[AMG]: This page exists as a temporary repository for a file that the SourceForge bug tracker won't let me attach.  Will some SF administrator please file a way to get the file into the bug tracker, then delete this page?  Thanks.  The bug URL is: [https://sourceforge.net/tracker/index.php?func=detail&aid=3033307&group_id=10894&atid=110894]

Here's the patch:

======none
diff -ur tcl8.6b3~/generic/tclBinary.c tcl8.6b3/generic/tclBinary.c
--- tcl8.6b3~/generic/tclBinary.c        2012-09-17 07:25:44.000000000 -0500
+++ tcl8.6b3/generic/tclBinary.c        2012-10-16 19:04:02.000000000 -0500
@@ -2658,7 +2658,7 @@
     Tcl_Obj *const objv[])
 {
     Tcl_Obj *resultObj = NULL;
-    unsigned char *data, *datastart, *dataend, c;
+    unsigned char *data, *datastart, *dataend, c = '\0';
     unsigned char *begin = NULL;
     unsigned char *cursor = NULL;
     int strict = 0;
@@ -2691,43 +2691,82 @@
     while (data < dataend) {
         unsigned long value = 0;
 
-        for (i=0 ; i<4 ; i++) {
+        /*
+         * Decode the current block. Each base64 block consists of four input
+         * characters A-Z, a-z, 0-9, +, or /. Each character supplies six bits
+         * of output data, so each block's output is 24 bits (three bytes) in
+         * length. The final block can be shorter by one or two bytes, denoted
+         * by the input ending with one or two ='s, respectively.
+         */
+        for (i = 0; i < 4; i++) {
+            /*
+             * Get the next input character. At end of input, pad with at most
+             * two ='s. If more than two ='s would be needed, instead discard
+             * the block read thus far.
+             */
             if (data < dataend) {
                 c = *data++;
+            } else if (i > 1) {
+                c = '=';
+            } else {
+                cut += 3;
+                break;
+            }
 
-                if (c >= 'A' && c <= 'Z') {
-                    value = (value << 6) | ((c - 'A') & 0x3f);
-                } else if (c >= 'a' && c <= 'z') {
-                    value = (value << 6) | ((c - 'a' + 26) & 0x3f);
-                } else if (c >= '0' && c <= '9') {
-                    value = (value << 6) | ((c - '0' + 52) & 0x3f);
-                } else if (c == '+') {
-                    value = (value << 6) | 0x3e;
-                } else if (c == '/') {
-                    value = (value << 6) | 0x3f;
-                } else if (c == '=') {
+            /*
+             * Load the character into the block value. Handle ='s specially
+             * because they're only valid as the last character or two of the
+             * final block of input. Unless strict mode is enabled, skip any
+             * input whitespace characters.
+             */
+            if (cut) {
+                if (c == '=' && i > 1) {
                     value <<= 6;
-                    if (cut < 2) {
-                        cut++;
-                    }
-                } else {
-                    if (strict || !isspace(c)) {
-                        goto bad64;
-                    }
+                    cut++;
+                } else if (!strict && isspace(c)) {
                     i--;
-                    continue;
+                } else {
+                    goto bad64;
                 }
-            } else {
+            } else if (c >= 'A' && c <= 'Z') {
+                value = (value << 6) | ((c - 'A') & 0x3f);
+            } else if (c >= 'a' && c <= 'z') {
+                value = (value << 6) | ((c - 'a' + 26) & 0x3f);
+            } else if (c >= '0' && c <= '9') {
+                value = (value << 6) | ((c - '0' + 52) & 0x3f);
+            } else if (c == '+') {
+                value = (value << 6) | 0x3e;
+            } else if (c == '/') {
+                value = (value << 6) | 0x3f;
+            } else if (c == '=') {
                 value <<= 6;
                 cut++;
+            } else if (strict || !isspace(c)) {
+                goto bad64;
+            } else {
+                i--;
             }
         }
         *cursor++ = UCHAR((value >> 16) & 0xff);
         *cursor++ = UCHAR((value >> 8) & 0xff);
         *cursor++ = UCHAR(value & 0xff);
-    }
-    if (cut > size) {
-        cut = size;
+
+        /*
+         * Since = is only valid within the final block, if it was encountered
+         * but there are still more input characters, confirm that strict mode
+         * is off and all subsequent characters are whitespace.
+         */
+        if (cut && data < dataend) {
+            if (strict) {
+                goto bad64;
+            } else {
+                for (; data < dataend; data++) {
+                    if (!isspace(*data)) {
+                        goto bad64;
+                    }
+                }
+            }
+        }
     }
     Tcl_SetByteArrayLength(resultObj, cursor - begin - cut);
     Tcl_SetObjResult(interp, resultObj);
diff -ur tcl8.6b3~/tests/binary.test tcl8.6b3/tests/binary.test
--- tcl8.6b3~/tests/binary.test        2012-06-26 08:59:02.000000000 -0500
+++ tcl8.6b3/tests/binary.test        2012-10-16 18:43:50.000000000 -0500
@@ -2642,6 +2642,27 @@
 test binary-73.24 {binary decode base64} -body {
     string length [binary decode base64 " "]
 } -result 0
+test binary-73.25 {binary decode base64} -body {
+    list [string length [set r [binary decode base64 WA==\n]]] $r
+} -result {1 X}
+test binary-73.26 {binary decode base64} -body {
+    list [string length [set r [binary decode base64 WFk=\n]]] $r
+} -result {2 XY}
+test binary-73.27 {binary decode base64} -body {
+    list [string length [set r [binary decode base64 WFla\n]]] $r
+} -result {3 XYZ}
+test binary-73.28 {binary decode base64} -body {
+    list [string length [set r [binary decode base64 -strict WA==\n]]] $r
+} -returnCodes error -match glob -result {invalid base64 character *}
+test binary-73.29 {binary decode base64} -body {
+    list [string length [set r [binary decode base64 -strict WFk=\n]]] $r
+} -returnCodes error -match glob -result {invalid base64 character *}
+test binary-73.30 {binary decode base64} -body {
+    list [string length [set r [binary decode base64 -strict WFla\n]]] $r
+} -returnCodes error -match glob -result {invalid base64 character *}
+test binary-73.31 {binary decode base64} -body {
+    list [string length [set r [binary decode base64 WA==WFla]]] $r
+} -returnCodes error -match glob -result {invalid base64 character *}
 
 test binary-74.1 {binary encode uuencode} -body {
     binary encode uuencode
======

Hopefully the Wiki won't destroy all the tabs... *sigh*