1
vote
2answers
106 views

Encoding multiple fields with same AES key and vector?

I'd like some advice on cryptography best practices. I need to store on my web app the users' credentials (login & password) for 3rd-party sites. I'm using AES-256 with random key and ...
0
votes
1answer
112 views

Hashing/encrypting an integer to produce an unique integer in the same range

I'm looking for a function or set of functions that can produce a pseudo-random permutation on an input set of arbitrary size. Given a set of $M$ values in range $0..M-1$, where $M$ is positive ...
1
vote
1answer
147 views

Homomorphic Encryption, LWE, and Practical Applications

I've been reading Brakerski and Vaikuntanathan's Efficient Fully Homomorphic Encryption from (Standard) LWE. When the authors discuss a small modulus p used for the transition from (n, log q) to (k, ...
3
votes
1answer
65 views

Private Information Retrieval with a pre-prepared database

Informally, Private Information Retrieval (PIR) is a protocol between a user $U$ and a Server $S$, that meets the following conditions: $U$ obtains enough information about a database hosted by $S$, ...
2
votes
1answer
108 views

Authentication of machine before installation (to stop piracy) [closed]

I have a web-software project which needs a bit of protection in form of authencitation… This web based software runs on an intranet. There is no access to the internet from these terminal machines. ...
2
votes
0answers
78 views

how to truncate a PRF on n bits to PRF on t bits where t < n? [closed]

Dan Boneh's courersa lecture on format preserving encryption at 5.50 mins , talks about truncating a PRF on $n$ bits to PRF on $t$ bits. The approach is by appending $(n-t)$ zeros to input $t$ bits ...
1
vote
1answer
92 views

Applying differential cryptanalysis to ciphers with addition mod $2^{32}$

I am studying differential analysis and have a question. Consider the following simple cipher: $$c_1 = S(m_1 \oplus k_1) \oplus k_2$$ (Plaintext $m_1$ xor with key $K_1$, then result goes into an ...
1
vote
2answers
142 views

hill cipher encryption way 1x3 plaintext matrix

Can someone help me with a Hill cipher? When do I have to use: 1x3 plain text matrix (p1, p2, p3) * 3x3 key matrix 3x3 key matrix * 3x1 plain text matrix Or they are both correct? I tried to ...
0
votes
1answer
118 views

Is it possible to decrypt an RSA ciphertext with only $N$ and $e$?

Is it possible to perform RSA decryption if one only knows $N$ and $e$? Failing that, what is the maximum key length where brute-force is still practically feasible?
1
vote
1answer
99 views

How does the length of the plaintext affect the cipher strength?

Let the block length 64-bit, 256-bit key, cipher text accordingly - 64 bits. What is the strength of the block cipher, if any unknown attacks, which could reduce its strength. We can only brute force. ...
4
votes
2answers
209 views

Is the following symmetric design secure?

Assume: $O$ be a reversible random permutation oracle on a finite set and $O^{-1}$ the inverse permutation (pretty much equivalent to a random permutation: What is the difference between a bijective ...
3
votes
2answers
133 views

Merkle Tree High Tree

I'm reading Merkle tree signatures and I like know if the security of the that scheme depends, also, of the high of the tree or only depends that hash function is resistant to collision?
7
votes
2answers
495 views

When using Curve25519, why does the private key always have a fixed bit at 2^254?

When using Curve25519, the private key always seems to have a fixed bit set at position 2^254. Why is that? Is there any good reason to use a fixed positioned most-significant-bit in the private key? ...
2
votes
1answer
171 views

How to prove this LFSR equation?

I am having difficulty solving part (ii) of Excercise 5.17 from Becker's Cipher Systems. Exercise 5.17 $\quad$If $f(x),g(x)$ are any two polynomials over $GF(2)$ with $f(0)=g(0)=1$ show that ...
4
votes
2answers
288 views

How can I implement the elliptic curve MOV attack myself?

I understand and have implemented elliptic curve signatures in Python without the use of libraries like Sage, and would like to implement the MOV attack against certain weak types of elliptic curves. ...
1
vote
1answer
134 views

Secure function evaluation

Consider 3 parties, Alice, Bob and Charlie. Suppose each party has a bit as input, i.e. Alice, Bob and Charlie hold $a, b, c \in \{0, 1\}$ respectively. Show how construct a scheme with which ...
0
votes
0answers
72 views

hybrid PKE scheme CCA2 insecurity

I'm reading a paper here say: As we know, in hybrid PKE schemes XOR alone cannot perfectly hide challenge bit to the CCA2 adversary The author does not define hybrid PKE schemes. What is ...
1
vote
1answer
128 views

Perfect secrecy over Stirling numbers

For all $c_0\leftarrow m_0 \oplus k$ there exists a $k'$ such that $c_1 \leftarrow m_1 \oplus k'$, where $m_0 \neq m_1$ and $c_0 = c_1$. Assuming a truly random $k$, the first assignment is a ...
5
votes
1answer
328 views

Why does the crypto_box functionality in NaCl library exposes the nonce to the programmer

The idea of crypto_box API in NaCl library is to shield the programmer away from the technical details and provide an easy to use functions for encrypting and encrypting messages. Given what I've ...
2
votes
1answer
289 views

Is CBC mode encryption vulnerable to a reordering attack?

I know that, if CBC-MAC is used in such a way that the tag is the concatenation of all the blocks' outputs (and not just the last one), it's insecure under CPA for the simplest case of 2 blocks ...
1
vote
1answer
70 views

Is the following aggregation scheme private?

Is the following scheme private? By private i mean an untrusted aggregator (UA) cannot reveal anything other then an aggregate function output on plaintext data Each party holds a secret key $k_i$ ...
3
votes
1answer
96 views

Difference between computational and statistical indistinguishabilities

What is the difference between the two notions of computational and statistical indistinguishability?
2
votes
1answer
422 views

A key-derivation function that is as strong as the stronger of PBKDF2 and scrypt

I want a key stretching system that's as strong as the stronger of scrypt and PBKDF2. The consensus now is that scrypt is by far the better system, but that might change if in the future, a weakness ...
1
vote
1answer
68 views

Why send a challenge with CHAP?

I have been reading about the Challenge-Handshake authentication Protocol (aka CHAP). Why send a challenge to the user and ask them to hash that challenge with the symmetric key and send that hash ...
0
votes
3answers
199 views

Why not just generate random strings for one-time password (OTP)?

http://en.wikipedia.org/wiki/One-time_password#How_OTPs_are_generated_and_distributed Why have so many methods to generate OTPs, why not just use just random strings?
3
votes
2answers
95 views

Can we build authenticated encryption using Feistel Networks?

Most of the encryption modes of Feistel Networks especially the ones used to build fixed length block ciphers just provide confidentiality . Can we build authenticated encryption using Feistel ...
1
vote
1answer
107 views

Efficient parameters for group Diffie-Hellman

I want to implement a basic version of Diffie-Hellman key agreement for groups. So, my key is $K=g^{abc} \mod p$. Following this, the parameters I would need to transfer would be $K_a = g^{bc}$ etc. ...
0
votes
1answer
87 views

Biometric authentication [closed]

I see in some “Authentication schemes with biometric” papers that those authors wrote a phase as password and biometric update phase. My question is that why we may needs biometric update phase? Is ...
1
vote
1answer
75 views

What is the difference between a one-shot MPC protocol and a unbounded invocation of multiple MPC protocols?

In this paper on pg. 1236, under the section "Security Against Continual Leakage", the authors say: "We further remark that the weaker security notion previously achieved cannot be extended ...
2
votes
1answer
170 views

Bellovin 96' attack on IPsec ESP protocol on encryption only option

Can you explain the actual attack? Why does the attacker need to firstly send some arbitrary UDP packet? How can the attacker break the privacy between A and B? Link: The article of Bellovin I'm ...
1
vote
2answers
184 views

Why can we always assume that $Gen$ algorithm choose key uniformly?

Talking about a (symmetric) encryption scheme $\Pi=(Gen,Enc,Dec)$ where $Gen$ take the security parameter $n$ as input and generates a key $k$ of length $n$. Generally speaking we say that $Gen$ ...
2
votes
2answers
299 views

what is the state of the art algorithm to encrypt credit card data

We have a business requirement to keep credit card data. What is today's (Nov 2013) state of the art algorithm to encrypt credit card data that will be saved on disk? Additionally, I'd appreciate ...
3
votes
1answer
370 views

Elliptic Curve Encryption Ciphertext Size

I'd like to know how much bigger is the ciphertext when encrypting a message using ECC encrytpion? ECIES (or ElGamal)
1
vote
1answer
54 views

order between adversaries and type of resources given

We have a game $G0$ for which an adversary $A$ has access to a certain amount of ressources. Let us suppose that the maximum advantage for the adversary to win this game is $Adv_{G0}$. If we modify ...
1
vote
0answers
88 views

Could MITM securely identify identity?

Consider protocol like QUIC or MinimaLT which are essentially layer4 replacement for TCP/UDP (riding on top of UDP currently because of fear of random firewalls dropping new L4 protocol). From crypto ...
1
vote
2answers
141 views

Any real world implementation using message recovery?

During my master thesis I came across RSA-PSS-R which is in my opinion a quite elegant way of saving bandwidth and space using digital signature. But I didn't find any information about any real world ...
2
votes
2answers
214 views

Scalar Multiplication on Elliptic Curves

In the elliptic curve: $y^2 = x^3 + 20x + 13 \bmod{2111}$. Using the point $P=(3, 10)$ I am wondering how to multiply this point by the scalar $57$? I realize I can write $57*P$ as $2^5*P + 2^4*P + ...
1
vote
0answers
36 views

MD Construction Doesn't Propagate TCR

I'm reading a proof of the proposition of CollisionResistant Hashing Towards Making UOWHFs Practical Suppose there exists a compression function $F: \Sigma^k\times\Sigma^{c+m'} \rightarrow ...
6
votes
1answer
236 views

Blinding twice in RSA

I understand that if you have a message $m$, you can blind it by selecting a random $r$ and then multiplying $r^e\times m \pmod{n}$ Someone else then signs it with $d$, raising to the power of $d$: ...
3
votes
2answers
62 views

Why do the outputs of parties in an MPC protocol have to be indistinguishable from the ideal / real world (as opposed to inputs)?

In this paper at the bottom of pg. 1235 and beginning of pg. 1236 it says: An MPC protocol is said to be secure, if for every real world leakage adversary A there exists an ideal world ...
5
votes
2answers
466 views

Has threefish successfully been attacked (practically or theoretically)?

Reading Schneier's "The Doghouse: Crypteto" dated September 30, 2009, I noticed Bruce Schneier stating: Threefish, the block cipher inside Skein, encrypts data at 7.6 clock cycles/byte with a ...
1
vote
1answer
85 views

Why have 4th and 5th steps in Needham-Schroeder Protocol?

Why have 4th and 5th steps in Needham-Schroeder Protocol? It is said "These steps assure B that the original message it received (step 3) was not a replay.". But what is a replay here? And I don't ...
1
vote
1answer
144 views

Why routers don't just use Diffie-Hellman protocol?

Routers could just use Diffie-Hellman protocol with any computer who tries to connect to that router. Then it would be impossible to eavesdrop for anyone. Only router and computer would know the ...
2
votes
2answers
923 views

What happens when a root CA has its private key compromised?

What happens when a root CA has its private key compromised? Then all children in the tree are compromised too? And then all certificates are compromised? What needs to happen then? Related: - How ...
0
votes
1answer
162 views

I've got my private key compromised. How does CRL work?

How does certificate revocation list (CRL) work? How can I send a request to the CA to add my current private key to the CRL, so no one except me can add my certificate to the CRL? Related: - How can ...
1
vote
1answer
168 views

How can we get CA's public key?

To get a public key of some organization or someone we want to send an encrypted message to, we need to make a request to CA asking that organization's public key. CA then returns X509 certificate. It ...
7
votes
1answer
582 views

Homomorphic (encrypted) comparison to an integer

When working with an additive homomorphic encryption scheme (say Pallier's), is there an efficient way to get the encrypted value of a comparison test to an integer value (I realise that an ...
2
votes
2answers
348 views

How should we interpret the cryptanalysis results of SIMON and SPECK?

The NSA recently released SIMON and SPECK light weight block ciphers. Although initial spec release did not have much of cryptanalysis details, two works later appeared providing the cryptanalysis for ...
5
votes
4answers
266 views

Is there a proof for showing any cryptogram is crackable?

I commonly hear statements along the lines of "all cryptograms are crackable - it's only a matter of time". Is there a proof to show that any cryptogram is "crackable"? The proof may be of a more ...
0
votes
1answer
185 views

modfied man in the middle attack diffie hellman

I have been given a problem in preparation for my cryptography final that I'm not sure how to solve. It asks me to suppose a scenario where instead of where an attacker would intercept some message ...

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