Re-initializing a Duo device does not produce an entirely different string in the new QR
code, so just in case I'm not going to post a real decoded string on a public list.
However, I will attempt to describe what I see experimenting with this.
The three codes I have gotten so far are all mixed-case alpha-numeric strings 59
characters long, with a hyphen at character 21. The first 20 characters vary between QR
codes. The 38 characters after the hyphen are constant even when Duo believes it is
configuring different devices. I hope this means that the constant string is an account
identifier of some kind rather than key material, but so far I can't be certain.
The Duo soft token application also clearly phones home to activate the toek after
scanning a QR code, so it would be necessary to reverse engineer the phone-home protocol
to make FreeOTP work with QR codes intended for the Duo app. According to their web site
Duo allows for the use of HOTP tokens, so there's probably not much benefit to reverse
engineering their proprietary token type.