IAX2 RSA Authentication unable to locate key

I have just upgraded my 1.4.x asterisk box to the latest 1.8.3 by installing a new disk. IAX authentication using RSA and the appropriate keys works fine on the 1.4 config but I am unable to get this working on 1.8.3.

I have added the directory to asterisk.conf but keep getting the following message:

-- Called prince/80999

[Mar 14 07:56:47] NOTICE[2708]: chan_iax2.c:8075 authenticate: Unable to find private key ‘*******’
[Mar 14 07:56:47] WARNING[2708]: chan_iax2.c:10877 socket_process: I don’t know how to authenticate xxxxxx to x.x.x.x

I have copied the old keys to the keys directory of the new build but asterisk is unable to locate them. I have also set:

initcrypto = yes

in the asterisk.conf

I am not sure where to go from here - any help would be appreciated.