Hash function with Glibc

Question

I have been trying to encrypt strings in Swift, however I want it to work under linux. Answers like the below code (taken from these questions does not work, as they depend either on iOS or OSx libraries:

func sha256(data : Data) -> Data {
    var hash = [UInt8](repeating: 0,  count: Int(CC_SHA256_DIGEST_LENGTH))
    data.withUnsafeBytes {
        _ = CC_SHA256($0, CC_LONG(data.count), &hash)
    }
    return Data(bytes: hash)
}

How can that be done using Glibc in linux?

Andre Kampling · Accepted Answer · 2017-09-01T09:12:29.143

There is a crypt library in glibc, see manpage.
You need to include: #include <crypt.h>.

The function you have to use is:

char *crypt(const char *key, const char *salt);

According to that manpage the SHA-256 algorithm is integrated since glibc 2.7 and is selected via the salt argument:

The glibc2 version of this function supports additional encryption
algorithms.

If salt is a character string starting with the characters "$id$"
followed by a string terminated by "$":

       $id$salt$encrypted

then instead of using the DES machine, id identifies the encryption
method used and this then determines how the rest of the password
string is interpreted.  The following values of id are supported:

       ID  | Method
       ─────────────────────────────────────────────────────────
       1   | MD5
       2a  | Blowfish (not in mainline glibc; added in some
           | Linux distributions)
       5   | SHA-256 (since glibc 2.7)
       6   | SHA-512 (since glibc 2.7)

So $5$salt$encrypted is an SHA-256 encoded password and
$6$salt$encrypted is an SHA-512 encoded one.

"salt" stands for the up to 16 characters following "$id$" in the
salt.  The encrypted part of the password string is the actual
computed password.  The size of this string is fixed:

MD5     | 22 characters
SHA-256 | 43 characters
SHA-512 | 86 characters

The characters in "salt" and "encrypted" are drawn from the set
[a-zA-Z0-9./].  In the MD5 and SHA implementations the entire key is
significant (instead of only the first 8 bytes in DES).

On this official GNU page and this wikipedia article are explanations and examples:

Example salt argument for SHA-256:
$5$9ks3nNEqv31FX.F$gdEoLFsCRsn/WRN3wxUnzfeZLoooVlzeF4WjLomTRFD

Hash function with Glibc

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