Bitcoin Forum

With BTC we can save additional data in various ways.
This was already done before, for example to upload pictures.

I am looking for software that can retrieve this data, or lets me enter it myself.

How about an owner of an Onion site, that stores the current Onion link within the blockchain?
This way everyone can verify that it is the same site operator.

Is this already being done?

There is no such software for entering keywords or data into the bitcoin blockchain. The method used to enter the data into the bitcoin blockchain forever is called as OP_RETURN or Null Data Transaction. But using the OP_RETURN is literally bad to the bitcoin network since they certainly burn the coins and the outputs of such are completely unspendable thus reducing the total supply of the coins significantly. Since including data into the blockchain with the help of OP_RETURN is very inexpensive, they have been seen as a viable way of polluting the blockchain. We need to understand that, the blockchain is for storing transaction information and not for arbitary data.

You can include the arbitary data by using a bitcoin full node and probably a BitcoinJs library. I am also quite sure there is no such way for entering them without the help of a full node. BTW if you are mentioning about the one which satoshi included (coinbase parameter), it can be only done by a miner when he mines and adds the block to the chain.

As for enter data into transaction (which will be included on block of BTC's blockchain), you can use wallet such as Bitcoin Core using createrawtransaction command by adding data column.

As for obtain information/data from BTC's blockchain, you can check these article/website :
1. http://www.righto.com/2014/02/ascii-bernanke-wikileaks-photographs.html (http://www.righto.com/2014/02/ascii-bernanke-wikileaks-photographs.html)
2. http://coinsecrets.org/ (http://coinsecrets.org/)

But, please take note that :
1. Script which used on righto article is outdated and might not detect all non-transaction information on BTC's blockchain
2. coinsecrets.org detect all non-transaction information which uses OP_RETURN, but you still need to know format/numeral system used to get actual information.

If you want to store data, I think using blockchain is a bit overkill (well, depends on what kind of data though). You mentioned pictures, I think storing it on IPFS is much better no? Unless you want to store the hash of the pictures.

Blockcypher can do this (but not automatically) without a HEX Converter but the "data" must be less than or equal to 40bytes (or 80?).
Here's the link: https://live.blockcypher.com/btc/embed-data/ (https://live.blockcypher.com/btc/embed-data/)
The address must have a balance for the transaction fee and the minimum BTC transaction amount.

Take note: [1] OP_return is mainly for "phrases", not images, if you wan't to check the embedded data in the blocks,
you can visit this site: https://bitcoinstrings.com/ (https://bitcoinstrings.com/) and select any block cluster.
[2] Its pretty hard to convert an image into HEX with 40byte file size without compromising the quality, but some claimed that they've done it.

Thank you everyone for your answers! :)

I will research into the listed options from you all!