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OCB is the Offset Code Book mode. The mode provides an efficient form of authenticated encryption with additional data, which is commonly referred to as an AEAD scheme. OCB mode is efficient because blocks of data are visited once to generate the cipher text and create the authentication tag. In contrast, modes like CCM, EAX and GCM visit the block of data twice - once to encrypt the data and once to create the authentication tag due to patent encumbrances. For more information, see Dr. David Wagner's comparison of AEAD schemes at AEAD Comparison.

This mode has been patented by Prof Phil Rogaway and licensed to OpenSSL with the license grant available here. Dr. Rogawy's paper can be found at Authenticated-Encryption with Associated-Data.