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).