请写一段ed25519的python实现

这是一段简单的 ed25519 加密算法的 Python 实现：

import hashlib
import os

b = 256
q = 2**255 - 19
l = 2**252 + 27742317777372353535851937790883648493

def H(m):
    return hashlib.sha512(m).digest()

def expmod(b,e,m):
    if e == 0: return 1
    t = expmod(b,e//2,m)**2 % m
    if e & 1: t = (t*b) % m
    return t

def inv(x):
    return expmod(x,q-2,q)

def generate_keypair(seed=None):
    if seed is None:
        seed = os.urandom(32)
    sk = H(seed)
    pk = sk[32:]
    return (sk,pk)

def sign(sk,m):
    h = H(sk[:32] + m)
    a = int.from_bytes(h, 'big')
    r = expmod(b,a,l) % q
    R = expmod(b,r,q)
    S = (r + int.from_bytes(h, 'big') * int.from_bytes(sk[32:], 'big')) * inv(a) % q
    return R.to_bytes(32, 'big') + S.to_bytes(32, 'big')

def verify(pk,m,sig):
    R = int.from_bytes(sig[:32], 'big')
    S = int.from_bytes(sig[32:], 'big')
    h = H(pk + m)
    a = int.from_bytes(h, 'big')
    v = expmod(b,a,q)
    z = int.from_bytes(h, 'big')
    u = (S * inv(a)) % q
    u1 = (z * inv(v)) % q
    u2 = (R * inv(v)) % q
    return (expmod(b,u1,q) * expmod(R,u2,q)) % q == R

此代码仅作为参考，不建议在生产环境中使用。请仔细阅读 ed25519 的官方文档并使用已被证明的实现。