为什么双重哈希字符串（加密）是不好的？ [重复]

i heard on another site that doing something like is a bad idea:

$string = "hello";
sha1(hash("sha512", $string));

... as far as i am concerned its a good, clever idea! why is it not? (i did not quite understand the explanation on the other site)

Thanks

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There's nothing wrong with hashing multiple times. There are a number of cases where this is actually a good idea.

1. You want to slow down brute force attacks.
2. You have a databases full of hashed passwords that use a broken hashing scheme. Upgrading from MD5 to SHA1 for example.
3. You're implementing a challenge/response scheme.

The example you pasted is bad because you're hashing a 512-bit hash with a 160-bit hash.