A writing method of a crypto device includes receiving a write request from a central processing unit, determining a write attribute of the write request, and performing one of a partial write operation and a full write operation according to the write attribute. In the full write operation, a random number for a version count is generated, a key stream is generated using the version count, the key stream and write data are encrypted in a first logical operation, and the encrypted data and the version count are stored in a memory device.