# All Questions

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### Strength of a cryptography algorithm [duplicate]

I'm just read this article from Atmel corporation in comparing RSA with ECC cryptography algorithms. First of all please read these two paragraphs quoted from the article: P1: Strength of an ...
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### Is a hash “reset sequence” possible?

Is there such thing as "hash reset sequence" that brings hash to the same known state? A hash function has a state. In MD5 and others vulnerable to the length extension attack, the attack works by ...
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### Can the Smart-Vercauteren Homomorphic Encryption be implemented as a practical fully homomorphic encryption scheme?

Smart and Vercauteren proposed a homomorphic encryption scheme (PKC 2010) following Gentry's principles. ​ Though their scheme can achieve fully homomorphism theoretically, they admitted that "for ...
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### IV larger than block size of cipher?

Is there any advantage to using an IV larger than a cipher's block size? Say I'm using AES-256 in Galois Counter Mode (GCM). My file format looks like this: ...
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### Help with pseudorandom functions

I started a new lecture this year about crypto and I've had a few problems in the first few weeks with the exercises we got. I would appreciate a "guide" on how to solve problems like the following ...
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### Are all encryption tools made equal?

I'd like to encrypt work-related data backups stored on an external drive. The backups contain proprietary company information and trade secrets. My current backup program, Comodo Backup, allows me ...
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### Is there any relation between two strings with the same MD5 hash?

Is there any relation between two strings with the same MD5 hash? For example these two strings: ...
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### what problem happen if we always use IV Zero in CBC Mode? [duplicate]

i have a question about CBC operating mode. what problem happen if we always use IV Zero in CBC Mode?
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### Why we use operating modes in cryptography? [duplicate]

Why we use operating modes (CBC , . . ) in cryptography? i mean what is the essential of use it ?
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### Relationship between exponent and modulus in RSA (as RSA properties as listed in X.509)

In one of my assignments, I had the following question (Please read on. Not a homework assignment :) ): X.509 (1998 version) lists properties that RSA keys must satisfy to be secure. One such ...
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The below equation is mentioned in Homomorphic Encryption and the BGN Cryptosystem (pdf, page 4): Mult(pk, $C_1$, $C_2$): Choose $u \xleftarrow{R} [1, n]$ and output $D = \hat{e}(C_1, C_2) \cdot e(... 1answer 267 views ### Confused with 128-bits AES round numbers The key size used for an AES cipher specifies the number of repetitions of transformation rounds that convert the input, called the plaintext, into the final output, called the ciphertext. The number ... 0answers 70 views ### Calculating Phi(M) and D for RSA decryption given p,q,e I've been given a message in 4 digit numbers but it doesn't seem to work when I use my calculated values for phi(m) and d. If I'm correct phi(m) = (p-1)(q-1) correct? And d is given by ed = 1mod(phi(m)... 1answer 41 views ### Test vectors representation The test vectors listed in https://github.com/cantora/avr-crypto-lib/blob/master/testvectors/Mickey-v2-128-unverified.test-vectors Are represented as Hex strings? I'm sure that the key is, because ... 1answer 298 views ### RSA vs ECDSA/ECDH So we've been using RSA for a while now. I wonder, is it still secure? I'm wondering whether surveillance agencies can at this point read traffic encrypted with say, TLS_ECDH_RSA_WITH_AES_128_GCM? ... 2answers 201 views ### Why do 1st and 3rd key have to be the same in 2 key 3DES? From this answer: However, that wasn't the only situation they had in mind; they also considered the case where someone implemented 2 key 3DES in hardware (which the first and last subkeys ... 1answer 149 views ### Encryption that can only be decrypted once? I want to encrypt a message so that it can only be decrypted once. I know that we can use nonces to do this when we exchange data over a communication channel, but in my case the cipher text will be ... 1answer 89 views ### If a PRG is constructed by iterating a PRF, is it backtracking-resistant? Say we construct a pseudo-random generator from a pseudo-random function$f$(using some constant key$k$and some initial value$v_0$). We do this by feeding the output block of the PRF back into the ... 3answers 216 views ### Does complicating a flawed algorithm make it secure? It's a basic question I guess, but I don't know the answer and I don't think that anyone asked it. Suppose I built a system, it relies on MD5 for security, suddenly I read am article on how easy are ... 0answers 65 views ### Which precautions to protect against side-channel attacks on ARX ciphers? In recent crypto there has been a trend to design ciphers using only the ARX set of instructions - i.e. additions (modulo$2^{32}$or$2^{64}$), rotations (by a fixed constant) and XORs, examples ... 1answer 64 views ### MD5 Hash outputs not produced Are there any hash outputs not produced by the MD5 hash function? I need to use it as a placeholder for an empty hash while comparing two hashes. 1answer 53 views ### How many Hashes to find a digest of 2^{n−k}? Given an n-bit cryptographic hash function$H$, how many messages should we expect to hash before finding a message$x$such that$H(x) < 2^{n−k}$for some integer k? My take: Shouldn't it simply ... 3answers 613 views ### Prevent replay attack without storage I am designing a set of low-power sensing motes. Each mote will communicate to a central base over RF. I want the mote to be able to authenticate itself with the base, and send its data without being ... 1answer 80 views ### Where can I get the correct and precise algorithms for elliptic curve cryptography? I have been asked to implement cryptographic operations using elliptic curves. I would like to get precise algorithms for various processes like key generation, digital signature and verification. I ... 1answer 76 views ### If PRF is non invertible, why does Ctr-mode work? I am currently taking Dan Boneh's Cryptography I course online. I have just finished the segment discussing the Cipher Block Chaining and Counter-Mode. I am having difficulty understanding why Ctr-... 1answer 88 views ### How can one protect against a Man In The Middle attack during key exchange without using certificates? Besides using a certificate to defend against a man in the middle attack during a key exchange, what methods can be used? 0answers 81 views ### Private and public keys, how do they relate mathematically? [closed] Assuming everyone here knows the basics of a public private key, then you know the keys are mathematically related. My question is, what is the math? I really would like an in detail algorithm of the ... 1answer 117 views ### Why is RSA encryption version 1.0 not specified in an RFC? I saw that there are three RSA encryption versions, as follows: RSA encryption version 1.5 (RFC 2313) RSA cryptography specification version 2.0 (RFC 2347) RSA cryptography specification version 2.1 ... 0answers 63 views ### What is the DK function when generating an AES key for Kerberos 5? I'm trying to understand RFC3962. So far I understood everything, except for the last part: generating the derived key from the PBKDF2 output. To generate an encryption key from a pass phrase and ... 0answers 22 views ### Proving for a single input vs several inputs. Is there any difference? I am working on a new protocol for pattern matching. While defining the ideal functionality, it is possible to say: Alice sends$p$and Bob sends$T$to the ideal functionality. i.e. searching for ... 1answer 46 views ### Is it possible to have self-authenticated message? Consider a text with$n$words which$m$words of it can be replaced with an equivalent word. Therefore we have$2^m$different texts with the same meaning. For example "You are {so/very} {keen/eager}... 1answer 76 views ### Is a nonce considered 'secret' enough for HMAC? We're implementing a mobile application which communicates with a web service, and on first install the user sends its deviceid to the web service (over TLS, unauthenticated at this point) - which ... 2answers 88 views ### Easy hash function collision I'm designing a cryptography-breaking assignment for a college-level introductory security course, and I'm looking for a hash function which is reasonably easy (but not too easy) to generate ... 1answer 125 views ### TLS RSA Certificate with Elliptic Curve Negotiation When I look at Google's Public Key in Firefox I get PKCS #1 SHA-256 With RSA Encryption Yet according to SSL Labs Google has EC 256 bits / SHA256withRSA and supports ... 0answers 30 views ### How do I check if the secret key polynomial of the ATV-FHE (NTRU based) scheme is invertible? I want to implement the ATV-FHE scheme as described here https://eprint.iacr.org/2014/039.pdf. To generate the secret key polynomial I compute f = 2*u + 1 , with scheme parameters chosen such that ... 0answers 19 views ### Encrypted vs unencrypted digital signatures [duplicate] Is there a difference in terms of security between An encrypted stream of bits obtained by encrypting a plaintext concatenated with it's signature$ C = E_k(plaintext||\sigma)$where$E$is a ... 1answer 128 views ### A “one time pad” can be thought of a Vigenere cipher with A "One Time Pad" can be thought of a Vigenere cipher with... An infinitely long key A secure symmetric key Multiple ciphertext alphabets A columnar transposition I had this question come up ... 3answers 205 views ### Hash email address with a non-random salt for moderate security? Crypto noob here. I have an application that has only moderate security requirements (internal use, limited audience). I want to send a unique url with a token in it to each user. If anyone has the ... 7answers 4k views ### Why can't I reverse a hash to a possible input? I'm going to provide “proof” why a hash function can be reversed, and I hope you can tell my why I'm wrong So, a hash function can be implemented as a series of logic gates. All logic gates can be ... 0answers 100 views ### When would you use KP-ABE over CP-ABE? When would you use KP-ABE over CP-ABE or vice versa? I started this question as I am having problems abstracting the pros and cons between KPABE and CPABE. This is not an assignment question. I have ... 0answers 33 views ### References for combining LFSRs with different lengths and taps? [closed] I started learning streams ciphers with the use of LFSRs. Therefore, I tried to simulate what I am learning in a C sourcecode, which combines three registers and ... 0answers 55 views ### How to sign an elliptic curve point using an ECC signature scheme? In Schnorr based ECC signature scheme, a message$M$is signed with the private key$\mathit{sk}$as $$s=\mathit{sk}\cdot h(M,R)+k$$ where$R=k\cdot P$and$P$is a base point. If$M$is a point$(x,y)...
The hash function in AES-GCM takes encrypted and unencrypted parts. Can the two parts be provided in either order? I'm wondering whether there is any fun with $GF(2^{128})$ that can be done to ...