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This question has been asked previously, here. However, none of the answers there address the question. The accepted answer conflates Authy's multi-device mode with its cloud backup feature. I am specifically not asking about the cloud backup feature, but about the multi-device mode which is available even without having enabled cloud backups or entered a backup password.

In the documentation for the cloud backup feature, Authy claims that the cloud backups are encrypted client-side using the backup password, and if cloud backups are disabled then none of the user keys are stored server-side at all. However, I just installed Authy on a new device, and after verifying my phone number via SMS (which is, as is well known, an insecure and un-securable method), I was able to see my configured accounts and generate OTPs for them. This seems to imply that Authy's servers have access to my keys in order to send them to the new device. Note that I have never enabled cloud backups nor entered any backup password. Nor did I use the "inherited trust" mechanism to enable the new account; all I did was enter a passcode received over SMS.

This answer suggests that the Authy server does indeed have my key available, since new devices can be added (and get access to existing accounts) using only an SMS verification.

Note that I am aware that multi-device can be disabled and that this is a necessary security measure. My concern is specifically about Authy servers having access to OTP secrets.

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