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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 and searching the given id, you can find the first transaction 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

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 or contact me opening an issue.