Symmetric cryptosystems assume two communicating entities share a pre-established secret key.
19
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3answers
939 views
Information leakage from the ecryptfs filesystem
I'm wondering what information might be leaked from the ecryptfs filesystem. This is what Ubuntu uses if you check the box for "encrypted home directory" when using the desktop installer, so is ...
14
votes
2answers
1k views
How can we reason about the cryptographic capabilities of code-breaking agencies like the NSA or GCHQ?
I have read in Applied Cryptography that the NSA is the largest hardware buyer and the largest mathematician employer in the world.
How can we reason about the symmetric ciphers cryptanalysis ...
11
votes
3answers
3k views
Why is public-key encryption so much less efficient than secret-key encryption?
I'm currently reading Cryptography Engineering. After giving a high level explanation of the difference between secret-key encryption and public-key encryption, the book says:
So why do we bother ...
10
votes
1answer
3k views
How to choose a padding mode with AES
Depending on the framework you are using, there are various padding modes that can be used with AES encryption. For example, with .NET we can choose PKCS7, ISO10126, ANSIX923, Zeros or None.
I ...
9
votes
1answer
2k views
Why do we use encrypt-decrypt-encrypt (EDE) in 3DES, rather than encrypting three times?
I'm wondering why we use encrypt-decrypt-encrypt sequence in 3DES with three keys instead of three times encryption with three different keys?
7
votes
4answers
301 views
Can I determine if a user has the wrong symmetric encryption key?
We're using the Objectivity/DB object database with a custom encryption plugin that encrypts serialized objects on disk. Encryption uses AES with a shared secret key held by all users. I would like to ...
6
votes
5answers
2k views
Why do we need asymmetric algorithms for key exchange?
In SSL protocols, both symmetric and asymmetric algorithms are used. Why is it so? The symmetric algorithms are more secure and easier to implement. Why are asymmetric algorithms usually preferred in ...
6
votes
2answers
323 views
Deriving Keys for Symmetric Encryption and Authentication
So here's the concept. Rather than storing 2 keys and using a random IV, which presents its own problems (key rotation, ensuring no key is used in more than 2^32 cycles, sharing the keys, etc), is it ...
4
votes
3answers
406 views
CBC - a canonical mode, even though there are streaming modes
Why is CBC considered the canonical mode when there are streaming modes available such as CFB and OFB? One thing that I can think of is that in CBC you can easliy do range-based decryption. All you ...
4
votes
1answer
323 views
Are there two-way encryption algorithms that include a work factor?
I recently learned about the hashing algorithm bcrypt, which allows you to specify a "work factor" for the hash which can be incremented to stay ahead of Moore's Law. I understand there are some other ...
4
votes
1answer
259 views
Why is Diffie-Hellman considered in the context of public key cryptography?
In all textbooks I used the Diffie-Hellman key exchange is under "public key cryptography".
As far as I can see it is a method to exchange a key to be used with a symmetric cryptographic algorithm, ...
3
votes
3answers
625 views
What is the importance of Modular arithmetic in cryptography?
Why do we use modular arithmetic so often in Cryptography?
3
votes
2answers
342 views
Difference between symmetric and asymmetric hash function?
The Linux kernel supports symmetric and asymmetric hash functions. E.g. sha1, sha256, ...
See tcrypt.c and search for test_hash_speed and ...
3
votes
3answers
483 views
Which of these 3 AES 128 symmetric encrypt/decrypt routines is most secure?
I am developing a symmetric en-/decryption routine written in c# for a database containing user-specific, sensitive information.
I have narrowed down the implementation to 3 different approaches, ...
3
votes
1answer
95 views
When confusion is applied during encryption?
I know confusion is used in cryptography to make it more difficult to identify any
relationship between the ciphertext and the symmetric key. I want to know when the confusion is applied during ...
3
votes
2answers
162 views
Is storing the hash of a key together with ciphertext encrypted with that key secure?
Is it secure to store the hash of a symmetric cipher key along with data encrypted with that key? Are there any circumstances or algorithms in which this combination could lead to potential weaknesses ...
3
votes
3answers
181 views
Derived Shared Key vs Distinct Keys?
I've seen a lot of 2-party applications that derive a shared key from distinct keys created by each party.
Why is this technique employed? Would it not be better to use those two distinct keys for ...
3
votes
4answers
226 views
symmetric-key cryptography based 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 ...
3
votes
2answers
280 views
are CFB and OFB really meant for streaming?
CFB, OFB and other modes are meant for streaming and don't require padding. Are there still limitations such as the text needs to be greater than key length?
3
votes
2answers
201 views
What is the best way to store sensitive information on the client?
I am very inexperienced with cryptography and would like some advice on how to store some sensitive information on client machines.
I have some information stored in a central database on my server. ...
3
votes
1answer
260 views
Is this a good way to encrypt a file?
I need to encrypt a file, distribute it over an insecure channel, and decrypt it later. Using a symmetric-key algorithm. Here's what I intend to do:
Get a password P from the user. Generate a 16-byte ...
3
votes
1answer
87 views
uniquely identify a symmetric key
Is there a safe way to uniquely identify a symmetric key? I know with asymmetric keys you typically use a hash of the public key, but I assume that using a hash of a symmetric key would reveal too ...
3
votes
1answer
143 views
Is a changing public truecrypt container secure?
I have a Truecrypt container which I want to synchronise between computers (i.e. different people, that I want to share the data with).
If I used Dropbox for synchronisation and someone downloaded ...
2
votes
2answers
387 views
Why is asymmetric cryptography bad for huge data?
I've been told that asymmetric cryptography requires that the message to be encrypted be smaller than its key length.
Why is this?
I know about hybrid encryption, which uses symmetric encryption to ...
2
votes
2answers
422 views
Are there standards for transport layer security using only symmetric keys?
I have an embedded system with AES-128 implemented in hardware. There is very little flash/RAM and the (8-bit) CPU runs relatively slowly. Public key crypto isn't a viable option. This system is ...
2
votes
1answer
643 views
Sending KCV (key check value) with cipher text
I was wondering why it is not more common to send the KCV of a secret key together with the cipher text. I see many systems that send cipher text and properly prepend the IV to e.g. a CBC mode ...
2
votes
1answer
149 views
Does an attacker gain an advantage if they have the initialisation vector?
Does an attacker gain any advantage by knowing which initialisation vector was used for encryption?
2
votes
2answers
187 views
Why does the recommended key size between symmetric and assymetric encryption differ greatly?
In various articles it is mentioned that for secure communications, the recommended key sizes are 128-bit key size for symmetric encryption (which makes it $2^{128}$ possible keys?) and 2048-bit key ...
2
votes
1answer
142 views
What is a tweakable block cipher?
Pretty simple question - but I can't seem to find much information about it.
What exactly is a tweakable block cipher?
How do they differ from traditional block ciphers?
What is the 'tweak'? Is it ...
2
votes
2answers
248 views
What is the best way to send two floating point numbers to a server on the internet from a small device such as an arduino?
I was thinking about sending some sensitive data (temperature) from an arduino (AVR 8-bit processor) to a server on the internet.
But I would like to make sure that it would not be possible for Eve ...
2
votes
2answers
912 views
How to distribute session keys in public key cryptography?
In public key cryptography we can also use session keys which are symmetric. How do the sender (say a server) provides this session key information to its clients?
If the sender (here server) ...
2
votes
1answer
116 views
Why is RSA usually limited to messages up to 1 block
I'm wondering why RSA encryption usually is only used for messages that fit into one block.
For larger messages hybrid encryption in combination with symmetric ciphers like AES seem to be the solution ...
2
votes
1answer
87 views
efficient authentication with broadcast encryption?
(I am not a crypto expert, so I apologize if the terminology I use is incorrect.)
Suppose I have the following simple broadcast encryption scheme for securely sending content to $n$ authorized ...
2
votes
1answer
187 views
Is symmetric encryption vulnerable to plain-text-attacks?
Imagine that Bob sends a message to Alice for symmetric encryption to send to Charlie. (Only Alice and Charlie know the key.) Alice sends the encrypted message back to Bob to send to Charlie. Can Bob ...
2
votes
1answer
62 views
Separate Read and Write Keys in TLS Key Material
Why does the TLS protocol use different symmetric keys for receiving and sending data?
Isn't it enough to have a single key used for both reading and writing?
2
votes
2answers
143 views
How can mega store my login details and still be secure?
I understand how Mega's encryption works. For a quick summary of all those in the future looking for an answer on this... here is how it works:
Upon first signing up for an account you make a ...
2
votes
2answers
203 views
Encryption scheme with equivalent keys?
I've long been looking for a symmetric encryption scheme (or algorithm) with equivalent keys. Let me define what I want:
Symmetric encryption algorithm with encryption function $E_k$ and inverse ...
2
votes
1answer
274 views
Self-expiring symmetric keys, or: cryptography in absence of secure deletion
I can encrypt some data D using a random symmetric key K, obtaining a ciphertext C, and then encrypt K with my public key Pub and obtain H. So far so good: I can only decrypt C if I have H and my ...
2
votes
1answer
444 views
Is this design of client side encryption secure?
I want to build a secure file storage web application. Users should be sure that server doesn't know how to decrypt files so encryption should take place at client side (i.e. in Javascript) and TLS ...
1
vote
3answers
489 views
How does one scale encryption strength upwards from 256-bit?
I have seen many examples of encryption up to about 256-bit. But how does one programmatically scale the logic upwards in a language such as PHP or Java to say 1024-bit or even 4096-bit and higher?
...
1
vote
3answers
180 views
Realize a MAC using a Pseudo-random function?
Given a pseudo-random function and assuming that we do not have any other tools,
How can we construct a MAC?
I believe this can be done. Would like to know if there is more than one way of doing ...
1
vote
3answers
119 views
Requiring a “supervisor” key pair and a “user” key pair to decrypt multiple-recipient messages
I've been toying with some encryption scenarios recently. One of the hard ones I came across is a multi-party system.
So we have
Bob -- The person who sends the message (and knows it's recipients)
...
1
vote
1answer
73 views
Ciphers in CBC mode reveal place of change in plaintext
Theoretically, when using a symmetric block cipher in CBC mode, the current block is dependent on the previous block. Suppose one plaintext is encrypted using CBC, and then one bit of it is changed, ...
1
vote
2answers
706 views
Symmetric vs. Asymmetric cryptographic approaches to data security
Ok. So, I now know the basic differences between them. But I'd love to know the deeper things, like:
Exactly why is the asymmetric approach slower than the symmetric?
Why does it make use of ...
1
vote
1answer
240 views
Can I secure my key by XORing it with a hashed password?
I'd like to build a simple password-protected symmetric key system. The key-creation process in my system operates as follows:
The system creates a 256-bit key purely at random.
The user chooses a ...
1
vote
2answers
57 views
IV Security Clarification
After doing lots of reading on SO and other websites relating to AES cryptography, I am trying to understand the security issues surrounding IV's.
There seems to be a lot of confusion and ...
1
vote
2answers
240 views
Would a “Triple AES” (in the sense of how Triple Des works) serve for a dramatic increase in safety?
The system requires to be as paranoid as possible regarding security. One of the few contemplated changes to the current design is to use multiple encryption. First proposal was to use Serpent on top ...
1
vote
2answers
84 views
How can I protect against the failure of a block or symmetric cipher?
Can I protect against the failure of a block or symmetric cipher by chaining different techniques together? If so what implementation details should I be aware of?
Are some combination of ciphers ...
1
vote
1answer
105 views
Is this method for exchanging private key using RSA sound?
Bob know's Alice's public key, and he wants to make sure he's connecting to the one which has that key. Furthermore, Alice wants to verify when she gets a connection from Bob who'll give his public ...
1
vote
1answer
89 views
Symmetric encryption mode where ciphertext size is plain text size
I've had many questions on Stackoverflow on how to minimize the output of a cipher - during encryption of course - to the same size as the input. Obviously this is possible for a single block of ...
