They used the first 64 bytes of key1 and the first 65 bytes of key2. key 1 and 2 are the 300D keys which are originally 512 and 513 bytes long.
The algorithm works in a cycle with two counterrotating inner cycles, as follows:
do
decode 64 bytes, begin with key1[ 0] and key2[ 0]
decode 1 bytes, begin with key1[ 0] and key2[64]
decode 63 bytes, begin with key1[ 1] and key2[ 0]
decode 2 bytes, begin with key1[ 0] and key2[63]
decode 62 bytes, begin with key1[ 2] and key2[ 0]
decode 3 bytes, begin with key1[ 0] and key2[62]
decode 61 bytes, begin with key1[ 3] and key2[ 0]
decode 4 bytes, begin with key1[ 0] and key2[61]
.
.
.
decode 2 bytes, begin with key1[62] and key2[ 0]
decode 63 bytes, begin with key1[ 0] and key2[ 2]
decode 1 bytes, begin with key1[63] and key2[ 0]
decode 64 bytes, begin with key1[ 0] and key2[ 1]
while decodedbytes != 0
If you just blindly calculate it, the resulting one-dimensional cipher block is 4160 bytes long, which is c-letter's size (0x1040 = 4160).
