56.4.1 Operating Modes

The TDES supports the following operating modes:

• ECB—Electronic Code Book
• CBC—Cipher Block Chaining
• OFB—Output Feedback
• CFB—Cipher Feedback
  • CFB8 (CFB where the length of the data segment is 8 bits)
  • CFB16 (CFB where the length of the data segment is 16 bits)
  • CFB32 (CFB where the length of the data segment is 32 bits)
  • CFB64 (CFB where the length of the data segment is 64 bits)

The data pre-processing, post-processing and data chaining for each mode are automatically performed. Refer to the FIPS Publication 81 for more complete information.

These modes are selected by setting TDES_MR.OPMOD.

In CFB mode, four data sizes are possible (8, 16, 32 and 64 bits), configurable in TDES_MR.CFBS (see TDES Mode Register).