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$(1$ point$)$ A cryptography student has accidentally used the same keystream to encrypt two different messages, $M_1$ and $M_2.$ We know that the ciphertexts are $C_1=0x6d4b77945914e2c5$ and $C_2=0x71681d7da4fc4f3e.$ We also happen to know that $M_1=0x19867a7e0fdd3ab6.$

a$)$ What is $M_2$ in hexadecimal? Please give your answer a leading $0x$ and use lowercase letters only.

b$)$ What is the keystream? Please give your answer a leading $0x$ and use lowercase letters only.

Hint: These are all $8$ byte numbers and will fit in an unsigned long type on most systems.