A C JWT Implementation

cjwt is a small JWT handler designed to allow consumers of JWTs of the JWS variant the ability to securely and easily get claims and data from a JWT. This particular JWT implementation uses cJSON and is designed to support multiple different crypto libraries in the future.

The API is meant to be fairly small & leverage what cJSON already provides nicely.

Here are the details

There are 3 function:

• cjwt_decode() that decodes successfully or fails with a more detailed reason
• cjwt_destroy() that destroys the cjwt_t object cleanly
• cjwt_print() that prints the cjwt_t object to a stream (generally for debugging)

Otherwise you get a simple C struct to work with in your code.

• cJSON
• openssl
• trower-base64

To help adopters not make costly security mistakes, cjwt tries to default to secure wherever possible. If you must use an insecure feature there are option flags that let you do so, but use them sparingly and with care.

• HS
• RS / PS
• ES

Using the decoder:

#include <stdint.h>
#include <stddef.h>
#include <string.h>

#include <cjwt/cjwt.h>

int main( int argc, char *argv[] )
{
    cjwt_t *jwt = NULL;
    cjwt_code_t rv;

    const char *hs_text = 
        /* header */
        "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9."
        /* payload */
        "eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaGVsbG8i"
        "OiJ3b3JsZCIsImJvYiI6WyJkb2ciLDEyM10sImNhdCI6eyJtb3VzZSI6eyJj"
        "aGVlc2UiOiJsb3RzIn19LCJpYXQiOjE1MTYyMzkwMjJ9."
        /* signature */
        "mJYSucD6RRg6zdPcSKvb5-LKFDJzRvdKqTlqAvDBknU";

    const char *hs_key = "hs256-secret";

    rv = cjwt_decode( hs_text, strlen(hs_text), 0, (uint8_t*) hs_key, strlen(hs_key), 0, 0, &jwt );
    if( CJWTE_OK != rv ) {
        printf( "There was an error processing the text: %d\n", rv );
        return -1;
    }

    cjwt_print( stdout, jwt );

    cjwt_destroy( jwt );

    return 0;
}

Gives you this output:

=====================
header
---------------------
   alg: HS256

payload
---------------------
   iat: 1516239022

   exp: NULL
   nbf: NULL

   iss: NULL
   sub: 1234567890
   jti: NULL
   aud: NULL

private claims
---------------------
{
     "name":     "John Doe",
     "hello":    "world",
     "bob":      ["dog", 123],
     "cat": {
         "mouse": {
             "cheese":   "lots"
         }
     }
}

meson setup --warnlevel 3 --werror build
cd build
ninja all test coverage
firefox ./meson-logs/coveragereport/index.html