**Background**:

I am trying to get an understanding of using a hash of a `passphrase` as a `secret`.

Example:

```
sha256("EasyToRememberPassphrase") -> secret.

```

I am assuming a properly designed hash function used this way above will yield a `secret` with the same or fairly similar entropy as the input `passphrase` (or the size of the hash, whatever is smaller), thus as far as being used as a secret goes, the `passphrase` and `secret` are equally hard to guess.

**On to the question:**

Say I generate two secrets, `secretA` and `secretB` in the following way:

```
passphrase = "EasyToRememberPassphrase"
sha256(passphrase + "a") -> secretA
sha256(passphrase + "b") -> secretB

```

Assuming an attacker gets access to two pieces of information:

* `secretA`
* the fact that `secretB` was hashed from data very similar to `secretA`

Does this give the attacker an easier time figuring out `secretB`? And if so, to what degree? Or are good hash functions immune/sufficiently resistant to an attack using this particular set of information?