user1878: I have several Yubikeys and would like to use one or more with MtGox.... however it seems that I can only use keys purchased from Mt Gox on their site.

Why did Mt Gox choose to implement security this way? 
Suppose I'm creating my own secure website based on Yubikey.. what are the conditions that I would choose to implement a "closed" system like this (and not allow other Yubikeys)?

I asked Mt Gox support this very question and all they told me was:

Hello, Thank you for the email. Please note that the yubikeys
  has to be pre-configured with our systems, so unfortunately it is not
  possible to add yubikeys that are purchased outside. Thanks, MtGox.com
  Team

... so I'm hoping to get a better understanding on why this choice was made.

user2748: The only problem with this is that someone at Mt. Gox could potentially know or discover the symmetric key for the YubiKey you purchased through them.  The keys loaded onto YubiKeys purchased directly from Yubico are neither available to Yubico employees nor to you, the user, and are stored securely with Yubico via their YubiHSM devices.  Mt. Gox may be using similar hardware to secure the keys for their YubiKeys, but we don't know how securely their YubiKeys are programmed or exactly how the keys are stored by Mt. Gox.  Without this knowledge, which we have with reasonable certainty in the case of YubiKeys straight from Yubico, we can assume that the YubiKeys sold my Mt. Gox are either as secure as those sold by Yubico or less secure.
My thought is that Mt. Gox insists on programming their branded YubiKeys themselves so that they do not depend on Yubico's YubiCloud service for their customers' ability to authenticate with their servers.  Right now, YubiCloud authentication is free, and Yubico offers a paid option that guarantees YubiCloud uptime and availability, but IMHO the SLA price is high (right now $3/YubiKey/year).  This way, Mt. Gox can take it upon themselves to guarantee availability of the YubiKey authentication service and not depend on Yubico at all.
I agree that it would be nice if we could use our own (non-MtGox-branded) YubiKeys with their service.  One of the problems with regular old passwords is that you have to have so many of them to make sure that the compromise of one doesn't affect all the others.  Strong two-factor authentication devices like YubiKeys were supposed to partially solve this problem, but now we're seeing an increasing number of services that require you to purchase a specially-branded YubiKey to work with their servers.  The resulting keychainfull of YubiKeys is arguably worse (and definitely more expensive) than a USB drive full of passwords.

user940: Properties of a yubikey:

A yubikey contains a secret symmetric key.
Knowledge of this key allows emulating that yubikey.
The server needs to know that key.

The YubiKey AES Key information can never be extracted from a YubiKey device – only programmed to it


So Mt.Gox's insistence on sending the key to you ensures several important properties:

You're not tempted to use the same key to authenticate to other websites, which would allow those websites to break into your Mt.Gox account
You don't need to send the key to the Mt.Gox server through your potentially insecure computer
Nobody but Mt.Gox knows the key stored in your yubikey

So pre-programming yubikeys and sending them to you only after the key was programmed into the device rules out several possible threats, and thus seems reasonable risk management.

1878: <p>I have several Yubikeys and would like to use one or more with MtGox.... however it seems that I can only use keys purchased from Mt Gox on their site.</p>

<ul>
<li><p>Why did Mt Gox choose to implement security this way? </p></li>
<li><p>Suppose I'm creating my own secure website based on Yubikey.. what are the conditions that I would choose to implement a "closed" system like this (and not allow other Yubikeys)?</p></li>
</ul>

<p>I asked Mt Gox support this very question and all they told me was:</p>

<blockquote>
  <p>Hello, Thank you for the email. Please note that the yubikeys
  has to be pre-configured with our systems, so unfortunately it is not
  possible to add yubikeys that are purchased outside. Thanks, MtGox.com
  Team</p>
</blockquote>

<p>... so I'm hoping to get a better understanding on <em>why</em> this choice was made.</p>
 2748: <p>The only problem with this is that someone at Mt. Gox could potentially know or discover the symmetric key for the YubiKey you purchased through them.  The keys loaded onto YubiKeys purchased directly from Yubico are neither available to Yubico employees nor to you, the user, and are stored securely with Yubico via their YubiHSM devices.  Mt. Gox may be using similar hardware to secure the keys for their YubiKeys, but we don't know how securely their YubiKeys are programmed or exactly how the keys are stored by Mt. Gox.  Without this knowledge, which we have with reasonable certainty in the case of YubiKeys straight from Yubico, we can assume that the YubiKeys sold my Mt. Gox are either as secure as those sold by Yubico or less secure.</p>

<p>My thought is that Mt. Gox insists on programming their branded YubiKeys themselves so that they do not depend on Yubico's YubiCloud service for their customers' ability to authenticate with their servers.  Right now, YubiCloud authentication is free, and Yubico offers a paid option that guarantees YubiCloud uptime and availability, but IMHO the SLA price is high (right now $3/YubiKey/year).  This way, Mt. Gox can take it upon themselves to guarantee availability of the YubiKey authentication service and not depend on Yubico at all.</p>

<p>I agree that it would be nice if we could use our own (non-MtGox-branded) YubiKeys with their service.  One of the problems with regular old passwords is that you have to have so many of them to make sure that the compromise of one doesn't affect all the others.  Strong two-factor authentication devices like YubiKeys were supposed to partially solve this problem, but now we're seeing an increasing number of services that require you to purchase a specially-branded YubiKey to work with their servers.  The resulting keychainfull of YubiKeys is arguably worse (and definitely more expensive) than a USB drive full of passwords.</p>
 940: <p>Properties of a yubikey:</p>

<ul>
<li>A yubikey contains a secret symmetric key.</li>
<li>Knowledge of this key allows emulating that yubikey.</li>
<li><p>The server needs to know that key.</p></li>
<li><blockquote>
  <p>The YubiKey AES Key information can never be extracted from a YubiKey device – only programmed to it</p>
</blockquote></li>
</ul>

<p>So Mt.Gox's insistence on sending the key to you ensures several important properties:</p>

<ul>
<li>You're not tempted to use the same key to authenticate to other websites, which would allow those websites to break into your Mt.Gox account</li>
<li>You don't need to send the key to the Mt.Gox server through your potentially insecure computer</li>
<li>Nobody but Mt.Gox knows the key stored in your yubikey</li>
</ul>

<p>So pre-programming yubikeys and sending them to you only after the key was programmed into the device rules out several possible threats, and thus seems reasonable risk management.</p>
