Homework on Hash Functions - Questions

Blockchain and Bitcoin 101
#1607

  1. A hash function is a function which takes a unique input and turns it into a unique output. You cannot workout what the input was from the output.

  2. Hash functions are used in bitcoin as the take the transaction as an input and turn it into a unique hash. If somebody tries to change any of the data on the blockchain then this will be spotted as the hash will completely change.

  3. Collision resistance means that for the most part two hashes should never be the same. It is possible but unlikely and would take a very long time,

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#1608

  1. A hash function takes a unique input and produces a unique output that can not be deduced back to the input.

  2. Hash functions are used to create digital signitures that are almost impossible to change.

  3. Two different inputs can not give you the same output

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#1609

  1. Where we input a string of data and was run it though a hashing algorithm and we get a unique output.

  2. In bitcoin the hash function is used to has transaction data. When mining you need a nonce which is required to generate a specific has with combination of the data of that current block.

  3. It is possible to get the same output but the likely of having a collision is very low due to the number of possibilities.

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#1610

1.) Hash functions are one way transactions, you are unable to find out the input from the output. Each input has a unique fingerprint hash output, if anything in the input changes the fingerprint hash will also change.
2.)Hash functions are used to create the blocks in the bitcoin blockchain, they contain list of merkle tree transactions, they also contain the previous block address and many other details which the block is made up of hashes.
3.)there needs to be a mechanism in which 2 parties or more, do not submit or have the possibility to submit the same hash at the same time. (Which ones is right?) collision resistances helps mitigate that from happening

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#1611

  1. Describe hash functions with your own words
    Hash functions are an encryption form of the input data.

  2. How are hash functions used in cryptocurrencies like bitcoin? (Try to research this on your own, we will cover this later in the course but challenge yourself and see if you can find information on this already now).
    Hash functions are used in cryptocurrencies in order to secure the initial input. SHA256 is one of the most secure hash functions that transforms the input into bits and is basically impossible to hack. The hash functions are used to transform the bitcoin transactions into hashes that are stored on the ledger.

  3. What does it mean when we say that hash functions need to be collision resistant? (We didn’t use the term “collision resistant” in the lecture, but you will easily find this on Google, we add this question intentionally to make you research information on your own, that’s how you learn best).
    Collision resistant means that once hashed, each input will have it’s own unique hash. It is very unlikely that the input will be the same as the hash.

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#1612

You are mostly correct :slight_smile: except Merkle trees don’t have much to do with collision resistance. Collision resistance is the property of hash functions itself. Merkle trees are a bit more advanced usage of hashes used in the blockchain (that is why the course doesn’t really mention them).
But in short: Merkle tree is a tree of hashes (not the ones outside but a data structure that resembles a tree :stuck_out_tongue: ). Blockchain uses this by constructing a tree of hashes from transaction ids and only uses the root hash to calculate the block hash.
Merkle tree structures have a nice property where you can easily verify if the leaf of the tree (in this case a tx hash) is included in the root (hash used in the blockhash).

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#1613

Correct! :stuck_out_tongue:

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#1614

It is unlikely that two inputs would produce the same hash. Not sure if its possible for an input to be the same as the output :slight_smile:

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#1615

  1. Unique & Irreversible when speaking of more reliable hash functions such as SHA256.

  2. From what I gather so far the blockchain is like a linked list in Java for example. Each block is composed of 2 part. One part for the new data and the other is the pointer to the previous block. Here is where the hash function come in use and makes the blockchain so reliable. Instead of a simple pointer, Blockchain you have a hash pointer which essentially is the SHA256 hash function of the data from the previous block.

  3. If you have two identical output will cause a collision. Therefore, collision-resistant basically means that each input will produce a unique output.

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#1616

Thanks Ales, for clarifying

On Sun, 12 Jul 2020 at 05:23, AleĹĄ Ferlan via Ivan on Tech Blockchain Academy Forum

Benjamin Lopez

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#1617
  1. Irreversible function that creates a unique output .
  2. To add a block of transactions to the blockchain, the header of the block is hashed with a nonce (guessed) value to produce an output hash having a predetermined value. This value has a bunch of most significant digit leading zeros. The number of leading zeros sets the difficulty level in the bitcoin mining system.Trillions of nonce values are guessed , concatenated with the header data and rehashed until a nonce value is guessed that produces a hash value numerically less than the difficulty value. This nonce is then incorporated into the data block header for the new block…(or something close to this!)
  3. When the hash of two different inputs creates the same hash value output.
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#1618

No problem! I’m glad they’re able to help.

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#1619

Describe hash functions with your own words
A. Hash function takes the input of a function and totally individualizes it in a unique way so that it is
essentially a carbon copy of the input, at output

How are hash functions used in cryptocurrencies like bitcoin?
A. One way hash functions can be used is the use of a nounce in P O work
B. Another way hash functions are used in Bitcoin is in the area of addresses (public keys and smart
contracts).

What does it mean when we say that hash functions need to be collision resistant?
A.WE mean duplicate blocks created at the same time needs to avoided.

#1620

  1. A hash function takes any group of characters, text or value and maps it to a fixed length of characters, text or value of which the contents of each output depend on each particular input. No matter how many times you parse a particular input through a hash function you will always get the same result or output.

  2. How are hash functions used in crypto currencies like BTC?

Hash functions are used cryptographically for mining, transactions, addresses and keys.

  1. Collision resistant is a required property in order to have a secure cryptographic hash function.
    It means it is extremely unlikely that two different inputs result in generating an identical output.
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#1621

  1. Hash functions are one way functions where each input produces its own unique output.

  2. Hash functions in Bitcoins are used for mining new transactions on the blockchain.

3.Collision resistance means that is very difficult to find 2 inputs that produce the same output in a hash function. This is a very important property in cryptographic hash functions.

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#1622

  • Describe hash functions with your own words
    They are special functions that always have the same output for a specific input but there is no way to derive the input given the output.

  • How are hash functions used in cryptocurrencies like bitcoin? (Try to research this on your own, we will cover this later in the course but challenge yourself and see if you can find information on this already now).
    Its the way how blocks are linked in the blockchain. The hash the is calculated over one block is included in the next block.
    Has functions are also used to sign transactions with your private key.

  • What does it mean when we say that hash functions need to be collision resistant? (We didn’t use the term “collision resistant” in the lecture, but you will easily find this on Google, we add this question intentionally to make you research information on your own, that’s how you learn best).
    It is collision resistant if it is hard to find two inputs that hash to the same output.

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#1623

Describe hash functions with your own words

Think about a machine where you put a unique input inside of the machine and get a unique output as return. You can go from input to output but you never go from output to input. Hash function is a one way function. There are many different hash functions in this course we working with SHA256.

How are hash functions used in cryptocurrencies like bitcoin? (Try to research this on your own, we will cover this later in the course but challenge yourself and see if you can find information on this already now).

Think about the machine i write about above, the machine takes an input and return output. The bitcoin transaction is the input, when input is given the machine starts the hashing algorithm in bitcoin is SHA256 and give the user an output of fixed length. In bitcoin protcol hash function is also used to finding a occasion to finding number that generates a specified hash combination with data unique to that block

What does it mean when we say that hash functions need to be collision resistant? (We didn’t use the term “collision resistant” in the lecture, but you will easily find this on Google, we add this question intentionally to make you research information on your own, that’s how you learn best).

The meaning of collision resistant is how two inputs not getting the same hash

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#1624
  1. Hash Function is a mathematical algorithm that converts an input data (let’s say a message) into an output of numbers and letters (the hash). Unlike a normal function, it is one way ie. Only input to output…cannot be inverted (output to input)
  2. Hash functions are used in cryptocurrencies like bitcoin by providing fingerprinting of data inputs. This enables verification and confirmation of data integrity without the need for a central third party.
  3. Collision resistance means that it is extremely unlikely (infeasible) that 2 different inputs will have the same output (hash).
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#1625

We want to avoid blocks being mined at the same time, but this question was regarding the hash functions in general.

A hash function is considered to be collision resistant if you are unable to find two different inputs producing the same output.

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#1626

Homework on Hash Functions - Questions
1. Describe hash functions with your own words
A cryptographic function to secure transaction, bitcoin uses the SHA-256 and it’s always returns a 64 characters hexadecimal(base16) string as an unique fingerprint. If one single bit is changed the hash function will return a totally different number a new unique fingerprint. It’s secure as it’s a one way function (cannot go from output to get the input)
2. How are hash functions used in cryptocurrencies like bitcoin?
The blockchain is a linked list that contains data and a hash pointer that points to its previous block, the previous block also have a hash of the data inside that block.
3. What does it mean when we say that hash functions need to be collision resistant?
Collision resistance doesn’t mean that no collisions exist, simply that they are hard to find
No hash functions is collision free, so It’s possible that two users will use the exact same hash but will not likely happened, If a collision will happened but remember this is a very unlikely event to occur, but, in the case it would happen nothing strange will go on. Also if a block is mined by two miners at the same time, some nodes will see and accept one of the two “twins” blocks first, and the second will be simply ignored.

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