Data Encryption Keys during upgrade from CRM 2013

I've been doing some upgrades from CRM 2013 the past couple of month and one thing I've run into is the encryption keys.

The funny thing is that an encryption key that is valid in CRM 2013 isn't valid in CRM 2015 and later. It seems that Microsoft has changed the requirement of the encryption key sometime after the lauch of CRM 2013. So what are the requirements of the encryption key. I started looking for it and it isn't all that clear, however  this blog: http://blog.cobalt.net/blog/microsoft-crm-data-encryption-key-%E2%80%93-one-time-you-can-just-make-sh-up say the following:  "The value you enter should be between 10 and 100 characters in length, have at least one numeral, at least one letter and one symbol or special character."

That sounds like a sound idea and the question is if the "at least one symbol or special character" is what have changed. When I use an encryption key which follows this rule set it works and the key that was used on the CRM 2013 which didn't work in later versions didn't have a special character, it did however contain an upper case letter, a lower case letter, a digit and was more than 10 characters long. 
After upgrade you need to re-enter the encryption key from the previous version again and if that key doesn't match you can't save and if the key doesn't match the above criteria you can't save either so you need to go back to the CRM 2013, change the key to something that will be accepted in the more modern version and then to the entire upgrade process again.

Hope this helps someone.

Rickard Norström
Developer at CRM-Konsulterna
www.crmkonsulterna.se