At 11:10 AM -0600 12/22/97, David Honig wrote:
At 06:46 PM 12/20/97 -0700, Johnson, Michael P (Mike) wrote:
cfb Ciphertext feeback mode c[i] = f1(K, c[i-1]) ^ p[i] p[i] = f1(K, c[i-1]) ^ c[i]
Suppose instead of c[i-1] you use c[i-N] where N is say 10. How would you prove that this has no security implications? That 10-way interleaved cfb streams are security-equivalent to a single cfb stream interleaved with the immediately previous block?
It's kind of obvious. The encryption of a single plaintext stream interleaved ten times is the same as the encryption of ten multiplexed plaintexts. If one is insecure, the other is insecure. Bruce ********************************************************************** Bruce Schneier, President, Counterpane Systems Phone: 612-823-1098 101 E Minnehaha Parkway, Minneapolis,MN 55419 Fax: 612-823-1590 http://www.counterpane.com