Writeup bài Bánh cuốn - by qt: Crypto 100 This is basically a substitution cipher. You lookup the characters corresponding to your numbers using the table. For some numbers that are multiplication of 2 other numbers, use them as row/column indices. I solved this one by replacing the high-frequency numbers with characters and used SCBSolvr to do the rest. Writeup bài Bánh tráng cuốn thịt heo - by qt: Crypto 300 The key is generated from a 8-byte seed so it has a very big weakness: it's repeating after every 72 bytes. Using known plain-text attack you can recover the key and decrypt the text.