How to convert hex string to CString in Swift?

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I'm new to swift and i need to convert a hex string in swift to a CString to use in argon2 binding in c.

I have seen:

let saltCString = context.salt.cString(using: .utf8)

argon2id_hash_raw(UInt32(context.iterations), UInt32(context.memory), UInt32(context.parallelism), passwordCString,
                                          passwordLength, saltCString, saltLength, &hashResult, hashLength)

But I want to pass a key instead of passpharse, does it work if I pass a salt of hex string and change .utf8 to .hex?

argon2-ffi c-strings swift hash c
1 Answers

argon2id_hash_raw does not expect a C string (terminated with NUL byte), but a uint8_t array. Therefore the length is a separate parameter, see https://github.com/P-H-C/phc-winner-argon2/blob/master/src/argon2.c lines 136 - 137:

context.salt = CONST_CAST(uint8_t *)salt;
context.saltlen = (uint32_t)saltlen;

Since you have a hexadecimal encoded string in Swift, you can convert it to a UInt8 array using this nice answer https://stackoverflow.com/a/43360864.

Test

If we define a small C function just to test the conversion, it might look like this:

#include <stdio.h>
#include "some_cfile.h"

void print_salt(const uint8_t *salt, uint32_t len) {
    for(uint32_t i = 0; i < len; i++) {
        printf("%x ", salt[i]);
    }
    printf("\n");
}

For the sake of completeness, the test header file would then look like this:

#ifndef some_cfile_h
#define some_cfile_h

#include <inttypes.h>

void print_salt(const uint8_t *salt, uint32_t len);

#endif /* some_cfile_h */

From Swift side this would then be called as follows:

struct Context {
    let salt: String
}
...
let context = Context(salt: "687568")
...
let salt = context.salt.hexaBytes
print_salt(salt, UInt32(salt.count))

This test finally gives the expected output:

68 75 68

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