3
votes
1answer
146 views

AES VS PRNG+HASH+XOR

I was thinking of doing symmetric encryption using just hashing and simple XOR encryption. It would work as: Alice and Bob share a symmetric key Alice manages to send an (authenticated) IV to Bob ...
2
votes
1answer
53 views

TLS - sharing 1792 bits of unidirectional keys

I've just watched a video on the TLS protocol and learnt that it uses unidirectional keys (meaning keys for both sides, from browser to server and from server to browser). Given that the server uses ...
3
votes
2answers
106 views

Basic attacks on McEliece; finding S and P

Take a McEliece cryptosystem with public generator matrix $G' = S G P$ where $G$ is a generator of a secret code with known fast decoding (not necessarily a Goppa code over $\mathbb{F}_2$), $S$ is ...
8
votes
2answers
205 views

Trial divisions before Miller-Rabin checks?

I'm trying to understand prime number generation (more correctly, the primality checking) as described in Handbook of Applied Cryptography. The context is circa pages 145 - 150, and specifically ...
3
votes
1answer
159 views

How to calculate inverse affine transform in AES from forward affine transform?

In AES the forward affine tranformation matrix is defined as: $$ \begin{bmatrix} 1& 0& 0& 0& 1& 1& 1& 1 \\ 1& 1& 0& 0& 0& 1& 1& 1 \\ 1& ...
7
votes
1answer
124 views

curve25519 weak points for contributory behaviour

The Diffie-Hellman on curve25519 is usually calculated using the base point $(9,…)$ which induces a cyclic subgroup of $G:=\{\infty\}\cup(E(F_{p^2})\cap(F_p\times F_p))$ with index 8, i.e. there is a ...
0
votes
1answer
122 views

Functions that are only second-preimage resistant?

Are there functions that satisfy the second-preimage resistance property without satisfying the others (preimage and collision resistance) ? I'm looking about such functions that compress data (exit ...
3
votes
2answers
132 views

What are good combinations of public key algorithms or primitives for long term security?

There has been talk in literature about doing multiple encryption with different block ciphers or stream ciphers several times before, and the benefits and risks of such efforts. There may be hidden ...
2
votes
1answer
75 views

Strategy for random CTR initial counter values

Alice has a secret key which she uses to encrypt messages in CTR mode. CTR mode is critically vulnerable to counter reuse, and Alice has a problem: she has terrible memory and no pen to write anything ...
0
votes
2answers
124 views

RSA decrypting of a huge file by parts [closed]

I need to decrypt a huge file that I own previously encoded by myself with a RSA public key (it's possible for this step using a symmetric algorithm key). Problem is that I can't load it in my ...
1
vote
1answer
60 views

Algorithm accepting every passphrase to fool unlegit user [closed]

I'm looking for specific names and literature on crypto algorithms that accept every passphrase input by the user, in order to 'decrypt' - even with false passphrase - and present the unlegit user ...
3
votes
1answer
159 views

Why does AES-GCM need MAC? (TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256)

"TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256" is just one example for a cipher suite, as far as I'm concerned it means this: it uses the TLS protocol, exchanges keys with Elliptic Curve Diffie Hellman ...
0
votes
0answers
54 views

What are the consequences of not checking the server mac in a TLS connection?

The context for this question (it's quite involved) can be found in this document: https://docs.google.com/file/d/0B15itpMWb2ztWTJrYTgzX204OVU However, I want to make it clear that you don't have to ...
-2
votes
2answers
74 views

Python KDF - Master Key and Derivatives

The encrypted file can be opened by following combinations (either any 2 DK, or 1 MK): DK1 + DK2 DK1 + DK3 DK2 + DK3 DK1 + DK2 + DK3 MK Any heads-up with Python ...
2
votes
2answers
105 views

Is a collision attack possible on MD5 if message is known partially

Is it possible to find $m'$ such that $\operatorname{MD5}(m'||m_1)=\operatorname{MD5}(m_1||m_2)$ if I know $m_2$ and $\operatorname{MD5}(m_1||m_2)$? Is this a collision or a preimage attack? Also in ...
1
vote
1answer
107 views

Brute force RSA cracking

Suppose one had a complete list of primes up to $2^{n+1}-1$. Then wouldn't one be able to crack an $n$-bit RSA public key in time $O(\pi(2^{n+1}-1))$, making RSA insecure? Thanks, René
4
votes
2answers
96 views

Use of less than secure random numbers for 'a' during an SRP proof of password

With Secure Remote Password protocol SRP6a random numbers are used for s, a and b. Where ...
3
votes
0answers
76 views

Compact digital signature for noisy data

The question asks for a signature scheme, with a public verification procedure, that is tolerant to minor alterations of the signed data during its transmission [possibly in analog form over some ...
2
votes
1answer
97 views

Does this protocol provide Perfect Forward Secrecy / are there potential security flaws?

We have a HMAC key (128 bit/16 bytes) an AES key (128 bit/16 bytes) an AES IV (128 bit/16 bytes) a random salt (128 bit/16 bytes) Each packet is signed with a HMAC (SHA256) After each packet is ...
4
votes
4answers
240 views

Given $n$ bits, how many “truly random” sequences/numbers can be constructed?

Suppose we have $n$ bits, so we can have $2^n$ different bit sequences. Some sequences don't look random, say, all $1$ or $0$. There are also other patterns like $10101010…$, $11001100…$ and such. ...
4
votes
1answer
144 views

Mixing Entropy Sources by XOR?

Assume that I have two sources of entropy (say, a hardware RNG and a CSPRNG) and that the two sources are independent (do not know anything about the internal state of the other RNG). Can I mix them ...
1
vote
1answer
65 views

How can a digital signature algorithm incorporate a random data if it is to be authenticated on a separate system

I am trying to understand how RSA digital signatures can incorporate any data from a PRNG into the final signature. Does this mean that separate implementations might fail to verify each others ...
1
vote
1answer
57 views

Fiat-Shamir security problem question

I am trying to understand the Fiat-Shamir identification protocol, but have a problem with understanding how this protocol is supposed to be save, even if you repeat it several times. I am using ...
3
votes
2answers
151 views

Generate a unique 64 bit numeric database key from a string without collisions at $2^{32}$ birthday threshold?

What is the best way to generate a unique 64 bit numeric database key from a string? For example: if the string is a URL, you could generate a 64 bit key using SipHash or by truncating a SHA256 hash ...
3
votes
2answers
103 views

Is ”generating transcripts that are indistinguishable from actual messages in a signature scheme” not universal forgery?

In the Pointcheval paper “Security arguments for digital signatures and blind signatures”, they have proved the security of Schnorr signature for existential forgery under adaptively chosen-message ...
2
votes
0answers
151 views

Elliptic Curve based blind signature implementation

I want to use Elliptic Curve based blind signature scheme for my research. There is no proper implementation of ECC-based blind signatures. Can someone describe to me which things I need to follow ...
5
votes
1answer
128 views

How can one detect if a hash function doesn't have a full image?

Suppose we have a hash function $H: \{0,1\}^* \mapsto \{0,1\}^n$, that doesn't get all the $n$ length bitstrings as a value for any input, but is otherwise secure. Can that fact be detected? For ...
1
vote
1answer
67 views

Familiar key signing over video call

Say that a friend of mine and I have both generated new PGP keys, and we want to use a video call to verify each other's new keys because we live on opposite sides of the planet. While we could both ...
3
votes
1answer
120 views

AES-128-CTR message integrity: Construction of HMAC

(Note: This post is not about "rolling my own", but rather trying to deepen my understanding of the underlying structures.) I'm having difficulties finding clear answers to some questions about the ...
3
votes
2answers
72 views

How do I construct variable length input (VIL) in a block cipher mode?

I've just been skimming NIST's "Five Confidentiality Modes". I understand those block cipher modes are used to securely encrypt input longer that the block cipher length. However, the common block ...
1
vote
1answer
113 views

Are those RSA keys flawed?

I used rsa-json.native to generate RSA keys for a node.js application that will use secure-peer later to connect two clients with each other. Now I have 2 questions: In secure-peer/index.js I've ...
5
votes
1answer
142 views

Significance of rotation constants in SHA-512?

In a single round of SHA-512, in the operations $\Sigma_0(A)$, $\Sigma_1(E)$ why are the constants $28$, $34$, and $39$ used for the number of rotations? What significance do these numbers have? What ...
3
votes
1answer
124 views

ECDSA: How to retrieve a non-random k

I have a question regarding the random $k$ number of ECDSA encryption. As far as I know, it is possible to retrieve $k$ (and thus the private key) from two signed messages if both used the same $k$. ...
0
votes
1answer
88 views

ECDSA: Retrieving non-random k [duplicate]

I have a question regarding the random $k$ number of ECDSA encryption. As far as I know, it is possible to retrieve $k$ (and thus the private key) from two signed messages if both used the same $k$. ...
-1
votes
1answer
134 views

What does it mean that a protocol realizes “strong mutual authentication”?

What does it mean that a protocol realizes "strong mutual authentication"? And What does it mean that a protocol realizes "weak mutual authentication"? Can you define them formally ? I have found a ...
2
votes
1answer
95 views

Why are there only positive value points on an elliptic curve?

I read about elliptic curve cryptography $E$ over $Z_p$ where $p$ is prime number and $G$ is a base point on the curve. I noticed the points resulting from multiplication e.g. $2G$,$3G$,.....,$(N-1)G$ ...
5
votes
2answers
184 views

Obfuscate an “I'll reveal if you do” function

Is there any cryptographic obfuscation scheme that will let me obfuscate a program that reveals my secret, if you reveal yours? Formalization. Let $C(\cdot),V(\cdot)$ be a secure commitment scheme. ...
1
vote
2answers
72 views

Sender and receiver having different moduli conflicts with encryption and signing in RSA

I should implement a security protocol as a part of which I need to: Encrypt the message with receiver public key. Sign it with my private key. Send it to receiver. Suppose that the system uses a ...
1
vote
1answer
52 views

Must the ciphertext be longer than the key for symmetric encryption?

I am planning to encrypt the different fields of a database independently, using a symmetric encryption scheme (probably 3DES). Since some of the fields are as small as an integer, may that be ...
2
votes
1answer
82 views

What differences between Menezes–Vanstone ECC and ElGamal ECC?

After researching ECC encryption, I found that we can use ElGamal cryptosystem with elliptic curve and can we use Menezes-Vanstone cryptosystem with elliptic curve. What is the essential difference ...
1
vote
2answers
161 views

Attack on a key-exchange,symmetric-key cryptography protocol

This is an exam question in Oxford 's Computer Security course: Here is the start of a protocol, based on a long-term secret key $k_{AB}$ previously shared between Alice and Bob, designed to offer ...
3
votes
1answer
74 views

Multiplication-homomorphic schemes

I'm looking into multiplication-homomorphic schemes now and basically I see that there are 3 options: RSA, Boneh-Goh-Nissim and ElGamal. RSA was proved to be insecure unless message is randomly ...
1
vote
0answers
112 views

RSA encryption and signature padding flaw

This is Oxford's computer security exam question: Suppose that Bob has published an RSA encryption key $ke$ (retaining in secret the corresponding decryption key $kd$), and Alice has published a ...
2
votes
1answer
112 views

What are differences between $E(F_p)$ and $E(Z_p)$?

When I read some books about elliptic curve cryptography noticed that. sometimes symbolized elliptic curve over $F_p$ is $E(F_p)$ and sometimes symbolized elliptic curve over $Z_p$ is $E(Z_p)$. I ...
1
vote
0answers
97 views

Commutative Encryption with RSA scheme?

I wanted to know how I could manage to do what I'm going to tell you next, with the RSA encryption/decryption scheme. So Alice and Bob each have a public key $(n, e)$ and a private key $(p, q, d)$; ...
2
votes
1answer
117 views

Pick faster private exponent

I recently tried to send 1536-bit modulus CSR to COMODO. They refused to sign the certificate. I later found out that it's because NIST mandated 2048-bit modulus on the SSL certificate. I think it's ...
1
vote
1answer
80 views

How (in)secure is a signature based on DSA with DSS1 (SHA1) in reality?

As I found out the OpenSSL implementation provided by PHP (openssl_sign) only allows DSS1 as the hashing algorithm for DSA signatures (see also my related question). I’d prefer DSA over RSA in my ...
1
vote
2answers
117 views

Three different numbers with x³=x mod p

p is a prime greater than 2 and $a \in \mathbb{Z}_p$. Why are there exactly three solutions for a³ = a mod p? Obviously 0 and 1 are both in $\mathbb{Z}$ and valid solutions, but that still means, ...
1
vote
1answer
89 views

Modulo properties of two prime numbers

I am supposed to prove that x = y mod (p*q) <=> x = y mod p and x = y mod q with p and q are prime numbers. It somewhat sounds reasonable to me, but unfortunately I don't have any clue how to prove ...
1
vote
1answer
39 views

server aid secure equality test

The scenario is like this: $P_1$ has $m_1$, $P_2$ has $m_2$, they want to compare whether $m_1=m_2$ via the server $S$. After the protocol, only $S$ knows the result, and there is no interaction ...

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