Bitcoin Wiki - User contributions [en]

VarInt

2023-10-25T13:33:55Z

Benjamin Watts: Changed example hex values from big endian to little endian.

A VarInt or "Variable Integer" is an [[integer]] format used widely in Bitcoin to indicate the lengths of fields within transaction, block and peer-to-peer network data.

A VarInt is a variable length field 1, 3, 5 or 9 bytes in length dependent on the size of the object being defined. The VarInt format is used as it is space efficient over simply using an 8-byte field where variable length objects are used. VarInt values are almost always represented in little endian.

==Using VarInts==

When expressing an integer between zero and <code>0xFC</code> (252) the value itself can be used and expressed in uint8_t format.

When expressing an integer value greater than <code>0xFC</code> but less than or equal to <code>0xFFFF</code> (65,535), the varint is <code>0xFDXXXX</code> where XXXX represents the two byte integer in uint16_t format.

When expressing an integer value greater than <code>0xFFFF</code> but less than or equal to <code>0xFFFFFFFF</code> (4,294,967,295), the varint is <code>0xFEXXXXXXXX</code> where XXXXXXXX represents the 4 byte integer in uint32_t format.

When expressing an integer value greater than <code>0xFFFFFFFF</code> but less than or equal to <code>0xFFFFFFFFFFFFFFFF</code> (18,446,744,073,709,551,615), the varint is <code>0xFFXXXXXXXXXXXXXXXX</code> where XXXXXXXXXXXXXXXX represents the 8 byte integer in uint64_t format.

===References with Examples===

{| class="wikitable"
|-
! Value Range Example Value
! Size Range Example Size
! VarInt Format Example VarInt
|-
| 0 <= Value <= 252 (<code>0xFC</code>) e.g. 187 (<code>0xBB</code>)
| 1 byte 1 byte
| uint8_t <code>0xBB</code>
|-
| 253 <= Value <= 65,535 (<code>0xFFFF</code>) e.g. 255 (<code>0xFF</code>) e.g. 13,337 (<code>0x3419</code>)
| 1 - 2 bytes 1 byte but greater than <code>0xFC</code> 2 bytes
| 0xFD + number as uint16_t <code>0xFDFF00</code> <code>0xFD1934</code>
|-
| 65,536 <= Value <= ‭4,294,967,295‬ (<code>0xFFFFFFFF</code>) e.g. 14,435,729 (<code>0xDC4591</code>) e.g. 134,250,981 (<code>0x80081E5</code>)
| 3 - 4 bytes 3 bytes 4 bytes
| 0xFE + number as uint32_t <code>0xFE9145DC00</code> <code>0xFEE5810008</code>
|-
| 4,294,967,296 <= value <= ‭18,446,744,073,709,551,615 (<code>0xFFFFFFFFFFFFFFFF</code>) e.g.198,849,843,832,919 (<code>0xB4DA564E2857</code>) e.g. 5,473,425,651,754,713,432 (<code>0x‭4BF583A17D59C158‬</code>)
| 5 - 8 bytes 6 bytes 8 bytes
| 0xFF + number as uint64_t <code>0xFF57284E56DAB40000</code> <code>0xFF58C1597DA183F54B</code>
|}

ECDSA Example

2023-04-05T15:40:29Z

Benjamin Watts: Created page with "This page will walk you through a demonstration of the Elliptic Curve Digital Signature Algorithm using secp256k1. ==ECDSA Signing Algorithm in Python== {| class="wikitable"..."

This page will walk you through a demonstration of the Elliptic Curve Digital Signature Algorithm using secp256k1.

==ECDSA Signing Algorithm in Python==
{| class="wikitable"
|-
! Parameter
! Value
! Descrption
|-
|p
|115792089237316195423570985008687907853269984665640564039457584007908834671663
|The elliptic curve is defined over this value. P defines the size of the finite field.
|-
|n
|115792089237316195423570985008687907852837564279074904382605163141518161494337
|This value is the order of the group. It defines the number of possible elliptic curve points given the generator point.
|-
|Gx
|55066263022277343669578718895168534326250603453777594175500187360389116729240
|This value is the x coordinate for the generator point.
|-
|Gy
|32670510020758816978083085130507043184471273380659243275938904335757337482424
|This value is the y coordinate for the generator point.
|-
|}

def ecdsa_sig(m, d): # ecdsa_sig function takes a message and private key as an integer
k = random.randint(1, n - 1) # Create the ephemeral key, k
m1 = hashlib.sha256() # Create a hash object
m1.update(m)
hashed_message = m1.hexdigest() # Hash the message

eph_point = Point_Multiplication(Gx, Gy, k, p) # Calculate ephemeral point using k
eph_point_x = eph_point[0]

s_temp = (int(hashed_message, 16) + (d * eph_point_x))

s = (Mod_Inv(k, n) * (s_temp)) % n # Create s value
r = eph_point_x # Create r value

print("Signing Complete")
print("Eph Point x: " + str(eph_point[0]))
print("Eph point y: " + str(eph_point[1]))
print(" ")
print("R: " + str(r))
print("S: " + str(s))

"""
1. 0x30 indicates start of DER
2. One byte to encode length of data
3. 0x02 header byte indicating integer
4. One byte to encode length of r
5. The r value as big-endian integer
6. 0x02 header byte to indicate integer
7. One byte to include length of the following s value
8. The s value as big-endian integer
"""

# DER Format Code Begin
#------------------------------------------------------------
r_hex = "00"
r_hex = r_hex + str(hex(r)[2:])
s_hex = str(hex(s)[2:])

serial_1 = "30"
serial_2 = hex(int(((get_length(r_hex) / 2) + (get_length(s_hex) / 2) + 4)))[2:]
serial_3 = "02"
serial_4 = hex(int(get_length(hex(r)[2:]) / 2) + 1)[2:]
serial_5 = "00" + hex(r)[2:]
serial_6 = "02"
serial_7 = hex(int(get_length(hex(r)[2:]) / 2))[2:]
serial_8 = hex(s)[2:]

sig = serial_1 + str(serial_2) + serial_3 + str(serial_4)
sig = sig + str(serial_5) + serial_6 + str(serial_7) + str(serial_8)
sig = sig + str(binascii.hexlify(HASH_TYPE))[2:4]

sig = bytes.fromhex(sig)

return sig