[One (M8) Q&A, Help & Troubleshooting] xda012:
For anyone who encounters this situation in the future, here is my experience. The short version: I couldn't confirm the APK I downloaded was the authentic HTC add-on, but I installed it, it works, and I see no signs of malware (though malware still could be there).

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Here's the long version:

NOTE: To make any of the links below work, you'll need to remove the extra space I inserted. The forum software won't let me post links.

The phone

- HTC

- Model: Remix (Verizon's version of fthe Mini 2), though I doubt the model matters much

- Android v4.4.2, as provided by HTC

- Not rooted

1) I tried the QS Add-on for Samsung (even though I'm using an HTC phone), which I could download directly from TeamViewer.com (see link below). It installed, App Manager saw it, but TeamViewer QuickSupport (TVQS) did not utilize it.

Link to the Samsung add-on at teamviewer.com:

download.teamviewer. com/download/QuickSupport_Add-On_Samsung.apk

TVQS's Advanced screen still displayed the option to download and install the add-on (which requires a Google Play account AFAICT)

I saw these messages in TVQS's log (accessible via the Advanced screen). I could have overlooked other messages:

I/RcAddonInfo.isAddonInstalled(): name not found

I/MainActivity.onCreate(): installable addon found and not installed​

The Samsung add-on's version number was 8.x; TVQS's is 10.x and the HTC add-on's is 9.x, so perhaps it's a simple version incompatibility.

2) I downloaded the APK for QS Add-on for HTC, but from a website which gave me no idea of its integrity (i.e., maybe it was repackaged with malware). Here are two sources I found, which appeared to be the same file:

oppo.apk. vc/Android/App/com.teamviewer.quicksupport.addon.htc

apk4fun.m5f. net/download-sv1/com.teamviewer.quicksupport.addon.htc.html

3) I tried to verify the APK's integrity by comparing it to the one from Google Play. I didn't have the Google Play APK, so I used the information on the webpage.

Google Play link: play.google. com/store/apps/details?id=com.teamviewer.quicksupport.addon.htc

The version numbers (9.0.2045) and release dates (26 Aug 2014) were the same at Google and at the websites of the APK downloads.

Google Play website reported size: 128K; Downloaded APK size: 128K (131,252 bytes)

4) I tried to verify the APK's integrity by looking at the cert used to sign it. I didn't see any red flags, but nothing confirmed HTC as the signer:

The cert (in all APK's, AFAIK) is found here: /meta-inf/cert.rsa

I didn't have a tool to view the cert info, so I opened the file in a hex editor and extracted these strings:

TW1

Taoyuan1

Android1

android [MENTION=1849816]htc.[/MENTION]com​

HTC is headquartered in Taoyuan, Taiwan, so there is nothing suspicious in those strings, but they certainly don't confirm authenticity. I don't think they would be hard to spoof.

For comparison, I did the same with the Samsung add-on. They match the questionable HTC APK in that the phone manufacturer and not TeamViewer signed that APK; on the other hand the Samsung signature mentions Samsung by name while the questionable HTC APK does not mention HTC.

KR1

South Korea1

Suwon City1

Samsung Corporation1

DMC1

Samsung Cert1

android.os [MENTION=381438]samsung[/MENTION].com​

* FWIW I also found this in TVQS's log, I think from its startup routine (to be clear, the following is for the TVQS app and not for any add-on AFAICT). The APK for the TVQS is signed by TeamViewer; perhaps this is HTC's signature for the Android distribution? Then where is HTC mentioned? If it's Google's signature, why is the location HTC's headquarters (Taoyuan, Taiwan)?

CN = Android

OU = Android

O = Android

L = Taoyuan

ST = Taoyuan

C = TW​

5) Not seeing any red flags and not having other options, I installed the questionable HTC APK. It works fine and I see no signs of malware, though certainly malware could be hidden or the signs might not have appeared (I installed it only a few hours ago).