Highly secure and easy to use OTP client written in C/GTK3 that supports both TOTP and HOTP and has the following features:

integration with the OS' secret service provider via libsecret
support both TOTP and HOTP
support setting custom digits (between 4 and 10 inclusive)
support setting a custom period (between 10 and 120 seconds inclusive)
support SHA1, SHA256 and SHA512 algorithms
support for Steam codes
import and export encrypted/plain andOTP backup
import and export encrypted/plain Aegis backup
import and export plain FreeOTPPlus backup (key URI format only)
import of Google's migration QR codes
local database is encrypted using AES256-GCM (PBKDF2 with SHA512 and 100k iterations) and, while decrypted, it's stored in a secure memory area allocated by GCrypt.

OTPClient