Solving linear congruences

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In summary, a linear congruence is an equation in the form of ax ≡ b (mod m), where a, b, and m are integers. It can be solved using the Euclidean algorithm, and can have multiple solutions. The Chinese remainder theorem relates to linear congruences by providing a way to find solutions to systems of congruences with different moduli. Linear congruences are also used in cryptography, particularly in the RSA encryption algorithm, to ensure the security of encrypted messages.
  • #1
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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  • #2
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.
 
  • #3



To find the decryption function, we need to use the inverse of 7 in mod 26. This means finding a number that, when multiplied by 7, gives a result of 1 when taken mod 26. In this case, the inverse of 7 is 15 (since 7x15 = 105 and 105 mod 26 = 1).

So, the decryption function would be D = (15C + 9) mod 26. This means that to decrypt a ciphertext (represented by C), we would multiply it by 15 and add 9, and then take the result mod 26. This will give us the original message (represented by M).

In summary, to solve linear congruences and find the decryption function, we need to find the inverse of the coefficient in mod 26 and use it in the formula D = (inverse of coefficient * C + constant) mod 26. This will give us the decryption function that we can use to decrypt any ciphertext.
 

1. What is a linear congruence?

A linear congruence is an equation in the form of ax ≡ b (mod m), where a, b, and m are integers. It represents a relationship between two numbers, where the remainder of their division by a third number is the same.

2. How do you solve a linear congruence?

To solve a linear congruence, you can use the Euclidean algorithm to find the greatest common divisor (GCD) of the coefficients a and m. Then, if b is divisible by the GCD, the solution is the remainder of b divided by the GCD. Otherwise, the linear congruence has no solution.

3. Can a linear congruence have multiple solutions?

Yes, a linear congruence can have multiple solutions. This is because the solutions are congruent modulo m, meaning they are equivalent in terms of their remainder when divided by m. Therefore, there can be multiple numbers that satisfy the linear congruence.

4. What is the Chinese remainder theorem and how does it relate to linear congruences?

The Chinese remainder theorem is a mathematical theorem that states that if we have a set of congruences with pairwise relatively prime moduli, then there is a unique solution modulo the product of the moduli. This theorem is often used to find solutions to systems of linear congruences with different moduli.

5. How are linear congruences used in cryptography?

Linear congruences are used in cryptography to ensure the security of encrypted messages. In particular, they are used in the RSA encryption algorithm, which relies on the difficulty of solving linear congruences with large numbers to protect sensitive information.

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