An asymmetric cipher is an encryption scheme using a pair of keys, one to encrypt and a second to decrypt a message. This way the encrypting key need not be kept secret to ensure a private communication. Similarly in public key authentication, the verification key can be public and the signing key ...

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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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224 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 ...
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671 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} \mathbb{...
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418 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
641 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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177 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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50 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
61 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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78 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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191 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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95 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? ...
2
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86 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
524 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 ...
2
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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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140 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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166 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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1answer
505 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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146 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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465 views

Verification of identity by certification authority

I have question about how a user's identity is verified by a Certification Authority. As I understand it, the process works as follows: User generates his private key and (with the help of private ...
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2answers
182 views

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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1answer
444 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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448 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 ...
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582 views

When making public key fingerprints - is a sha1 hash still a good idea?

I'm thinking about trying to save some space (and readability) when referencing 2k and 4k public keys (millions of them) by storing the fingerprint in some places instead of the full public key. ...
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103 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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240 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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358 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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1answer
307 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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506 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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212 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 ...
2
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1answer
508 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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40 views

How does Bluetooth pairing (SSP with numeric comparison) work?

I have a question concerning the pairing of two Bluetooth devices using Simple Secure Pairing with numeric comparison. The NIST document I am looking at states on page 14: Numeric Comparison was ...
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27 views

Why SSH does not use a certificate authority in the same way as SSL [closed]

SSL requires the use of a trusted third party -- the certificate authority to verify a public key presented by a server to a client. SSH on the other hand relies on the client having the server's ...
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196 views

Where and how to store private keys in web applications for private messaging with web browsers

I am working on a web application enabling users to communicate over private messages which is just one part of the whole system. The main focus during my development process is to protect the privacy ...
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34 views

Some confusions about Repeated Doubling Algorithm?

The following repeated point doubling algorithm is taken from the book Guide to Elliptic Curve Cryptography by D. Hankerson, A. Menezes, and S. Vanstone on page#93. Clearly, this algorithm is ...
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61 views

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 ...
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71 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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45 views

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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40 views

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 ...
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66 views

EdDSA Verification vs. Cofactorless Verification

In the EdDSA for more curve paper the authors defines: Keys An EdDSA secret key is a $b$-bit string $k$. The hash $H(k) = (h_0, h_1, ... , h_{2b−1})$ determines an integer $s = 2^n+\sum_{c≤i<n}...
2
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2answers
136 views

Possible to guess RSA public key $e$ using $n$ and $\phi$

We know that if $n$ and $\phi$ is known, then it is possible to find $p$ and $q$ (initial prime numbers). I have following the questions: Assume the public key $e$ is not known, how difficult would ...
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36 views

Can I expand the modular of an encryption scheme?

Take the lifted ElGamal as an example: suppose the original ciphertext is $<g^y ~mod~q,~g^m~mod~q>$. After some calculations, $m$ could be large enough to do modular. But I don't want $m$ to be ...
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46 views

Signing integers and UUIDs and restoring them from a third party server without tampering

I'm educating myself about cryptography. I know this is a long one, but please, bear with me. :) Now I have a scenario in my mind and I'd like to ask if what I've thought is sound (enough), what more ...
2
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2answers
88 views

Semantic Security and Equal Message Length in the Context of Public Key Cryptography

A lot of definitions for semantic security make use an experiment $\text{Exp}$ which is performed between a challenger $\mathcal{C}$ and an adversary $\mathcal{A}$ that begins as follows: $\mathcal{C}...
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62 views

Most Secure Exchange of Keys [closed]

Given both Alice and Bob must exchange IP addresses to perform such an operation, what is the most secure way Alice and Bob can exchange public keys between two mobile devices without having to rely ...
2
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1answer
224 views

RSA - PKCS#1 padding - overall correction of algorithm

From PKCS#1, page 24: After we create the EM byte string and convert it to an integer, we then encrypt that integer with RSAEP (page 10) to get the ciphertext (as an integer). My question arrives ...
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69 views

Ciphertext size of any CPA-secure public-key encryption scheme

I know that, for any CPA-secure public-key encryption scheme, the size of the ciphertext after encrypting a single bit is superlogarithmic in the security parameter. That is, for $(pk,sk) \leftarrow ...
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676 views

Challenge-response based on public-key decryption, why send public key

Quoting the handbook of applied cryptography, chapter 10.3.3 (i): Identification based on PK decryption and witness. Consider the following protocol: $A \leftarrow B: h(r), B, P_A(r,B)$ $...
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How can I make a message into a blocks for NTRU?

I have more bits than the degree of the NTRU polynomial. For example: $x^{11}-1$ message: Hello (I am using ascii, 8 bits for 1 symbol) got: ...