How to save SHA256 object to a file?

Question

(I'm doing all this in python 3.10.4 using pycryptodome)

I'm trying to do this process:

1. Get a hash of a file
2. Save that hash somewhere
3. Load that hash and perform RSA signing using a private key

I'm having a problem in step 3 where to save the hash, I have to save it as a string which doesn't work in Step 3.

I've tried using pickle but I'm getting "ctypes objects containing pointers cannot be pickled"

Code generating the hash:

sha256 = SHA256.new()
with open(fileDir, 'rb') as f:
    while True:
        data = f.read(BUF_SIZE)
        if not data:
            break
        sha256.update(data)

Code to perform the signing:

get_file(fileName + '.hash', directory)
with open(currentDir + '/client_files/downloaded/' + fileName + '.hash', 'r') as f:
    hash_data = f.read()    
with open(currentDir + '/client_files/private_key.pem', 'rb') as f:
    private_key = RSA.importKey(f.read())
print(private_key)    
signer = PKCS1_v1_5.new(private_key)
signature = signer.sign(hash_data)

The error I'm getting:

Traceback (most recent call last):
  File "c:\Users\User\Documents\Coding\VSCode Projects\practiceGround\sec_cloud_project\client\client.py", line 168, in <module>
    main()
  File "c:\Users\User\Documents\Coding\VSCode Projects\practiceGround\sec_cloud_project\client\client.py", line 163, in main
    sign(fileName, 'worker_test_files')
  File "c:\Users\User\Documents\Coding\VSCode Projects\practiceGround\sec_cloud_project\client\client.py", line 120, in sign
    signature = signer.sign(hash_data)
  File "C:\Users\User\anaconda3\envs\nscc_project\lib\site-packages\Crypto\Signature\pkcs1_15.py", line 77, in sign
    em = _EMSA_PKCS1_V1_5_ENCODE(msg_hash, k)
  File "C:\Users\User\anaconda3\envs\nscc_project\lib\site-packages\Crypto\Signature\pkcs1_15.py", line 191, in _EMSA_PKCS1_V1_5_ENCODE
    digestAlgo = DerSequence([ DerObjectId(msg_hash.oid).encode() ])
AttributeError: 'str' object has no attribute 'oid'

Note that I'm currently saving the original hash as a string to a text file. If I try to use pickle to save the object as a whole I get this error

with open(currentDir + '/worker_files/sha256.pickle', 'wb') as f:
    pickle.dump(sha256, f)

Traceback (most recent call last):
  File "c:\Users\User\Documents\Coding\VSCode Projects\practiceGround\sec_cloud_project\worker\worker.py", line 188, in <module>
    main()
  File "c:\Users\User\Documents\Coding\VSCode Projects\practiceGround\sec_cloud_project\worker\worker.py", line 179, in main
    hash_file(fileName, 'worker_test_files')
  File "c:\Users\User\Documents\Coding\VSCode Projects\practiceGround\sec_cloud_project\worker\worker.py", line 55, in hash_file
    pickle.dump(sha256, f)
ValueError: ctypes objects containing pointers cannot be pickled

Answer 1

Thanks to @Topaco. Changing to using Cyptography for both hashing and signing seemed to work.

Hashing with Cryptography, dumping to a file with pickle, then load and sign with Cryptography again.