# The writeup may contain external images or files that I loaded in my github repository, please read it from the [original source](https://francesco-scar.github.io/CTF-writeups/24h@CTF/2022-02-05/Transmitter/) for a better viewing.
-----
# 24h@CTF Transmitter Writeup
## Category
Blockchain
## Description
The challenge gives the text:
`0x7BB2B3F29faC32dd86b2B760ca180462b2E08e6E@Ropsten`
The description gives some hints about some important messages mixed with junk messages, communication between sailors during an emergency and the needing to convert some hex to get the flag.
## Writeup
Searching the given text in a search engine we find out that it is the id of a Ropsten Ethereum (crypto-currency based on blockchain) contract, so the blocks are publicly accessible.
Using the website [ropsten.etherscan.io](https://ropsten.etherscan.io) and [searching](https://ropsten.etherscan.io/address/0x7bb2b3f29fac32dd86b2b760ca180462b2e08e6e) the given id, you can find the [first transaction](https://ropsten.etherscan.io/tx/0x4268e5c9b9c87f869753a0cb33078872a84f6860f7d8f2dbbbedf74f910b75fd) to the specified account.
Checking the Logs for each transaction you can find that sometimes the data sections contains a message with only dots and dashes.
![First morse transaction](Ropsten_Transaction_Etherscan.png?raw=true)
Passing through all the transactions and writing down only the payload with dots and dashes you will get the following morse message:
```....- -.... ....- -.-. ....- .----....- --... --... -... ..... --......-- ...-- ....- -.-. ....- ...--...-- ----- ....- -.. ...-- ...--..... ..-. ..... ....- ...-- -----..... ..-. ....- ..... ...-- --.......- ---.. ...-- ...-- ..... ..---....- ..... ..... ..... ....- -....... ..-. ..... ---.. ..... ---.....-- .---- ....- ...-- ..... --...-.... ---.. -.... ----. ..... --......-- ----- -.... ....- ..... -....-.... -.... --... -..```
Which decoded into text gives:
`464C41477B57334C43304D335F54305F453748335245554D5F5858314357686957306456667D`
Which decoded from hex to ascii chars give the flag:
`FLAG{W3LC0M3_T0_E7H3REUM_XX1CWhiW0dVf}`
## Flag
`FLAG{W3LC0M3_T0_E7H3REUM_XX1CWhiW0dVf}`
-----
If you find errors or you want to contribute to this writeup go to the [GitHub repository](https://github.com/francesco-scar/CTF-writeups/tree/main/24h%40CTF/2022-02-05/Transmitter) or contact me [opening an issue](https://github.com/francesco-scar/CTF-writeups/issues).