0. The message to decode :

K Q O W E F V J P U J U U N U K G L M E K J I
N M W U X F Q M K J B G W R L F N F G H U D W
U U M B S V L P S N C M U E K Q C T E S W R E
E K O Y S S I W C T U A X Y O T A P X P L W P
...
The complete message can be found here.

1. Finding the key length:

Clear : T H I S I S A C L E A R T E X T T H I S C A N B E E A S L Y D E C
Key :   C O D E C O D E C O D E C O D E C O D E C O D E C O D E C O D E C
Crypt : V V L W K G D G N S D V V S A X V V L W E O Q F G S D W N M G I E

Update :

No they are not ;), You can use Heathcliff74's tool from XDA here to gain root access to the Registry and change whatever you want

Most of the WP7 users who unlocked their devices using the ChevronWP7 tool have applied a registry hack to prevent their devices from relocking again, something that I applied myself, but had never thought it would unlock the device forever (till now at least).

For HTC users, they can use provxml to do this, and the Samsung users have the ability to do the same with a little app that can be found here.

The hack is pretty simple actually, it just clears the URLs the phone uses to ring back Microsoft and asks if it should be unlocked or not, there URLs can be found in the Registry at this location :
By setting these two values to null (a void string), the phone will never be able to synchronize with Microsoft servers and hence won’t relock again.

Now if you want  to relock your phone again for whatever reason, with HTC device it is just applying a provxml that will revert the process and insert the URLs back, but on Samsung devices it is not that easy.

Writing on Samsung registry (take a look at my previous post about this) is done using the CHybridClass_FCRProxy class that comes with the native Samsung DLL FCRouterProxy.dll (this DLL has access to some privileged Registry keys).

To write a value on the registry, you basically call something like this :

Strangely, the native Samsung methods don’t allow writing values that are more than 30 characters length, which makes it impossible to write back the original URLs that point to Microsoft servers.

So if a Samsung user locks its phone after this, he will never be able to unlock it again even with a valid developer account (just happened to me).

Heathcliff74 over XDA forums has promised me that his future Registry utility for Samsung devices will have root access, so let’s hope it will come soon. Another solution would be putting a less than 30 chars url that redirects to 127.0.0.1 or https://developerservices.windowsphone.com something I didn’t try (won’t risk yet) and that would require more work (domain certificates?…)