“Why not a specific sequence of alphanumeric characters that happens to be an encryption key?”
Because the encryption key is nothing but a number. An integer, to be exact, albeit a rather lengthy one. Speaking of which, has anyone managed to convert this string to a decimal integer, yet. I gave it a quick try with Windows calculator and some online converters, but none of them support enough digits to do it.
FYI, the decimal integer 13,256,278,887,989,457,651,018,865,901,401,704,640 has a hexadecimal representation of 09F911029D74E35BD84156C5635688C0