Design of cryptographic protocols, i.e. ways of using algorithms (primitives) to achieve one or more security goals like integrity, confidentiality, authenticity (maybe together with non-security-related goals). If you ask about a specific protocol, tag with its name instead (or additionally, if ...

learn more… | top users | synonyms

1
vote
0answers
88 views

Should layered use of crypto always have a cryptographic binding between the layers?

Reading the latest TLS weakness, it seems that multiple weaknesses have been found over the years caused by not ensuring that messages are sent over the same channel, and in general a lack of ...
3
votes
3answers
128 views

Asymmetric algorithm for one time signing of small cleartext

I want to generate a key pair such that the private key can be used once to sign a small message (1024 bytes) at some indeterminate point in the future and the public key can be used to verify that ...
3
votes
1answer
75 views

Secure Deduplication and Encryption in a data store

Assume: We have a centralized data store that can store whatever we need Every user is online Function $f : File \to Key$ gets access to the store, and is the same for all users. $g : Key \to File$ ...
13
votes
1answer
827 views

Why do new versions of TLS use an explicit IV for CBC suites?

SSL 3.0 and TLS 1.0 used an insecure scheme to generate implicit IVs when encrypting records in CBC mode: they used the last part of the previous record, a value that can be predicted by the attacker. ...
1
vote
0answers
65 views

ECC cryptography with shorter signature when not needing high security?

I am new here and fairly new to cryptography, so if I say something wrong, let me know. I am trying to set up a system where a user can receive a temporary license key over the phone, put it into ...
0
votes
1answer
37 views

Detect desynchronization in HOTP authentication [duplicate]

In HOTP authentication, is there a way for a system to detect that the token has been desynchronized? If so how does it know that the OTP was generated from a desynchronized (but valid) token and is ...
4
votes
1answer
234 views

Why does the SRP-6 calculation of B include a multiplier k = 3?

In SRP-6 $B$ is calculated as $B=kv+ g^b, k=3$. What is the purpose of $k$, and why was it fixed as $3$? (In SRP-6a, this value $3$ is replaced by $k = H(N,g)$, but this question is about SRP-6.)
7
votes
2answers
2k views

Why is TLS susceptible to protocol downgrade attacks?

A recent blog post from Ivan Ristić (expert extraordinaire on all things SSL) says: all major browsers are susceptible to protocol downgrade attacks; an active MITM can simulate failure conditions ...
3
votes
4answers
425 views

Is there a symmetric-key cryptography based on key establishment techniques?

Most of the current key exchange techniques are based on public-key cryptography. Are there any key exchange/establishment techniques based on symmetric-key cryptography too? In my setup primary ...
5
votes
2answers
196 views

What does well-typed mean?

I am currently studying Cryptography and I don't really understand what does 'well-typed' mean when talking about secure cryptographic protocols. I can't find any reasonable explanation on the ...
4
votes
1answer
63 views

Diffie-Hellman when $p$ and $g$ are pre-agreed and hidden from Eve

During a regular Diffie–Hellman public key exchange, Alice and Bob agree to use a prime number $p$ and a base $g$… which are both made public and are therefore also assumed to be known by Eve. Alice ...
2
votes
2answers
81 views

Skein or Keccak stream cipher construction

I am wondering if using Skein or the Keccak hash algorithm in this construction (as a stream cipher) is secure: $H$ = Keccak or Skein hash with a 256 bit output $K$ = The main 256 bit random key, ...
2
votes
1answer
56 views

Secure MultiParty Computation with secret inputs for secret outputs

I want to know if it’s possible to use a SMPC (Secure Multi-Party Computation) to have $N$ entities compute the outcome of a known mathematical operation with two or more secret inputs, where each ...
2
votes
1answer
41 views

Symmetric-key login protocol

I'm trying to analyze a login protocol based on symmetric key cryptography. I'm just starting out, so this may very likely be a very bad idea, but nonetheless I'd like to hear your thoughts on it. ...
4
votes
1answer
78 views

OTT service using FPE

Would it be possible to create an Over-The-Top communication utility that will encrypt voice using format preserving encryption (voice clear-text to audio encrypted stream) and send that over an ...
8
votes
2answers
340 views

What current authenticated key exchange standards exist?

If neither of the 'big two' of TLS Handshake and IKE are appropriate in a given situation, what alternative Authenticated Key Exchange (AKE) standards exist and are recommended? Many protocols have ...
5
votes
2answers
521 views

How does a rolling code work?

I have general questions regarding rolling codes. Basically there is a sender and a receiver. Both have a sequence generator. The receiver checks if the received sequence matches the newly generated. ...
4
votes
1answer
120 views

Implementing CD serial key system

I am trying to create a system where to unlock the application one needs to enter a serial code. I have read many articles on the theme but there are two problems bugging me. One is, If I have a ...
6
votes
4answers
179 views

Authenticating data generated by a particular build of an open source program

[I was torn between posting here or security.stackexchange.com. In the end, I felt that this was more of a design question, rather than an implementation question and so chose this forum.] My ...
3
votes
0answers
107 views

Lowest number challenge scheme

Suppose Alice knows a secret number $a$, and Bob knows a secret number $b$. Is there a simple way for Alice and Bob to know who has the lowest number, without Alice & Bob exchanging their numbers ...
4
votes
2answers
110 views

Allowing multiple password reset tokens, and a question on random number predictability

I have a few requirements for a password reset system, along with some questions at the end as to its security. Requirements The aim is to provide a password reset mechanism satisfying the following ...
-3
votes
2answers
127 views

pairing-based schemes

some authors claimed that computational performance of a pairing-fee scheme (based on scalar multiplication over an elliptic curve group) is about 1000% more efficient than a pairing based one I would ...
5
votes
3answers
318 views

Leak-proof protocol: is such a thing possible?

Is it possible to design a protocol that by itself guarantees that a malicious implementation cannot leak secret data without breaking the protocol? Setting: Alice and Bob have a pre-shared secret ...
6
votes
2answers
984 views

RSA-based authentication and key-agreement protocol

An authentication and key-agreement protocol between devices shall mutually demonstrate their identity, and establish a shared random secret $R$ suitable for securing later communications. To that ...
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 ...
1
vote
0answers
52 views

A question or few about Mental Poker

I have spent some time studying the "Mental Poker" protocol (sometimes called SRA), initially proposed by Shamir, Rivest, and Adleman -- ...
3
votes
3answers
289 views

Third-party security issues with Shamir's secret sharing scheme

In Shamir's secret sharing scheme, we are trusting a third party who generates the secret polynomial. How can we ensure security here when we are involving a third party?
5
votes
1answer
149 views

What functions allow for practical indistinguishability obfuscation?

Recently, there has been a major theoretical breakthrough in program obfuscation, in the area of indistinguishability obfuscation. Background: Indistinguishability obfuscation is a form of program ...
5
votes
1answer
98 views

Universal Circuits in Indistinguishability Obfuscation Candidate Construction

I am currently working on an implementation of the candidate construction for an indistinguishability obfuscator that was recently proposed by Garg et al. The relevant paper can be found here. ...
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 ...
5
votes
2answers
185 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 ...
6
votes
1answer
916 views

Zero knowledge proof protocol example?

Alice is color blind. She never knows if her gloves are matched. Her brother Bob always teases her saying her gloves are mismatched and she should go change them. Alice wants to know if ...
1
vote
1answer
86 views

Are formal methods used for cryptographic protocol design?

I decided to learn more about the application of formal methods to cryptographic protocols and found this interesting talk by Cathy Meadows at YouTube. After doing research about how people are using ...
5
votes
1answer
293 views

Is it a requirement to understand mathematics when implementing (or breaking) cryptography?

Related: What is the lowest level of mathematics required in order to understand how encryption algorithms work? and Recommended skills for a job in cryptology In the context of putting crypto into ...
1
vote
1answer
82 views

Use additional keys to thwart key compromise?

Is it good or bad practice to design crypto protocols for key compromise by using additional keys? Argument for bad practice would be: When you have a key you should trust it and not throw more keys ...
11
votes
4answers
1k views

Why is plain-hash-then-encrypt not a secure MAC?

It seems that even in MAC-then-encrypt systems like SSL, something like HMAC is used rather than a plain hash. Why? Suppose we use some stream cipher; then why can't we use $Encrypt(m | H(m))$ as ...
3
votes
1answer
92 views

Need to generate new DH keys for every message in OTR

Going through the OTR specification I don't quite get why a new DH keys (and hence new symmetric key and hmac key) are generated for each message to be sent. Why is this necessary? I don't think it ...
2
votes
0answers
32 views

Common pitfalls of session key activation?

Assume that initiator $I$ and responder $R$ of a key agreement protocol have agreed on new symmetric session keys $K'_{auth}$ and $K'_{enc}$, e.g. by way of Diffie-Hellman and key derivation, possibly ...
3
votes
3answers
163 views

Does ECIES imply authenticity?

Assume: Alice and Bob both generate separate EC keypairs Alice obtains Bob's public key, and together with her private key creates a shared secret key Alice encrypts a message using the shared key ...
1
vote
1answer
96 views

How does this SSL-related flaw work?

I just saw a video at CNN in which it is claimed that, if the server isn't generating a random public key, the server can be hacked. First how HTTPS works… to be sure I’m on the same page: Server ...
1
vote
2answers
106 views

What are the major flaws and limitations of this hash-based stream cipher design?

K is a 224-bit key. C is a 32-bit counter; it is publicly known. the same value is never used twice. ...
1
vote
1answer
76 views

How is padding oracle attack mitigated by encrypt-then-MAC?

Let us suppose Alice sends a message to Bob. As far as I know, the most popular scheme of MAC-then-encrypt is as follows: Alice computes the HMAC of the plaintext using her private key, and then ...
1
vote
0answers
49 views

Is container format relevant to security of encrypted message inside?

Still trying to design a fully binary cryptography container format for my mobile app, I am here asking if container is ever relevant. Thanks to Apple, I cannot use GPG directly because I can neither ...
0
votes
3answers
355 views

Would it be possible to create a game where one would need to decrypt a simple encryption?

This might sound pretty weird, but I was thinking of a game a person could play alone with a pen and paper. I got this idea where the person would write a random four letter string on a paper, and ...
3
votes
2answers
70 views

Group membership with unique anonymous claims

Is it possible to accomplish the following scheme? There is a Server (S) and many clients. Each client enrolls with S, exposing its real identity; S saves the real identity information in its ...
1
vote
1answer
95 views

Zero-knowledge proof of a product

I have non-negative integers $x,y,z$. I'm going to give you commitments $C(x),C(y),C(z)$ to them. Then, I would like to prove in zero knowledge that $xy=z$. I can choose the commitment scheme to ...
2
votes
0answers
88 views

encrypted file sharing protocol

I am trying to design a file exchange protocol for learning purposes, inspired by PGP, but fixing some of its shortcomings. Assuming the machine where the creation and decryption of the file happens ...
10
votes
4answers
854 views

Why should I make my cipher public?

As I understand it, the less people know about the internals of my protocol or cipher, the more secure the protocol is. However Kerckhoffs's principle states that A cryptosystem should be secure ...
3
votes
1answer
124 views

Zero-knowledge proof for committing a choice?

Let's say there are 100 choices (which are publicly known), each represented as a different string, and today you have to choose one of them. You need not reveal what that choice is right now, though. ...