An asymmetric cipher is an encryption scheme using a pair of keys, one to encrypt and a second to decrypt a message. A special class of asymmetric ciphers are public-key ciphers, in which the encrypting key need not be kept secret to ensure a private communication.

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N users must approve a request

I have a locked-down production system that we'd like to process specific requests, picked-up (poll) from a less-secure location. For security and "go slow, this is production" reasons, a request ...
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123 views

Eavesdropping experiment with private key

I am extremely new to cryptography, still trying to learn basics. I got stuck in the following example of an eavesdropping experiment with a private key. $$\begin{array} {|r|r r r|r|} \hline C & ...
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160 views

Key Check Value standard pratice for asymetric cryptography?

I'm looking for a standard, or sound industry practice, for the equivalent of a Key Check Value, applied to the private key of a public key algorithm, like ECDSA. For DES or 3DES keys, practice (as ...
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201 views

Given only RSA public key parameters, what security issues might I be able to detect?

I'm attempting to automate some checks against a large list of .NET assemblies, and want to include a test to see if the RSA parameters used to sign the assemblies are sensible. However, since I don't ...
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197 views

RSA public key exponent generation confusion

Quote from Wiki: $e$ having a short bit-length and small Hamming weight results in more efficient encryption — most commonly $2^{16} + 1 = 65537$. However, much smaller values of $e$ (such as $3$) ...
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186 views

Attribute-based broadcast encryption with respect to ABE

What is the exact difference between attribute-based encryption (ABE) and attribute-based broadcast encryption (ABBE)? Is ABBE equivalent to broadcast encryption on top of ABE?
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2answers
120 views

Group description in pairing based cryptography

Suppose we have a public key encryption scheme, in which public parameter contains $(p, G, G_t, e, g)$, where $p$ is prime number, $G$ is a (cyclic) group of prime order, $e:G \times G \mapsto G_t$. ...
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419 views

Can public key be recovered from ciphertext & encrypted private key?

I'd like to implement something like a write-once public/private encrypted shared secret (no better quick description for the lack of terminology knowledge). I guess, I'm trying to implement HSM. The ...
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380 views

SSL-like protocol with public-key hard-coded in the client

Imagine a SSL-like protocol that instead of using certificates signed by a trusted CA, has the server's public key hard-coded in the client. My question is: what happens if the server's private key is ...
2
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1answer
223 views

Designing a secure IM protocol

I haven't been programming much lately, so I figured I would make an encrypted instant-messaging program (read: Skype clone) with groups to hone my skills. That's not important though, I can do that. ...
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2answers
2k views

Digital Signature using symmetric key cryptography

Generally digital signature is a public key cryptography concept.But it needs high overhead. So is there any publication or link available where 'digital signature using symmetric key' has been ...
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189 views

How to design a cryptographically secure file hosting service

How one would go about design something like Google Drive/Dropbox providing the following features: files are encrypted and decrypted on the client files can be synced between devices the service ...
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260 views

How vulnerable is RSA when using it to encode ~1000s of datasets with 500bytes each? How easy can the private key passphrase be hacked?

I know that this is not the best use case for RSA, but for a customer of mine I have to encode user data on the server in a way so that it can be decoded/downloaded by the customer through a "secure" ...
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353 views

Bandwidth and block size for Paillier cryptosystem

Can someone clarify what is meant by the terms cryptosystem bandwidth and block size for public key cryptosystems; The context is the Paillier cryptosystem and its Damgard-Jurik generalisation. My ...
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220 views

How can a key pair be derived from an arbitrary hash?

If I correctly understand the concept of a "brain wallet" in BitCoin, you start with a passphrase, generate the hash of the passphrase, then somehow derive a public / private key from that to use as ...
2
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651 views

Why is proof-by-reduction needed (for Elgamal proof of security, for example)?

The textbook proof for Elgamal encryption basically reduces to the Decisional Diffie-Hellman assumption (DDH). Elgamal: $Gen(.): x \xleftarrow{R} \mathbb{Z}_p$; $Enc(m,g^x): r \xleftarrow{R} ...
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409 views

implications of SSH server key compromission when authenticating users against a public key

Friday we had a disagreement with a colleague of mine about the implications of SSH server key compromission The question was stated as this : What could a hacker do while provided with the SSH ...
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2answers
590 views

Encryption with private key?

we normally always encrypt by public key and decrypt with private key. If i encrypt with private key, then its still secure as normal PKI ? i mean known-plain-text will not take private key on the ...
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142 views

McEliece for streaming data

Under the assumption that there exists a real-world implementation of the McEliece scheme, could it be applied to streaming data as is? By that I mean in 'block cipher mode'? I've read that McEliece ...
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174 views

Is there a way to provide proof of batch RSA security?

Suppose we have two encrypted messages with two different public key issued from the one server. There is a client who wants to send these to messages to the server. In the middle there is an ...
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48 views

How to convert roots of Weber polynomial to Hilbert class polynomial over modulo prime?

Using any non square root discriminant $D$, we should be able to find the Weber class polynomial. How can I convert the roots of a Weber polynomial to a Hilbert class polynomial over modulo prime?
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1answer
52 views

Perfect Forward Secrecy in PGP/asynchronous email

Looking at the RFC for PGP email/asynchronous chat with PFS here https://tools.ietf.org/html/draft-brown-pgp-pfs-03 can anyone tell me why the following scheme is no good? Alice and Bob retrieve each ...
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66 views

PublicKey Unknown Encoding

I have a public key: ...
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52 views

What is the most an adversary can do in the context of public key masquerading?

Suppose two parties A and B are using a public-key encryption scheme in order to communicate securely. A and B both have a public/private key pair: $$ A: (P_A, S_A), B: (P_B, S_B)$$ and A already ...
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51 views

Establish Trust By Signing Random Seed

I'm trying to get my server to establish trust with a client. Neither is on the Internet, so there are no certificate authorities. The typical way to establish trust in my domain is for a client to ...
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1answer
173 views

Asymmetric cipher with multiple public keys

Asymmetric cryptography is well known. You generate a private key. Using the private key you generate a public key. Then you can sign a message with the private key and check signatures using the ...
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92 views

Authenticated EC key exchange without a signing/signature scheme?

From my little understanding of EC-based authenticated key exchange protocols, I believe that it is not possible for authenticated key exchange without a signing/signature scheme. Is this correct? ...
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81 views

Algebraic cryptanalysis on public-key schemes

The book Algebraic Cryptanalysis (Bard, G.V.; Springer, 2009) speaks about transforming a symmetric scheme into a system of polynomial equations and solving these equations to break the scheme. But ...
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3answers
477 views

Key derivation of public key without knowledge of private key

I'm looking for a convenient way to share a set of public keys across multiple services, while preserving privacy by preventing these services from tying the identity of these public key sets together ...
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3answers
2k views

Difference between Symmetric and Asymmetric. Is this answer correct, based on a written test?

I asked the following question on a final exam: Symmetric Ciphers (e.g. AES, DES, Blowfish) use up to 256 bit on the best case, which are considered strong against brute-force attacks. On ...
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1answer
269 views

Is it possible to perform a rudimentary asymmetric key encryption algorithm by hand?

I have at least three, distinct motivations for having a simple way to use asymmetric key encryption by hand or without modern computers. First, I was recently demonstrating to a 12-year-old how ...
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138 views

Question about dual system encryption

Recently, I have been reading some dual system encryption papers. In these systems, is the adversary able to generate “semi-functional ciphertexts”?
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155 views

Is there need for message authentication if a public key is transmitted and then used to encrypt a symmetric key?

First I'd like to note that I'm really a beginner at cryptography, and while this interests me and while I have researched it in the past few months, there is still a lot of things that simply aren't ...
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478 views

Hierarchical Encryption

Is there a way to setup a hierarchical encryption using public key encryption? Let's say a higher level user can decrypt messages encrypted by lower level users. Is that possible? I guess it is only ...
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142 views

hybrid PKE scheme CCA2 insecurity

I'm reading "An Efficient CCA2-Secure Variant of the McEliece Cryptosystem in the standard model" by Rastaghi which states: As we know, in hybrid PKE schemes XOR alone cannot perfectly hide ...
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Can you give me an example of any PKC encrytion algorithm with coins?

I'm reading a Definition of Public Key Encryption and here say: ... Enc is a (possibly) probabilistic polynomial time encryption algorithm which takes as input a public key $pk$, a $m\in M$ and ...
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429 views

Why does Merkle's Puzzle requires Eve quadratic complexity of effort to break the system?

The way Applied cryptography 2ED explains the puzzle is as follows (I paraphrase it): Bob generates 2^20 messages of the form x,y where ...
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436 views

Asymmetric format preserving encryption?

I would like to encrypt a string asymmetrically. I would like the ciphertext to be as short as as plaintext. (or shorter) I've read about format preserving encryption functions - are there ...
2
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102 views

What happen if a asymmetric crypto-system deals with only one key

Assume there is a crypto algorithm that deals with matrices to encrypt and decrypt. Regardless of the specification of such algorithm, what if the algorithm assumes that two parties can securely agree ...
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233 views

EC equivalent for RSA-OAEP

I have some questions regarding aforementioned subject: Is there a EC equivalent of RSA-OAEP key transport/encryption algorithm ? Is ECIES-KEM sufficient ?
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346 views

Asymmetric password encryption - Viable? Which algorithm?

We have an application that requires cleartext passwords for user authentication because of the authentication mechanism in use (RADIUS/CHAP), which unfortunately we cannot change. Since we don't want ...
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297 views

DLP based crypto systems with multiple independent generators

One example of a DLP based crypto system (or rather DDH based crypto system) where the public key parameters include two independent generators of the subgroup, is Cramer Shoup. Since the security ...
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499 views

Generate an insecure public / private key pair

I am looking for a way to generate an "insecure" public key pair. and by insecure I actually mean a pair that is breakable using brute-force (or other encryption) methods. As far as I know PGP ...
2
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1answer
197 views

In S/MIME, are the same certificates always used to sign and encrypt messages?

My assumption is that S/MIME almost always utilizes certificates as follows: My certificate can be used to allow people to encrypt messages and send them to me. My certificate (the same ...
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491 views

DGK Cryptosystem Encryption Speedup

Following @poncho's nice clarification of the RSA speedup here, let's see if I'm able to do the same in the case of the DGK cryptosystem: We have pk = (n, g, h, u), sk = (p, q, $v_p$, $v_q$) which ...
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Convert messages to elliptic curve points [duplicate]

Let $E$ be an elliptic curve; $\alpha,\beta$ two points of $E$; and $a$ a private key such that $\beta=a\cdot\alpha$. We choose random integer $k$ and plain text $x\in E$. Encryption and decryption ...
2
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1answer
146 views

end-to-end encrypted chat app with symmetric + public key encryption, TLS and cert pinning

A big drawback of end-to-end encrypted messaging services is that, as soon as you lose your device, you lose all your messages. Your private key is gone after all. I was wondering though, wouldn't it ...
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67 views

MITM Attack on Key Exchange Protocols

I was doing research on this attack and I am wondering, why is Diffie-Hellman always associated with such an attack? ​ Aren't other public key cryptography schemes vulnerable to the same kind of ...
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How do I prove that a bit by bit PKE scheme (that is IND-CPA secure) for an n-bit message is not IND-CCA-2 secure?

Gen generates Pk and Sk (l bits each), Enc encrypts m (length l) but by bit using each bit of Pk respectively, Dec decrypts ciphertext bit by bit using each bit of Sk respectively. How do I prove this ...
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David Chaum's anonymous credential

I am a newbie in cryptography (I know the basic RSA, AES stuffs) and am interested in anonymous authentication systems. Looks like David Chaum is the godly figure in the field. I see lots of people ...