bson/src/main/org/bson/internal/Base64.java
public static byte[] decode(final String s) {
        int delta = s.endsWith("==") ? 2 : s.endsWith("=") ? 1 : 0;
        byte[] buffer = new byte[s.length() * BYTES_PER_UNENCODED_BLOCK / BYTES_PER_ENCODED_BLOCK - delta];
        int mask = 0xFF;
        int pos = 0;
        for (int i = 0; i < s.length(); i += BYTES_PER_ENCODED_BLOCK) {
            int c0 = DECODE_TABLE[s.charAt(i)];
            int c1 = DECODE_TABLE[s.charAt(i + 1)];
            buffer[pos++] = (byte) (((c0 << 2) | (c1 >> 4)) & mask);
            if (pos >= buffer.length) {
                return buffer;
            }
            int c2 = DECODE_TABLE[s.charAt(i + 2)];
            buffer[pos++] = (byte) (((c1 << 4) | (c2 >> 2)) & mask);
            if (pos >= buffer.length) {
                return buffer;
            }
            int c3 = DECODE_TABLE[s.charAt(i + 3)];
            buffer[pos++] = (byte) (((c2 << 6) | c3) & mask);
        }
        return buffer;
    }
public static byte[] decode(final String s) {
        int delta = s.endsWith("==") ? 2 : s.endsWith("=") ? 1 : 0;
        byte[] buffer = new byte[s.length() * BYTES_PER_UNENCODED_BLOCK / BYTES_PER_ENCODED_BLOCK - delta];
        int mask = 0xFF;
        int pos = 0;
        for (int i = 0; i < s.length(); i += BYTES_PER_ENCODED_BLOCK) {
            int c0 = DECODE_TABLE[s.charAt(i)];
            int c1 = DECODE_TABLE[s.charAt(i + 1)];
            buffer[pos++] = (byte) (((c0 << 2) | (c1 >> 4)) & mask);
            if (pos >= buffer.length) {
                return buffer;
            }
            int c2 = DECODE_TABLE[s.charAt(i + 2)];
            buffer[pos++] = (byte) (((c1 << 4) | (c2 >> 2)) & mask);
            if (pos >= buffer.length) {
                return buffer;
            }
            int c3 = DECODE_TABLE[s.charAt(i + 3)];
            buffer[pos++] = (byte) (((c2 << 6) | c3) & mask);
        }
        return buffer;
    }
Decodes the given Base64-encoded string.

@param s the Base64-encoded string
@return the decoded byte array