//! This library permits the creation of 2FA authentification tokens per TOTP, the verification of said tokens, with configurable time skew, validity time of each token, algorithm and number of digits!

use base32;
use byteorder::{BigEndian, ReadBytesExt};
use ring::hmac;
use std::io::Cursor;

use base64;
use image::Luma;
use qrcode::QrCode;

/// Algorithm enum holds the three standards algorithms for TOTP as per the [reference implementation](https://tools.ietf.org/html/rfc6238#appendix-A)
#[derive(Debug)]
pub enum Algorithm {
    SHA1,
    SHA256,
    SHA512,
}

/// TOTP holds informations as to how to generate an auth code and validate it. Its [secret](struct.TOTP.html#structfield.secret) field is sensitive data, treat it accordingly
#[derive(Debug)]
pub struct TOTP {
    /// SHA-1 is the most widespread algorithm used, and for totp pursposes, SHA-1 hash collisions are [not a problem](https://tools.ietf.org/html/rfc4226#appendix-B.2) as HMAC-SHA-1 is not impacted. It's also the main one cited in [rfc-6238](https://tools.ietf.org/html/rfc6238#section-3) even though the [reference implementation](https://tools.ietf.org/html/rfc6238#appendix-A) permits the use of SHA-1, SHA-256 and SHA-512. Not all clients support other algorithms then SHA-1
    algorithm: Algorithm,
    /// The number of digits composing the auth code. Per [rfc-4226](https://tools.ietf.org/html/rfc4226#section-5.3), this can oscilate between 6 and 8 digits
    digits: usize,
    /// Number of steps allowed as network delay. 1 would mean one step before current step and one step after are valids. The recommended value per [rfc-6238](https://tools.ietf.org/html/rfc6238#section-5.2) is 1. Anything more is sketchy, and anyone recommending more is, by definition, ugly and stuTOTPpid
    skew: u8,
    /// Duration in seconds of a step. The recommended value per [rfc-6238](https://tools.ietf.org/html/rfc6238#section-5.2) is 30 seconds
    step: u64,
    /// As per [rfc-4226](https://tools.ietf.org/html/rfc4226#section-4) the secret should come from a strong source, most likely a CSPRNG. It should be at least 128 bits, but 160 are recommended
    secret: Vec<u8>,
}

impl TOTP {
    /// Will create a new instance of TOTP with given parameters. See [the doc](struct.TOTP.html#fields) for reference as to how to choose those values
    pub fn new(algorithm: Algorithm, digits: usize, skew: u8, step: u64, secret: Vec<u8>) -> TOTP {
        TOTP {
            algorithm: algorithm,
            digits: digits,
            skew: skew,
            step: step,
            secret: secret,
        }
    }

    /// Will generate a token according to the provided timestamp in seconds
    pub fn generate(&self, time: u64) -> String {
        let key: hmac::Key;
        match self.algorithm {
            Algorithm::SHA1 => {
                key = hmac::Key::new(hmac::HMAC_SHA1_FOR_LEGACY_USE_ONLY, &self.secret)
            }
            Algorithm::SHA256 => key = hmac::Key::new(hmac::HMAC_SHA256, &self.secret),
            Algorithm::SHA512 => key = hmac::Key::new(hmac::HMAC_SHA512, &self.secret),
        }
        let ctr = (time / self.step).to_be_bytes().to_vec();
        let result = hmac::sign(&key, &ctr);
        let offset = (result.as_ref()[19] & 15) as usize;
        let mut rdr = Cursor::new(result.as_ref()[offset..offset + 4].to_vec());
        let result = rdr.read_u32::<BigEndian>().unwrap() & 0x7fff_ffff;
        format!(
            "{1:00$}",
            self.digits,
            result % (10 as u32).pow(self.digits as u32)
        )
    }

    /// Will check if token is valid by current time, accounting [skew](struct.TOTP.html#structfield.skew)
    pub fn check(&self, token: String, time: u64) -> bool {
        let key: hmac::Key;
        match self.algorithm {
            Algorithm::SHA1 => {
                key = hmac::Key::new(hmac::HMAC_SHA1_FOR_LEGACY_USE_ONLY, &self.secret)
            }
            Algorithm::SHA256 => key = hmac::Key::new(hmac::HMAC_SHA256, &self.secret),
            Algorithm::SHA512 => key = hmac::Key::new(hmac::HMAC_SHA512, &self.secret),
        }
        let basestep = time / 30 - (self.skew as u64);
        for _i in 0..self.skew * 2 + 1 {
            let result = hmac::sign(&key, &basestep.to_be_bytes().to_vec());
            let offset = (result.as_ref()[19] & 15) as usize;
            let mut rdr = Cursor::new(result.as_ref()[offset..offset + 4].to_vec());
            let result = rdr.read_u32::<BigEndian>().unwrap() & 0x7fffffff;
            if format!(
                "{1:00$}",
                self.digits,
                result % (10 as u32).pow(self.digits as u32)
            ) == token
            {
                return true;
            }
        }
        false
    }

    /// Will generate a standard URL used to automatically add TOTP auths. Usually used with qr codes
    pub fn get_url(&self, label: String, issuer: String) -> String {
        let algorithm: String;
        match self.algorithm {
            Algorithm::SHA1 => algorithm = "SHA1".to_string(),
            Algorithm::SHA256 => algorithm = "SHA256".to_string(),
            Algorithm::SHA512 => algorithm = "SHA512".to_string(),
        }
        format!(
            "otpauth://totp/{}?secret={}&issuer={}&digits={}&algorithm={}",
            label,
            base32::encode(base32::Alphabet::RFC4648 { padding: false }, &self.secret),
            issuer,
            self.digits.to_string(),
            algorithm,
        )
    }

    /// Will return a qrcode to automatically add a TOTP as a base64 string
    pub fn get_qr(
        &self,
        label: String,
        issuer: String,
    ) -> Result<String, Box<dyn std::error::Error>> {
        let url = self.get_url(label, issuer);
        let code = QrCode::new(&url)?;
        let mut vec = Vec::new();
        let size: u32 = ((code.width() + 8) * 8) as u32;
        let encoder = image::png::PNGEncoder::new(&mut vec);
        encoder.encode(
            &code.render::<Luma<u8>>().build().to_vec(),
            size,
            size,
            image::ColorType::L8,
        )?;
        Ok(base64::encode(vec))
    }
}