# Tag Info

6

Rick Demer already wrote the answer in the very first comment, but without explanation: Hybrid encryption. But since you asked for a real practical example to encrypt your word document, this is how: Your file is on your disc, and it is 100,000 byte large. You can then do: First, you start up a random number generator. Preferably you should either have ...

4

The key is not stored with the encrypted data, the encrypted key is. This is part of the header. In short, when the volume is created a random key (the master key) is generated and this random key is encrypted using a key derived from the passphrase, and spread out (using an algorithm called 'afsplit'). This key derivation is designed to take a relatively ...

3

Some people define "encryption" in such a way as to exclude analog techniques -- there is no such thing as "analog encryption", they would say, because any analog method of obscuring the contents of a transmission is, by their definition, "scrambling", not encryption. There have been several such scrambling ("analog encryption") techniques, including: ...

3

There are a bunch of problems with this protocol. First of all, the way you generate your RC4 key (concatenate a secret key with a public nonce) is known to be weak. The one thing that saves you is that you only do it 256 times before generating a fresh secret key; however it is known that if you were to do it, say, 2000 times with a secret key, you would ...

2

AES-GCM uses single block cipher operation and can be processed in parallel, therefore it should be faster. CTR+HMAC requires block cipher and hash function, which usually can't be processed in parallel. Also it requires 2 keys. It is often miss-implemented (MAC-than-encrypt or MAC-and-encrypt, using single key). Cipher-text length is the same for same ...

2

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 ...

2

Let $n$ be the modulus and $d = \gcd(a,n)$. Then, $ax \mod n$ will be a multiple of $d$. If $d > 1$, this implies that there is no $x$ such that $ax \equiv 1 \pmod n$, and so there is no $x$ such that $f(x) = ax+b \equiv b+1 \pmod n$. So $f$ is not a bijection since it is not surjective. P.S.: I am not certain how you got the idea of using the word ...

2

Yes, absolutely. Here is the standard construction to address this problem. Let $pk_1,\dots,pk_n$ be the public keys of the $n$ recipients. We pick a random symmetric key $k$, encrypt the message $m$ (using authenticated encryption) under key $k$ to get $c=AE_k(m)$, and then encrypt $k$ under each of the public keys. Finally, we form the whole ciphertext ...

1

Well, exponentiating a word document is rather easy. As you've said, it's $100,000$ bytes, or $800,000$ bits. This word document can thus be interpreted as a number between $0$ and $2^{800,000}$. Sure this number may be large, but it can be exponentiated. However, more commonly symmetric encryption is used with a $128$ or $256$ bit key to encrypt the word ...

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Simply put, you need to first understand why that code is bad. You also need to know what preimage resistance is. The comments tell you the algorithm, you don't even need to read the code. // Pad the String with spaces so that it is a multiple of 4 characters // XOR each consecutive 4 byte block This is not cryptographically secure, or even secure from a ...

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Selection of the Diffie-Hellman parameters If you are asking about the TLS cipher suites that use a Diffie-Hellman exchange (basically the ones containing "DH" or "DHE"), it depends on whether static or ephemeral Diffie-Hellman certificates are used. The ephemeral cipher suites (DHE_DSS, DHE_RSA, DHE_anon and the various elliptic counterparts) transmit ...

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I am not sure if I understand your requirement correctly, but from the first part of your description I think you want the following (I skipped the second part since I do not understand the meaning of "$+$") : Set up a public key $pk$ which can be used to encrypt a message $m$ and you want to split the corresponding private key $sk$ into two shares $sk_1$ ...

1

A PRNG with a seed $S$ whose output is combined with the plaintext is called a stream cipher with the key $S$. So assuming that the details are filled in correctly, what you're describing is a stream cipher algorithm with two session keys, one used in each direction. Each seed/key must be unique, and the combination must ensure that each output bit has a ...

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I think the very first question before diving into designing the protocol would be to ask yourself what kind of secure communication do you want? Namely, there are two options to consider when it comes to communication between the IM users: For the client-server secure communication, there are two directions to consider: From “sender to server”, you ...

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Proof of security: Let (x_1, ..., x_n) be the messages you caught. Let (p_1, ...) and (q_1, ...) be three streams of random messages known to both communication partners. Let these be from a true random number source generating non-autocorrelated data. Let (n_1, ...) be another stream of data from the same source. Since there is no correlation, it is ...

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I don't have a full answer yet but here is what I have so far. Since you know what $p^5 \bmod N$ is and what $q^5 \bmod N$ is then you know what $m^5 \bmod N$ is. Since we also know what $N$ is then we should be able to find out how many times we need multiply $m^5 \bmod N$ by $m^5 \bmod N$ till we get a full cycle (you can calculate it through $\phi(N)$ or ...

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