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15

For a real-world example of precisely the same ECB weakness leading to a massive password compromise, see the Adobe password database leak, as memorably illustrated in the xkcd web comic: $\hspace{83px}$ While there were several issues contributing to the scale of the compromise, one of them was that Adobe, instead of properly hashing the passwords, ...


13

The main good reasons to change keys periodically are: to mitigate the risk of a key compromise; at least some ways keys get compromised are a one-time event, e.g. eavesdropping of the keyboard protecting the passphrase; changing the key thus restores security until the next leak, and only messages enciphered with the leaked key are compromised (and only ...


12

The crucial difference between plain encryption and authenticated encryption (AE) is that AE additionally provides authenticity, while plain encryption provides only confidentiality. Let's investigate in detail these two notions. In the further text, we assume $K$ to be a secret key, which is known to authorized parties, but unknown to attackers. Goals ...


12

It illustrates the point that the same plaintext going in to the cipher will result in the same ciphertext. It just happens to be a lot better example than showing someone abc387af de7231ab abc387af abc387af a129867e Now, what does this mean in the real world? If I gave you an email encrypted with AES-128 ECB, could you look at it and figure out the ...


11

Very short answer: No Quite Short answer: No, because a scheme can only be a One-Time-Pad if the entire pad is perfectly random and secret. Concise answer: It sounds like you're trying to build a stream cipher. The security of it really comes down to how much of the scheme you think can be kept secret. If I listen in to your wifi and hear you requesting a ...


11

@fgrieu has written a good answer but there's slightly more to be said on the topic. It is entirely possible that a paper will someday be published that shows a practical attack on 2048-bit RSA. Equally, a new attack could be discovered that breaks AES in a reasonable amount of time. Finally, someone might find a collision in SHA-256! This has happened ...


11

Yes, it's the same XOR. It gets used inside most of the algorithms, or just to merge a stream cipher and the plaintext. Everything is just bits, even text. The word "hello" is in ASCII "01101000 01100101 01101100 01101100 01101111". Just normal bits, grouped in 5 bytes. Now you can encrypt this string with a random string of 5 bytes, like an One-time pad. ...


10

I try to provide a brief intro. ABE Attribute-based encryption (ABE) is a relatively recent approach that reconsiders the concept of public-key cryptography. In traditional public-key cryptography, a message is encrypted for a specific receiver using the receiver’s public-key. Identity-based cryptography and in particular identity-based encryption (IBE) ...


10

This depends on the public-key system (algorithm). For RSA, technically the private and public key (i.e. the exponents, the keys share the same modulus) are symmetric, you can swap them, and it still works. But you usually don't want to do this: The public exponent is usually a small number (like $3$ or $2^{16} + 1$) in order to speed up ...


10

Yes, of course there is a benefit to signing unencrypted emails. The article you cite is solely about the combination of signature and encryption; it doesn't directly say anything about signing unencrypted emails. There is an important concern raised by the article which does apply to unencrypted emails, but that's because that concern applies equally ...


9

In terms of marketing hype, that statement rates about a 9 in a scale from 0-10. The reason is that we don't choose the encryption algorithm based on how many bits the CPU can handle at once. Instead, we choose a secure algorithm, and then implement it using the resources that the CPU provides us. There aren't any algorithms we cannot implement on a 32 ...


9

The general scheme is called Three-pass protocol and works for all commutative ciphers. It is secure for some of them, but xor (and modular addition) are insecure choices. Your scheme: A->B: $c_1 = m \oplus a$ B->A: $c_2 = c_1 \oplus b$ A->B: $c_3 = c_2 \oplus a$ B computes $m = c_3 \oplus b$ an attacker sees all of $c_1$, $c_2$ and $c_3$. So they can ...


9

First, your use of 'echo' gets you: ~ % echo 'Attack at dawn!!' | hexdump -C 00000000 41 74 74 61 63 6b 20 61 74 20 64 61 77 6e 21 21 |Attack at dawn!!| 00000010 0a |.| 00000011 Note that there are 17 bytes there, not 16. echo adds a newline character. To stop that, use the -n flag: ~ % echo -n 'Attack ...


9

Diffie Hellman Diffie Hellman is a key exchange protocol. It is an interactive protocol with the aim that two parties can compute a common secret which can then be used to derive a secret key typically used for some symmetric encryption scheme. I take the notation from the link above and this means we have a group $\mathbb{Z}_p^*$ for prime $p$ ...


9

XXTEA (also known as Corrected Block TEA) is a block cipher with $128$-bit key and block width parameterizable to $n\cdot32$ bits for $n\ge2$. It is an Unbalanced Feistel Cipher making $q=6+\lfloor52/n\rfloor$ passes over the block, with $q\cdot n$ rounds each modifying $32$ bits of the block. In Cryptanalysis of XXTEA, it is presented a chosen-plaintext ...


9

Perceptual encryption is a term used to describe various applications of encryption methods intended for audio, speech, image and video data. The basic idea is that one performs encryption for multimedia content in a way that only a certain amount of "perceptual information" is touched by the encryption. It may be considered as intentionally degrading the ...


8

When encrypting something with RSA, using PKCS#1 v1.5, the data that is to be encrypted is first padded, then the padded value is converted into an integer, and the RSA modular exponentiation (with the public exponent) is applied. Upon decryption, the modular exponentiation (with the private exponent) is applied, and then the padding is removed. The core of ...


8

This is the Shamir Three Pass protocol; it turns out the attacker can deduce some information about $m$; whether that information is meaningful depends on exactly what you are sensitive to. Exactly what information is leaked turns out to depend on the factorization of $p-1$ (assuming, of course, that $p$ is large enough to make solving the discete problem ...


8

Yes. Modern cryptosystems are designed and analysed under the assumption that the key is never used for anything else. If you use your encryption keys for digital signatures, you are violating that assumption, and it is very easy to construct schemes where this violation will compromise security. It is possible to construct schemes that can use the same ...


8

The article you linked to predates the S/MIME 3.2 spec. If your client is sending S/MIME 3.2 messages, it should support header protection. Refer to RFC 5751 Section 3.1: In order to protect outer, non-content-related message header fields (for instance, the "Subject", "To", "From", and "Cc" fields), the sending client MAY wrap a full MIME message ...


7

You first need to consider your adversary and what are your goals for this mechanism. This kind of mechanism appears less effective than proper cryptographic means: having secure PRNG means that both ends of the message exchange have access to some proper cryptographic means Adding noise means that the information exchange is less efficient: there is much ...


7

Did you try Wikipedia? DES consists of 16 rounds of the form: $$L_{i+1} = R_{i}, \quad R_{i+1} = L_i \oplus F(R_i, K_i),$$ which are identical except for the round subkeys $K_i$. (The last round is slightly different, in that the half-blocks $L$ and $R$ are not swapped as they are after all other rounds, but that makes no cryptanalytic difference.) The ...


7

When $n$ is prime, solving for $e$-th roots modulo $n$ is easy, since it suffices to compute $d = e^{-1} \pmod {n-1}$ and then $s = m^d \pmod n$. If $n$ is not prime, but is instead a RSA modulus (a composite integer that is the product of two big primes), then the problem becomes apparently hard (in the sense that we don't have a clue how to do it ...


7

AES does not operate on or produce characters — it has no knowledge or care of any particular character encoding. AES and other modern block ciphers accept and output arrays of bytes. The same concept applies to the key, and (in block modes that require one), the initialization vector. How (and if) you choose to encode the output is up to you. For storing ...


7

The two last equations don't directly give you the value of $C_i$, they are telling you the values of the remainder of Ci when divided by $P$ and $Q$. You then use the Chinese Remainder Theorem with this information to produce the value of $C_i$ (modulo $N$) that you are looking for. See en.wikipedia.org/wiki/Chinese_remainder_theorem (there is an algorithm ...


7

ECDSA is a digial signature algorithm ECIES is an Intergrated Encryption scheme ECDH is a key secure key exchange algorithm. First you should understand what are the purpose of these algorithms. Digital signature algorithms are used to authenticate a digital content.A valid digital signature gives a recipient reason to believe that the message was created ...


7

The XXTEA cipher is badly broken. Even though the paper is not published at a conference, the author verified it on reduced versions of XXTEA. You should never ever use a cipher or a hash function, that has been broken in academic terms, in particular if you are not a cryptographer. Attacks always get better, and a cipher does not attract much attention ...


7

You could use HMAC for this. HMAC is available in pretty much every crypto library out there. The process would work like this. Randomly pick A and C. For simplicity, let's assume they are strings (of any length). Compute $B=HMAC(A,C)$. Publish $B$. Once someone guesses $A$, you publish $C$. Anyone can then verify that $B=HMAC(A,C)$. As long as a good hash ...


6

Using a MAC on the plaintext may potentially leak information about the plaintext (MAC algorithms do not necessarily ensure confidentiality of the data they are applied to, although some MAC algorithms like HMAC seem pretty safe). If you want to avoid this (theoretical) problem, then you should encrypt the MAC on the plaintext (i.e. MAC-then-encrypt, not ...


6

OpenPGP as defined by RFC 4880 knows two different encodings. Binary encoding Obviously, there is no reasonable limitation to an (ASCII) character subset in binary encoding. Radix 64 Radix 64 is also often entitled ASCII armored. In the end, it is a base64 encoding with a checksum. The content may consist of [a-zA-Y0-0+/=]. ASCII-armored OpenPGP ...



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