Bitcoin Forum

I've never seen a bitcoin address begin with a 3.  Is that a legit address?

It is a legit adress indeed , Bitcoin adresses can begin with either 1 or 3
you can have a lot more informations here on the Bitcoin Wiki : https://en.bitcoin.it/wiki/Address

That's Multi-signature addresses (https://en.bitcoin.it/wiki/Address#Multi-signature_addresses).
Address with begin with 1 just have 1 private key, but an address begin with 3 have more than 1 private key.
It's provided by several online Bitcoin wallets.

It's also provided by Bitcoin Core

Oh sorry, I don't know about that, I never using the Bitcoin Core till now :D

I started using Bitcoin Core recently but never relized that at all . I believe it's not possible to do it simply , correct ? only possible by using some commands and using Debug window if I'am not wrong

Yes, now only RPC commands, debug window...
In the future, I'm sure will be GUI.

Bitcoin addresses beginning with 3 are a multisignature address. This type of addresses is used by the service in a 2-of-3 configuration. It takes two signatures to transfer bitcoins. This illustration sort of illustrates their use by the service:
http://www.dcmagnates.com/wp-content/uploads/2014/04/BitGo-2-of-3-key.png


A blurb from bitgo "the aforementioned multi-signature wallet that leverages both BIP16 and BIP32 standards found with hierarchical deterministic wallets; and a “2-of-3-key” configuration, whereby three keys are issued and any two can be used to sign a transaction. It ensures against loss or theft while allowing for relatively easy access."

I realize that my answer is probably more technical than the OP is ready to understand, but since this is the "Technical Support" board, I'd just like to mention that bitcoin addresses that start with a 3 are "Pay-to-script-hash" (P2SH).  One of the most common uses for a P2SH transaction is to implement an M-of-N multisig requirement (such as 2-of-3 or 3-of-5, etc).  However, it is possible to have an address that starts with a 3 that is not a 2-of-3 multisig (or M-of-N multisig).

Additionally, addresses that start with a 1 are "Pay-to-public-key-hash" (P2PKH).

Note that at the protocol level there are no "addresses" at all.  There are simply requirements that an output is encumbered with by way of a script that must be satisfied in order to use that output in the funding of a transaction.  "Addresses" are just a shorthand (and user friendly) way to indicate to the wallet what type of script to use (along with a checksum to allow the wallet to double-check your typing for mistakes).  The first 8 bits (first byte) of every address is a "version" number that tells the wallet software which type of script to use to build the transaction.

A byte of 0x00 indicates that the wallet should build a P2PKH transaction.  When converted to Base58Check notation, any leading 0x00 is converted to a 1.

A byte of 0x05 indicates that the wallet should build a P2SH transaction.  When converted to Base58Check notation, a leading 0x05 byte will result in an address that starts somewhere between '51' (for a script hash of 0x0000000000000000000000000000000000000000) and '5R' (for a script hash of 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF).

Yes, its multi-sig addr.
After so many hacks and steals, 3xxxxx addresses are more commonly occuring now.
It is said top 3 btc addresses with biggest balance now starts with 3.

Bitstamp seems to be making use of BitGo now, saw this quote on the home page of BitGo:

BitGo is the only company in the industry we trust to secure our hot wallet. The integration was very straightforward, and now I can sleep better at night knowing that my customers’ holdings are secured with BitGo. - Nejc Kodrič, CEO Bitstamp

BitGo addresses are multi-signature and starts with a 3.

I wonder how it provide extra security over normal P2PKH ? Because, previously it required one private key to be online, now two will be online. But, to run a hot wallet, it has to be online at the end of the day. How come it provides complete security apart from adding an extra lock which can also be stolen like the previous ?

Before:
1 key at Bitstamp server
Now:
1 key at Bitstamp server + 1 key at BitGo server.(+1 key at Bitstamp backup to ensure receipt of funds if BitGo closes. But this key is offline)
So now hackers need to have access to bitstamp and bitgo...

Well... I understand that 2 keys are on 2 server (I am not considering the third as it is not required to sign a Tx, if the first 2 are available). Now, how things work for a withdrawal ? Withdrawal request hits the Bitstamp server, it signs with its own private key and notifies the BitGo server to sign with its private key. In this scenario, if Bistamp server is compromised, it would not be difficult for the attacker to notify BitGo with its private key. So, how come the outcome is different as it was before ? The central point of operation is still the Bitstamp server, is not it ?

Any way of making a multisig address using electrum? or bitaddress?

With the Electrum 2.0, you can create and control a multisig wallet easily.
For details: https://github.com/spesmilo/electrum/blob/master/RELEASE-NOTES and https://electrum.orain.org/wiki/Multisig_Wallets

Or Bitcoin Armory. Bitcoin Armory Website (https://bitcoinarmory.com/) / Lockbox Tuto (https://bitcoinarmory.com/tutorials/armory-advanced-features/lockbox/create-lockbox/)

Thanks. How about greenaddress? Is it safe, can user control their keys there?