PyCrypto RSA失败了很长的字符串

Question

我尝试将RSA与PyCrypto一起使用来加密/解密长字符串。我非常有信心我的代码适用于短字符串（大约200个字符），但对于比这更长的字符串，我无法成功解密。

这就是我所拥有的：

random_generator = Random.new().read

def encrypt_RSA(message):
    key = open('public_key.der', "r").read()
    rsakey = RSA.importKey(key)
    encrypted = rsakey.encrypt(message, random_generator)
    return encrypted

def decrypt_RSA(package):
    key = open('private_key.der', "r").read() 
    rsakey = RSA.importKey(key)
    decrypted = rsakey.decrypt(package) 
    return decrypted

alphabet = 'abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz'
small_alphabet = 'abcdefghijklmnopqrstuvwxyz'

encrypted = encrypt_RSA(small_alphabet)
print base64.b64encode(encrypted[0])
print "========================"
print decrypt_RSA(encrypted[0])

这适用于字符串small_alphabet，但不适用于alphabet。那是为什么？

我正在使用PyCrypto 2.6.1，Python 2.7.6。