Hi! We recently found ourselves on a weird situation where strings like !Y@Q#=$=^ validated as base64 without issues. We understand that these strings are valid per RFC2045 but ended up puzzled on the XML Schema description for base64Binary.

Let me quote both specifications:

XML Schema Part 2

The lexical forms of base64Binary values are limited to the 65 characters of the Base64 Alphabet defined in [RFC 2045], i.e., a-z, A-Z, 0-9, the plus sign (+), the forward slash (/) and the equal sign (=), together with the characters defined in [XML 1.0 (Second Edition)] as white space. No other characters are allowed.

RFC2045

               Table 1: The Base64 Alphabet

Value Encoding  Value Encoding  Value Encoding  Value Encoding
    0 A            17 R            34 i            51 z
    1 B            18 S            35 j            52 0
    2 C            19 T            36 k            53 1
    3 D            20 U            37 l            54 2
    4 E            21 V            38 m            55 3
    5 F            22 W            39 n            56 4
    6 G            23 X            40 o            57 5
    7 H            24 Y            41 p            58 6
    8 I            25 Z            42 q            59 7
    9 J            26 a            43 r            60 8
   10 K            27 b            44 s            61 9
   11 L            28 c            45 t            62 +
   12 M            29 d            46 u            63 /
   13 N            30 e            47 v
   14 O            31 f            48 w         (pad) =
   15 P            32 g            49 x
   16 Q            33 h            50 y

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.

Relevant code

Base64BinaryType.java:119

/**
 * computes the length of binary data.
 * 
 * This function also performs format check.
 * @return    -1        if format is illegal.
 * 
 */
private static int calcLength( final char[] buf ) {
    final int len = buf.length;
    int base64count=0, paddingCount=0;
    int i;

    for( i=0; i<len; i++ ) {
        if( buf[i]=='=' )    // decodeMap['=']!=-1, so we have to check this first.
            break;
        if( buf[i]>=256 )
            return -1;      // incorrect character
        if( decodeMap[buf[i]]!=-1 )
            base64count++;
    }

    // once we saw '=', nothing but '=' can be appeared.
    for( ; i<len; i++ ) {
        if( buf[i]=='=' ) {
            paddingCount++;
            continue;
        }
        if( buf[i]>=256 )
            return -1;      // incorrect character
        if( decodeMap[buf[i]]!=-1 )
            return -1;
    }

    // no more than two paddings are allowed.
    if( paddingCount > 2 )        return -1;
    // characters must be a multiple of 4.
    if( (base64count+paddingCount)%4 != 0 )    return -1;

    return ((base64count+paddingCount)/4)*3-paddingCount;
}

I do think that some facility for requesting stricter validation is needed. On the spec-compliance side of things I don't know how to position myself given these conflicting accounts (XML Schema depends on the RFC).