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To take a concrete example of encryption using a key derived from a passphrase: SSH keys. OpenSSH uses MD5 as a key derivation function, so if someone steals a passphrased SSH key file you'd better hope that the passphrase is very strong.

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Using a KDF such as scrypt for that example does make a lot of sense, and after considering some others, I must admit, makes a lot of sense for most circumstances.

I also had a quick look at the tarsnap source code, and I see you do exactly that for the passphrased keyfile.

Yes -- in fact, that's why I invented scrypt, because I was adding passphrase encryption to tarsnap key files and wanted to do it as securely as possible.

Note that it's possible to convert your keys to use PBKDF2: http://martin.kleppmann.com/2013/05/24/improving-security-of...

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