sorry for jumping in late here, but I just wanted to throw in a little more on M110A.
For RX only, you can use the same MS-DMT software that MARS ops apparently use. I don't remember exactly where I found it lately, but I remember a year or so ago MARS made MS-DMT and an unencrypted version of their chat terminal available to amateur radio operators for either Armed Forces Day or one of their quarterly communications exercises. A Google search should turn up links to both.
It's finicky to get M110A to decode. Set your audio filter to the full 3 kHz channel, or as wide as your radio will allow.
Even if everything is setup just right, you might not see unencrypted headers anymore. At least I haven't lately. I can get a decode with few or no errors that still prints nothing but garbled text. Reading between the lines of some Facebook posts I've seen, I think they're using online encryption, now.
All my experience with M110A has been unencrypted, so I don't know exactly what encrypted messages should look like at the raw data level other than the modem showing no errors.