Base64 decoder supports missing padding

This makes the C implementation compatible with the Go one, which
generates unpadded base64-encoded tokens.

src/sso/base64.c

@@ -27,6 +27,8 @@
 
 #include <sys/types.h>
 #include <inttypes.h>
+#include <memory.h>
+#include <stdlib.h>
 #include "sso.h"
 
 static const unsigned char base64_enc_map[64] = {
@@ -117,15 +119,8 @@ int sso_base64_encode(unsigned char *dst, size_t *dlen,
   return 0;
 }
 
-/**
- * Decode a base64-formatted buffer.
- *
- * Supports both standard and "web-safe" encodings.
- *
- * The destination buffer is *not* zero-terminated.
- */
-int sso_base64_decode(unsigned char *dst, size_t *dlen,
-                      const unsigned char *src, size_t slen) {
+static int sso_base64_decode_real(unsigned char *dst, size_t *dlen,
+                                  const unsigned char *src, size_t slen) {
   size_t i, n;
   uint32_t j, x;
   unsigned char *p;
@@ -199,3 +194,29 @@ int sso_base64_decode(unsigned char *dst, size_t *dlen,
 
   return 0;
 }
+
+/**
+ * Decode a base64-formatted buffer.
+ *
+ * Supports both standard and "web-safe" encodings, and it can
+ * deal with missing padding.
+ *
+ * The destination buffer is *not* zero-terminated.
+ */
+int sso_base64_decode(unsigned char *dst, size_t *dlen,
+                      const unsigned char *src, size_t slen) {
+  unsigned char *srcbuf;
+  int n, r;
+
+  n = 4 - (slen % 4);
+  if (n < 4) {
+    srcbuf = (unsigned char *)malloc(slen + n);
+    memcpy(srcbuf, src, slen);
+    memset(srcbuf + slen, '=', n);
+    r = sso_base64_decode_real(dst, dlen, srcbuf, slen + n);
+    free(srcbuf);
+  } else {
+    r = sso_base64_decode_real(dst, dlen, src, slen);
+  }
+  return r;
+}

src/sso/test/sso_unittest.cc

@@ -61,6 +61,34 @@ TEST(Base64, DecodeStdOk) {
   EXPECT_EQ(0, memcmp(alphabet, out, sz));
 }
 
+static const char *unpadded_src[] = {
+  "aA",
+  "aGU",
+  "aGVs",
+  "aGVsbA",
+  "aGVsbG8",
+};
+
+static const char *unpadded_res[] = {
+  "h",
+  "he",
+  "hel",
+  "hell",
+  "hello",
+};
+
+TEST(Base64, DecodeUnpaddedOk) {
+  for (int i = 0; i < 5; i++) {
+    char out[256] = {0};
+    size_t sz = sizeof(out);
+    EXPECT_EQ(0, sso_base64_decode((unsigned char *)out, &sz,
+                                   (unsigned char *)unpadded_src[i],
+                                   (size_t)strlen(unpadded_src[i])));
+    EXPECT_EQ(std::string(unpadded_res[i]), std::string(out, sz))
+      << "Error decoding '" << unpadded_src[i] << "'";
+  }
+}
+  
 class SSO : public testing::Test {
 protected:
   virtual void SetUp() { sso_generate_keys(public_key, secret_key); }