![]() The server and authenticator app are not communicating with one another. Clunkiness of keeping the Counter in sync Of TOTP flavour, I’ll describe how that works in details in the TOTP part. Thos QR Codes are structured almost identically in both, HOTP, and TOTP. They contain the key and some metadata about the account to which the 2nd factor relates. QR Codes in both HOTP, and TOTP are just printable ASCII strings encoded in a graphical image. Ok, our code integer ( 0x482edb0a) in decimal notation is 1211030282, so our code (last 6 But if it was 1, it would be setting it to 0 when The most significant bit (the far left one) is alreadyĠ, so we don’t have to do anything. In our case the most significant byte isĠx48. Omits this bit and takes only the remaining 31 bits. To avoid any potential confusion around that the algorithm simply The most significant bit is typically a sign indicator The 2nd twist is that on various architecture the most significant bit might be treated differentlyĪnd yield actually different integer values. Thus, in our example, the number would be 0x482edb0a. Index at which the 4-byte sequence starts_」 And that number is the offset within the hashįrom where we get the 4 bytes that get converted into the code. This number isĪlways between 0 and 15 ( 0x0 – 0xf). (2nd hex digit of the last byte: 0x83 in our example gives us 3). Namely, we’re looking at last 4 bits of the hash It is always taking 4 bytes, but from where exactly is actuallyĬalculated using super-simple approach. When you convert this to theĭecimal notation, you’ll get: 1357872921. That as a number (represented in hex: 0x5340a683). Take final 4 bytes from the HMAC-SHA-1 result (e.g.Those 6 digits are derived always from that HMAC-SHA-1 hash. ![]() We’re being asked to always enter a sequence of (usually 6) decimal digits. This is not the way it works in Google Authenticator, though. Ok, so far we’ve been pondering the idea of asking user to enter for example last 3 bytes It may also contain zero ( 0x00) bytes anywhere in the middle.Īfter the server generates the key, it shares with the app typically through a QR code. The key is just sequence of random bytes generated by the server when you’re adding 2ndĪuthentication factor. The range is from 0x0 all the way up to 0xFFFFFFFFFFFFFFFF. If you’re wondering how many possible values can the counter have, well, a lot. Thus, HOTP calls HMAC-SHA-1 with the key and the above array of bytes representing the So the number becomes the following array of bytes: That the most significant byte goes into the lowest address (the 1st element in the array). If you don’t know what Big-endian is, well, it’s nothing more than just ordering the bytes so ![]() Means that the table of bytes is padded with zeros up to 8 bytes overall. In addition to that, HOTP is using a large – 8-byte long – integer. The most significant byte is 03, the least significant For example, let’s take some largeĬounter value here: 53115673. HOTP went with keeping counter as a Big-Endian integer. ![]() Since my wife and I have already implementedĪnd we already have some example test data, I’ll explain this in part 3, where we’ll discuss This is common to both, HOTP, and TOTP and they work the same way.
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