Is it possible to reverse a SHA-1?
I'm thinking about using a SHA-1 to create a simple lightweight system to authenticate a small embedded system that communicates over an unencrypted connection.
Let's say that I create a sha1 like this with input from a "secret key" and spice it with a timestamp so that the SHA-1 will change all the time.
sha1("My Secret Key"+"a timestamp")
Then I include this SHA-1 in the communication and the server, which can do the same calculation. And hopefully, nobody would be able to figure out the "secret key".
But is this really true?
If you know that this is how I did it, you would know that I did put a timestamp in there and you would see the SHA-1.
Can you then use those two and figure out the "secret key"?
secret_key = bruteforce_sha1(sha1, timestamp)
Note1:
I guess you could brute force in some way, but how much work would that actually be?
Note2:
I don't plan to encrypt any data, I just would like to know who sent it.
No, you cannot reverse SHA-1, that is exactly why it is called a Secure Hash Algorithm.
What you should definitely be doing though, is include the message that is being transmitted into the hash calculation. Otherwise a man-in-the-middle could intercept the message, and use the signature (which only contains the sender's key and the timestamp) to attach it to a fake message (where it would still be valid).
And you should probably be using SHA-256 for new systems now.
sha("My Secret Key"+"a timestamp" + the whole message to be signed)
You also need to additionally transmit the timestamp in the clear, because otherwise you have no way to verify the digest (other than trying a lot of plausible timestamps).
If a brute force attack is feasible depends on the length of your secret key.
The security of your whole system would rely on this shared secret (because both sender and receiver need to know, but no one else). An attacker would try to go after the key (either but brute-force guessing or by trying to get it from your device) rather than trying to break SHA-1.
SHA-1 is a hash function that was designed to make it impractically difficult to reverse the operation. Such hash functions are often called one-way functions or cryptographic hash functions for this reason.
However, SHA-1's collision resistance was theoretically broken in 2005. This allows finding two different input that has the same hash value faster than the generic birthday attack that has 280 cost with 50% probability. In 2017, the collision attack become practicable as known as shattered.
As of 2015, NIST dropped SHA-1 for signatures. You should consider using something stronger like SHA-256 for new applications.
Jon Callas on SHA-1:
It's time to walk, but not run, to the fire exits. You don't see smoke, but the fire alarms have gone off.
The question is actually how to authenticate over an insecure session.
The standard why to do this is to use a message digest, e.g. HMAC.
You send the message plaintext as well as an accompanying hash of that message where your secret has been mixed in.
So instead of your:
sha1("My Secret Key"+"a timestamp")
You have:
msg,hmac("My Secret Key",sha(msg+msg_sequence_id))
The message sequence id is a simple counter to keep track by both parties to the number of messages they have exchanged in this 'session' - this prevents an attacker from simply replaying previous-seen messages.
This the industry standard and secure way of authenticating messages, whether they are encrypted or not.
(this is why you can't brute the hash:)
A hash is a one-way function, meaning that many inputs all produce the same output.
As you know the secret, and you can make a sensible guess as to the range of the timestamp, then you could iterate over all those timestamps, compute the hash and compare it.
Of course two or more timestamps within the range you examine might 'collide' i.e. although the timestamps are different, they generate the same hash.
So there is, fundamentally, no way to reverse the hash with any certainty.
In mathematical terms, only bijective functions have an inverse function. But hash functions are not injective as there are multiple input values that result in the same output value (collision).
So, no, hash functions can not be reversed. But you can look for such collisions.
Edit
As you want to authenticate the communication between your systems, I would suggest to use HMAC. This construct to calculate message authenticate codes can use different hash functions. You can use SHA-1, SHA-256 or whatever hash function you want.
And to authenticate the response to a specific request, I would send a nonce along with the request that needs to be used as salt to authenticate the response.
It is not entirely true that you cannot reverse SHA-1 encrypted string.
You cannot directly reverse one, but it can be done with rainbow tables.
Wikipedia:
A rainbow table is a precomputed table for reversing cryptographic hash functions, usually for cracking password hashes. Tables are usually used in recovering a plaintext password up to a certain length consisting of a limited set of characters.
Essentially, SHA-1 is only as safe as the strength of the password used. If users have long passwords with obscure combinations of characters, it is very unlikely that existing rainbow tables will have a key for the encrypted string.
You can test your encrypted SHA-1 strings here:
http://sha1.gromweb.com/
There are other rainbow tables on the internet that you can use so Google reverse SHA1.
Note that the best attacks against MD5 and SHA-1 have been about finding any two arbitrary messages m1 and m2 where h(m1) = h(m2) or finding m2 such that h(m1) = h(m2) and m1 != m2. Finding m1, given h(m1) is still computationally infeasible.
Also, you are using a MAC (message authentication code), so an attacker can't forget a message without knowing secret with one caveat - the general MAC construction that you used is susceptible to length extension attack - an attacker can in some circumstances forge a message m2|m3, h(secret, m2|m3) given m2, h(secret, m2). This is not an issue with just timestamp but it is an issue when you compute MAC over messages of arbitrary length. You could append the secret to timestamp instead of pre-pending but in general you are better off using HMAC with SHA1 digest (HMAC is just construction and can use MD5 or SHA as digest algorithms).
Finally, you are signing just the timestamp and the not the full request. An active attacker can easily attack the system especially if you have no replay protection (although even with replay protection, this flaw exists). For example, I can capture timestamp, HMAC(timestamp with secret) from one message and then use it in my own message and the server will accept it.
Best to send message, HMAC(message) with sufficiently long secret. The server can be assured of the integrity of the message and authenticity of the client.
You can depending on your threat scenario either add replay protection or note that it is not necessary since a message when replayed in entirety does not cause any problems.
Hashes are dependent on the input, and for the same input will give the same output.
So, in addition to the other answers, please keep the following in mind:
If you start the hash with the password, it is possible to pre-compute rainbow tables, and quickly add plausible timestamp values, which is much harder if you start with the timestamp.
So, rather than use
sha1("My Secret Key"+"a timestamp")
go for
sha1("a timestamp"+"My Secret Key")
I believe the accepted answer is technically right but wrong as it applies to the use case: to create & transmit tamper evident data over public/non-trusted mediums.
Because although it is technically highly-difficult to brute-force or reverse a SHA hash, when you are sending plain text "data & a hash of the data + secret" over the internet, as noted above, it is possible to intelligently get the secret after capturing enough samples of your data. Think about it - your data may be changing, but the secret key remains the same. So every time you send a new data blob out, it's a new sample to run basic cracking algorithms on. With 2 or more samples that contain different data & a hash of the data+secret, you can verify that the secret you determine is correct and not a false positive.
This scenario is similar to how Wifi crackers can crack wifi passwords after they capture enough data packets. After you gather enough data it's trivial to generate the secret key, even though you aren't technically reversing SHA1 or even SHA256. The ONLY way to ensure that your data has not been tampered with, or to verify who you are talking to on the other end, is to encrypt the entire data blob using GPG or the like (public & private keys). Hashing is, by nature, ALWAYS insecure when the data you are hashing is visible.
Practically speaking it really depends on the application and purpose of why you are hashing in the first place. If the level of security required is trivial or say you are inside of a 100% completely trusted network, then perhaps hashing would be a viable option. Hope no one on the network, or any intruder, is interested in your data. Otherwise, as far as I can determine at this time, the only other reliably viable option is key-based encryption. You can either encrypt the entire data blob or just sign it.
Note: This was one of the ways the British were able to crack the Enigma code during WW2, leading to favor the Allies.
Any thoughts on this?
SHA1 was designed to prevent recovery of the original text from the hash. However, SHA1 databases exists, that allow to lookup the common passwords by their SHA hash.
Is it possible to reverse a SHA-1?
SHA-1 was meant to be a collision-resistant hash, whose purpose is to make it hard to find distinct messages that have the same hash. It is also designed to have preimage-resistant, that is it should be hard to find a message having a prescribed hash, and second-preimage-resistant, so that it is hard to find a second message having the same hash as a prescribed message.
SHA-1's collision resistance is broken practically in 2017 by Google's team and NIST already removed the SHA-1 for signature purposes in 2015.
SHA-1 pre-image resistance, on the other hand, still exists. One should be careful about the pre-image resistance, if the input space is short, then finding the pre-image is easy. So, your secret should be at least 128-bit.
SHA-1("My Secret Key"+"a timestamp")
This is the pre-fix secret construction has an attack case known as the length extension attack on the Merkle-Damgard based hash function like SHA-1. Applied to the Flicker. One should not use this with SHA-1 or SHA-2. One can use
HMAC-SHA-256 (HMAC doesn't require the collision resistance of the hash function therefore SHA-1 and MD5 are still fine for HMAC, however, forgot about them) to achieve a better security system. HMAC has a cost of double call of the hash function. That is a weakness for time demanded systems. A note; HMAC is a beast in cryptography.
KMAC is the pre-fix secret construction from SHA-3, since SHA-3 has resistance to length extension attack, this is secure.
Use BLAKE2 with pre-fix construction and this is also secure since it has also resistance to length extension attacks. BLAKE is a really fast hash function, and now it has a parallel version BLAKE3, too (need some time for security analysis). Wireguard uses BLAKE2 as MAC.
Then I include this SHA-1 in the communication and the server, which can do the same calculation. And hopefully, nobody would be able to figure out the "secret key".
But is this really true?
If you know that this is how I did it, you would know that I did put a timestamp in there and you would see the SHA-1. Can you then use those two and figure out the "secret key"?
secret_key = bruteforce_sha1(sha1, timestamp)
You did not define the size of your secret. If your attacker knows the timestamp, then they try to look for it by searching. If we consider the collective power of the Bitcoin miners, as of 2022, they reach around ~293 double SHA-256 in a year. Therefore, you must adjust your security according to your risk. As of 2022, NIST's minimum security is 112-bit. One should consider the above 128-bit for the secret size.
Note1: I guess you could brute force in some way, but how much work would that actually be?
Given the answer above. As a special case, against the possible implementation of Grover's algorithm ( a Quantum algorithm for finding pre-images), one should use hash functions larger than 256 output size.
Note2: I don't plan to encrypt any data, I just would like to know who sent it.
This is not the way. Your construction can only work if the secret is mutually shared like a DHKE. That is the secret only known to party the sender and you. Instead of managing this, a better way is to use digital signatures to solve this issue. Besides, one will get non-repudiation, too.
Any hashing algorithm is reversible, if applied to strings of max length L. The only matter is the value of L. To assess it exactly, you could run the state of art dehashing utility, hashcat. It is optimized to get best performance of your hardware.
That's why you need long passwords, like 12 characters. Here they say for length 8 the password is dehashed (using brute force) in 24 hours (1 GPU involved). For each extra character multiply it by alphabet length (say 50). So for 9 characters you have 50 days, for 10 you have 6 years, and so on. It's definitely inaccurate, but can give us an idea, what the numbers could be.
Related
Wikipedia says:
preimage resistance: for essentially all pre-specified outputs, it is computationally infeasible to find any input which hashes to that output, i.e., it is difficult to find any preimage x given a "y" such that h(x) = y.
second-preimage resistance: it is computationally infeasible to find any second input which has the same output as a specified input, i.e., given x, it is difficult to find a second preimage x' ≠ x such that h(x) = h(x′).
Yet, I don't understand it. Doesn't h(x′) (where x' is input) generate that y (the output), which is then compared to the same h(x)?
Say, I have a string "example". It generates the MD5 "1a79a4d60de6718e8e5b326e338ae533". Why is it different to just use the MD5 compared to doing the MD5(example)?
Ideal hashing is like taking the fingerprint of a person, it is unique, it is non-reversible (you can't get the whole person back just from the fingerprint) and it can serve as a short and simple identifier for the given person.
If we bring some of the terminology you introduced into our analogy, we see that preimage resistance refers to the hash function's ability to be non-reversible. Imagine if you could generate the likeness of a whole person from their fingerprint, aside from being really cool, this could also be very dangerous. For the same reason, hash functions must be made so that an attacker cannot find the original message that generated the hash. In that sense, hash functions are one-way in that the message generates the hash and not the other way round.
Second preimage resistance refers to a given hash function's ability to be unique. Forensic fingerprinting would be a gross waste of time if any number of individuals could share the same fingerprint (lets exclude identical twins for now. Edit: See Det's comment below). If a given hash was used for verification against data corruption, it would quite pointless if there is a good chance corrupt data can generate the same hash.
To have both preimage resistance and second preimage resistance hash functions adopt several traits to help them. One trait very common for hash functions is where the given input has no correspondence to the output. A single bit change can produce a hash that has completely no bytes shared with the hash of the original input. For this reason, a good hash function is commonly used in message authentication.
Whilst you are right comparing the original message directly would be functionally equivalent to comparing the hashes, it is simply not feasible in the majority of cases. For example:
If party A wanted to reliably send a message to party B, party A/B would need to agree upon a scheme to detect data corruption during transfer. Note: party B does not have the original message until party A sends it.
A possible scheme of transfer could be to transfer the message twice such that party B can verify if the second message equals the first. The problem with this is that there is a chance that corruption can occur twice in the same place (as well as the significantly higher bandwidth). This can only be reduced by sending the messages even more times, incurring severe bandwidth costs.
As an alternative, party A can pass his/her long message into a hash function and generate a short hash which he/she sends to party B, followed by the original message. Party B can then take the received message and pass it into the hash function and match the hashes. If either the message or the hash got corrupted even by a single bit during transfer, the resultant hashes will not match, thanks to second preimage resistance (no two plaintext should have same hash).
Preimage Resistance in this case would be useful if the message is encrypted during transfer but the hash was taken prior encryption (whether this is appropriate is another discussion). If the hash was reversible, a eavesdropper could intercept the hash and reverse to find the original message.
All hash functions are not equal, that's why its important to consider their preimage resistance/second preimage resistance when choosing which ones to use, which ones are secure and which ones should be deprecated and replaced.
You understood preimage and second preimage resistance? It says the output of a hash function is unique, at least in theory.. And obtaining the original string from a hash is "computationally" in-feasible. It is possible (brute-force) though but takes up a lot of time and resources.
Now, output of a hash function and the string itself are different.. For example, consider a website with a dashboard. You provide your user_id and password at the time of signing up. If the website stores your password as such in their database, it is accessible to a hacker. He can access your account. But if a hash of your password is stored, even if he manages to hack down the server, that hash is of no use to him. Because, he cannot access your account without your password, and it is computationally in-feasible to obtain your password from the hash (preimage resistance). Comparing md5 (yourpassword) with the hash stored in the db is different. Each time you enter your password, it is hashed with the sampe hash function and compared to the existing hash. According to second-preimage resistance, if you entered an incorrect password, the hashes won't match.
Another example of hashing is in the version control or source control mechanisms. To track down changes in a file, hashing is used. They hash the entire file and keeps it. If a file is modified, its hash changes accordingly.
These are all examples explaining what you asked.
I store some sensitive data. Data is divded into parts and I want to have separate accees to each part. Let's assume that I have 1000 files. I want to encrypt each file by the same symetric encryption algorithm.
I guess that breaking a key is easier when hacker has got 1000 cryptogram than he has only one cryptogram, so I think that I should use separate key for each file.
My question is following:
Should I use separate key for each file?
If I should, there is problem with storing 1000 keys. So I want to have one secret key and use some my own algorithm to calculate separate key for each file from secret key. Is it good idea?
If you consider passive adversary and use CPA-strong cipher (like AES), it is sufficient to use only one key for all files. Supposing adversary knows the cipher you use, and even knows plaintexts, he cannot reconstruct the key with non-negligible probability. Here is more detailed answer.
If you consider also active adversary (which can replace ciphertexts) you should use Authenticated Encryption. But as I understand this is not your case.
So I want to have one secret key and use some my own algorithm to calculate separate key for each file from secret key. Is it good idea?
In general, developing your own algorithm or scheme is bad idea. You can easily make some unseen mistake in algorithm or implementation and you data will be vulnerable. It is better to use well-known algorithms and implementations peer-reviewed by lots of people and proved to be secure.
A friend of me ask this, and i was thinking of asking this here too..
"What kind of data are this, how are they encrypted, or decrypted?"
My friend told me he got this from facebook.
d9ca6435295fcd89e85bd56c2fd51ccc
It looks like it could be an md5 hash.
Basically a hash is a one-way function. The idea is that you take some input data and run it through the algorithm to create a value (such as the string above) that has a low probability of collisions (IE, two input values hashing to the same string).
You cannot decrypt a hash because there is not enough information in the resultant string to go back. However, it may be possible for someone to figure out your input values if you use a 'weak' hashing algorithm and do not do proper techniques such as salting a hash, etc.
I don't know how FaceBook uses hashes, but a common use for a hash might be to uniquely identify a page. For example, if you had a private image on a page, you might ask to generate a link to the image that you can email to friends. That link might use a hash as part of the URL since the value can be computed quickly, is reasonably unique, and has a low probability of a third party figuring it out.
This is actually a large topic that I am by no means doing justice to. I suggest googling around for hash, md5, etc to learn more, if you are so inclinded.
It is a sequence of 128 bits, encoded as a lower-case hex string.
If you are talking about a Facebook API key, there is no deeper meaning to decode from the bits. The keys are created at random by Facebook and assigned to a particular application to identify it. Each application gets a different set of random bits for its API key.
This appears the be the...
hexadecimal representation for...
- ... a 16 bytes encryption block or..
- ... some 128 bits hash code or even
- ... just for some plain random / identifying number.
(Hexadecimal? : note how there are only 0 thru 9 digits and a thru f letters.)
While the MD5 Hash guess suggested by others is quite plausible, it could be just about anything...
If it is a hash or a identifying / randomly assigned number, its meaning is external to the code itself.
For example it could be a key to be used to locate records in a database, or a value to be compared with the result of the hash function applied to the user supplied password etc.
If it is an encrypted value, its meaning (decrypted value) is directly found within the code, but it could be just about anything. Also, assuming it is produced with modern encryption algorithm, it could take a phenomenal amount of effort to crack the code (if at all possible).
I'm currently experimenting with both public-key and personal file encryption. The programs I use have 2048 bit RSA and 256 bit AES level encryption respectively. As a newbie to this stuff (I've only been a cypherpunk for about a month now - and am a little new to information systems) I'm not familiar with RSA algorithms, but that's not relevant here.
I know that unless some secret lab or NSA program happens to have a quantum computer, it is currently impossible to brute force hack the level of security these programs provide, but I was wondering how much more secure it would be to encrypt a file over and over again.
In a nutshell, what I would like to know is this:
When I encrypt a file using 256-bit AES, and then encrypt the already encrypted file once more (using 256 again), do I now have the equivalent of 512-bit AES security? This is pretty much a question of whether or not the the number of possible keys a brute force method would potentially have to test would be 2 x 2 to the 256th power or 2 to the 256th power squared. Being pessimistic, I think it is the former but I was wondering if 512-AES really is achievable by simply encrypting with 256-AES twice?
Once a file is encrypted several times so that you must keep using different keys or keep putting in passwords at each level of encryption, would someone** even recognize if they have gotten through the first level of encryption? I was thinking that perhaps - if one were to encrypt a file several times requiring several different passwords - a cracker would not have any way of knowing if they have even broken through the first level of encryption since all they would have would still be an encrypted file.
Here's an example:
Decrypted file
DKE$jptid UiWe
oxfialehv u%uk
Pretend for a moment that the last sequence is what a cracker had to work with - to brute-force their way back to the original file, the result they would have to get (prior to cracking through the next level of encryption) would still appear to be a totally useless file (the second line) once they break through the first level of encryption. Does this mean that anyone attempting to use brute-force would have no way of getting back to the original file since they presumably would still see nothing but encrypted files?
These are basically two questions that deal with the same thing: the effect of encrypting the same file over and over again. I have searched the web to find out what effect repeated encryption has on making a file secure, but aside from reading an anecdote somewhere that the answer to the first question is no, I have found nothing that pertains to the second spin on the same topic. I am especially curious about that last question.
**Assuming hypothetically that they somehow brute-forced their way through weak passwords - since this appears to be a technological possibility with 256-AES right now if you know how to make secure ones...
In general, if you encrypt a file with k-bit AES then again with k-bit AES, you only get (k+1) bits of security, rather than 2k bits of security, with a man-in-the-middle attack. The same holds for most types of encryption, like DES. (Note that triple-DES is not simply three rounds of encryption for this reason.)
Further, encrypting a file with method A and then with method B need not be even as strong as encrypting with method B alone! (This would rarely be the case unless method A is seriously flawed, though.) In contrast, you are guaranteed to be at least as strong as method A. (Anyone remembering the name of this theorem is encouraged to leave a comment; I've forgotten.)
Usually you're much better off simply choosing a single method as strong as possible.
For your second question: Yes, with most methods, an attacker would know that the first layer had been compromised.
More an opinion here...
First, when computer are strong enough to do a brute-force attack on AES-256 for example, it will be also for iterations of the same... doubling or tripling the time or effort is insignificant at that level.
Next, such considerations can be void depending on the application you are trying to use this encryption in... The "secrets" you will need to carry become bigger (number of iterations and all the different keys you will need, if in fact they are different), the time to do the encryption and the decryption will also need to increase.
My hunch is that iterating the encryption does not help much. Either the algorithm is strong enough to sustain a brute-force attach or it is not. The rest is all in the protection of the keys.
More practically, do you think your house is more protected if you have three identical or similar locks on your front door ? (and that includes number of keys for you to carry around, don't loose those keys, make sure windows and back door are secured also...)
Question 1:
The size of the solution space is going to be the same for two passes of the 256-bit key as the 512-bit key, since 2^(256+256) = 2^512
The actual running time of each decrypt() may increase non-linearly as the key-size grows (it would depend on the algorithm), in this case I think brute forcing the 256+256 would run faster than the 2^512, but would still be infeasible.
Question 2:
There are probably ways to identify certain ciphertext. I wouldn't be surprised if many algorithms leave some signature or artifacts that could be used for identification.
I am working on adding hash digest generating functionality to our code base. I wanted to use a String as a hash salt so that a pre-known key/passphrase could be prepended to whatever it was that needed to be hashed. Am I misunderstanding this concept?
A salt is a random element which is added to the input of a cryptographic function, with the goal of impacting the processing and output in a distinct way upon each invocation. The salt, as opposed to a "key", is not meant to be confidential.
One century ago, cryptographic methods for encryption or authentication were "secret". Then, with the advent of computers, people realized that keeping a method completely secret was difficult, because this meant keeping software itself confidential. Something which is regularly written to a disk, or incarnated as some dedicated hardware, has trouble being kept confidential. So the researchers split the "method" into two distinct concepts: the algorithm (which is public and becomes software and hardware) and the key (a parameter to the algorithm, present in volatile RAM only during processing). The key concentrates the secret and is pure data. When the key is stored in the brain of a human being, it is often called a "password" because humans are better at memorizing words than bits.
Then the key itself was split later on. It turned out that, for proper cryptographic security, we needed two things: a confidential parameter, and a variable parameter. Basically, reusing the same key for distinct usages tends to create trouble; it often leaks information. In some cases (especially stream ciphers, but also for hashing passwords), it leaks too much and leads to successful attacks. So there is often a need for variability, something which changes every time the cryptographic method runs. Now the good part is that most of the time, variability and secret need not be merged. That is, we can separate the confidential from the variable. So the key was split into:
the secret key, often called "the key";
a variable element, usually chosen at random, and called "salt" or "IV" (as "Initial Value") depending on the algorithm type.
Only the key needs to be secret. The variable element needs to be known by all involved parties but it can be public. This is a blessing because sharing a secret key is difficult; systems used to distribute such a secret would find it expensive to accommodate a variable part which changes every time the algorithm runs.
In the context of storing hashed passwords, the explanation above becomes the following:
"Reusing the key" means that two users happen to choose the same password. If passwords are simply hashed, then both users will get the same hash value, and this will show. Here is the leakage.
Similarly, without a hash, an attacker could use precomputed tables for fast lookup; he could also attack thousands of passwords in parallel. This still uses the same leak, only in a way which demonstrates why this leak is bad.
Salting means adding some variable data to the hash function input. That variable data is the salt. The point of the salt is that two distinct users should use, as much as possible, distinct salts. But password verifiers need to be able to recompute the same hash from the password, hence they must have access to the salt.
Since the salt must be accessible to verifiers but needs not be secret, it is customary to store the salt value along with the hash value. For instance, on a Linux system, I may use this command:
openssl passwd -1 -salt "zap" "blah"
This computes a hashed password, with the hash function MD5, suitable for usage in the /etc/password or /etc/shadow file, for the password "blah" and the salt "zap" (here, I choose the salt explicitly, but under practical conditions it should be selected randomly). The output is then:
$1$zap$t3KZajBWMA7dVxwut6y921
in which the dollar signs serve as separators. The initial "1" identifies the hashing method (MD5). The salt is in there, in cleartext notation. The last part is the hash function output.
There is a specification (somewhere) on how the salt and password are sent as input to the hash function (at least in the glibc source code, possibly elsewhere).
Edit: in a "login-and-password" user authentication system, the "login" could act as a passable salt (two distinct users will have distinct logins) but this does not capture the situation of a given user changing his password (whether the new password is identical to an older password will leak).
You are understanding the concept perfectly. Just make sure the prepended salt is repeatable each and every time.
If I'm understanding you correctly, it sounds like you've got it right. The psuedocode for the process looks something like:
string saltedValue = plainTextValue + saltString;
// or string saltedalue = saltString + plainTextValue;
Hash(saltedValue);
The Salt just adds another level of complexity for people trying to get at your information.
And it's even better if the salt is different for each encrypted phrase since each salt requires its own rainbow table.
Its worth mentioning that even though the salt should be different for each password usage, your salt should in NO WAY be computed FROM the password itself! This sort of thing has the practical upshot of completely invalidating your security.