Well, let me eat my words. Unless all layers turn on encryption at the same time, and there is not predictable text that passes from one layer to the next, adding encryption at each layer cannot substantially improve the size of the key space. Consider two layers each of which has a verifiable header and a body of encrypted text. By "verifyable", I mean that it contains enough redundancy to recognize a correct decryption of the cipher added by the lower layer. For example, a header that included a content type field and a length field could be examined to see if it looked reasonable, and thus confirm a guess at the lower level's cipher. Plaintext-Body-1 | Layer-1-cipher | Header-1, Encrypted-Body-1 | Layer-2-cipher | Header-2, Encrypted-Body-2 To crack this system, an attacker does brute force search of the keyspace for the layer-2-cipher, for each key check the decrypted Header-1 value to see if it looks OK, if not, continue, otherwise start searching the keyspace for the Layer-1-cipher given the candidate for the Encrypted-Body-1 produced by the guess at the Layer-2-cipher key. Clearly, if you have several layer 2 blocks and they all have good looking values for the Header-1, then the Layer-2-cipher key is correct. The summary is that two layers of 40 bit ciphers with the first layer adding some verifiable information, has the effect of adding at most one bit to the effective keysize (doubling the amount of work). It DOES NOT increase the keysize to 80 bits. --Bob Baldwin ______________________________ Reply Separator _________________________________ Subject: 80 bit security from 40 bit exportable products Author: "baldwin" <baldwin@RSA.COM (Robert W. Baldwin)> at INTERNET Date: 10/24/95 10:52 AM Long ago vendors should have put encryption into network layer products, but for a variety of reasons that effort was delayed or discouraged. One effect of this lack is that almost every layer of the network stack is adding its own encryption. For example, the HTTP session layer added S-HTTP and the TCP transport layer added SSL. Soon we will have network layer encryption with IPsec. The vendors for each layer can export a product that uses ciphers with 40 bit keys. A user can then combine multiple products to get more than 40 bits worth of security. For example, a web client might fetch an S-HTTP page over an SSL protected link via a firewall that supports IPsec tunnels. That's three 40 bit keys protecting the data over the internet link (of course, this may not be equivalent to a 120 bit cipher, that depends on the details of the cipher systems and independence of the key setups). Interesting possibilities. --Bob Baldwin