Converts pubkey to Nostr address

npub1.py

#!/usr/bin/env python3
"""
% python3 npub1.py 0350dbc4dfd2313112fa6d1ac3effed7d1428b3c8dcc7357c0b84bac8f356a1894
Compressed BTC pubkey (hex): 0350dbc4dfd2313112fa6d1ac3effed7d1428b3c8dcc7357c0b84bac8f356a1894
Nostr x-only pubkey (hex)  : 50dbc4dfd2313112fa6d1ac3effed7d1428b3c8dcc7357c0b84bac8f356a1894
Nostr npub                 : npub12rdufh7jxyc397ndrtp7llkh69pgk0yde3e40s9cfwkg7dt2rz2qjdzd2t
"""

import argparse
import binascii
import sys

# ---------- Bech32 (BIP-173) for NIP-19 ----------
CHARSET = "qpzry9x8gf2tvdw0s3jn54khce6mua7l"

def bech32_polymod(values):
    GENERATORS = [0x3b6a57b2, 0x26508e6d, 0x1ea119fa, 0x3d4233dd, 0x2a1462b3]
    chk = 1
    for v in values:
        b = chk >> 25
        chk = ((chk & 0x1ffffff) << 5) ^ v
        for i in range(5):
            if (b >> i) & 1:
                chk ^= GENERATORS[i]
    return chk

def bech32_hrp_expand(hrp):
    return [ord(x) >> 5 for x in hrp] + [0] + [ord(x) & 31 for x in hrp]

def bech32_create_checksum(hrp, data):
    values = bech32_hrp_expand(hrp) + data
    polymod = bech32_polymod(values + [0, 0, 0, 0, 0, 0]) ^ 1
    return [(polymod >> 5*(5-i)) & 31 for i in range(6)]

def bech32_encode(hrp, data):
    combined = data + bech32_create_checksum(hrp, data)
    return hrp + '1' + ''.join([CHARSET[d] for d in combined])

def convertbits(data, frombits, tobits, pad=True):
    acc = 0
    bits = 0
    ret = []
    maxv = (1 << tobits) - 1
    max_acc = (1 << (frombits + tobits - 1)) - 1
    for value in data:
        if value < 0 or (value >> frombits):
            return None
        acc = ((acc << frombits) | value) & max_acc
        bits += frombits
        while bits >= tobits:
            bits -= tobits
            ret.append((acc >> bits) & maxv)
    if pad:
        if bits:
            ret.append((acc << (tobits - bits)) & maxv)
    elif bits >= frombits or ((acc << (tobits - bits)) & maxv):
        return None
    return ret

def nip19_encode_npub(pubkey_bytes: bytes) -> str:
    """Encode raw 32-byte pubkey into NIP-19 'npub' (Bech32, not Bech32m)."""
    data5 = convertbits(list(pubkey_bytes), 8, 5, True)
    if data5 is None:
        raise ValueError("convertbits failed")
    return bech32_encode("npub", data5)

def is_compressed_pubkey(pub: bytes) -> bool:
    return len(pub) == 33 and pub[0] in (0x02, 0x03)

def main():
    ap = argparse.ArgumentParser(
        description="Convert a compressed Bitcoin pubkey (02/03 + 32 bytes) into a Nostr pubkey (hex x-only and npub)."
    )
    ap.add_argument("pubkey_hex", help="Compressed secp256k1 pubkey in hex (starts with 02/03, 33 bytes).")
    args = ap.parse_args()

    try:
        pub = bytes.fromhex(args.pubkey_hex.strip())
    except ValueError:
        print("Error: invalid hex for pubkey.", file=sys.stderr)
        sys.exit(1)

    if not is_compressed_pubkey(pub):
        print("Error: pubkey must be 33 bytes and start with 0x02 or 0x03 (compressed).", file=sys.stderr)
        sys.exit(1)

    # Extract x-only (drop the first byte)
    x_only = pub[1:]
    x_only_hex = x_only.hex()
    npub = nip19_encode_npub(x_only)

    print("Compressed BTC pubkey (hex):", args.pubkey_hex.lower())
    print("Nostr x-only pubkey (hex)  :", x_only_hex)
    print("Nostr npub                 :", npub)

if __name__ == "__main__":
    main()