Re: Double Encryption Q
If your original mode of operation is secure, then this should be secure. The reduction: Consider algorithm A that tries to break the double encryption mode of operation (DM) in the IND-CPA setting. We can construct an algorithm B that tries to break the original mode of operation (OM) in the IND- CPA setting. B simply runs A and responds to A's queries by querying B's oracle twice to simulate A's oracle, and returning the result. B returns the output of A. If A breaks the encryption, so does B. So if the original mode is IND- CPA secure, this double encryption should be okay. Note that the examples given, OCB and CTR with repeated counters, are not IND-CPA secure. Martin Cochran On Apr 11, 2008, at 8:30 AM, COMINT wrote: Quick system scenario: You have packet [A]. It gets encrypted using an AES algo in a particular mode and we are left with [zA]. More data [B] is added to that encrypted packet. Now I have [zA]+[B] in one packet and I re-encrypt it with the same algo/key/mode. Have I just compromised the security somehow? I wasn't aware of anything but something about this double encryption made something ring in my mind so I wanted to double check... Many thanks, Mr Pink - The Cryptography Mailing List Unsubscribe by sending "unsubscribe cryptography" to [EMAIL PROTECTED] - The Cryptography Mailing List Unsubscribe by sending "unsubscribe cryptography" to [EMAIL PROTECTED]
Re: Double Encryption Q
There are some situations when this can be dangerous. It's a matter of implementation. I can directly come up with one trivial scenario that will end you up in trouble: Assume that you are using AES-CTR (AES in Counter mode) and do not change the IV between the two encryptions. In this case you will correctly encrypt the data, but the second encryption will leave A unprotected. /Pehr Söderman COMINT wrote: > Quick system scenario: > > You have packet [A]. > > It gets encrypted using an AES algo in a particular mode and we are > left with [zA]. > > More data [B] is added to that encrypted packet. > > Now I have [zA]+[B] in one packet and I re-encrypt it with the same > algo/key/mode. > > Have I just compromised the security somehow? I wasn't aware of > anything but something about this double encryption made something > ring in my mind so I wanted to double check... > > Many thanks, > > Mr Pink > > - > The Cryptography Mailing List > Unsubscribe by sending "unsubscribe cryptography" to [EMAIL PROTECTED] > > - The Cryptography Mailing List Unsubscribe by sending "unsubscribe cryptography" to [EMAIL PROTECTED]
Re: Double Encryption Q
On Fri, Apr 11, 2008 at 04:30:47PM +0200, COMINT wrote: > Quick system scenario: > > You have packet [A]. > > It gets encrypted using an AES algo in a particular mode and we are > left with [zA]. > > More data [B] is added to that encrypted packet. > > Now I have [zA]+[B] in one packet and I re-encrypt it with the same > algo/key/mode. > > Have I just compromised the security somehow? I wasn't aware of > anything but something about this double encryption made something > ring in my mind so I wanted to double check... This would certainly cause problems in if "particular mode" == OFB or counter, since (if you also reuse the IVs), you could have E(zA) == A. If you use a different (independent, unrelated) key/IV, then the existence of a weakness in this scheme would seem to provide evidence of an attack on AES, regardless of mode choice. -Jack - The Cryptography Mailing List Unsubscribe by sending "unsubscribe cryptography" to [EMAIL PROTECTED]
Double Encryption Q
Quick system scenario: You have packet [A]. It gets encrypted using an AES algo in a particular mode and we are left with [zA]. More data [B] is added to that encrypted packet. Now I have [zA]+[B] in one packet and I re-encrypt it with the same algo/key/mode. Have I just compromised the security somehow? I wasn't aware of anything but something about this double encryption made something ring in my mind so I wanted to double check... Many thanks, Mr Pink - The Cryptography Mailing List Unsubscribe by sending "unsubscribe cryptography" to [EMAIL PROTECTED]
