<blockquote>
  <p>1) Are the transactions 100% secure?</p>
</blockquote>

<p>No.</p>

<blockquote>
  <p>2) Suppose there are two identical transactions happening at the same time, are those transactions are going to get differentiated based on the Hashes?</p>
</blockquote>

<p>Yes. It's difficult, though, because each transaction must include the hash of the previous transaction, and so on.</p>

<p>It used to be much easier. A while back, it was pointed out that two coinbases could have identical hashes, and so create identical transactions down the line. See also: <a href="https://bitcoin.stackexchange.com/questions/30764/why-block-height-is-required-in-coinbase">Why block height is required in coinbase?</a></p>

CC BY-SA 3.0
2015-04-30T04:13:55.607Z
Can you explain why its not 100% secure? Any example?
John
CC BY-SA 3.0
2015-04-30T04:26:06.303Z
@John For example, a transaction that has a confirmation can lose that confirmation if a longer blockchain that does not include it appears.
Nick ODell
CC BY-SA 3.0
2015-04-30T00:41:46.490Z
2015-04-30T00:41:46.490Z
2017-04-13T12:47:33.610Z
Nick ODell
<p>As I was reading <a href="https://en.bitcoin.it/wiki/Block_hashing_algorithm" rel="nofollow">Bitcoin hashing algorithm</a>, I see that a Block Header contains hashes in the form of fields like 
<code>hashPrevBlock</code> , <code>hashMerkleRoot</code> . I have following questions:</p>

<p>1) Are the transactions 100% secure? If yes, then I am wondering if Hash is the reason behind it? Since moving forward in a hash is
easy than backwards.</p>

<p>2) Suppose there are two identical transactions happening at the same time, are those transactions are going to get differentiated
based on the Hashes ? </p>

<p>Please elaborate. </p>

CC BY-SA 3.0
2015-04-30T14:51:41.153Z
2 identical transactions is impossible. A transaction by definition includes inputs and amounts and outputs and signatures. If all of that is identical then it's actually one and the same transaction. But yes as the transactions won't be identical the hashes won't be either so they are easily differentiated by the hashes.
Jannes
CC BY-SA 3.0
2015-05-20T01:29:09.147Z
2 identical transactions is both possible and has happened in the live blockchain before. for example, block [91842](http://blockexplorer.com/block/00000000000a4d0a398161ffc163c503763b1f4360639393e0e4c8e300e0caec) and block [91812](http://blockexplorer.com/block/00000000000af0aed4792b1acee3d966af36cf5def14935db8de83d6f9306f2f) both have the exact same coinbase tx. once one of these is spent then the other becomes unspendable.
mulllhausen
<encryption><algorithms>
Two identical transactions and hashes in Block Hashing algorithm
John
# Two identical transactions and hashes in Block Hashing algorithm

(Asked by: username_0 on 2015-04-30)

As I was reading [Bitcoin hashing algorithm](https://en.bitcoin.it/wiki/Block_hashing_algorithm), I see that a Block Header contains hashes in the form of fields like
`hashPrevBlock` , `hashMerkleRoot` . I have following questions:

1) Are the transactions 100% secure? If yes, then I am wondering if Hash is the reason behind it? Since moving forward in a hash is
easy than backwards.

2) Suppose there are two identical transactions happening at the same time, are those transactions are going to get differentiated
based on the Hashes ?

Please elaborate.

**Tags:** encryption, algorithms

> **Comments:**
> **username_1**: 2 identical transactions is impossible. A transaction by definition includes inputs and amounts and outputs and signatures. If all of that is identical then it's actually one and the same transaction. But yes as the transactions won't be identical the hashes won't be either so they are easily differentiated by the hashes.  
> **username_2**: 2 identical transactions is both possible and has happened in the live blockchain before. for example, block [91842](http://blockexplorer.com/block/00000000000a4d0a398161ffc163c503763b1f4360639393e0e4c8e300e0caec) and block [91812](http://blockexplorer.com/block/00000000000af0aed4792b1acee3d966af36cf5def14935db8de83d6f9306f2f) both have the exact same coinbase tx. once one of these is spent then the other becomes unspendable.  

---

## 1 answers

### Answer by username_3 on 2015-04-30. Score: 1 | Accepted answer

> 1) Are the transactions 100% secure?

No.

> 2) Suppose there are two identical transactions happening at the same time, are those transactions are going to get differentiated based on the Hashes?

Yes. It's difficult, though, because each transaction must include the hash of the previous transaction, and so on.

It used to be much easier. A while back, it was pointed out that two coinbases could have identical hashes, and so create identical transactions down the line. See also: [Why block height is required in coinbase?](https://bitcoin.stackexchange.com/questions/30764/why-block-height-is-required-in-coinbase)

> **Comments:**
> **username_0**: Can you explain why its not 100% secure? Any example?  
> **username_3**: @username_0 For example, a transaction that has a confirmation can lose that confirmation if a longer blockchain that does not include it appears.  

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