Encrypted string doesn't give same value as identical string python

Question

I'm writing a password manager program, and I need to store an encrypted string in a file. (The "service" variable) When the user needs to retrieve the information, they enter the same string as before. When I encrypt the same string, I get a different output. I've checked multiple times, and the string is the same. Anyone know of a fix for this?

Code I'm using to encrypt and write to the file:

def new_account(service, username, password, filepath, key):
    # Encrypting the account details #
    encrypted_service = key.encrypt(bytes(service, "utf-8"))
    encrypted_username = key.encrypt(bytes(username, "utf-8"))
    encrypted_password = key.encrypt(bytes(password, "utf-8"))

    encrypted_service = encrypted_service.decode("utf-8")
    encrypted_username = encrypted_username.decode("utf-8")
    encrypted_password = encrypted_password.decode("utf-8")

    password_file = open(f"{filepath}\passwords.txt", "a")
    password_file.close()
    password_file = open(f"{filepath}\passwords.txt", "r")
    password_file_content = password_file.read()
    password_file.close()
    password_file = open(f"{filepath}\passwords.txt", "a")

    if f"{encrypted_service},{encrypted_username},{encrypted_password}" in password_file_content:
        password_file.close()
        return("The account already exists.")
    else:
        password_file.write(f"{encrypted_service},{encrypted_username},{encrypted_password}\n")
        password_file.close()
    
    return f"Account saved for {service.title()} with username {username} and password {password}."

Code I'm using to retrieve the information:

# The account retrieval function #
def get_account(service, filepath, key):
    encrypted_service = key.encrypt(bytes(service, "utf-8"))
    encrypted_service = encrypted_service.decode("utf-8")
    password_file = open(f"{filepath}\passwords.txt")
    lines = password_file.read().split("\n")
    word = f"{encrypted_service}"

    # Getting the line with the account details #
    for i in lines:
        index = lines.index(i)
        myline = lines[index]

        encrypted_account_content = myline.split(",")
        print(encrypted_account_content)
        print(f"service is: {encrypted_service}")


        if encrypted_account_content.count(encrypted_service) != 0:    
            # Decrypting the account details #
            username = encrypted_account_content[1]
            decrypted_username = key.decrypt(bytes(username, "utf-8"))
            decrypted_username = decrypted_username.decode("utf-8")
            password = encrypted_account_content[2]
            decrypted_password = key.decrypt(bytes(password, "utf-8"))
            decrypted_password = decrypted_password.decode("utf-8")

            return f"Service: {service.title()}\nUsername: {decrypted_username}\nPassword: {decrypted_password}"
        else:
            return "Account not found. Please try again."

I am failing to understand the problem. Is it retrieval that returns broken data? Also, what library/algorithms are you using? — bereal
– bereal, Commented Jul 16, 2021 at 17:56
It would help to write a smaller working test function. You don't need multiple things to encrypt or that open a file only to close it stuff. Ideally, the test program will not use a file at all. Just a script that encodes and decodes that demonstrates the same error. — tdelaney
– tdelaney, Commented Jul 16, 2021 at 18:00
encrypted_service = encrypted_service.decode("utf-8") ... isn't this an encrypted bytes string? Nothing says its valid UTF-8. If you want to store bytes as a string in a file, something like a hex encoding is a good idea. encrypted_service = key.encrypt(bytes(service, "utf-8")).hex() for example. — tdelaney
– tdelaney, Commented Jul 16, 2021 at 18:04
When I encrypt the same string, I get a different output Show us a complete example, that we can run ourselves, that demonstrates this behavior. As it is, you've shown us some function definitions, but you haven't shown us examples of how those functions are used. — John Gordon
– John Gordon, Commented Jul 16, 2021 at 18:05

bereal · Accepted Answer · 2021-07-16 18:18:58Z

1

Any proper encryption will use randomization, so that the result will always be different, and you won't be able to tell that the plaintext was the same. That's needed to achieve semantic security. In practice, initialization vectors and modes of operation like CBC or CTR are used. The cryptography library you're using does it out of the box (with CBC mode).

You will either have to store service as a plaintext, which shouldn't significantly reduce the overall security, or decrypt each service field in order to find the needed record.

answered Jul 16, 2021 at 18:18

bereal

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