3
votes
1answer
373 views

Advantages/disadvantages of different message numbering schemes

For a stream of packets, where each packet is individually encrypted with a block cipher, it's desirable to have each encrypted packet only valid for that position in the stream. A message number ...
3
votes
1answer
104 views

Signing an integer, which may be increased

I have a 64 bit integer. It has some sort of signature so that one can be sure it is valid. Anyone may increase the integer, update the signature, and remove all trace of the old signature. But it is ...
6
votes
1answer
1k views

ECDSA Compressed public key point back to uncompressed public key point

From the ECDH demo here http://www-cs-students.stanford.edu/~tjw/jsbn/ecdh.html If I generate a private key for alice I can get ...
2
votes
1answer
125 views

Is Triple-DES a group?

I know for a fact that DES is not a group, but are any of the Triple-Des versions a group? Why, or why not?
2
votes
1answer
149 views

Offline app authentication - embedding hashed password inside an encrypted file

I am working on an occasionally connected app. To allow the users to authenticate I created a profile which is encrypted and placed on the users machine. The first byte of the file contains the IV ...
1
vote
1answer
125 views

How to build a secure AE scheme with generic composition?

I am actually looking for using a secure Encrypt-Then-MAC AE scheme , and consider using either an existing "ready-to-use" dedicated AEAD mode (GCM, OCB, CCM, EAX, etc ..) or an alternate composed ...
1
vote
0answers
92 views

Rating a steganographic system using Blum Blum Shub to locate hidden bits

Currently I'm building a toy steganography system based in a web browser. Besides all the obvious issues with building a system requiring extreme privacy into a browser (this is a toy!), I was curious ...
1
vote
1answer
152 views

What should the size of the private exponents in Diffie-Hellman key exchange for TLS be? [duplicate]

I'm using TLS with a Diffie-Hellman key exchange to negotiate the symmetric keys. How long should the exponents be?
4
votes
1answer
159 views

Does EAX use the same keys for encryption and authentication?

I am actually checking use of EAX AEAD mode, and following EAX spec review (scheme definition for my part), my question is: what about the derivation of the authentication and encryption keys? In the ...
4
votes
3answers
368 views

Why are elliptic curves better than cyclic groups?

The set of points of an elliptic curve over a finite field is isomorphic to the direct product of two cyclic groups (i.e. $E(F_{p^n}) \cong Z_{s} \times Z_{t})$. What is the advantage of representing ...
5
votes
2answers
4k views

Finding a keylength in a repeating key XOR cipher

In an old cryptography FAQ, I found the following step described for determining a the length of the key a cipher was repeatedly XORed against: Discover the length of the key by counting ...
1
vote
1answer
127 views

What is meant by this notation in the ElGamal key generation process?

Alice chooses i) A large prime $p_A$ (say 200 to 300 digits) ii) A primitive element $\alpha_A$ modulo $p_A$, iii) A (possibly random) integer $d_A$ with $2 \le d_A \le p_A-2$. Alice ...
5
votes
2answers
657 views

uniqueness of the RSA public modulus

What is the probability that two separate RSA public moduli are the same? For example, consider a 2048-bit modulus. The number seems to be huge, but the choice for prime factors p and q is much more ...
8
votes
5answers
1k views

Encrypt a file once with 50 characters password or twice with 25 characters?

What better Encrypt the file once with 50 characters password or encrypt it twice each time with 25 characters password.
1
vote
0answers
382 views

Non-malleable file encryption using AES XTS 256?

I'm looking to do file encryption of a bunch of individual files, some small, some quite large. The files will be write-once/read-many, so I could use CBC, however, since the read-access needs to ...
30
votes
2answers
26k views

How does one attack a two-time pad (i.e. one time pad with key reuse)?

My question might appear the same as the question Taking advantage of one-time pad key reuse?, but actually I did read all the answers and none of them helped me with the details I need. I am new to ...
2
votes
0answers
83 views

Is this any recognizable encoding or cipher? [closed]

I've been watching the Marble Hornets videos on YouTube (basically, an independent film series about a group of kids chased by the "Slender Man"), and in this video I saw something interesting. ...
1
vote
0answers
144 views

Best Asymmetric Encryption Standard for Communications? [closed]

I'm new to the standards of encryption. I understand the math behind RSA really well. I'm wanting to design an application for private peer to peer communications using RSA and wanted to know what ...
5
votes
1answer
303 views

md5: is reverse length-extension attack possible?

If I know H = md5( SECRET_KEY | DATA ) then I can calculate H' = md5( SECRET_KEY | DATA | DATA' ) That's length-extension ...
1
vote
2answers
186 views

Digital Signature Scheme Count

My question is How I will be able to count digital signature Scheme that can sign many documents with one private key. There are signature scheme as long as exists private keys in the keys spaces? or ...
2
votes
0answers
122 views

Is it possible to derive the midstate of a sha256 hash?

Let's say I have an unknown string with the known sha256-hash of it. I was wondering if it was possible to now calculate the sha256 of the concatenation of the unknown string and "abc". (In PHP: ...
2
votes
1answer
599 views

Is a PBKDF2-derived master key easier cracked if very many Data Protection Keys are derived from it?

I'm referring the NIST document about PBKDF2, NIST Special Publication 800-132, Recommendation for Password-Based Key Derivation, Part 1: Storage Applications, Page 8 and 9, section 5.4 Using the ...
3
votes
2answers
335 views

Map Bytes to Number

Using node.js crypto library I can generate cryptographically strong bytes. How can I use this to generate a random number between 0 and 9999 (both inclusive)?
0
votes
1answer
176 views

Signature with appendix

I see sometimes signatures schemes with appendix. This is about signatures schemes in which the message is needed in the verification algorithm, that is, the ouput of the signature algorithm is of the ...
1
vote
1answer
234 views

Using AES-CBC and providing the IV in file header -> security hole? [duplicate]

currently I audit some code I've written in the past to use KeePass 1.x databases. I stumbled over the following question: The database itself is decrypted with AES-CBC and inside the file header the ...
1
vote
0answers
181 views

How is key rotation defined?

I'm aware that it's advisable to rotate cryptographic keys periodically. HKDF accepts high-entropy input (IKM - Input Key Material), and expands it to an arbitrary length. It also accepts a salt, ...
1
vote
1answer
548 views

Help with example RSA problem

While learning about RSA, I found this example problem. The answer is supposed to be "a 4-digit number that is a pattern of digits." I have computed it to be 16657 twice. OK, now to see if you ...
2
votes
1answer
74 views

How does the possible attack on passwords and salts mentioned in PKCS#5 v2.1 work?

From PKCS#5 v2.1, 4.1 Salt (page 6, emphasis by me), also in v2.0, RFC 2898: In password-based encryption, the party encrypting a message can gain assurance that these benefits are realized ...
3
votes
3answers
819 views

Proper Way to Encrypt Data with Two Keys?

I'm going to develop a system that will have two keys. One supplied and generated by the system that's specific to an individual user and one from the user. I plan on using AES. Does it make sense ...
3
votes
2answers
306 views

How to calculate leaked information from a check digit

Given a check digit calculated using mod 11, of an encrypted numeric sequence of 8 digits, how much information can be known about the plaintext? More specifically, a single 3DES encrypted CBC block ...
10
votes
1answer
357 views

Which MACs can be converted into a secure unkeyed hash function?

It is known that setting the secret key to a fixed, public value does not make MACs like CBC-MAC or GMAC into secure unkeyed cryptographic hash functions that could be used - for instance - for ...
3
votes
2answers
173 views

Is this exponentation or bitwise XOR

I'm reading about SRP from this page and came accross the line that says a party computes v = g^x I am unfamiliar with reading cryptography texts. Does this mean gx, or g ⊕ x?
4
votes
1answer
137 views

low-exponent RSA

I have questions from http://link.springer.com/chapter/10.1007%2F3-540-68339-9_1 Suppose we have 2 messages $m_1$ and $m_2$ related by a known relation $m_2=m_1+1$. Suppose further the messages are ...
0
votes
2answers
316 views

Recommended authenticated stream cipher for minimum overhead?

I'm running my TCP/IP protocol secured by encrypting each packet with AES128/CBC, bundling a SHA256 HMAC over that packet. This causes quite a bit of space overhead for small packets, so at first I ...
5
votes
2answers
2k views

Difference between “one-way function” and “cryptographic hash function”

What's the difference between a one-way function and a cryptographic hash function?
3
votes
1answer
1k views

What is U2FsdGVkX1?

Using CryptoJS 3.1 I noticed that using 3Des, the encrypted message always start with U2FsdGVkX1 Why is the first part of the encryption always the same? What information does this hold and ...
9
votes
1answer
573 views

How to build an electro-mechanical public key cipher machine?

It is generally assumed that asymmetric encryption schemes were invented in 1973 at GCHQ in Britain and, independently, in 1976 at the MIT. Imagine, if the abstract idea of having a public key and a ...
4
votes
1answer
259 views

How hard is it to generate a partial RSA fingerprint collision?

When I use SSH to log into a new system, I get asked to verify that the fingerprint (a 32 hex digits string) of the hosts's RSA key is correct. How much if it must I actually compare (by hand/eye) to ...
2
votes
2answers
234 views

Is it possible to cryptographically prove when was the last time a ciphertext was decrypted/encrypted?

I want to provide a service that encrypts and decrypts documents and I want to provide the users with proof that I haven't stolen their secret key and read their documents. I know I can prove when a ...
4
votes
1answer
513 views

Has GMAC mode a future outside GCM?

Once gcm has been implemented for providing both encryption & authentication solution , it appears obvious to consider that such implementation can also provide (if required by new user) a Mac ...
1
vote
0answers
154 views

Hash function with values in a multiplicative group of prime order [closed]

I have to implement a cryptographic protocol which involves a cryptographic hash function $H: \{0,1\}^* \to G$. It is viewed as random oracle. $G$ is a multiplicative group of prime order. I want to ...
3
votes
1answer
758 views

Security of authenticated encryption modes gcm & ccm

I have two questions for Clarification for AE mode choice criteria GCM : it appears to be actually the most popular and widely used AE mode of operation. however it is also well-known to be highly ...
3
votes
1answer
231 views

Anonymity in end-to-end encryption

Considering a scenario of anonymous data transfer between seller and buyer using proxies. If we consider that there are three proxies that are being selected by the buyer to obtain data from different ...
3
votes
1answer
2k views

How do I decrypt ciphertext with this cipher?

I have an encryption function $$E(p) = (ap + b) \mod{26}$$ where the parameters are $a=7$, $b = 10$, and $p$ is the plaintext letter. I was asked to decrypt the ciphertext ...
2
votes
3answers
5k views

OpenSSL AES 256-bit Key Management

I am using C and OpenSSL to encrypt files. After experimenting with the OpenSSL command line utility, it makes you enter a passphrase that can be any length, but uses that to create a 256-bit key. ...
16
votes
2answers
4k views

understanding a length extension attack

I have been trying to understand exactly how a length extension attack works on SHA-1. I'll detail below what I've understood so far, so that I can convey my understanding of the same and hopefully ...
2
votes
2answers
298 views

Encrypting and MACing different data with same key

It's a well-known best practice to not use one key to both encrypt and MAC data. In my application, there are instances where I MAC a piece of data without having encrypted it first. Do I still need a ...
1
vote
1answer
218 views

Scrypt as a KDF with one-time high-entropy input

I'm looking at using Scrypt as a KDF. Assume the following: the input will always be high-entropy random bytes generated by a CSPRNG the length of the input can vary from between 8 to 32 bytes the ...
2
votes
2answers
126 views

KDF with low-entropy salts

I need to derive a key from a username and a password. These are the only two things I have access to. What I thought is using PBKDF2 with username as the salt and password as the master password. ...
3
votes
2answers
653 views

Reason(s) for using a KDF for encryption keys

Assuming I already have a 256-key (32 char password) for AES encryption, comprised of random alpha-numeric characters and punctuation (95 possible ascii chars), generated by a decent PRNG, is there ...

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