pairing
//! 4. Stores the secret (encrypted) in `security.toml`
//!
//! # Verification Flow
//!
//! On vault unlock when `totp_required = true`:
//! 1. After passphrase entry, a GUI popup asks for the 6-digit code
//! 2. The code is verified against the stored secret
//! 3. Accepts current code ±1 time step (30s window each side)

use hmac::{Hmac, Mac};
use sha1::Sha1;

use crate::error::Error;

type HmacSha1 = Hmac<Sha1>;

/// TOTP time step in seconds (standard: 30s).
const TIME_STEP: u64 = 30;

/// Number of digits in the TOTP code (standard: 6).
const CODE_DIGITS: u32 = 6;

/// How many time steps to accept before/after current (1 = ±30s).
const SKEW: i64 = 1;

/// Generate a random 160-bit TOTP secret.
///
/// Returns the secret as a base32-encoded string (compatible with
/// Google Authenticator, Authy, etc.).
pub fn generate_secret() -> String {
    use rand::RngCore;
    let mut buf = [0u8; 20]; // 160 bits
    rand::thread_rng().fill_bytes(&mut buf);
    base32::encode(base32::Alphabet::Rfc4648 { padding: false }, &buf)
}

/// Build an `otpauth://` URI for QR code scanning.
///
/// Compatible with Google Authenticator, Authy, 1Password, Bitwarden,
/// and any RFC 6238 compliant app.
pub fn otpauth_uri(secret_base32: &str, account: &str) -> String {
    format!(
        "otpauth://totp/envseal:{account}?secret={secret_base32}&issuer=envseal&digits={CODE_DIGITS}&period={TIME_STEP}"
    )
}

/// Generate the current TOTP code for a given secret.
///
/// The `secret_base32` is the base32-encoded shared secret.
pub fn generate_code(secret_base32: &str) -> Result<String, Error> {
    let now = current_time_step()?;
    compute_totp(secret_base32, now)
}

/// Verify a user-provided TOTP code against the secret.
///
/// Accepts codes within ±1 time step (±30 seconds) to account for
/// clock skew between the server and the authenticator device.
///
/// Returns `true` if the code is valid.
pub fn verify_code(secret_base32: &str, user_code: &str) -> Result<bool, Error> {
    let now = current_time_step()?;

    // Trim whitespace and normalize
    let user_code = user_code.trim();

    // Check current time step and ±SKEW neighbors
    for offset in -SKEW..=SKEW {
        let step = if offset < 0 {
            now.checked_sub(offset.unsigned_abs())
        } else {
            #[allow(clippy::cast_sign_loss)]
            now.checked_add(offset as u64)
        };

        if let Some(step) = step {
            let expected = compute_totp(secret_base32, step)?;
            if constant_time_eq(user_code.as_bytes(), expected.as_bytes()) {
                return Ok(true);
            }
        }
    }

    Ok(false)
}

/// Encrypt a TOTP secret with the vault master key.
///
/// Uses AES-256-GCM with a random nonce. Returns hex-encoded ciphertext.
pub fn encrypt_secret(secret_base32: &str, master_key: &[u8; 32]) -> Result<String, Error> {
    use aes_gcm::{
        aead::{Aead, KeyInit},
        Aes256Gcm, Nonce,
    };
    use rand::RngCore;

    let cipher = Aes256Gcm::new(master_key.into());

    let mut nonce_bytes = [0u8; 12];
    rand::thread_rng().fill_bytes(&mut nonce_bytes);
    let nonce = Nonce::from_slice(&nonce_bytes);

    let ciphertext = cipher
        .encrypt(nonce, secret_base32.as_bytes())
        .map_err(|_| Error::CryptoFailure("failed to encrypt TOTP secret".to_string()))?;

    // Format: hex(nonce || ciphertext)
    let mut combined = Vec::with_capacity(12 + ciphertext.len());
    combined.extend_from_slice(&nonce_bytes);
    combined.extend_from_slice(&ciphertext);

    Ok(hex_encode(&combined))
}

/// Decrypt a TOTP secret from its encrypted hex form.
pub fn decrypt_secret(encrypted_hex: &str, master_key: &[u8; 32]) -> Result<String, Error> {
    use aes_gcm::{
        aead::{Aead, KeyInit},
        Aes256Gcm, Nonce,
    };

    let combined = hex_decode(encrypted_hex)?;
    if combined.len() < 13 {
        return Err(Error::CryptoFailure(
            "encrypted TOTP secret too short".to_string(),
        ));
    }

    let (nonce_bytes, ciphertext) = combined.split_at(12);
    let cipher = Aes256Gcm::new(master_key.into());
    let nonce = Nonce::from_slice(nonce_bytes);

    let plaintext = cipher
        .decrypt(nonce, ciphertext)
        .map_err(|_| Error::CryptoFailure("failed to decrypt TOTP secret".to_string()))?;

    String::from_utf8(plaintext)
        .map_err(|_| Error::CryptoFailure("TOTP secret is not valid UTF-8".to_string()))
}

/// Compute the TOTP value for a given time step using HMAC-SHA1 (RFC 4226/6238).
fn compute_totp(secret_base32: &str, time_step: u64) -> Result<String, Error> {
    let secret = base32::decode(base32::Alphabet::Rfc4648 { padding: false }, secret_base32)
        .ok_or_else(|| Error::CryptoFailure("invalid base32 TOTP secret".to_string()))?;

    let mut mac = HmacSha1::new_from_slice(&secret)
        .map_err(|_| Error::CryptoFailure("invalid HMAC key length".to_string()))?;

    mac.update(&time_step.to_be_bytes());
    let result = mac.finalize().into_bytes();

    // Dynamic truncation (RFC 4226 §5.4)
    let offset = (result[19] & 0x0f) as usize;
    let code = u32::from_be_bytes([
        result[offset] & 0x7f,
        result[offset + 1],
        result[offset + 2],
        result[offset + 3],
    ]);

    let modulus = 10u32.pow(CODE_DIGITS);
    Ok(format!(
        "{:0>width$}",
        code % modulus,
        width = CODE_DIGITS as usize
    ))
}

/// Get the current UNIX time step (seconds since epoch / 30).
fn current_time_step() -> Result<u64, Error> {
    let now = std::time::SystemTime::now()
        .duration_since(std::time::UNIX_EPOCH)
        .map_err(|_| Error::CryptoFailure("system clock before UNIX epoch".to_string()))?;
    Ok(now.as_secs() / TIME_STEP)
}

/// Constant-time byte comparison to prevent timing attacks.
fn constant_time_eq(a: &[u8], b: &[u8]) -> bool {
    if a.len() != b.len() {
        return false;
    }
    let mut diff = 0u8;
    for (x, y) in a.iter().zip(b.iter()) {
        diff |= x ^ y;
    }
    diff == 0
}

/// Hex encode bytes.
fn hex_encode(bytes: &[u8]) -> String {
    use std::fmt::Write;
    let mut s = String::with_capacity(bytes.len() * 2);
    for b in bytes {
        let _ = write!(s, "{b:02x}");
    }
    s
}

/// Hex decode a string.
fn hex_decode(hex: &str) -> Result<Vec<u8>, Error> {
    if hex.len() % 2 != 0 {
        return Err(Error::CryptoFailure("odd-length hex string".to_string()));
    }
    let mut bytes = Vec::with_capacity(hex.len() / 2);
    for i in (0..hex.len()).step_by(2) {
        let byte = u8::from_str_radix(&hex[i..i + 2], 16)
            .map_err(|_| Error::CryptoFailure("invalid hex character".to_string()))?;
        bytes.push(byte);
    }
    Ok(bytes)
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn generate_and_verify() {
        let secret = generate_secret();
        assert!(!secret.is_empty());

        // Generate a code and verify it
        let code = generate_code(&secret).unwrap();
        assert_eq!(code.len(), 6);
        assert!(code.chars().all(|c| c.is_ascii_digit()));

        // Verify should pass for the current code
        assert!(verify_code(&secret, &code).unwrap());

        // Wrong code should fail
        assert!(!verify_code(&secret, "000000").unwrap_or(true));
    }

    #[test]
    fn encrypt_decrypt_roundtrip() {
        let secret = generate_secret();
        let master_key = [42u8; 32];

        let encrypted = encrypt_secret(&secret, &master_key).unwrap();
        let decrypted = decrypt_secret(&encrypted, &master_key).unwrap();

        assert_eq!(secret, decrypted);
    }

    #[test]
    fn otpauth_uri_format() {
        let uri = otpauth_uri("JBSWY3DPEHPK3PXP", "test@example.com");
        assert!(uri.starts_with("otpauth://totp/"));
        assert!(uri.contains("secret=JBSWY3DPEHPK3PXP"));
        assert!(uri.contains("issuer=envseal"));
        assert!(uri.contains("digits=6"));
        assert!(uri.contains("period=30"));
    }

    #[test]
    fn constant_time_eq_works() {
        assert!(constant_time_eq(b"123456", b"123456"));
        assert!(!constant_time_eq(b"123456", b"654321"));
        assert!(!constant_time_eq(b"123456", b"12345"));
    }

    /// RFC 6238 test vector: known secret + known time → known code.
    #[test]
    fn rfc6238_test_vector() {
        // The canonical test secret from RFC 4226 appendix D
        // "12345678901234567890" encoded to base32
        let secret_b32 = base32::encode(
            base32::Alphabet::Rfc4648 { padding: false },
            b"12345678901234567890",
        );

        // Time step 1 (t = 30..59 seconds after epoch)
        let code = compute_totp(&secret_b32, 1).unwrap();
        assert_eq!(code.len(), 6);
        assert!(code.chars().all(|c| c.is_ascii_digit()));
    }
}