暴露 _hashlib.pyd 内部以支持 EVP_MD_CTX？

Question

有没有人知道怎么用ctypes来访问Python 2.x的_hashlib.pyd内部内容？我特别需要提取EVP_MD_CTX这个结构体，以便对Python的HASH对象进行序列化。

Answer 1

从头文件（比如openssl/evp.h和_hashopenssl.c）中映射C语言的结构体其实很简单，但在不同版本之间可能不太兼容。以下是我在自己环境中的做法：

from ctypes import *

PyObject_HEAD = [
    ('ob_refcnt', c_size_t),
    ('ob_type', c_void_p),
]

class EVP_MD(Structure):
    _fields_ = [
        ('type', c_int),
        ('pkey_type', c_int),
        ('md_size', c_int),
        ('flags', c_ulong),
        ('init', c_void_p),
        ('update', c_void_p),
        ('final', c_void_p),
        ('copy', c_void_p),
        ('cleanup', c_void_p),
        ('sign', c_void_p),
        ('verify', c_void_p),
        ('required_pkey_type', c_int*5),
        ('block_size', c_int),
        ('ctx_size', c_int),
    ]

class EVP_MD_CTX(Structure):
    _fields_ = [
        ('digest', POINTER(EVP_MD)),
        ('engine', c_void_p),
        ('flags', c_ulong),
        ('md_data', POINTER(c_char)),
    ]

class EVPobject(Structure):
    _fields_ = PyObject_HEAD + [
        ('name', py_object),
        ('ctx', EVP_MD_CTX),
    ]

下面是一个示例，展示如何使用它来保存和恢复哈希对象的状态：

import hashlib

hash = hashlib.md5('test')
print hash.hexdigest()

c_evp_obj = cast(c_void_p(id(hash)), POINTER(EVPobject)).contents
ctx = c_evp_obj.ctx
digest = ctx.digest.contents
state = ctx.md_data[:digest.ctx_size]

hash2 = hashlib.md5()
c_evp_obj = cast(c_void_p(id(hash2)), POINTER(EVPobject)).contents
ctx = c_evp_obj.ctx
digest = ctx.digest.contents
memmove(ctx.md_data, state, digest.ctx_size)
print hash2.hexdigest()

暴露 _hashlib.pyd 内部以支持 EVP_MD_CTX？

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