How are addresses computed?
13 minutes of reading
As you know, there are two kinds of accounts in Ethereum: External Accounts & Contract Accounts. Their addresses are computed in a very different way.
External Accounts
There are 3 steps in total to generate an address for an external account. We will be using the bash shell, if you are using a Windows machine, you could use Git Bash or Windows Subsystem for Linux.
Firstly, a random private key of 64 hexadecimal characters (32 bytes) is generated. We can use the OpenSSL ecparam command to generate an elliptic curve private key. Ethereum standard is to use the secp256k1 curve, which is the same curve used by Bitcoin. The following command will print the private key in PEM format to a file called private_key_pem:
$ openssl ecparam -name secp256k1 -genkey -noout > private_key_pem
$ cat private_key_pem
-----BEGIN EC PRIVATE KEY-----
MHQCAQEEIN8rObQpsiL9wrQb2P/LUEssAwyIEbr+Itvxy+1I4TwsoAcGBSuBBAAK
oUQDQgAEhXxq9EAtIjxjNj8U1sToOT7lw/sMDRR9E/QwttbtEjxQ4X39K6w9QzlH
ENTmU32pnGmy+oHV/W7DJsGkH2XPhA==
-----END EC PRIVATE KEY-----Secondly, a 128 hexadecimal character (64 bytes) public key is then derived from the generated private key using Elliptic Curve Digital Signature Algorithm (ECDSA). We can derive the public key from the private key and display both private and public key in hexadecimal format by using the following command:
$ cat private_key_pem | openssl ec -text -noout > key
read EC key
$ cat key
Private-Key: (256 bit)
priv:
df:2b:39:b4:29:b2:22:fd:c2:b4:1b:d8:ff:cb:50:
4b:2c:03:0c:88:11:ba:fe:22:db:f1:cb:ed:48:e1:
3c:2c
pub:
04:85:7c:6a:f4:40:2d:22:3c:63:36:3f:14:d6:c4:
e8:39:3e:e5:c3:fb:0c:0d:14:7d:13:f4:30:b6:d6:
ed:12:3c:50:e1:7d:fd:2b:ac:3d:43:39:47:10:d4:
e6:53:7d:a9:9c:69:b2:fa:81:d5:fd:6e:c3:26:c1:
a4:1f:65:cf:84
ASN1 OID: secp256k1The public key is what we need in order to derive the Ethereum address. Every EC public key begins with the 0x04 prefix. You should remove this leading 0x04 byte and also the semicolons in order to hash it correctly. Using the following bash manipulations we can obtain the 128 hexadecimal character public key we want.
$ cat key | grep pub -A 5 | tail -n +2 | tr -d '
[:space:]:' | sed 's/^04//' > public_key
$ cat public_key
857c6af4402d223c63363f14d6c4e8393ee5c3fb0c0d147d13f430b6d6ed123c50e17dfd2bac3d43394710d4e6537da99c69b2fa81d5fd6ec326c1a41f65cf84Finally, we can then apply the Keccak-256 hash function to the public key to obtain a 64 hexadecimal character (32 bytes) hash string, the last 40 characters (20 bytes) of the resulting hash output prefixed with 0x become the final Ethereum address. Since not every OS has a preinstall command for computing the keccak-256 hash, for convenience, you can use any keccak256 online tool to compute the result.
By using the above online tool with the public key 857c6af4402d223c63363f14d6c4e8393ee5c3fb0c0d147d13f430b6d6ed123c50e17dfd2bac3d43394710d4e6537da99c69b2fa81d5fd6ec326c1a41f65cf84 and be sure you have set the input type as hex otherwise it will be treated as a normal string, then you will obtain the result 34566a44f7b0bfcb378a519129a76cee76805d1cfaff8a027846a621cfad9cbb. The address will be the last 40 characters plus the 0x prefix, i.e. 0x29a76cee76805d1cfaff8a027846a621cfad9cbb.
Now, as a bonus, here we’ll learn how to create an external account using the private key we just generated! First of all, let’s retrieve the private key from the key file we created above, we format the private key the same way we did for the public one above. The only exception is removing the leading 0x00 instead of the 0x04.
$ cat key | grep priv -A 3 | tail -n +2 | tr -d '
[:space:]:' | sed 's/^00//' > private_key
$ cat private_key
df2b39b429b222fdc2b41bd8ffcb504b2c030c8811bafe22dbf1cbed48e13c2cNow, we’ll be using Geth to run an Ethereum node on our computer so as to connect to the Ethereum network and create an external account.
$ geth account import private_key
Your new account is locked with a password. Please give a password. Do not forget this password.
Password:
Repeat password:
Address: {29a76cee76805d1cfaff8a027846a621cfad9cbb}The address returned by geth is the same we computed. Note that geth will ask you a passphrase to encrypt your private key.
You’re now ready to use the new account with geth as the account is on chain. Of course, it would be easier to take advantage of the geth account new command in order to quickly setup an Ethereum account. But manually doing it gives you the power of knowing your public and unencrypted private keys. You can even now try to import this account using its private key in MetaMask!
Select the circular icon at the top right corner and choose Import Account.
Next, enter the private key you have generated and click on Import.
Tada! You now have access to this account in MetaMask.
Contract Accounts
The address for an Ethereum contract is deterministically computed from the address of its creator (sender) and how many transactions the creator has sent (nonce). The sender and nonce are RLP encoded and then hashed with Keccak-256.
For more information, you can check out How is the address of an ethereum contract computed.