So let's say I have a md5 hash of a five digits long PIN. It is given as string.
The only way is cracking it, means try every combination, hash it and compare it with the hash you want to crack. I'm trying to return the cracked PIN as string. Given the initial hash input is the md5 digest of the PIN (in a string). Here's my code:
require 'digest'
def crack_PIN(hash)
md5 = Digest::MD5.new
permutations = [0,1,2,3,4,5,6,7,8,9].permutation(5).to_a
perm_digest = permutations.map{|element| md5.hexdigest(element.map{|num| "#{num}"}.join)}
x = perm_digest.index(hash)
permutations[x].map{|num| "#{num}"}.join
end
The problem with this code is that it only uses each number once (it can't repeat each number as many times as it wants). I'm trying to figure out what I can do to alter this code that would make it work for any amount of repetition of each number. Thanks :-)
Minimal change:
permutations = '00000'..'99999'
That will get you the full list of all strings in the range. You can then just try running md5.hexdigest on each of them.
Also note: Your method is going to perform the exact same series of calculations every time you run it!! You would improve the performance by caching the calculated result somehow, (i.e. generating a lookup table of PIN --> md5 sum) and just referencing this in your code.
Combining Tom and Cary's suggestions into a simplified version that needs less memory than your example because it does not store all hashes in memory. Furthermore, it returns the PIN as soon as it was found and does not generate all hashes upfront.
require 'digest'
def crack_PIN(hash)
md5 = Digest::MD5.new
('00000'..'99999').find { |pin| md5.hexdigest(pin) == hash }
end
crack_PIN("c4ded2b85cc5be82fa1d2464eba9a7d3")
#=> "45678"
Related
I need unique tokens for users stored in my DB. At the moment when I generate a token I check it for uniqueness in the DB before using it. Is this a test I actually need to perform or am I wasting time?
I've looked at the Ruby 2.0.0 API for SecureRandom and it doesn't clarify if I can "trust" the uniqueness or not.
I know no random value can really be "unique" and there's a finite number of possibilities. But with 32 bits of hex values I feel confident I will never run into the same value again in my app but wanted to ask if anyone knew of a "gotcha" with this situation.
Another consideration is using SecureRandom.uuid but that will essentially be the same situation.
# usage
user.password_reset_token = Generator.unique_token_for_user(:password_reset_token)
# Performs DB query to ensure uniqueness
class Generator
def self.unique_token_for_user(attribute)
begin
token = SecureRandom.urlsafe_base64(32)
end while User.exists?(attribute => token)
token
end
end
SecureRandom.uuid generates uuids. A UUID is 128 bits long, and can guarantee uniqueness across space and time. They are designed to be globally unique, unlike urlsafe_base64. See RFC4122.
It doesn't ensure uniqueness but, as svoop said, it's extremely unlikely that you'll get the same result twice.
My advice is: if all you need are random, unique and unguessable tokens, and you don't have hundreds of thousands of users, then use it without worrying.
If you absolutely want unique tokens (e.g. there is some legal requirement), then combine a unique field associated with the user (e.g. the user email) and a random salt, and hash the result.
A naive implementation would be:
require 'securerandom'
require 'digest/md5'
def generate_user_token(user)
digest(user.email + random_salt)
end
def random_salt
SecureRandom.urlsafe_base64
end
def digest(string)
Digest::MD5.hexdigest string
end
No, you won't see a duplicate in your lifespan.
32 is the length (in bytes) of the random number generated before it get's converted to an urlsafe base64 string, so chances of a duplicate are roughly 1 to 10'000'000'000'000'000'000'000'000'000'000. That's 10e31 and the universe is only 43e17 seconds old.
I was reading a tutorial on how to salt a key to make your encryption secure, but couldn't make much of it. I don't know a lot about cryptography, and need some help. I am using commoncrypto to encrypt files, and am done, except for the fact that it isn't secure...
This is what I have:
- (NSData *)AES256EncryptWithKey:(NSString *)key
{
// 'key' should be 32 bytes for AES256, will be null-padded otherwise
char keyPtr[kCCKeySizeAES256 + 1]; // room for terminator (unused)
bzero( keyPtr, sizeof( keyPtr ) ); // fill with zeroes (for padding)
NSLog(#"You are encrypting something...");
// fetch key data
[key getCString:keyPtr maxLength:sizeof( keyPtr ) encoding:NSUTF8StringEncoding];
NSUInteger dataLength = [self length];
//See the doc: For block ciphers, the output size will always be less than or
//equal to the input size plus the size of one block.
//That's why we need to add the size of one block here
size_t bufferSize = dataLength + kCCBlockSizeAES128;
void *buffer = malloc( bufferSize );
size_t numBytesEncrypted = 0;
CCCryptorStatus cryptStatus = CCCrypt( kCCEncrypt, kCCAlgorithmAES128, kCCOptionPKCS7Padding,
keyPtr, kCCKeySizeAES256,
NULL /* initialization vector (optional) */,
[self bytes], dataLength, /* input */
buffer, bufferSize, /* output */
&numBytesEncrypted );
if( cryptStatus == kCCSuccess )
{
//the returned NSData takes ownership of the buffer and will free it on deallocation
return [NSData dataWithBytesNoCopy:buffer length:numBytesEncrypted];
}
free( buffer ); //free the buffer
return nil;
}
If someone can help me out, and show me exactly how I would implement salt, that would be great! Thanks again!
dYhG9pQ1qyJfIxfs2guVoU7jr9oniR2GF8MbC9mi
Enciphering text
AKA jumbling it around, to try and make it indecipherable. This is the game you play in cryptography. To do this, you use deterministic functions.
Encrypting involves using a function which takes two parameters: usually a short, fixed length one, and an arbitrary length one. It produces output the same size as the second parameter.
We call the first parameter the key; the second, the plaintext; and the output, the ciphertext.
This will have an inverse function (which is sometimes the same one), which has the same signature, but given instead ciphertext will return the plaintext (when using the same key).
Obviously the property of a good encryption function is that the plaintext is not easily determinable from the ciphertext, without knowing the key. An even better one will produce ciphertext that is indistinguishable from random noise.
Hashing involves a function which takes one parameter, of arbitrary size, and returns an output of fixed size. Here, the goal is that given a particular output, it should be hard to find any input that will produce it. It is a one-way function, so it has no inverse. Again, it's awesome if the output looks completely random.
The problem with determinism
The above is all very well and good, but we have a problem with our ultimate goals of indecipherability when designing implementations of these functions: they're deterministic! That's no good for producing random output.
While we can design functions that still produce very random-looking output, thanks to confusion and diffusion, they're still going to give the same output given the same input. We both need this, and don't like it. We would never be able to decipher anything with a non-deterministic crypto system, but we don't like repeatable results! Repeatable means analysable... determinable (huh.). We don't want the enemy to see the same two ciphertexts and know that they came from the same input, that would be giving them information (and useful techniques for breaking crypto-systems, like rainbow tables). How do we solve this problem?
Enter: some random stuff inserted at the start.
That's how we defeat it! We prepend (or sometimes better, append), some unique random input with our actual input, everytime we use our functions. This makes our deterministic functions give different output even when we give the same input. We send the unique random input (when hashing, called a salt; when encrypting, called an Initialisation Vector, or IV) along with the ciphertext. It's not important whether the enemy sees this random input; our real input is already protected by our key (or the one-way hash). All that we were actually worried about is that our output is different all the time, so that it's non-analysable; and we've achieved this.
How do I apply this knowledge?
OK. So everybody has their app, and within it their cryptosystem protecting parts of the app.
Now we don't want to go reinventing the wheel with cryptosystems (Worst. Idea. Ever.), so some really knowledgable people have already come up with good components that can build any system (i.e, AES, RSA, SHA2, HMAC, PBKDF2). But if everyone is using the same components, then that still introduces some repeatability! Fortunately, if everyone uses different keys, and unique initial random inputs, in their own cryptosytem, they should be fine.
Enough already! Talk about implementation!
Let's talk about your example. You're wanting to do some simple encryption. What do we want for that? Well, A) we want a good random key, and B) we want a good random IV. This will make our ciphertext as secure as it can get. I can see you haven't supplied a random IV - it's better practice to do so. Get some bytes from a [secure/crypto]-random source, and chuck it in. You store/send those bytes along with the ciphertext. Yes, this does mean that the ciphertext is a constant length bigger than the plaintext, but it's a small price to pay.
Now what about that key? Sometimes we want a remember-able key (like.. a password), rather than a nice random one that computers like (if you have the option to just use a random key - do that instead). Can we get a compromise? Yes! Should we translate ASCII character passwords into bytes to make the key? HELL NO!
ASCII characters aren't very random at all (heck, they generally only use about 6-7 bits out of 8). If anything, what we want to do is make our key at least look random. How do we do this? Well, hashing happens to be good for this. What if we want to reuse our key? We'll get the same hash... repeatability again!
Luckily, we use the other form of unique random input - a salt. Make a unique random salt, and append that to your key. Then hash it. Then use the bytes to encrypt your data. Add the salt AND the IV along with your ciphertext when you send it, and you should be able to decrypt on the end.
Almost done? NO! You see the hashing solution I described in the paragraph above? Real cryptographers would call it amateurish. Would you trust a system which is amateurish? No! Am I going to discuss why it's amateurish? No, 'cus you don't need to know. Basically, it's just not REALLY-SUPER-SCRAMBLED enough for their liking.
What you need to know is that they've already devised a better system for this very problem. It's called PBKDF2. Find an implementation of it, and [learn to] use that instead.
Now all your data is secure.
Salting just involves adding a random string to the end of the input key.
So generate a random string of some length:
Generate a random alphanumeric string in cocoa
And then just append it to the key using:
NSString *saltedKey = [key stringByAppendingString:salt];
Unless salt is being used in a different way in the article you read this should be correct.
How a random salt is normally used:
#Ca1icoJack is completely correct in saying that all you have to do is generate some random data and append it to the end. The data is usually binary as opposed to alphanumeric though. The salt is then stored unencrypted alongside each hashed password, and gets concatenated with the user's plaintext password in order to check the hash every time the password gets entered.
What the is the point of a SALT if it's stored unencrypted next to the hashed password?
Suppose someone gets access to your hashed passwords. Human chosen passwords are fairly vulnerable to being discovered via rainbow tables. Adding a hash means the rainbow table needs to not only include the values having any possible combination of alphanumeric characters a person might use, but also the random binary salt, which is fairly impractical at this point in time. So, basically adding a salt means that a brute force attacker who has access to both the hashed password and the salt needs to both figure how how the salt was concatenated to the password (before or after, normally) and brute force each password individually, since readily available rainbow tables don't include any random binary data.
Edit: But I said encrypted, not hashed:
Okay, I didn't read very carefully, ignore me. Someone is going to have to brute-force the key whether it's salted or not with encryption. The only discernable benefit I can see would be as that article says to avoid having the same key (from the user's perspective) used to encrypt the same data produce the same result. That is useful in encryption for different reasons (encrypted messages tend to have repeating parts which could be used to help break the encryption more easily) but the commenters are correct in noting that it is normally not called an salt in this instance.
Regardless, the trick is to concatenate the salt, and store it alongside each bit of encrypted data.
I'm trying to generate UUIDs with the same style as bit.ly urls like:
http://bit [dot] ly/aUekJP
or cloudapp ones:
http://cl [dot] ly/1hVU
which are even smaller
how can I do it?
I'm now using UUID gem for ruby but I'm not sure if it's possible to limitate the length and get something like this.
I am currently using this:
UUID.generate.split("-")[0] => b9386070
But I would like to have even smaller and knowing that it will be unique.
Any help would be pretty much appreciated :)
edit note: replaced dot letters with [dot] for workaround of banned short link
You are confusing two different things here. A UUID is a universally unique identifier. It has a very high probability of being unique even if millions of them were being created all over the world at the same time. It is generally displayed as a 36 digit string. You can not chop off the first 8 characters and expect it to be unique.
Bitly, tinyurl et-al store links and generate a short code to represent that link. They do not reconstruct the URL from the code they look it up in a data-store and return the corresponding URL. These are not UUIDS.
Without knowing your application it is hard to advise on what method you should use, however you could store whatever you are pointing at in a data-store with a numeric key and then rebase the key to base32 using the 10 digits and 22 lowercase letters, perhaps avoiding the obvious typo problems like 'o' 'i' 'l' etc
EDIT
On further investigation there is a Ruby base32 gem available that implements Douglas Crockford's Base 32 implementation
A 5 character Base32 string can represent over 33 million integers and a 6 digit string over a billion.
If you are working with numbers, you can use the built in ruby methods
6175601989.to_s(30)
=> "8e45ttj"
to go back
"8e45ttj".to_i(30)
=>6175601989
So you don't have to store anything, you can always decode an incoming short_code.
This works ok for proof of concept, but you aren't able to avoid ambiguous characters like: 1lji0o. If you are just looking to use the code to obfuscate database record IDs, this will work fine. In general, short codes are supposed to be easy to remember and transfer from one medium to another, like reading it on someone's presentation slide, or hearing it over the phone. If you need to avoid characters that are hard to read or hard to 'hear', you might need to switch to a process where you generate an acceptable code, and store it.
I found this to be short and reliable:
def create_uuid(prefix=nil)
time = (Time.now.to_f * 10_000_000).to_i
jitter = rand(10_000_000)
key = "#{jitter}#{time}".to_i.to_s(36)
[prefix, key].compact.join('_')
end
This spits out unique keys that look like this: '3qaishe3gpp07w2m'
Reduce the 'jitter' size to reduce the key size.
Caveat:
This is not guaranteed unique (use SecureRandom.uuid for that), but it is highly reliable:
10_000_000.times.map {create_uuid}.uniq.length == 10_000_000
The only way to guarantee uniqueness is to keep a global count and increment it for each use: 0000, 0001, etc.
I need a technique (an a pointer to sample code if you have) for generating conformation numbers for web payment. I don't want the customer to write down a long sequence like a GUID but I don't want it easily predictable as well.
Using C#
Thanks for all the tips.
I decided on a format like this:
TdddRROOO
T = 2009 (next year will be U = 2010)
ddd = days this year
RR = two random numbers
000 = order number (I'll offset this so folks can't know the order number that day)
So the confirmation number will be something like
P23477098
You could do something with a mixture. Generate the first half of the key as a known, predictable value (e.g. 00001, 00002, 00003, etc.) and then generate the second half as a randomly generated value so it won't be predictable. Then, increment the "known, predictable" value so that you will never get a match.
Your unique code would then become: 00001-53481, 00002-43853, 00003-54511, etc.
Of course, I am sure there are libraries out there that probably do this already. (It might help if you specify what language you are using.)
I recent did same thing in PHP. We use random function in this class,
https://github.com/kohana/core/blob/3.3/master/classes/Kohana/Text.php
We use random('distinct', 8) to generate confirmation number. It generates strings like this,
4CFY24HJ
JH5AYL7J
2TVWTMJ5
As you can see, it has no confusing numbers/letters like (1/l, 0/O etc) so it makes it much clearer when customers have to read the numbers over the phone.
Decide on the characters (char[] chars) that you want in your confirmation code, decide on the length of confirmation code (n), generate n random numbers (i_1, i_2, ... i_n) in the range [0..chars.Length) and return the string chars[i_1]chars[i_2]...chars[i_n].
In C#:
public string ConfirmationCode(char[] chars, int length, Random rg) {
StringBuilder codeBuilder = new StringBuilder();
for(int i = 0; i < length; i++) {
int index = rg.Next(chars.Length);
codeBuilder.Append(chars[index]);
}
return codeBuilder.ToString();
For uniqueness, prepend the current time in yyyyMMddhhmmss format.
Just generate a random number between 100000 and 999999, for example. Also a good idea is to put some letters in front that identify that it is a confirmation number, such as CONF-843682 so that people will recognize it more easily when you ask for it.
Store the number in the database, together with an ID for the order and an expiry date (say 1 year).
You could do something like get a random number of a specified length, convert to base64 and add a checksum character.
How about something like Amazon's PayPhrase? Use a library like Faker (Ruby) or Data::Faker (Perl) to generate random phrases, or write your own utility. Then just use a simple hash function to convert the "confirmation phrase" into a number you can index.
As for C# there exists a port Ruby's Faker gem at http://github.com/slashdotdash/faker-cs
I'm looking to create a simple short-lived reservation system, and I'd like to generate confirmation numbers that are
unique
random-looking
alphanumeric
short-ish, at least much shorter than 32 character-long strings returned by sha1
I'm only looking to have ~500 reservations, so I don't imagine high likelyhood of collissions.
One idea I had is generate an sha1 hash based on a date-time stamp and username, then truncating it to its first 10 characters. Would something like that be reliably unique enough for the purposes of processing ~500 reservations?
There should be no difference in the randomness of any given bit of a SHA-1 hash, so that's possible. Another way would be to fold the hash into itself using XOR until you have 60 bits worth of data, then encode it using Base 64 to get a mostly alpha-numeric result.
This is only necessary if you want to be able to generate the same Id repeatedly for the same input data. Otherwise, if a random id that you generate once, and hold onto after that, use Anders' suggestion. If you get a conflict, just generate another one.
You can use whatever, even a plain random number generator; however, you should check that the reservation code isn't already present. If this is the case, add characters ('x') to the string (date+user) until you get a new random/sha1/etc.
I'm only looking to have ~500 reservations, so I don't imagine high likelyhood of collissions.
Another stupid idea: generate 1000 or 2000 unique random numbers with the desired properties, store them somewhere, and assign them to the users as they register :)
Here's one way to do it in Perl:
sub get_random_name()
{
my #chars=('a'..'z','A'..'Z');
my $random_string;
foreach (1..22)
{
# rand #chars will generate a random
# number between 0 and scalar #chars
$random_string .= $chars[rand #chars];
}
return $random_string . "-" . time();
}
I don't remember how long the time() part is, so you may have to adjust the numbers to fit your length. You can also remove that part if you don't need it.
If it's really just 500, then pre-generate 20,000 of them, into a table, then get the "next unused one" when you need it.
Some good tips on this question: How do I create a random alpha-numeric string in C++?
I'd avoid including characters like "1", "l", and "O", "0" and "5", "S", and "Z", "2" in your string, to make it easier for customers when they need to read your reservation code over the phone. The algorithm presented at that link should help you do this.
use a guid? 16 characters, though if you really don't care about collision, you could just choose the first n characters.
In C# you can use http://www.dotnetfunda.com/forums/thread1357-how-do-generate-unique-alpha-numeric-random-number-in-aspnet.aspx (the super easy way, they say)