From 48c33ae0a208e6b637475ec8c68fe0e5e0741f10 Mon Sep 17 00:00:00 2001
From: Benjamin Valentin <benpicco@googlemail.com>
Date: Tue, 15 Dec 2020 12:18:45 +0100
Subject: [PATCH] sys/base64: drop padding for base64url encoding

The URLsafe base64 encoding variant does not require padding, using
'=' as padding is in fact an error as it is not a URL-safe character.
---
 sys/base64/base64.c                         | 15 +++++++++++++--
 tests/unittests/tests-base64/tests-base64.c |  4 ++--
 2 files changed, 15 insertions(+), 4 deletions(-)

diff --git a/sys/base64/base64.c b/sys/base64/base64.c
index 1e885cc5f2..4e7b565606 100644
--- a/sys/base64/base64.c
+++ b/sys/base64/base64.c
@@ -90,6 +90,7 @@ static int base64_encode_base(const void *data_in, size_t data_in_size,
                               void *base64_out, size_t *base64_out_size,
                               bool urlsafe)
 {
+    const uint8_t padding = urlsafe ? 0 : '=';
     const uint8_t *in = data_in;
     const uint8_t *end = in + data_in_size;
     uint8_t *out = base64_out;
@@ -131,14 +132,24 @@ static int base64_encode_base(const void *data_in, size_t data_in_size,
         encode_three_bytes(out, in[0], 0, 0, urlsafe);
         /* Replace last two bytes with "=" to signal corresponding input bytes
          * didn't exist */
-        out[2] = out[3] = '=';
+        out[2] = out[3] = padding;
+
+        /* padding is not required for urlsafe application */
+        if (urlsafe) {
+            *base64_out_size -= 2;
+        }
         return BASE64_SUCCESS;
     }
 
     /* Final case: 2 bytes remain for encoding, use zero as third input */
     encode_three_bytes(out, in[0], in[1], 0, urlsafe);
     /* Replace last output with "=" to signal corresponding input byte didn't exit */
-    out[3] = '=';
+    out[3] = padding;
+
+    /* padding is not required for urlsafe application */
+    if (urlsafe) {
+        *base64_out_size -= 1;
+    }
 
     return BASE64_SUCCESS;
 }
diff --git a/tests/unittests/tests-base64/tests-base64.c b/tests/unittests/tests-base64/tests-base64.c
index 573686d4d0..80a5403907 100644
--- a/tests/unittests/tests-base64/tests-base64.c
+++ b/tests/unittests/tests-base64/tests-base64.c
@@ -411,7 +411,7 @@ static void test_base64_10_decode_empty(void)
 static void test_base64_11_urlsafe_encode_int(void)
 {
     uint32_t data_in = 4345;
-    unsigned char expected_encoding[] = "-RAAAA==";
+    unsigned char expected_encoding[] = "-RAAAA";
 
     size_t base64_out_size = 0;
     char base64_out[sizeof(expected_encoding)];
@@ -448,7 +448,7 @@ static void test_base64_11_urlsafe_encode_int(void)
 
 static void test_base64_12_urlsafe_decode_int(void)
 {
-    static const char encoded_base64[]  = "_____wAA==";
+    static const char encoded_base64[]  = "_____wAA";
     static const uint8_t expected[]     = {0xFF, 0xFF, 0xFF, 0xFF, 0x0, 0x0};
 
     size_t base64_size = strlen(encoded_base64);