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    <p>Symmetric algorithms are limited, in that any novice cracker with a disassembler can find your key (or the algorithm used to generate one) and make a "keygen".</p> <p>For this reason, asymmetric cryptology is the way to go. The basic premise is something like this:</p> <ul> <li>When the user purchases a license from you, you collect certain identifying details about the user and/or their environment (typically, this is just a full name; sometimes a company, too).</li> <li>You make a 128-bit MD5 hash of this information.</li> <li>Using a 128-bit <a href="http://en.wikipedia.org/wiki/Elliptic_curve_cryptography" rel="nofollow noreferrer">Elliptic Curve</a> crypto, encrypt this hash using the <em>private</em> key on the server.</li> <li>The 128-bit cipher text can be represented to the user as a 25-character string consisting of letters and digits (plus separating dashes for readability). Notice that 26 letters + 10 digits = 36 discrete values, and that 36^25 > 2^128.</li> <li>The user types this product key into your registration dialog. The client software converts it back to a 128-bit number (16 bytes), decrypts that using the public key of your EC crypto, and compares the result to an MD5 hash of the user's personal information, which must match what was used for registration.</li> </ul> <p>This is just the basic idea, of course. For more details and source code, see <a href="http://www.codeguru.com/cpp/cpp/algorithms/general/article.php/c12799/Product-Keys-Based-on-Elliptic-Curve-Cryptography.htm" rel="nofollow noreferrer">Product Keys Based on Elliptic Curve Cryptography</a>.</p>
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    1. COthanks a lot for the infos. Suppose that I cannot use EEC for some reason....RSA wouldn't lend itself for the 25-character key method, because RSA keys have a min length of 1024 bits, right?
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    2. CORSA *can* be only 384 bits, but that's still too long for a user-entered key. Even if you differentiated upper- and lower-case (yielding 62 discreet characters), you'd have to have a 65-length key. And RSA at 384 bits is *very* weak, so your private key would likely be compromised anyway.
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    3. COI see a free implementation of ECC here: http://www.codeproject.com/KB/security/Elliptic_Curves.aspx. I haven't checked to see how good it is. A little digging would surely turn up others. Here's one that's pretty cheap: http://ellipter.com/.
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