I want to decrypt SHA256, Is it possible?
From many forums I saw that the SHA256 data cannot be decrypted? If that's really true then how is the data verified? What's the use of just encrypting the data? The same question goes for the digital signatures (which I believe is the hashed value and private key)?
The significance of SHA256 is that SHA-256 stands for Secure Hash Algorithm 256-bit and it's used for cryptographic security. Cryptographic hash algorithms produce irreversible and unique hashes. The larger the number of possible hashes, the smaller the chance that two values will create the same hash.
To know how to Decrypt SHA256, you must understand that it is not an encryption function but a hash function. The fundamental difference is that while encryption is a two way function (given the key) hash is only a one way function: given some data you can compute the hash, given the hash it is difficult (and mathematically impossible) to have the data back.
I said it is mathematically impossible to find the data from the hash because typically a hash function has a small codomain (for example 256bit for SHA256) but a big domain (you can hash any string), so there will be collisions: different strings with the same hash. For this reason if your password is saved in a hashed form then there exists an infinite password (but they can be very long) that unlocks your account. The good news is that collisions are rare when you use cryptographic hash functions, so your account is still safe.
Answering the fist part of your question, data verification is easy: if Alice sends to Bob a file with the hash checksum Bob can easily compute the hash of the file he has received and compare it with the hash received from Alice. This is usually enough to find out if there has been any error during the transmission (so the file is corrupt), but isn't enough if the transmission has been altered by some attacker that also altered the hash. So Alice and Bob need a secure channel to transmit the hash (for example a https page with a valid certificate) or they need to sign the hash in some way.
So we move to the answer to your second question: Alice can sign the hash using her private key before sending it to Bob, in this way an attacker can't tamper with it without invalidating the signature. Now you could ask why Alice signs with her RSA (or similar) key only the hash and not all the message, this is because computing RSA is slower than computing a hash (so she has to do the slow thing only on a small string: the hash). This was true especially when PGP was created and computers were slower.
Answer (1)
SHA-256 is a cryptographic hash function, which means it takes an input (or 'message') and returns a fixed-size string of bytes. The output, often referred to as the hash or digest, is unique to each unique input. SHA-256 is designed to be a one-way function, meaning you can't "decrypt" a hash back to its original input because:
Hash functions are irreversible: They are designed to be a one-way process. You can hash data, but you can't go backward to retrieve the original data.
Fixed Output Size: SHA-256 always produces a 256-bit hash value, regardless of the size of the input data. This means the original data can't be recovered from the hash alone because there's no information about the input's length or content.
Understanding SHA-256
- One-Way: The algorithm is designed such that it is computationally infeasible to reverse it.
- Collision Resistance: It’s hard to find two different inputs that produce the same hash output.
- Deterministic: The same input will always produce the same hash output.
What Can Be Done Instead?
1. Brute Force Attack
A brute force attack involves hashing potential inputs and comparing the resulting hash with the target hash. This is computationally expensive and practically infeasible for strong hash functions like SHA-256, especially with long and complex input data.
2. Rainbow Tables
Rainbow tables are precomputed tables for reversing cryptographic hash functions. However, they are limited by the size and complexity of the table. Modern cryptographic practices often use salts to mitigate the effectiveness of rainbow tables.
3. Lookup Tables
For simple and commonly known inputs (e.g., passwords), people sometimes create lookup tables (dictionaries) of inputs and their corresponding hash outputs. However, this is only practical for small, commonly used inputs.
Practical Approach
1. Hash Cracking Tools
There are tools and services that can help crack SHA-256 hashes, such as:Hashcat: An advanced password recovery tool that can use the power of GPUs.
John the Ripper: Another popular tool for cracking hashes.
These tools work by trying a large number of possible inputs (dictionary attacks, brute-force, etc.) and comparing their hash outputs to the target hash.
2. Using a Hashing Service
Online services can sometimes help crack hashes by using extensive databases of precomputed hashes. However, this is usually limited to weak passwords or common phrases.
Example Using Hashcat
- Install Hashcat: Download and install Hashcat from the official website.
- Prepare Your Dictionary: Obtain or create a dictionary file containing potential inputs.
- Run Hashcat: Use the following command to start cracking.
- hashcat -a 0 -m 1400 yourhashes.txt yourdictionary.txt
Here:
-a 0 specifies a dictionary attack.-m 1400 specifies SHA-256.Conclusion
Direct decryption of SHA-256 hashes is not possible due to their cryptographic nature. However, you can attempt to crack the hash by trying all possible inputs using tools like Hashcat, especially if the input is likely to be simple or commonly used. For secure systems, always use strong, unique inputs and add salts to hashes to enhance security.