super naïve bolt11 decoder.

LNbits/bolt11.py

@@ -0,0 +1,106 @@
+import bitstring
+import re
+from bech32 import bech32_decode, CHARSET
+
+
+class Invoice(object):
+    def __init__(self):
+        self.payment_hash: str = None
+        self.amount_msat: int = 0
+        self.description: str = None
+
+
+def decode(pr: str) -> Invoice:
+    """ Super naïve bolt11 decoder,
+    only gets payment_hash, description/description_hash and amount in msatoshi.
+    based on https://github.com/rustyrussell/lightning-payencode/blob/master/lnaddr.py
+    """
+    hrp, data = bech32_decode(pr)
+    if not hrp:
+        raise ValueError("Bad bech32 checksum")
+
+    if not hrp.startswith("ln"):
+        raise ValueError("Does not start with ln")
+
+    data = u5_to_bitarray(data)
+
+    # Final signature 65 bytes, split it off.
+    if len(data) < 65 * 8:
+        raise ValueError("Too short to contain signature")
+    data = bitstring.ConstBitStream(data[: -65 * 8])
+
+    invoice = Invoice()
+
+    m = re.search("[^\d]+", hrp[2:])
+    if m:
+        amountstr = hrp[2 + m.end() :]
+        if amountstr != "":
+            invoice.amount_msat = unshorten_amount(amountstr)
+
+    # pull out date
+    data.read(35).uint
+
+    while data.pos != data.len:
+        tag, tagdata, data = pull_tagged(data)
+
+        data_length = len(tagdata) / 5
+
+        if tag == "d":
+            invoice.description = trim_to_bytes(tagdata).decode("utf-8")
+        elif tag == "h" and data_length == 52:
+            invoice.description = trim_to_bytes(tagdata)
+        elif tag == "p" and data_length == 52:
+            invoice.payment_hash = trim_to_bytes(tagdata)
+
+    return invoice
+
+
+def unshorten_amount(amount: str) -> int:
+    """ Given a shortened amount, return millisatoshis
+    """
+    # BOLT #11:
+    # The following `multiplier` letters are defined:
+    #
+    # * `m` (milli): multiply by 0.001
+    # * `u` (micro): multiply by 0.000001
+    # * `n` (nano): multiply by 0.000000001
+    # * `p` (pico): multiply by 0.000000000001
+    units = {
+        "p": 10 ** 12,
+        "n": 10 ** 9,
+        "u": 10 ** 6,
+        "m": 10 ** 3,
+    }
+    unit = str(amount)[-1]
+
+    # BOLT #11:
+    # A reader SHOULD fail if `amount` contains a non-digit, or is followed by
+    # anything except a `multiplier` in the table above.
+    if not re.fullmatch("\d+[pnum]?", str(amount)):
+        raise ValueError("Invalid amount '{}'".format(amount))
+
+    if unit in units:
+        return int(amount[:-1]) * 100_000_000_000 / units[unit]
+    else:
+        return int(amount) * 100_000_000_000
+
+
+def pull_tagged(stream):
+    tag = stream.read(5).uint
+    length = stream.read(5).uint * 32 + stream.read(5).uint
+    return (CHARSET[tag], stream.read(length * 5), stream)
+
+
+def trim_to_bytes(barr):
+    # Adds a byte if necessary.
+    b = barr.tobytes()
+    if barr.len % 8 != 0:
+        return b[:-1]
+    return b
+
+
+def u5_to_bitarray(arr):
+    ret = bitstring.BitArray()
+    for a in arr:
+        ret += bitstring.pack("uint:5", a)
+    return ret