Một số kết quả thí nghiệm cho thấy không an toàn của Khối Cơ yếu Chaining (CBC) - một kỹ thuật phổ biến để mở rộng các thuật toán mã hóa vượt quá kích thước khối tăng lên khi O 2 / 2 n, trong đó n là kích thước khối bit, và s là số khối được mã hóa [20]. | 194 Chapter 10 Grid Networks and Layer 3 Services Some experimental results indicate that the insecurity of Cipher Block Chaining CBC - a popular technique to extend ciphers beyond block size - increases as O s2 2n where n is the block size in bits and s is the number of blocks encrypted 20 . Intuitively this insecurity results from the dependency of the ciphertext of one block upon both the plain text for that block and the ciphertext of the preceding block. Should two blocks yield the same ciphertext and the plaintexts of the next block are also identical then the ciphertexts of the next block will be identical. This situation creates a vulnerability to certain types of malicious attacks. As a matter of common practice a rekeying event should occur any time B bytes have been sent through an encrypted security association whose crypto-transform uses a CBC mode of operation . 3DES 21 . This event sets B to n 8 2n 2l wherein n is the block size in bits. A security association that uses 3DES n 64 at 1 Gbps requires a keying event every seconds. At 10 Gbps it requires a keying event every seconds. As speeds increase designers will need to take into account the challenge of executing a rekeying event at shorter intervals. It should be noted that a keying event typically requires a very large integer exponentiation which is a very demanding challenge when compared with ordinary message crypto-processing. IP MULTICAST The IP multicast extensions 22 were introduced to relieve the network from forwarding as many copies of a set of data as there are receivers. Furthermore its receiver-driven style of operation is meant to relieve the sender or publisher from tracking the subscribers to the data. In general the promise to mitigate traffic volumes and complexity is likely to appeal to Grid communities especially when multicast aligns with a push style of data diffusion in the Grid. In practice however few networks have IP multicast enabled on the scale .
