How exactly are you trying to import it?
The key file is simply a text file, you should be able to open it in a text editor like notepad and read the key. So your decode.key file should look something like like this:
# KEEP YOUR PRIVATE KEYS SAFE !
# Anyone who can read this file can spend your bitcoin.
#
# Format:
# <Base58 encoded private key>[<whitespace>[<key createdAt>]]
#
# The Base58 encoded private keys are the same format as
# produced by the Satoshi client/ sipa dumpprivkey utility.
#
# Key createdAt is in UTC format as specified by ISO 8601
# e.g: 2011-12-31T16:42:00Z . The century, 'T' and 'Z' are mandatory
#
KyRmhgasglarh389hskhqkhrkhakhskfafsfsdfsfhf3hhhsfkjhszhjfT 2009-10-27T22:04:57Z
# End of private keys
if however you see something like:
U2FsdGVkX1+F2I5kp4D+8Hf1xUzasfh39rf13hfhkjsahkjkdsafsdg32eVYkNy5hJD+pCu0qxY3
kTGafssafsafasfsafsyM//sPCejwafsdsdgsfsn+gu3tgsdghsfdfdA
Then the file is still encrypted.