![]() 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. How do server and the app exchange the secret? You probably already know that: using. ![]() How is the hash being transformed into that sequence? Result of HMAC-SHA-1 is a SHA-1 hash – 20 bytes long sequence of bytes.What really is the key? We’ve been using the password: $3cr3tP4$$ throughout, but itĬould be any binary sequence of bytes (not necessarily printable ASCII or even UTF).How does HOTP represent the counter? It’s not the ASCII string representing the number that.We take the keyĪnd counter, calculate HMAC-SHA-1, process the resulting hash into a (typically) 6-digit long code. We’ll tackle TOTP inĪll that I’ve explained so far is enough to understand all the details around HOTP. Here we’ll focus on the details of implementation of HMAC. In my previous post I explained the gist of the approach used That is predictable only to the server and the authenticator app. They obviously are different, but both are centered around the same basic idea: using a rolling hash value, HOTP ( HMAC-based One Time Password), and.Authenticator apps like Google Authenticator use 2 authenticaion protocol centered around What you have
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |