Affine cipher gets the same key in C language

Question

I tried to encrypt an affine cipher reading from text file. My .txt files include two key: alpha and beta . Alpha should be an odd number because gcd(alpha, 26)==1 and generate random key. Here is my code:

beta = rand() % 26; 

but encryption doesn't work correctly. For example: the encrypted word "bahar" , when encrypted, turns out in the "XWDWN" word but it should be start "M" not "X" . Keys are alpha = 3 and beta = 9 but my code selected key (1, 22) alpha=1 beta=22 . Constantly alpha has been selected "1" . Another encryption for example (1, 17) .. etc.

Answer 1

这是加密的方式

putc(((toupper(p) - 'A')*alpha + beta) % 26 + 'A', cipher);