Breaking the 40 bit DVD CSS stream cipher
There is a Berlekamp-Massey algorithm that constructs the shortest LFSR for a given binary sequence.
For an LFSR of length $L$ given $2L$ output sequence from the LFSR, it is enough to construct the LFSR. To construct a 25-bit LSFR, 50 bits is enough. Note that, the algorithm doesn't need to know the taps. It just constructs the minimum LFSR that can produce the given sequence.
Looking at the 20-byte (160-bit) you can make sure that the remaining 110-bits are output from the LFSR. We don't expect that with a negligible $1/2^{110}$ probability the LFSR will succeed to produce the random bits.
To Study the LFSR's

Solomon W Golomb's book Shift Register Sequences: Secure And Limited-access Code Generators, Efficiency Code Generators, Prescribed Property Generators, Mathematical Models (Third Revised Edition), is advised.