Custom implementation of Base64 decoding function.

Compared to the previous implementation based on OpenSSL, the new implementation
has these advantages:

 1. Strict and reliable detection of invalid strings, including strings with
    less than 4 bytes of garbage at the end;

 2. Allows to use Base64 strings without '=' padding.

1 files changed, 97 insertions, 0 deletions

diff --git a/src/nxt_string.c b/src/nxt_string.c
index ab568990..b7aef79e 100644
--- a/src/nxt_string.c
+++ b/src/nxt_string.c
@@ -745,3 +745,100 @@ nxt_is_complex_uri_encoded(u_char *src, size_t length)
 
     return 1;
 }
+
+
+ssize_t
+nxt_base64_decode(u_char *dst, u_char *src, size_t length)
+{
+    u_char   *end, *p;
+    size_t   pad;
+    uint8_t  v1, v2, v3, v4;
+
+    static const uint8_t  decode[] = {
+        77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77,
+        77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77,
+        77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 62, 77, 77, 77, 63,
+        52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 77, 77, 77, 77, 77, 77,
+        77,  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, 77, 77, 77, 77, 77,
+        77, 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, 77, 77, 77, 77, 77,
+
+        77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77,
+        77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77,
+        77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77,
+        77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77,
+        77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77,
+        77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77,
+        77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77,
+        77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77, 77
+    };
+
+    end = src + length;
+    pad = (4 - (length % 4)) % 4;
+
+    if (dst == NULL) {
+        if (pad > 2) {
+            return NXT_ERROR;
+        }
+
+        while (src < end) {
+            if (decode[*src] != 77) {
+                src++;
+                continue;
+            }
+
+            if (pad == 0) {
+                pad = end - src;
+
+                if ((pad == 1 || (pad == 2 && src[1] == '=')) && src[0] == '=')
+                {
+                    break;
+                }
+            }
+
+            return NXT_ERROR;
+        }
+
+        return (length + 3) / 4 * 3 - pad;
+    }
+
+    nxt_assert(length != 0);
+
+    if (pad == 0) {
+        pad = (end[-1] == '=') + (end[-2] == '=');
+        end -= (pad + 3) & 4;
+
+    } else {
+        end -= 4 - pad;
+    }
+
+    p = dst;
+
+    while (src < end) {
+        v1 = decode[src[0]];
+        v2 = decode[src[1]];
+        v3 = decode[src[2]];
+        v4 = decode[src[3]];
+
+        *p++ = (v1 << 2 | v2 >> 4);
+        *p++ = (v2 << 4 | v3 >> 2);
+        *p++ = (v3 << 6 | v4);
+
+        src += 4;
+    }
+
+    if (pad > 0) {
+        v1 = decode[src[0]];
+        v2 = decode[src[1]];
+
+        *p++ = (v1 << 2 | v2 >> 4);
+
+        if (pad == 1) {
+            v3 = decode[src[2]];
+            *p++ = (v2 << 4 | v3 >> 2);
+        }
+    }
+
+    return (p - dst);
+}