The scrypt command line utility uses the scrypt kdf to generate a 256 bit key for aes.
Both kdf and cipher are used during single file encryption with openssl and the scrypt command line utility. Openssl implicitly uses a md5 as a kdf during encryption [1.1][1.2]. Cast5 requires a 128bit key, and the kdf helps stretch the user's password to fit this key requirement.
I can understand the confusion, as scrypt is typically referenced in kdf discussions. It's actually somewhat difficult to extract the kdf functionality from the scrypt source code because the code is geared towards single file encryption. See this post for a confused q&a with scrypt's author [2]. Wrappers around scrypt like this python package[3] have made the "mistake" of using the entire encryption pipeline when they just wanted the kdf. Using scrypt in this manner should still be safe, but it will waste some cpu cycles on aes.
Both kdf and cipher are used during single file encryption with openssl and the scrypt command line utility. Openssl implicitly uses a md5 as a kdf during encryption [1.1][1.2]. Cast5 requires a 128bit key, and the kdf helps stretch the user's password to fit this key requirement.
I can understand the confusion, as scrypt is typically referenced in kdf discussions. It's actually somewhat difficult to extract the kdf functionality from the scrypt source code because the code is geared towards single file encryption. See this post for a confused q&a with scrypt's author [2]. Wrappers around scrypt like this python package[3] have made the "mistake" of using the entire encryption pipeline when they just wanted the kdf. Using scrypt in this manner should still be safe, but it will waste some cpu cycles on aes.
[1.1] http://www.openssl.org/docs/apps/enc.html
[1.2] http://www.openssl.org/docs/crypto/EVP_BytesToKey.html
[2] https://news.ycombinator.com/item?id=1350392
[3] http://pypi.python.org/pypi/scrypt/