TOTP/HOTP implementation for modern browsers (RFC 4226/6238)

otp.js

/** Compute a HOTP/TOTP for a base32 secret. */
export async function computeOTP(secret, digits = 6, counter = Math.floor(Date.now() / (30 * 1000))) {
    // parse the secret as base32, stripping whitespace and padding
    const parsedSecret = Uint8Array.from(Array
        .from(secret.replace(/[\s=]/g, "").toUpperCase())
        .reduce((state, c) => {
            const n = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567".indexOf(c)
            if (n < 0) throw new Error("Invalid base32")
            state.as = (state.as << 5) | n
            state.in += 5
            while (state.in > 7) {
                state.buf.push(state.as >> (state.in -= 8))
                state.as &= ~(0xff << state.in)
            }
            return state
        }, {buf: [], in: 0, as: 0}).buf
    ).buffer

    // get last 8 bytes (left padded with zeroes)
    const moving = Uint8Array.from(new Array(8)
        .fill(counter.toString(2).slice(-8*8).padStart(8*8, "0"))
        .map((s, i) => parseInt(s.slice(i*8, i*8+8), 2))
    ).buffer

    // sign it with the secret
    const key = await crypto.subtle.importKey(
        "raw", parsedSecret,
        { name: "HMAC", hash: "SHA-1" },
        false, ["sign"],
    )
    const hmac = new Uint8Array(await crypto.subtle.sign("HMAC", key, moving))

    // HOTP dynamic truncation
    const arr = hmac.subarray(hmac[hmac.length - 1] & 0xF)
    const truncated = (
        ((arr[0] & 0x7F) << 24) |
        ((arr[1] & 0xFF) << 16) |
        ((arr[2] & 0xFF) << 8) |
        ((arr[3] & 0xFF))
    )

    // HOTP code generation
    return (truncated % 10**digits).toString().slice(-digits).padStart(digits, "0")
}

otp.min.js

export async function computeOTP(d,b=6,e=Math.floor(Date.now()/3e4)){let f=Uint8Array.from(Array.from(d.replace(/[\s=]/g,"").toUpperCase()).reduce((a,c)=>{let b="ABCDEFGHIJKLMNOPQRSTUVWXYZ234567".indexOf(c);if(b<0)throw new Error("Invalid base32");for(a.as=a.as<<5|b,a.in+=5;a.in>7;)a.buf.push(a.as>>(a.in-=8)),a.as&=~(255<<a.in);return a},{buf:[],in:0,as:0}).buf).buffer,g=Uint8Array.from(new Array(8).fill(e.toString(2).slice(-64).padStart(64,"0")).map((b,a)=>parseInt(b.slice(8*a,8*a+8),2))).buffer,h=await crypto.subtle.importKey("raw",f,{name:"HMAC",hash:"SHA-1"},!1,["sign"]),c=new Uint8Array(await crypto.subtle.sign("HMAC",h,g)),a=c.subarray(15&c[c.length-1]),i=(127&a[0])<<24|(255&a[1])<<16|(255&a[2])<<8|255&a[3];return(i%10**b).toString().slice(-b).padStart(b,"0")}