strutl: Add Base64Decode

Lifted from the dpkg mtree code that mjg59 submitted in

https://lists.debian.org/debian-dpkg/2018/05/msg00005.html

Which as dpkg is GPL-2.0+ also is GPL-2.0+

2 files changed, 68 insertions, 0 deletions

diff --git a/apt-pkg/contrib/strutl.cc b/apt-pkg/contrib/strutl.cc
index ec61c7e2b..69a67188e 100644
--- a/apt-pkg/contrib/strutl.cc
+++ b/apt-pkg/contrib/strutl.cc
@@ -625,6 +625,73 @@ string Base64Encode(const string &S)
    return Final;
 }
 									/*}}}*/
+// Base64Decode - Base64 decoding routine for short strings		/*{{{*/
+// ---------------------------------------------------------------------
+// Taken from mjg59's dpkg code: https://lists.debian.org/debian-dpkg/2018/05/msg00002.html
+std::string Base64Decode(const std::string_view in)
+{
+   constexpr std::array<unsigned char, 256> mime_base64_rank{
+      255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+      255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+      255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 62, 255, 255, 255, 63,
+      52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 255, 255, 255, 0, 255, 255,
+      255, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,
+      15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 255, 255, 255, 255, 255,
+      255, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
+      41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 255, 255, 255, 255, 255,
+      255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+      255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+      255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+      255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+      255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+      255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+      255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+      255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255};
+   std::string out;
+   unsigned char rank;
+   unsigned char last[2];
+   unsigned int v;
+   int i;
+
+   /* convert 4 base64 bytes to 3 normal bytes */
+   v = 0;
+   i = 0;
+
+   last[0] = last[1] = 0;
+
+   /* we use the sign in the state to determine if we got a padding character
+   in the previous sequence */
+   if (i < 0)
+   {
+      i = -i;
+      last[0] = '=';
+   }
+
+   for (const unsigned char c : in)
+   {
+      rank = mime_base64_rank[c];
+      if (rank != 0xff)
+      {
+	 last[1] = last[0];
+	 last[0] = c;
+	 v = (v << 6) | rank;
+	 i++;
+	 if (i == 4)
+	 {
+	    out += v >> 16;
+	    if (last[1] != '=')
+	       out += v >> 8;
+	    if (last[0] != '=')
+	       out += v;
+	    i = 0;
+	 }
+      }
+   }
+
+   return out;
+}
+
+									/*}}}*/
 // stringcmp - Arbitrary string compare					/*{{{*/
 // ---------------------------------------------------------------------
 /* This safely compares two non-null terminated strings of arbitrary 
diff --git a/apt-pkg/contrib/strutl.h b/apt-pkg/contrib/strutl.h
index 90d9674fd..91f5fe44b 100644
--- a/apt-pkg/contrib/strutl.h
+++ b/apt-pkg/contrib/strutl.h
@@ -69,6 +69,7 @@ APT_PUBLIC std::string DeEscapeString(const std::string &input);
 APT_PUBLIC std::string SizeToStr(double Bytes);
 APT_PUBLIC std::string TimeToStr(unsigned long Sec);
 APT_PUBLIC std::string Base64Encode(const std::string &Str);
+APT_PUBLIC std::string Base64Decode(const std::string_view in);
 APT_PUBLIC std::string OutputInDepth(const unsigned long Depth, const char* Separator="  ");
 APT_PUBLIC std::string URItoFileName(const std::string &URI);
 /** returns a datetime string as needed by HTTP/1.1 and Debian files.