If it is legal to do so (and I can only assume you have the key via legitimate means), then the only two options are:
1. Assuming RAS isn't used, buy the same DMR equipment that's being used and then add the key to this to decrypt, or
2. If (and only if) the Motorola Basic Privacy flavor of encryption is being used, then acquire an ADCR25 receiver and it will decrypt it without the need for a key. Note you can no longer buy an ADCR25: they were withdrawn from sale shortly after some simple decryption features were added....I'm sure you can draw your own conclusions as to why it's no longer for sale !
I know the problem is discussed, but i would know if there a way to extract the encoded Basic privacy frame bits from DSD+ or previous version in order to check and try an offline manipulation to remove the encription?
Could be nice to have a binary output in DSD in order to perform some sperimentation, any idea if is possible to modify DSD source file to add this functionality?