The OpenTag crypto security module implements cryptographic transforms and key exchange primitive functions as needed by a subset of crypto-security methods supported in the DASH7 Mode 2 specification.
The crypto module is not required for Mode 2 usage, per se, because the same functionality is available in the Authentication Module. The Authentication Module is specifically set-up for Mode 2 usage, whereas the crypto module is simply a generic interface to the supported cryptographic primitives.
The primary type of cryptography in OpenTag is EAX. EAX is a cipher based on AES, which offers both privacy and authentication. The EAX specification supported by the OpenTag Crypto Module is specifically:
As such, it is best when used with relatively small payloads (<1280 bytes). Indeed, this is exactly what OpenTag tends to work with. The 128 bit key is not viewed as a security issue because the ability to attack OpenTag devices is heavily constrained by the relative slowness of its I/O.