Abstract: A method of Completely Automated Public Turing test to tell Computers and Humans Apart (CAPTCHA) includes: recording, by a voice CAPTCHA module, a speech spoken by a user; determining, by a voice biometric service (VBS), whether a voiceprint matching the user's speech exists; and if a voiceprint matching the user's speech exists, verifying the user as a human user by the VBS. If a voiceprint matching the user's speech does not exist, the VBS i) generates a unique voiceprint for the user based on the user's speech, and/or ii) determines whether the user's speech is at least one of a synthetically generated speech and a previously recorded audio being played back. The user can perform a guest checkout without logging into the voice CAPTCHA module, in which case the VBS compares previously used voiceprints to the user's speech.