13,536 reputation
33272
bio website paul-ebermann.tumblr.com
location Berlin, Germany
age
visits member for 3 years, 4 months
seen 3 hours ago

Don't fear to edit my posts: even if I have more reputation than you, I do make mistakes.

I'm now also a Moderator Pro Tempore (= until the first elections) at Cryptography Stack Exchange: feel free to come around and ask some cryptography questions.


My personal name is spoken as /ˈpawlo/ (IPA), in English this would be written similar to Powlo, I think (i.e. the vowels are ow and o), with an accent on the before-last syllable (which is the first in this case). It's the Esperanto form of my given name.

The photo shows my shadow, taken at night. My camera sometimes seems to forget all the other frequencies and only stores the green ones.

My current main private programming project is the game of fencing, an online abstract turn based strategy game. Implemented as a Java applet, using git as a version control system.


Some more links:


Nov
21
comment Trying to find a different DES encryption system explanation
Welcome to Cryptography Stack Exchange. Your two questions on security.stackexchange.com and math.stackexchange.com were migrated here because the questions related to the internals of a cryptographic algorithms, and thus are fully on-topic here. I then merged both questions. Please register your account here, too, to be able to comment and accept an answer.
Nov
21
comment Derived Shared Key vs Distinct Keys?
Actually, using the same stream cipher key (without an or with same initialization vector) for both directions looks like a two-times-pad, definitely not recommendable. And keeping track of two independent stream states seems to be easier than having a common crypto stream state used for communication in both directions. (Though maybe I am misunderstanding you.)
Nov
21
comment Derived Shared Key vs Distinct Keys?
Whether using the same key for authentication and encryption is a problem depends on your encryption and authentication algorithms/modes. HMAC combined with AES/CBC seems fine (since they use the key in quite different ways), but I would not use CBC-mode with CMAC with the same key. Either use different keys, or use an authenticated-encryption mode designed to do this with one key.
Nov
21
comment Derived Shared Key vs Distinct Keys?
When you use the same key for communication in both directions, a message sent from client to server can be sent back to the client, pretending it came from the server. This can also made impossible by other protocol measures (by including sender/receiver in the authenticated data), but using different keys is an easy way.
Nov
20
answered Derived Shared Key vs Distinct Keys?
Nov
20
revised Derived Shared Key vs Distinct Keys?
add the example from the comment
Nov
20
revised Can you help with that definition for a CCA?
add some details
Nov
20
answered Can you help with that definition for a CCA?
Nov
20
revised Can you help with that definition for a CCA?
formatting, tag
Nov
19
comment How does a chosen ciphertext attack work, with a simple example?
It depends on the cipher, really. A good cipher should not be suspectible to that kind of attack, but Henning's example shows that bad ciphers can be. And the XML-CBC example shows how this can even occur with a good cipher and a wrongly used good mode of operation.
Nov
19
comment How does a chosen ciphertext attack work, with a simple example?
I got this description from Wikipedia. But as all understand, all the IND-properties (indistinguishable) are like "attacker chooses two plaintexts and gets to see the ciphertext of one of them, then has to guess which one". (This actually correlates to a quite common scenario, where it is already narrowed down that Bob did send either "Yes" or "No".) They just differ about what means the attacker has to do this, like a ciphertext-only-attack, a chosen-plaintext attack, a chosen-ciphertext attack.
Nov
19
comment How does a chosen ciphertext attack work, with a simple example?
In a chosen-plaintext-attack, the attacker gets to chose plaintexts and sees their encryption. In a chosen-ciphertext attack, the attacker can only chose ciphertexts, and may see their decryption (when one has a decryption oracle), or only a bit less (for the validation oracle).
Nov
19
comment Is it possible to derive the encryption method from encrypted text?
Most stream ciphers are in practice also only used on the byte-level, even if they could have been used at the bit level. This will not help to distinguish.
Nov
19
revised Taking advantage of one-time pad key reuse?
formatting, spelling
Nov
19
revised Is the CBC weakness in XML Encryption a new discovery? Are other applications vulnerable?
edited tags
Nov
19
revised How does a chosen ciphertext attack work, with a simple example?
mention IND-CCA
Nov
19
answered How does a chosen ciphertext attack work, with a simple example?
Nov
18
comment Encryption technique performance evaluation
@mikeazo: I think this depends on the type of image. The original Tux image has a plain white background and quite some more one-colored areas, which of course causes each block in these to give the same result after encryption. I think it would work less good on a photographic image (at least with a block cipher in ECB mode - single XOR will be still worse, though). (The fact will be complicated that the cipher will likely destroy the image headers, not only change the pixels.)
Nov
18
comment Encryption technique performance evaluation
Welcome to Cryptography Stack Exchange. Your question was migrated here because of being not directly related to software development (the topic of Stack Overflow), and being fully on-topic here. Please register your account here, too, to be able to comment and accept an answer.
Nov
18
comment Encryption technique performance evaluation
What do you understand under "bitwise encryption scheme"? A stream cipher?