Disclosed herein are methods and systems for configuring and using one or more block ciphering techniques in order to encrypt/decrypt serial data streams while maintaining cryptographic synchronization and attempting to minimize the amount of overhead introduced into the stream. The techniques disclosed herein may be used to encrypt and decrypt serial data streams using a block cipher in a manner that can be substantially transparent to the devices involved in the serial communication session. For example, the serial user data may be left unframed by the encryption device while monitoring for opportunistic times to transmit framed cryptographic synchronization information during periods of relative inactivity in an asynchronous serial data stream. A cryptographic device implementing the techniques described herein may be configured to implement one or more of an encryption device or a decryption device.