# Cryptography in Bitcoin: How to Create an Address with C

Public key cryptography is integral to the Bitcoin network. Here I explain how we can generate Bitcoin private & public keys, as well as Bitcoin addresses from scratch using the C programming language. I also cover some design choices and features of the Bitcoin protocol implemented by Satoshi Nakamoto.

# Public Key Cryptography (Asymmetric)

The predecessor to public key cryptography is symmetric key cryptography, where you and I share a secret password and use it to encrypt and decrypt messages that we send eachother. Think of this like a cipher, where you might replace the letter A with B, and B with C and so on. To decrypt your secret message, I would do the reverse using the same key.

A fundamental flaw of this scheme is that we must share the secret key in order to use it, but as we are yet to establish a safe encrypted means of communication, an attacker may intercept our secret key!

Public key cryptography solves this, where each user has their own secret private key and public key. If I want to send you a message, I first obtain a copy of your public key and then use it to encrypt my message. Then I can transmit this encrypted message to you, where you then use your private key to decrypt the message. Notice here how you can openly share your public key rather than secretly, and as long as no one else knows your private key the message will be safe from prying eyes.

There are a number of different public key protocols, but in all of them you can generate a public key from a private key but never the reverse. These protocols rely on special cryptographic functions that are exclusively one way, ensuring that it is impossible to determine a private key from someone’s public key.

In the Bitcoin protocol, an address is where bitcoin is sent to. You can generate an address by taking a hash of your public key. A cryptographic hash function takes any input data and returns a fixed-length string, where it is impossible to calculate the reverse as these functions are one-way. For example, a commonly used hash function is SHA-256:

SHA256("nick")

SHA256("nicko")
= "17224492f2c70539c54b545f34f11a99e248c0aa200ec35b7ab66227a440e81b"

Notice how just adding a single letter to the input string completely changes the hash. Also see how the hash function outputs a string of fixed length regardless of input length.

In Bitcoin there are now multiple types of addresses (see /wiki/Address). The original “legacy” addresses implemented by Satoshi Nakamoto begin with the number 1: 1K4Y7MF8uXFu7GrwvDcUpwEoNkKcREFnh7. This address type is known as Pay-to-PubkeyHash or P2PKH for short.

1. Calculate the SHA-256 hash of the public key
• SHA256(public_key) = 0b7c28c9b7290c98d7438e70b3d3f7c848fbd7d1dc194ff83f4f7cc9b1378e98
2. Calculate the RIPEMD-160 hash of this SHA-256 hash
• RIPEMD160(SHA256(public_key)) = f54a5851e9372b87810a8e60cdd2e7cfd80b6e31
3. Add a version byte in front of this to complete our address (0x00 for main-net)
• 00f54a5851e9372b87810a8e60cdd2e7cfd80b6e31
4. To create a checksum we take the SHA-256 of this extended RIPEMD-160 hash twice and take the first 4 bytes
• SHA256(SHA256(0x00 & RIPEMD160(SHA256(public_key)))) = c7f18fe8fcbed6396741e58ad259b5cb16b7fd7f041904147ba1dcffabf747fd
• c7f18fe8 is our checksum
5. Add the checksum to the end of the original RIPEMD160 hash at step 3
• 00f54a5851e9372b87810a8e60cdd2e7cfd80b6e31c7f18fe8
6. Convert the byte result from hexidecimal into a base58 string
• 1PMycacnJaSqwwJqjawXBErnLsZ7RkXUAs
7. Remove any extra leading 1’s. In base58 a ‘1’ represents a value of zero and thus has no value at the front of address. However, a leading ‘1’ is included for each leading 00 byte.
• Nothing needs to be done here as my address above only has a single 1, matching the single 0x00.
• 1PMycacnJaSqwwJqjawXBErnLsZ7RkXUAs

It is a complicated scheme! But it includes several very useful features which are detailed below.

## Checksum

The checksum allows anyone to confirm that this public address is a legitimate address. If someone sends us an address, we can verify by performing steps 4 & 5 on the first 21 bytes, and then check that the last 4 bytes of the address are correct. This is most useful when entering a public address by hand where most wallets will prevent you from attempting to send bitcoin to an illegitimate address.

## Base 58

1K4Y7MF8uXFu7GrwvDcUpwEoNkKcREFnh7 Base58 might seem like a weird way to represent addresses at first, but it has some properties that make it advantageous over a decimal 0-9 representation. The first is that using a number system with a high base allows for shorter representation of large integers. For example, the number 1337 in binary would look like 10100111001 in binary (base 2) but looks like Q4 in base 58. As bitcoin addresses are technically just very large integers, this makes sending and reading addresses much simpler.

A computationally inclined person may ask, why not use something more natural like base64? Well base58 uses every number, lowercase letter, and upercase letter except for characters that can easily be mistaken (0, O, I, l). Note $$2\times26 + 10 - 4 = 58$$. Removing these confusing characters makes it much less likely for Bitcoin addresses to be entered incorrectly.

# Creating Bitcoin Addresses in C

Bitcoin uses Elliptic Curve Cryptography to create a public and private key pair. I will cover the details of how this works in a later post, but for now all we need to know is that Bitcoin uses an eliptic curve called secp256k1 to create a public key from a private key which have the properties of asymmetric cryptography outlined earlier.

To use this eliptic curve in C, we will be using the bitcoin-core/secp256k1 library, installation is trivial on Linux and MacOS.

In order to use this library, we have to set up a sep256k1_context with

#include <secp256k1.h>
static secp256k1_context *ctx = NULL;
ctx = secp256k1_context_create(
SECP256K1_CONTEXT_SIGN | SECP256K1_CONTEXT_VERIFY);

From the documentation of the library: “The purpose of context structures is to cache large precomputed data tables that are expensive to construct…“. This allows for addresses to be generated quickly and we will need a context in order to generate our keys.

## Generating a Private Key in C

A private key in Bitcoin is a 256-bit number, also represented as 32 bytes in hexidecimal: E9873D79C6D87DC0FB6A5778633389F4453213303DA61F20BD67FC233AA33262

In Unix systems, we can get randomness from /dev/urandom, if you cat /dev/urandom you will see a whole bunch of gibberish.

Using C we can load 32 random bytes by reading from /dev/urandom:

#include <secp256k1.h>
#include <stdio.h>
static secp256k1_context *ctx = NULL;

int main() {
ctx = secp256k1_context_create(
SECP256K1_CONTEXT_SIGN | SECP256K1_CONTEXT_VERIFY);
/* Declare the private variable as a 32 byte unsigned char */
unsigned char seckey[32];

/* Load private key (seckey) from random bytes */
FILE *frand = fopen("/dev/urandom", "r");

/* Read 32 bytes from frand */

/* Close the file */
fclose(frand);

/* Loop through and print each byte of the private key, */
printf("Private Key: ");
for(int i=0; i<32; i++) {
printf("%02X", seckey[i]);
}
printf("\n");
}

Saving as privkey.c, compiling with gcc privkey.c -o privkey, and running with ./privkey:

Private Key: 224877F96B66F4A114DDCE97085F5F1570EDF5EB1F1D7E6795673729A2E80B20

Due to limitations on the secp256k1 curve, not all private keys are valid. There is a $$1/2^{128}$$ chance that the private key is invalid, so we need to check our key is valid by including the following to our main function:

if (!secp256k1_ec_seckey_verify(ctx, seckey)) {
printf("Invalid secret key\n");
return 1;
}

## Creating a Public Key in C

Now that we have a 32 byte private key, we can use secp256k1 to calculate the corresponding public key. First, we need to declare a new variable, secp256k1_pubkey, to store our public key:

secp256k1_pubkey pubkey;

then we create the public key using a function from the secp256k1 library:

secp256k1_ec_pubkey_create(ctx, &pubkey, seckey);

Note that this function takes a pointer for the location of where the public key is to be written.

We now need to serialize this address to translate it into bytes. We use a size_t variable to store the length of the address in bytes:

size_t pk_len = 65;
char pk_bytes[34];

/* Serialize Public Key */
secp256k1_ec_pubkey_serialize(
ctx,
pk_bytes,
&pk_len,
&pubkey,
SECP256K1_EC_UNCOMPRESSED
);

This function takes a pointer for the length and location of the public key. It serializes the public key into bytes so that we can then manipulate it.

## Creating a Bitcoin Address in C

Currently, we have a private key and its corresponding public key in bytes. We wish to follow the scheme outlined earlier to create a bitcoin address. Here we will need to use the C openssl library (-lcrypto) to provide the required SHA256 and RIPEMD160 hashing functions.

At the top of our .c file we require:

#include <openssl/sha.h>
#include <openssl/ripemd.h>

To use these hashing functions we need several new variables, a char variable of length 34 to store our address, a byte variable rmd of length 5+RIPEMD160_DIGEST_LENGTH (5+20) to store our final hashes. We will also use a byte type variable s to store some temporary hashes in:

byte s[65];
byte rmd[5 + RIPEMD160_DIGEST_LENGTH];

We need to define the type byte at the top of our file with typedef unsigned char byte.

First, let us duplicate our public key into s by looping through each byte. Uncompresse public keys are 65 bytes:

int j;
for (j = 0; j < 65; j++) {
s[j] = pk_bytes[j];
}

Following the scheme outlined in the steps to create a legacy Bitcoin Address. We be taking the first SHA256 hash of our public key s with SHA256(s, 65, 0) and then take the RIPEMD160 of this hash with RIPEMD160(SHA_HASH, SHA256_DIGEST_LENGTH, md) where md is the location where we are storing the output.

We also need to set a version byte at the start of the address to 0x00:

/* Set 0x00 byte for main net */
rmd[0] = 0;
RIPEMD160(SHA256(s, 65, 0), SHA256_DIGEST_LENGTH, rmd + 1);

This completes steps 1-3, now we need to find the checksum.

To create the checksum, we take the SHA256 of this hash twice with:

SHA256(SHA256(rmd, 21, 0), SHA256_DIGEST_LENGTH, 0)

but we only need the last 4 bytes of the checksum, so we can use memcpy to copy these bytes to the end of the first 21 bytes of rmd:

memcpy(rmd + 21, SHA256(SHA256(rmd, 21, 0), SHA256_DIGEST_LENGTH, 0), 4);

To use memcpy we need to #include <string.h> at the top of our file.

Now we are very close to a usable bitcoin address, all that is left is to convert these bytes into base58.

We can use this base58 function to convert into base 58. Placing base58() above our main() function:

/* See https://en.wikipedia.org/wiki/Positional_notation#Base_conversion */
char* base58(byte *s, int s_size, char *out, int out_size) {
static const char *base_chars = "123456789"
"ABCDEFGHJKLMNPQRSTUVWXYZ"
"abcdefghijkmnopqrstuvwxyz";

byte s_cp[s_size];
memcpy(s_cp, s, s_size);

int c, i, n;

out[n = out_size] = 0;
while (n--) {
for (c = i = 0; i < s_size; i++) {
c = c * 256 + s_cp[i];
s_cp[i] = c / 58;
c %= 58;
}
out[n] = base_chars[c];
}

return out;
}

To store our address we need a char 34 characters long. We can now create our bitcoin address with these final lines in main:

Finally we need to remove any extra leading 1’s from the beginning of the address. We can find the number of leading 1s we need to remove by counting the number of 1s at the start of the address and subtracting by the number of 0x00 bytes at the beginning of our rmd hash. If a digit needs to be removed, we shift the memory storing the address accross by one, removing the first digit. We also shorten the string by the number of 1s removed.

/* Count the number of extra 1s at the beginning of the address */
int k;
for (k = 1; out[k] == '1'; k++);

/* Count the number of extra leading 0x00 bytes */
int n;
for (n = 1; rmd[n] == 0x00; n++);

When you compile with gcc file.c -o btcGen -lcrypto -lsecp256k1, and run with ./btcGen hopefully you will be greeted with a Bitcoin address starting with 1:

$gcc btc.c -o btcGen -lcrypto -lsecp256k1$ ./btcGen
Private Key: C616D1713C3BBE0627CEBBA2527C31E3304381D88BD470B27BE1318AFC3F1469

# Wallet Import Format (WIF)

Great now we have an address that we can send bitcoin to! But how do we spend it? Most software wallets WILL NOT ACCEPT raw byte private keys. Instead, the convention is to use Wallet Import Format; which like in bitcoin addresses, makes the private key easier to copy and transmit.

NEVER STORE LARGE AMOUNTS OF BTC ON SELF GENERATED WALLETS

To convert your private key into WIF:

1. Add a version byte (0x80 for mainnet) to the front of the private key
2. Calculate the SHA256 hash
3. Calculate the SHA256 hash again
4. Take the first 4 bytes from this hash, this is our checksum
5. Add these 4 bytes to the end of the extended private key in step 1.
6. Convert to base58 (you can reuse the function from earlier)

🧠 I will leave this as an excercise for now, as it is almost identical to the bitcoin address creation scheme. If everything goes right then your WIF private key should begin with a 5. For more details see wiki/Wallet_import_format.

In the Electrum/Electron wallets you can load your WIF like this: (I’m only using BCH here because the low fees are nice for testing 😷)