Hill cipher -- obtain matrix key I have to solve Hill's cipher. I have only cipher text: CGQIYN and key: CFDG. I don't know how to decrypt it, because in all videos I've watched they knew the Matrix key from the beginning. I don't want you to solve it, but I am really hopeless and I absolutely don't know how to do it. If you can provide me some procedure, I will be grateful. Thank you.
The key is 4 characters long, therefore it must be in a 2 × 2 matrix. The numbers in this matrix must be the inverse of the encryption key matrix, and there are various methods to work this out (see this link). Once the matrix inversion has been calculated, you multiple it through each part of the cipher text in their respective 2 × 1 matrices