rfc2047: skip whitespace everywhere during base64 decoding

> The encoded output stream must be represented in lines of no more
> than 76 characters each.  All line breaks or other characters not
> found in Table 1 must be ignored by decoding software.  In base64
> data, characters other than those in Table 1, line breaks, and other
> white space probably indicate a transmission error, about which a
> warning message or even a message rejection might be appropriate
> under some circumstances.

rfc2047.c

@@ -82,16 +82,25 @@ blaze822_decode_b64(char *s, char *e, char **deco, size_t *decleno)
 	*deco = buf;
 
 	while (s + 4 <= e) {
-		while (s < e && isfws((unsigned char)*s))
-			s++;
-		if (s >= e)
-			break;
-
 		uint32_t v = 0;
 		unsigned char t = 0;
 
-		unsigned char c0 = s[0], c1 = s[1], c2 = s[2], c3 = s[3];
-		s += 4;
+#define SKIP_WS \
+		while (s < e && isfws((unsigned char)*s)) \
+			s++; \
+		if (s >= e) \
+			break;
+
+		SKIP_WS;
+		unsigned char c0 = *s++;
+		SKIP_WS;
+		unsigned char c1 = *s++;
+		SKIP_WS;
+		unsigned char c2 = *s++;
+		SKIP_WS;
+		unsigned char c3 = *s++;
+
+#undef SKIP_WS
 
 		if ((c0 | c1 | c2 | c3) > 127)
 			goto error;

t/1701-mshow-regress.t

@@ -1,7 +1,7 @@
 #!/bin/sh -e
 cd ${0%/*}
 . ./lib.sh
-plan 2
+plan 3
 
 # Mail with \n\n and \r\n\r\n
 cr=$(printf '\r')
@@ -27,3 +27,27 @@ EOF
 
 check 'mail has 3 attachments' 'mshow -t ./tmp | wc -l | grep 4'
 check 'mail attachment foo has size 35' 'mshow -t ./tmp | grep size=35.*name=\"foo\"'
+
+# Mail with linebreaks in base64 quartets
+cat <<EOF >tmp
+Subject: base64
+MIME-Version: 1.0
+Content-Type: multipart/mixed; boundary="----_=_2f8f1e2243b55f8618eaf0d9_=_"
+
+This is a multipart message in MIME format.
+
+------_=_2f8f1e2243b55f8618eaf0d9_=_
+Content-Disposition: attachment; filename=base64
+Content-Type: application/binary
+Content-Transfer-Encoding: base64
+
+dGhp
+cyBpc
+yBzb21
+lIGJhc2
+U2NAo=
+
+------_=_2f8f1e2243b55f8618eaf0d9_=_--
+EOF
+
+check 'mail decodes correctly' 'mshow -O ./tmp 2 | grep -q "this is some base64"'