Adding a wrapper function to src/core/lib/security/credentials/jwt/jwt_verifier.c in order to partially resolve https://github.com/grpc/grpc/issues/10589.

There are a total of four files require modification and this is the only one in which a wrapper-type function is necessary.

src/core/lib/security/credentials/jwt/jwt_verifier.c

@@ -462,6 +462,36 @@ static BIGNUM *bignum_from_base64(grpc_exec_ctx *exec_ctx, const char *b64) {
   return result;
 }
 
+#if OPENSSL_VERSION_NUMBER < 0x10100000L
+
+// Provide compatibility across OpenSSL 1.02 and 1.1.
+int RSA_set0_key(RSA *r, BIGNUM *n, BIGNUM *e, BIGNUM *d)
+{
+    /* If the fields n and e in r are NULL, the corresponding input
+     * parameters MUST be non-NULL for n and e.  d may be
+     * left NULL (in case only the public key is used).
+     */
+    if ((r->n == NULL && n == NULL)
+        || (r->e == NULL && e == NULL))
+        return 0;
+
+    if (n != NULL) {
+        BN_free(r->n);
+        r->n = n;
+    }
+    if (e != NULL) {
+        BN_free(r->e);
+        r->e = e;
+    }
+    if (d != NULL) {
+        BN_free(r->d);
+        r->d = d;
+    }
+
+    return 1;
+}
+#endif
+
 static EVP_PKEY *pkey_from_jwk(grpc_exec_ctx *exec_ctx, const grpc_json *json,
                                const char *kty) {
   const grpc_json *key_prop;
@@ -478,18 +508,24 @@ static EVP_PKEY *pkey_from_jwk(grpc_exec_ctx *exec_ctx, const grpc_json *json,
     gpr_log(GPR_ERROR, "Could not create rsa key.");
     goto end;
   }
+  BIGNUM *tmp_n = NULL;
+  BIGNUM *tmp_e = NULL;
   for (key_prop = json->child; key_prop != NULL; key_prop = key_prop->next) {
     if (strcmp(key_prop->key, "n") == 0) {
-      rsa->n =
+      tmp_n =
           bignum_from_base64(exec_ctx, validate_string_field(key_prop, "n"));
-      if (rsa->n == NULL) goto end;
+      if (tmp_n == NULL) goto end;
     } else if (strcmp(key_prop->key, "e") == 0) {
-      rsa->e =
+      tmp_e =
           bignum_from_base64(exec_ctx, validate_string_field(key_prop, "e"));
-      if (rsa->e == NULL) goto end;
+      if (tmp_e == NULL) goto end;
     }
   }
-  if (rsa->e == NULL || rsa->n == NULL) {
+  if (tmp_e == NULL || tmp_n == NULL) {
+    gpr_log(GPR_ERROR, "Missing RSA public key field.");
+    goto end;
+  }
+  if (!RSA_set0_key(rsa, tmp_n, tmp_e, NULL)) {
     gpr_log(GPR_ERROR, "Missing RSA public key field.");
     goto end;
   }