1,993 reputation
716
bio website
location
age
visits member for 3 years, 1 month
seen 2 mins ago

I am a math/computer nerd. Nothing to see here, move along.


Sep
20
reviewed Excellent Combining two keys
Sep
20
reviewed Excellent Elliptic Curve Cryptography
Sep
20
reviewed Excellent The REACT transform and Replayable CCA
Sep
20
reviewed Excellent Point addition equation in projective co ordinates
Sep
20
reviewed Satisfactory Is SRP post-quantum secure?
Sep
20
reviewed Satisfactory Can you fake messages from recorded message-history?
Sep
20
reviewed Satisfactory Secure MultiParty Computation with secret inputs for secret outputs
Sep
19
comment Authenticating Very Short (Sub-Block) Data
Addendum: I should probably also note that this approach won't protect you against replay attacks. While this is true of authenticated encryption in general, it's a harder problem to solve if messages are too short to include a monotonically increasing counter or some other protocol-level countermeasure. Without knowing more context, it's hard to know what threats you should be concerned with.
Sep
19
comment Authenticating Very Short (Sub-Block) Data
Length-preserving schemes are by definition FPE. But if bandwidth is an issue, you can get very short authenticated ciphertexts by using encode-then-encipher (seclab.cs.ucdavis.edu/papers/Rogaway/encode.pdf). If you pad your 64-bit payload with 0s until it's 128 bits long and encrypt using AES (in ECB mode, no IV), then you can check authenticity by verifying that the padding is all 0s when you decrypt. Some warnings: (1) an attacker will be able to learn if a message is repeated (2) this only works if messages are significantly shorter than the AES block length (128 bits).
Sep
18
revised Authenticating Very Short (Sub-Block) Data
added 1 character in body
Sep
18
answered Authenticating Very Short (Sub-Block) Data
Sep
2
awarded  Custodian
Sep
2
reviewed Close Do any non-US ciphers exist?
Sep
2
reviewed Close How to make a document truly read only
Sep
2
reviewed Close How Proof all Subgroup point of prime order elliptic curve have prime order [#G=#E]?
Aug
25
reviewed Close Seemingly simple decryption question
Aug
25
reviewed Close Counter Mode: how to choose the nonce part of the counter?
Aug
25
reviewed Close Solution with high decryption cost and low encryption cost
Aug
23
comment Efficiently map $2^n$ unique 64-bit vectors to $2^n$ $n$-bit vectors where $n < 64$?
I'm not sure what you're arguing. For concreteness, let's use the example I gave earlier, and suppose that, say, n = 63, both x1 and x2 are at least 2^63, x2 = DES_K(x1), and suppose that y = DES_K(x2) < 2^63. My understanding of your proposal is that both x1 and x2 would map to y because this is the first value in [0, 2^63) we reach by repeated application of DES_K, regardless of whether we start from x1 or from x2. If this is incorrect, could you please clarify how the mapping is computed?
Aug
23
comment Efficiently map $2^n$ unique 64-bit vectors to $2^n$ $n$-bit vectors where $n < 64$?
The "cycle-walking" method wouldn't work here because your target space is not your source space. For example, you could have two inputs X1 and X2 in your source space such that AES_K(X1) = X2 (it would make slightly more sense to use DES here, but same thing would happen). In this case, X1 and X2 would then map to the same point in your target space, unless X2 was already in it.