How bad is the following strategy for a password manager on a local machine?

The user wants to sign up for somesite.com where he needs a password. The user has previously generated a secret key K which he remembers and isn't stored anywhere. Now using a good hash function the site's URL somesite.com is hashed together with K to form the password used for the site. This way passwords are never stored and can be recalled on a per-site basis from the secret key.

• How is somesite.com "hashed together with K"? $\;$ – user991 Oct 30 '14 at 6:29
• I would assume using the key in HMAC with the site name as the plaintext?. It should be noted I designed a scheme around this idea, there are a lot of complexities involved, like site password character and length restrictions, among other things – Richie Frame Oct 30 '14 at 6:37
• Related: crypto.stackexchange.com/questions/9035/… ... and I guess basically every question containing the words "password generation", as this is hardly a novel concept – Thomas Oct 30 '14 at 7:06