In the previous blog “The Many Flavors of AES – Part 1″ we gave a little background on the AES standard and its most used modes which are tied to specific applications. In the current blog we will dive deeper into the world of AES “modes”.
Some of the better known and widely used AES “modes” are: ECB/CBC/OFB/CFB/CTR/CCM/GCM/XTS.
A few words about ECB/CBC/OFB/CFB/CTR. These are approved modes of using a block cipher (like AES) and are covered in detail in NIST SP 800-38A (csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf). A nice picture of ECB/CBC/OFB/CFB can be found on page 229 of the Handbook of Applied Cryptography. ECB or Electronic Code Book is the base use of AES. A pure block cipher application where each transaction involves putting in a 128, 192, or 256-bit key and a 128-bit block of data, and sending it through the AES cipher with a 128-bit of data coming out. Either a 128-bit block of plaintext  data goes in with a key and a 128-bit block of ciphertext comes out, or the reverse. Nothing is saved between blocks. This is nice, but in truth AES-ECB is not very useful (safe) because it is susceptible to cryptographic attacks; it essentially breaks down to a simple substitution cipher, where a given 128-bit block of plaintext is always encrypted to the same 128-bit block of ciphertext, and thus can be statistically analyzed.
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