private int getLength() throws IOException {

        int i = in.read();
        if (i == -1)
            throw new IOException("Invalid DER: length missing"); //$NON-NLS-1$

        // A single byte short length
        if ((i & ~0x7F) == 0)
            return i;

        int num = i & 0x7F;

        // We can't handle length longer than 4 bytes
        if (i >= 0xFF || num > 4)
            throw new IOException("Invalid DER: length field too big (" //$NON-NLS-1$
                    + i + ")"); //$NON-NLS-1$

        byte[] bytes = new byte[num];
        int n = in.read(bytes);
        if (n < num)
            throw new IOException("Invalid DER: length too short"); //$NON-NLS-1$

        return new BigInteger(1, bytes).intValue();
    }
Decode the length of the field. Can only support length encoding up to 4
octets.

<p/>
In BER/DER encoding, length can be encoded in 2 forms,
<ul>
<li>Short form. One octet. Bit 8 has value "0" and bits 7-1 give the
length.
<li>Long form. Two to 127 octets (only 4 is supported here). Bit 8 of
first octet has value "1" and bits 7-1 give the number of additional
length octets. Second and following octets give the length, base 256,
most significant digit first.
</ul>

@return The length as integer
aws-java-sdk-cloudfront/src/main/java/com/amazonaws/auth/DerParser.java