How Do I Find the Decryption Function for C = (7M + 9) mod 26?

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SUMMARY

The decryption function for the equation C = (7M + 9) mod 26 can be derived by finding the multiplicative inverse of 7 in the group of integers modulo 26, denoted as U(26). The multiplicative inverse of 7 is 15, which satisfies the equation 7 * 15 ≡ 1 (mod 26). To decrypt, the formula can be rearranged to M = (15(C - 9)) mod 26, allowing for the recovery of the original message M from the ciphertext C.

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stunner5000pt
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another little bit of help and this is NOT a homework assignment this is for an exam i need to understand it

the qustion is find the decryption function for C = (7M + 9 ) mod 26

how do i fin the decryption function using the inverse?


that is 15 in this case
 
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the same way you do it for any other equation:
you've just got to find what the multiplicative inverse of 7 is in U(26)
I don't see where 15 comes from.
 

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