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    <p>HMAC is a construct for turning a hash function, like SHA1, into a <a href="http://en.wikipedia.org/wiki/Message_authentication_code" rel="nofollow noreferrer">Message Authentication Code</a> (MAC). </p> <p>Normal hash functions don't have any secret data associated with it. This means that anyone can compute the digest, assuming they have the original input. HMAC uses a secret key, so that only those in possession of the key can compute outputs. </p> <p>Suppose I have a file, file.txt. I want to send this to you, and we need to make sure nobody tampers with it. Sorry, I have no clever way to represent this with just text.</p> <pre><code>me -&gt; file.txt -&gt; you me -&gt; SHA1(file.txt) -&gt; you </code></pre> <p>Then you verify the result by computing your own SHA1 digest, and verifying it matches what I sent you. </p> <p>Now suppose an attacker was in the middle. Unfortunately, because there is no secret involved, the attacker can modify the file, and compute his own file/digest pair. When you compute your version, it'll match what he sent you, and you'll be none the wiser. </p> <pre><code>me -&gt; file.txt -&gt; attacker -&gt; modified.txt -&gt; you me -&gt; SHA1(file.txt) -&gt; attacker -&gt; SHA1(modified.txt) -&gt; you </code></pre> <p>With HMAC, we add a secret key to the computation. </p> <pre><code>me -&gt; file.txt -&gt; you me -&gt; SHA1_HMAC(file.txt, our_secret) -&gt; you </code></pre> <p>When you compute your version, you apply the secret key as well, and the result matches. The attacker, without knowledge of the key, can't replace the digest. </p> <pre><code>me -&gt; file.txt -&gt; attacker -&gt; modified.txt -&gt; you me -&gt; SHA1(file.txt) -&gt; attacker -&gt; SHA1_HMAC(modified.txt, // DOESN'T KNOW KEY) -&gt; you </code></pre> <p>HMAC is a very specific way of adding the secret key. Unfortunately, simple methods of just concatenating a key to the end of the file, or pre-pending it before hashing, are vulnerable to different attacks (length extension attacks, for example). </p> <p>The B64 is Base64 encoding the output, to make it pretty. </p> <p>What this code is ultimately doing is taking some input, and some secret key, and computing a 160-bit digest, and base64 encoding the result. </p> <p>There is an implementation of <a href="http://msdn.microsoft.com/en-us/library/system.security.cryptography.hmacsha1.aspx" rel="nofollow noreferrer">SHA1 HMAC in .NET</a></p> <p><a href="https://stackoverflow.com/questions/169907/how-do-i-base64-encode-a-string-efficiently-using-excel-vba">This</a> looks like an implementation of Base64 for VBA</p> <p>I hope this answers it well enough, or clear enough. If the text is confusing, please let me know. I tried a couple routes of how to express it, and none of them seemed that clear. </p>
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    1. POBase64 HMAC SHA1 String in VBA
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    1. COThanks for your help. I wrote my functions and posted them in my question under "Edit1".
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    2. COGlad it helped. Is everything working? The order looks correct, as best I can tell, though I've never written VB or VBA.
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    3. COI guess I don't know for sure if it's working correctly or not. I'll post an example.
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