Questions tagged [mac]

(or message authentication code), a short piece of information used to authenticate a message, and the algorithm to create and check such information, using a secret key.

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Why reveal MAC keys after using them?

As far as my understanding goes, MACs are used to determine whether the received message comes from a source with access to a MAC-key, which both parties know. Some crypto protocols reveal their used ...
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is a MAC defined by a pseudo random generator secure?

Let $ G:\{0,1\}^*\rightarrow\{0,1\}^*$ be a length doubling PRG, and let $ \Pi=(Gen,Mac,Vrfy)$ be the following MAC scheme: $ Gen $ on input $ 1^n$ uniformly samples and outputs $ k\leftarrow \{0,1\...
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Which compression functions are MACs?

The original HMAC paper based the security proof on the fact that the compression function of the Hash is already a MAC (appropriately keyed). How does this relate to common assumptions for ...
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uf-cma and uf-cmva probabilistic MAC

Is there any simple probabilistic MAC $\Pi=(\operatorname{Gen},\operatorname{Tag},\operatorname{Vrfy})$ with UF-CMA security but without UF-CMVA security? How can an attacker efficiently use the $\...
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Can a secure MAC break if the adversary is given oracle access to Verify?

In the Mac-forge experiment, the adversary is given access to a tag-generation oracle, which generates a tag for each message given by the adversary. The adversary's task is to generate a valid tag on ...
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ECIES: Purpose of optional shared information?

According to Wikipedia the ECIES algorithm has two optional shared information $S_1$ and $S_2$. They are used as follows: Generate a random shared secret $Z$ according to ECIES, which will never be ...
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What was used for symmetric message authentication before HMAC was invented?

Currently, Carter Wegman MACs are popular. Before that, HMAC was popular. What was popular before HMAC? Was it CBC-MAC? Something else? I know about NIST FIPS 113 "Data Authentication Algorithm" ...
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1-digit MAC with error-catching properties?

The well-known Luhn check digit (used in credit-card numbers) is a public method to append a check digit on the right of a decimal number, so that any 1-digit alteration and any exchange of two (...
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Authenticated encryption with Enc-Sign-Mac

I've given a CPA secure private key encryption scheme, an unforgeable private key MAC scheme and an unforgeable public key signature scheme. I want to combine them to protect confidentiality, ...
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Designated verifier signature from Diffie–Hellman and a MAC

Here is an idea for a designated verifier signature scheme. Suppose Alice and Bob know each other’s public keys and Alice wants to send a message to Bob, such that only he will be convinced of its ...
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Is the following derived MAC where the output is XOR'ed with the key secure?

Hey I'm wondering if the following scheme is secure or not , I tried reductions and some tries to prove that it not must be secure but I feel completely stuck . More details: It's just any reduction ...
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What is the original paper of the length extension attack on hash based MAC functions

A message authentication code (MAC) based on using a hash function with the secret prefix $H(k||m)$ technique goes back to two independent sources according to Tsudik's article on 1992; S. Crocker, S....
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43 views

Attackability of one-time MAC when using block mode

Consider an encrypted message m which is encrypted with AES-CBC, so in addition to the message there is an IV supplied initially....
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120 views

Security of MAC-Then-Encrypt without MAC key authentication

We have the following settings in our system: TLS 1.2 using legacy cipher suite - OFB mode (using 64 bit block cipher) and trunkated CBC-MAC (32 bit). The goal is to achieve authenticated encryption ...
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Do AEAD algorithms and encrypt-then-MAC mitigate external decryption oracles?

I'm curious about a possible attack scenario that I'm not sure exists. If it is possible, I'd like to know if and how AEAD algorithms and encrypt-then-MAC implementations protect against it. The ...
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Using NIST constructions based on Keccak like cSHAKE128 for other sponges

NIST defines a lot of interesting applications of sponges in FIPS 202 and NIST SP 800-185 like the XOF cSHAKE128 or the MAC scheme KMAC. All this schemes come with a security guarantee and are ...
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Why does OMAC2 exist?

The specification for OMAC has the following to say about OMAC2: For OMAC2, we adopted u−1 instead of u2 as Cst2. It requires one right shift to compute L· u−1 instead of one left shift to compute (L·...
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Proof check: Forgery for a MAC

I'm looking at the MAC defined as follows: $$\text{Mac}_k(m)=\langle r,f_k(r\oplus m)\rangle $$ where $r$ is uniformly chosen at random (each time) and $f_k$ is a PRF. Vrfy is the canonical one. I'm ...
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TLS 1.3 Handshake - Usage of MAC and Digital Signature for CertificateVerify and Finished Message

I was wondering, why the TLS 1.3 handshake employs two different 'messages' that compute an authentication code over the whole handshake. As far as I understand, the CertificateVerify "message&...
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Constructing a mac scheme secure to regular definition but insecure to special case

The mac-forge security game for mac schemes looks like this: $k \leftarrow \operatorname{Gen}(1^n)$; $(m,t) \leftarrow A^{\operatorname{MAC}_k(⋅)}(1^n)$; Let $Q$ denote the set of all queries that $A$...
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Forgery of the Carter-Wegman MAC

Consider the nonce-based Carter-Wegman MAC which works on key space $K=\{0,1\}^n \times \{0,1\}^n,$ message space $M=\{0,1\}^{mn}$, nonce space $N=\{0,1\}^n$ and the tag space $T=\{0,1\}^n$ as follows:...
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CBC-MAC variable-length weakness with MAC size greater than the encryption block size

The title mostly explains it; CBC-MAC variable-length weakness with MAC size greater than the encryption block size I've seen that by having the pairs $(M,T_m)$ and $(N,T_n)$, we can generate a ...
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Implementing MAC with PRF family $f$, why do we need $f_k$ to be invertible?

As per the title, say we have $\text{MAC}(k,m) = (m,f_k(m))$ where $f$ is a PRF family and every function $f_k$ is PRP, where $f_k(m)$ and $f_k^{-1}(m)$ are efficiently computable. I proved that this ...
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Circular Security - Encrypting the key with the key

I know this isn't the best practice, but what if I took a cheap/fast 128-bit stream cipher (assuming both the key and the stream blocks are 128 bits) and mixed in the plainkey with the plaintext (let'...
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Generating a MAC on a Polynomial

In the following, all the operations and polynomials are defined over a finite field of prime order, $\mathbb{F}_p$, where $p$ is a sufficiently large prime number. All polynomials and values below ...
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A modification of the NMAC construction

Consider the NMAC construction: This is a proposed exercise in my notes: Assume F is a PRP with $n = l(n)$. Is it secure to replace $k_0$ by $F_{0^n}(k_0)$ and $k_i$ by $F_{0^n}(k_i)$? In ...
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Parallel MAC vs doubled rounds for SipHash, KMAC and others

SipHash 4-8 is documented by the authors as being about half the speed of SipHash 2-4 and is obviously the more conservative option. I was wondering from a security perspective how this would compare ...
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PKI Challenge Response: why is $h(r) = x$ enough to protect against chosen text attacks?

I think the question was asked somewhere else but not answered very precisely. Quoting from the "Handbook of applied cryptography": Identification based on PK decryption and witness. Consider ...
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MAC with key supplied afterwards

Looking at HMAC and KMAC it seems that the key is always required up front, i.e. before starting to process any other data. However, in many protocols the key may not be known it advance. For instance,...
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Safe MAC proof check

Let $(Mac, Vrfy)$ be a safe defined $MAC$ over $(K,M,T)$ where $ M = \{0,1\}^n$ and $T = \{0,1\}^{128}$. Is the following $MAC$ safe? Show your proof. $Mac'(k,m) = Mac(k, m)$ $Vrfy'(k,m,t) = \{Vrfy(...
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How to normalize data of a generic HTTP request for the purpose of signing

I am working on PowerAuth 2.0, a solution for counter-based signing of HTTP requests (for the purpose of mobile banking). One of the tasks that I solved somehow but I am still not happy with the ...
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Which protocols / file formats use CBC with a random IV and without a MAC?

Disclaimer: I am a noob so this may well be a stupid question or one that's been asked before. I didn't see it come up after searching, though, so I'm going to assume it hasn't been asked here. The ...
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Confidentiality then Integrity with different keys

Can anyone describe how we can do what is said in the below sentence which is in Mark Stamp crypto slides (PPTX) on slide number 101 : Can do a little better - about 1.5 “encryptions” The whole ...
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Which of these is a secure MAC?

Let (S,V) be a secure MAC defined over $(K,M,T)$ where $M = \{0,1\}^n$ and $T =\{0,1\}^{128}$. Which of the following is a secure MAC: $S'(k,m) = S(k,m)[0,\ldots,126]\quad and \quad V'(k,m,t) = \big[...
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CBC MAC and DES combined question?

Suppose that we want to develop a MAC scheme which is as secure as Triple-DES CBC-MAC and at the same time as efficient as Single-DES CBC-MAC. We come up with the following idea: Except the last ...
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Security of .net AES implementation and security of a hash MAC

My question rather splits into two sections: Is AES correctly and security implemented in the .net Framework? Are there any reviews of implementation mistakes that would weaken the security of AES (...
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extend a MAC scheme to me unforgeable against unbounded queries

For some prime p we generate two keys $k_{1},k_{2} \leftarrow Z^2_{p}$ where $Z$ is group and the message space of the MAC is also $Z$. We generate a tag for ...
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Message Authentication Code (MAC) calculation using OpenSSL

I have (1) a cipher text (2) a key and (3) an IV. I need to find the authentication tag of the cipher text. It is known that the authentication tag is created with the HMAC-SHA256 message ...
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Hybrid Authentications

Is there a reason why we can't combine private key MAC with digital signature to get a hybrid authentication scheme? Is it because of the computational assumptions that digital signatures have? Edit: (...
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MAC Security - MAC verification queries

In the applied cryptography book by Boneh and Shoup, Chapter 6 on MACs, it is stated that an adversary that is also capable of requesting the challenger for verification queries (in addition to ...
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layout scheme to store encrypted files

I'm working on a little hobby project to better understand crypto. https://app.gitbook.com/@noojee/s/dvault/ The aim is to make it easy to encrypt a set of files into a 'vault' and decrypt them at a ...
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How to compose multiple values to MAC authenticate?

If the data I want to authenticate consists of multiple values and I compute a MAC simply concatenating the values, an adversary can "shift" characters within those values without ...
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Algorithm to validate 1 of N inputs is part of output hash

Is there an algorithm that allows proofing that an input x1 was used as 1 out of N inputs to create an output hash y, without ...
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Is 64-bit SHA-256 IV-based MAC for fixed-length messages reasonably secure?

I want to use truncate8(SHA256-nopad(IV=secret, message)) as a 64-bit MAC, where message is guaranteed to be exactly one hash ...
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Authentication tag length and post-quantum security

The recommended hash length for post-quantum security seems to be either 384-bits or 512-bits. 512-bits gives 256-bit collision resistance, and 256-bit security is obviously ideal for post-quantum ...
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Insecure pseudo random functions

Let $F$ be a pseudo-random function. Show that the following constructions are insecure as message authentication codes (in each case $K \in \{0,1\}^n$ is the private key): $(a)$ To authenticate a ...
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How secure is Encrypt-And-MAC?

From this popular Crypto.SE thread, the general feeling is that Encrypt-Then-MAC is the most secure. From the first answer, the poster says that Encrypt-And-MAC provides integrity of the plaintext. If ...
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Constructing a no-query secure MAC II

I'm trying to construct a no-query-secure MAC $II=(Gen,Mac,Vrfy)$ without using any cryptographic primitives such as PRF, PRG, has function, etc. My-attempt: I learned that a message authentication ...
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Using AES to create a secured MAC

Considering public message M and private key K, would it be safe to create the MAC tag this way: $$MAC(M,K) = AES(m_1) \oplus AES(m_2) \oplus \cdots \oplus AES(m_n)$$ when $m_1,m_2,\ldots,m_n$ are n ...
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Compare two protocols of keyed hash functions

Authentication protocol using a keyed hash function: Alice and Bob share a secret key for a keyed hash function. The protocol is: Alice $\Rightarrow$ $m||h_{k}(m)$ $\Rightarrow$ Bob. There are the ...