The SHA1 Password Calculator Pwcalc works as safely as a meat grinder: If you put in a pig and turn the crank, you will get minced meat. But pushing back the minced meat while turning the crank backwards, will not bring back the pig. That is quite secure.
In contrast to other apps strong passwords are calculated and not just generated. No confidential data is stored or does leave the device. No special app permissions are required. The network is not used. Source code is available on GitHub:
You chose a single secret word (your secret master password) and an alias for the service the password is needed for (the pig), e.g. mysecret + gmail.com. Next the cryptographic hash function SHA-1 is applied to this combined word which at the end produces a strong unique password (the minced meat). This calculated password is then copied to the clipboard to be used wherever it is needed. In addition a QR code is created.
In a simplified form it works like this:
secret word + alias | sha1 | base64 = password