Hashapass is a password generator which uses a mathematic formula to create new passwords based on a "master" password and a parameter.
The formula is calculated in the safety of your web browser, without any information being transmitted over the Internet.
The generated password is computed as the first 8 characters of the Base64 encoding of HMAC-SHA1(master password, parameter). HMAC-SHA1 is defined on byte arrays, so only the lowest-significant byte of the Unicode code point for each character of the master password and parameter is used in the calculation of the HMAC-SHA1.
There's no secret to this formula: SHA1, HMAC and Base64 are all well-known algorithms, which guarantees that other people can implement products that are compatible with Hashapass.