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We use this 3rd party services:
https://etherscan.io/ - free public blockchain explorer. they provide information for user balances (Ethereum cryptoccurrency and ER20 assets);
https://infura.io/ - (free 100,000 Requests/Day) ethereum endoint to broadcast transactions.
https://www.blockchain.com/btc,
https://api.blockcypher.com/v1/btc/main,
https://live.blockcypher.com/ - bitcoin explorers
https://ghostscan.io - GHOST blockchain explorer
https://explore.next.exchange - NEXT blockchain explorer
https://horizon.stellar.org - STELLAR blockchain exploreer
https://api.blocktrail.com/v1/BTC - Bitcoin mining fees data
https://api.bitcore.io/api/BTC/mainnet - free bitcoin endpoint where we send a transactions for broadcatst to the bitcoin network. Otherwise user must install own bitcoin node.
https://next.swaponline.io/mainnet - NEXT blockchain node endpoint
https://noxon.wpmix.net/currsAll.php - free cryptocurrency price feed