From d26f046fc9149693a6ebc28301ccc3581c0f144e Mon Sep 17 00:00:00 2001
From: Xavier Del Campo Romero <xavi.dcr@tutanota.com>
Date: Mon, 9 Jan 2023 01:22:54 +0100
Subject: Initial commit

---
 jwt.c | 147 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 147 insertions(+)
 create mode 100644 jwt.c

(limited to 'jwt.c')

diff --git a/jwt.c b/jwt.c
new file mode 100644
index 0000000..2587b91
--- /dev/null
+++ b/jwt.c
@@ -0,0 +1,147 @@
+#include "jwt.h"
+#include "base64.h"
+#include <dynstr.h>
+#include <cjson/cJSON.h>
+#include <openssl/evp.h>
+#include <openssl/hmac.h>
+#include <openssl/sha.h>
+#include <errno.h>
+#include <stddef.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <string.h>
+
+static const char jwt_header[] = "{\"alg\": \"HS256\", \"typ\": \"JWT\"}";
+
+static char *get_payload(const char *const name)
+{
+    char *ret = NULL;
+    struct dynstr d;
+
+    dynstr_init(&d);
+
+    if (dynstr_append(&d, "{\"name\": \"%s\"}", name))
+    {
+        fprintf(stderr, "%s: dynstr_append name failed\n", __func__);
+        goto end;
+    }
+    else if (!(ret = base64_encode(d.str, d.len)))
+    {
+        fprintf(stderr, "%s: base64 failed\n", __func__);
+        goto end;
+    }
+
+end:
+    dynstr_free(&d);
+    return ret;
+}
+
+static char *get_hmac(const void *const buf, const size_t n,
+    const void *const key, const size_t keyn)
+{
+    unsigned char hmac[SHA256_DIGEST_LENGTH];
+    const EVP_MD *const md = EVP_sha256();
+    char *ret = NULL;
+
+    if (!md)
+    {
+        fprintf(stderr, "%s: EVP_sha256 failed\n", __func__);
+        return NULL;
+    }
+    else if (!HMAC(md, key, keyn, buf, n, hmac, NULL))
+    {
+        fprintf(stderr, "%s: HMAC failed\n", __func__);
+        return NULL;
+    }
+    else if (!(ret = base64_encode(hmac, sizeof hmac)))
+    {
+        fprintf(stderr, "%s: base64 failed\n", __func__);
+        return NULL;
+    }
+
+    return ret;
+}
+
+char *jwt_encode(const char *const name, const void *const key, const size_t n)
+{
+    char *ret = NULL;
+    char *const header = base64_encode(jwt_header, strlen(jwt_header)),
+        *const payload = get_payload(name),
+        *hmac = NULL;
+    struct dynstr jwt;
+
+    dynstr_init(&jwt);
+
+    if (!header)
+    {
+        fprintf(stderr, "%s: base64_encode header failed\n", __func__);
+        goto end;
+    }
+    else if (!payload)
+    {
+        fprintf(stderr, "%s: get_payload failed\n", __func__);
+        goto end;
+    }
+    else if (dynstr_append(&jwt, "%s.%s", header, payload))
+    {
+        fprintf(stderr, "%s: dynstr_append header+payload failed", __func__);
+        goto end;
+    }
+    else if (!(hmac = get_hmac(jwt.str, jwt.len, key, n)))
+    {
+        fprintf(stderr, "%s: get_hmac failed\n", __func__);
+        goto end;
+    }
+    else if (dynstr_append(&jwt, ".%s", hmac))
+    {
+        fprintf(stderr, "%s: dynstr_append hmac failed\n", __func__);
+        goto end;
+    }
+
+    ret = jwt.str;
+
+end:
+    free(header);
+    free(payload);
+    free(hmac);
+
+    if (!ret)
+        dynstr_free(&jwt);
+
+    return ret;
+}
+
+int jwt_check(const char *const jwt, const void *const key, const size_t n)
+{
+    const char *const p = strrchr(jwt, '.');
+    const EVP_MD *const md = EVP_sha256();
+    unsigned char hmac[SHA256_DIGEST_LENGTH];
+    char *dhmac = NULL;
+    size_t hmaclen;
+
+    if (!md)
+    {
+        fprintf(stderr, "%s: EVP_sha256 failed\n", __func__);
+        return -1;
+    }
+    else if (!p)
+    {
+        fprintf(stderr, "%s: expected '.'\n", __func__);
+        return 1;
+    }
+    else if (!HMAC(md, key, n, (const unsigned char *)jwt, p - jwt, hmac, NULL))
+    {
+        fprintf(stderr, "%s: HMAC failed\n", __func__);
+        return -1;
+    }
+    else if (!(dhmac = base64_decode(p + 1, &hmaclen)))
+    {
+        fprintf(stderr, "%s: base64_decode failed\n", __func__);
+        return -1;
+    }
+
+    const int r = memcmp(dhmac, hmac, hmaclen);
+
+    free(dhmac);
+    return !!r;
+}
-- 
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