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In contrast to the short form, the long form length octets consist of an initial octet and one or more subsequent octets. According to the X.690 standard [1] the initial length octet shall be encoded as follows:

bit 8 shall be one;
bits 7 to 1 shall encode the number of subsequent octets in the length octets, as an unsigned binary integer with bit 7 as the most significant bit;
the value 111111112 shall not be used.

http://en.wikipedia.org/wiki/Basic_Encoding_Rules#Length_Octets