There is 1 message, it is encrypted with 6 different keys, each has E = 5. I tried various attacks, but they do not seem to work.   Is it possible to receive a private key / encrypted message? Is it safe to use a small E?
35258
I have three different 1024-bit public keys with common exponent $e$ but different moduli. A message $m$ is encrypted (without padding) using the three keys, which results in three different encrypted messages.  Given the three pairs of public keys $(N_i,e)$ and the encrypted messages $c_i=m^e\bmod N_i$ , how do I decipher the original message $m$ ?
123052
What are the increased possibilities (if any) of being able to crack a private key given the following:   The associated bitcoin (ECDSA Secp256k1-based) public key is known. The private key has been used to generate a public key from a different ECDSA curve and that public key is also known. It is known that the two public keys are derived from the same private key. It is known which ECDSA curves were used to generate the public keys. It is known which ECDSA curve was used to generate which public key.   Does it make any more difference if there is data that is known to have been signed by the private key and the signature(s) are known and the raw data is known?  EDIT: I should have pointed out that the conditions are ANDed, I.e. All conditions are known. Sorry, I should have been clearer the first time.
8545
I have some thoughts on this. First, I want to say I am no expert on cryptography, I just know some stuff, and I took a cryptography class in University. I am very interested in this topic.  I understand that many security experts out there might get angry just by the discussion of these things, but I hope you can spare me :)  My Questions:  Would it be possible to break an RSA key, in for example 1 week of time, if the cracker have already spent X number of years building an index of primes by performing every permutation of existing prime keys up to 2^2048 ?  I understand this would take an immense amount of time to do, but it would only be done once.  Given the public key, you would be able to look up what the two primes are, and hence retrieve the private key instantly.  When I consider who might break such a code, I am primarily thinking about NSA and other Governments that might have spent years in research and have the resources and interest in doing so.  Breaking an 256 AES would then be easier, as AES keys are often generated using the RSA private key.  Would it be possible to build such an index ? How long would that take in permutations, and perhaps with the fastest computer in the world ?  I hope we can have a nice chat about this. We are just discussion theory here. I hope you can offer me some insight on this topic.  Thanks!
98857
Here is my problem that I have to solve:  An agent will send a secret code made up of 12 different symbols across a secure wire. In addition to the 12 symbols, the agent will also send a total of 45 blank spaces between the symbols, with at least three spaces between each pair of consecutive symbols. How many ways are there for the agent to send such a message?  Just the wording of this problem has me thoroughly confused and lost.  Now, I figure that since the agent will put at least 3 spaces between a pair of consecutive symbols, then at least 36 spaces would be used. But that's all I have figured out, if I even did that correctly. Could someone help me solve this problem?  Thank you for any help.
90368
If I understand correctly in (unpadded) RSA the length of message M must be shorter than modulus n.  Encryption gives ciphertext that is also shorter than n.  Let's say I want to send a message of 2.5x length of n.  Is the split of message into chunks inferred somehow, or is that encoded somehow within ASN.1 keys? What are the different options/standards for this?  EDIT: Probably RSA is not used for messages longer than n but symmetric keys are generated with RSA and then they are used for longer messages?
35375
It is my understanding that the public key modulus $N$ for the RSA cryptosystem is presumed to be a semiprime. I have also read where it is not necessary that $N$ be a semiprime, but it could be some other type of composite number. My question is: If one wants to decrypt a message knowing $N$ and one of its prime factors, can the message still be decrypted if $N$ is not a semiprime?
45755
What if, instead of using CBC mode in the normal way with a random IV, I used this approach:   Use a fixed IV (like a block of 0's). Before encrypting, generate a random block and prepend it to the plaintext. After decrypting, ignore the first block.   Is this exactly the same in terms of security as using a random IV?  Also, would it make any difference if I used something like a message counter instead of a random block in step 2?
111358
I'm just repurposing  for CryptoWall:     "If we know exact contents of some of the files prior to them being encrypted, would it be feasible to use those files to discover the private key?"   There's an answer posted for the case of CryptoLocker, but what about for CryptoWall?  I've read it is not as thorough, for instance not using the type of RSA encryption meant for large blocks of data.
12711
Consider a block cipher with a key of size 128 bits but a small block size, say 32 bits.  Is this kind of block cipher secure?  I would like to know to what extent I can use a small block cipher, like a 32 bit block cipher. What are the constraints so that the use is acceptable in a real life application.  For instance, if we use a one-time key, that is, to encrypt only one message. Besides, the message could be small.  Suppose that when we want so send one file, we perform a key agreement so that we use the key for one message. This scenario is not efficient, but the CPA find-then-guess game is not adequate in this case.
111228
Am I missing something, or is RSA very very limiting when it comes to ecrypting data when it comes to the actual message size?  I have read that you can only encrypt a message of around 117 to 200 bytes in size (depending on your key size: 1024 or 2048 bits). Is this correct?  If so, I think I have read people suggesting on using it for larger files. I guess they are just repeating the process many times over and over in blocks of 117 bytes?
110421
I am new to crypto and trying to understand if it would be insecure to use a cipher such as AES to encrypt a key with the same key. If it is insecure, then why would it be insecure?  Basically, something like this:  encrypt(key, key)   What happens when both key and message are the same (128 bits)?  The obvious question would be why would I want to do that. If I have a secure channel to send the key, why encrypt it? My question is more for understanding.
89879
I am looking at the different attacks on RSA. I came across known-plaintext attack where the public key can be used to encrypt all possible messages until it finds a match on the sent encrypted message. However, on the book I found in the library it seems this form of attack is also called "Forward search attack".      If the message space is small or predictable, an adversary can decrypt a ciphertext c by simply encrypting all possible plaintext messages until c is obtained. Salting the message as described above is one simple method of preventing such an attack. -Handbook of Applied Cryptography   So are these attacks the same with different names or have I misunderstood it?
41822
I need to generate a 256 bit encryption key described by the adjectives in the title. Currently I intend to create the key using this .  Is this a secure manner of creating the key, given that it has the following properties:   Private key for symmetric encryption Used many times (of course with unique IV's)   And given the following facts:   I will literally just get the RNG to produce 32 random bytes not doing anything with (1) SALT (2) Iterations (3) Anything else   Please note that I am only asking if the key, directly after being generated, is secure. Thanks
76297
I just read this post here:  that asserts that in public key cryptography when asymmetric keys are used to secure communications, for parties to agree on a common shared secret, the following method is used:     So, say you want to send me a very long message, using asymmetric   encryption. Here's what you'd actually do:      (1) Pick a random key for use with a symmetric (fast) cipher. Encrypt   your message using that (fast) cipher and key. Send me that encrypted   message.      (2) Then use asymmetric (slow) encryption to encrypt the following   message and send it to me      "Hi, I just sent you a message encrypted using the BLAH   symmetric   cipher with key ABCDE123456."   So, you use your peer's public key to encrypt a shared secret, send the peer the encrypted secret, and then the peer decrypts the secret with his or her private key. In this way, you can use the big certificates to authenticate parties as well as use them to establish a shared secret.   This makes sense, but i have to be missing something. Why/how would DH be used here as well? I see that there are plenty of schemes like RSA2048 that have large keys, which i assume would be used in this way above, but also incorporate DH in their cipher suite:RSA_DHE, ECDHE.   What benefit does the DH component add? What can it do, now that we can establish a shared secret with the public keys in the certificates?
114381
Some systems, that operate over flaky bearers, need to send signed, encrypted messages that are entirely self contained - i.e. send a single 300 byte UDP rather than have a 10Kb, two-way, certificate conversation over TCP.  Assume the destination has a private key and already knows the source's public key, and the source has a private key and already knows the destination's public key.  Given this assumption, is there a standard for encrypting/signing a small, but arbitrary length, message?  (I'm well aware of the dangers of rolling my own encryption scheme.)  Would there be a minimum ciphertext size that is related to the public key size?  (I'm thinking plain PKI encryption for plaintexts shorter than the key, and a PKI encrypted AES key for longer plaintexts.)
27125
One thing cryptography articles never seem to explain is how the message actually gets encrypted. You get this long-winded lecture on number theory which ends with, Ta Da! and we have a public and private key. Then, they never explain the process of exactly how some 100,000 byte word document gets encrypted using the public key.  Other times the explanations seem downright misleading. For example, in my book Cryptography (by Meyer and Matyas) it says for RSA the ciphertext is the plain text to the power of key. How do you exponentiate a word document? I don't get it.
82331
From , there are $14$ bytes specifying the file format of the key. I am still quite confused about the format bytes. For example, the following public key 30 82 01 0a 02 82 01 01 00 8e a3 d1 c7 9c 86 05 52 3d 70 9a 5b 24 8a 6e ab 8f 5d 8d 9a 44 5f 25 78 c7 ba bd 3a a6 e1 36 b8 55 88 18 d7 ea e8 14 2c 68 8f e7 fe 94 4c f3 fd ad 0b e6 d2 eb 9e d2 66 b4 3a 3b d1 bb 5d d5 2a 53 7e 0f 1d ba ec 03 29 9d 47 50 3b 99 fb 4a 3a 80 a2 23 3e d7 11 e3 de a8 8d ab 7c 90 d0 92 af 36 b8 8b 28 fd 80 ec bc 37 6d 23 44 86 4e 28 19 1d 18 37 af 44 a9 40 b3 f6 e7 6c ad 56 5d 6f ff 3b e3 a5 cc 23 5c 54 2a 47 28 5b 29 f3 45 8e 69 98 ad 57 45 2e 60 bd ac 55 fc 35 e8 47 9f 98 0d f9 ea 9d 55 35 c9 db af 24 d2 bc 18 12 02 53 d6 aa ef 9c c9 11 c9 8e d7 7c 4f 2f 22 0f 66 b1 bf 06 a5 fa 87 22 9f ff f6 20 75 e7 51 87 26 30 c2 e1 a5 30 2c a1 fc 47 a5 f7 a5 38 d3 cc 8d 0e ee 5a 54 ee a2 f9 ff d0 0a 0f 18 7f 94 d2 04 5e 1f 25 ca be 4e 30 c3 40 00 ed a4 ce 58 ab 23 39 2d 02 03 01 00 01 is taken from google's certificate. The first byte $30$ means it is a sequence. Then I don't know what is the meaning from 2nd byte to 9th byte. Can anyone explain to me? UPDATE: The 2nd byte (82) means that the following 2 bytes give the length of the sequence. The 5th byte tells us that it is an integer. The byte following it tells us the length and the value of the integer (I guess). Then the rest I have no idea. Also what do the last 3 bytes stand for? I can't find format start with $01$.