authenticator-rs 0.1.0

The rust implementation for google authenticator with 2fa authentication
Documentation 
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use std::fmt::Error;
use std::result;
use std::time::{SystemTime, UNIX_EPOCH};
use ring::hmac;
use ring::hmac::{Algorithm, HMAC_SHA1_FOR_LEGACY_USE_ONLY, Key};

/// the base32 charset using generate secret
const ALPHABET: [char; 32] = [
    'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S',
    'T', 'U', 'V', 'W', 'X', 'Y', 'Z', '2', '3', '4', '5', '6', '7',
];

/// The Max pin code length
const MAX_PINCODE_LENGTH: u16 = 9;

const SECRET_MAX_LEN: usize = 128;
const SECRET_MIN_LEN: usize = 16;

const DIGITS_POWER: [i32; 10] = [1, 10, 100, 1000, 10000, 100000, 1000000, 10000000, 100000000, 1000000000];

pub trait Signer {
    fn sign(&self, data: &[u8]) -> Vec<u8>;
}

impl Signer for Key {
    fn sign(&self, data: &[u8]) -> Vec<u8> {
        let mut output = Vec::new();
        let tag = hmac::sign(&self, data);
        output = tag.as_ref().to_vec();
        output
    }
}

pub fn current_pin_code(secret: &str, code_len: u16) -> Result<String, String> {
    let otp_state = SystemTime::now().duration_since(UNIX_EPOCH).unwrap().as_secs() / 30;
    if secret.len() < SECRET_MIN_LEN || secret.len() > SECRET_MAX_LEN {
        return Err(String::from("Bad secret length. must be less than 128 and more than 16, recommend 32"));
    }
    let result = base32_decode(secret);
    match result {
        Ok(key) => {
            let authenticator = Authenticator::new(get_signer(key.as_slice()), code_len)?;
            Ok(authenticator.generate_response_code(otp_state)?)
        }
        Err(e) => { Err(e) }
    }
}

fn get_signer(secret_key: &[u8]) -> Box<dyn Signer> {
    Box::new(hmac::Key::new(HMAC_SHA1_FOR_LEGACY_USE_ONLY, secret_key))
}

fn base32_decode(secret: &str) -> Result<Vec<u8>, String> {
    match base32::decode(base32::Alphabet::RFC4648 { padding: true }, secret) {
        Some(_decode_str) => Ok(_decode_str),
        _ => Err(String::from("secret must be base32 decodeable.")),
    }
}

pub struct Authenticator {
    code_len: u16,
    signer: Box<dyn Signer>,
}

impl Authenticator {
    pub fn new(signer: Box<dyn Signer>, code_len: u16) -> Result<Self, String> {
        if code_len <= 0 || code_len > MAX_PINCODE_LENGTH {
            return Err(format!("PinCode Length must be between 1 and {} digits", MAX_PINCODE_LENGTH));
        }
        Ok(Authenticator { code_len, signer })
    }


    pub fn generate_response_code(&self, otp_state: u64) -> Result<String, String> {
        let msg_bytes = otp_state.to_be_bytes();
        let hash = &self.signer.sign(&msg_bytes)[..];
        let offset = hash[hash.len() - 1] & 0x0F;
        let mut truncated_hash: [u8; 4] = Default::default();
        truncated_hash.copy_from_slice(&hash[offset as usize..(offset + 4) as usize]);
        let mut code = i32::from_be_bytes(truncated_hash);
        code &= 0x7FFFFFFF;
        code %= DIGITS_POWER[self.code_len as usize];
        let mut code_str = code.to_string();
        for i in 0..(self.code_len as usize - code_str.len()) {
            code_str.insert(i, '0');
        }
        Ok(code_str)
    }
}