Custom Provider - EVP_CIPHER_fetch fails

[Matt Caswell]

On 13/12/2022 09:13, Hareesh Das Ulleri wrote:
> Hello OpenSSL users,
> 
>    I am in preparation of a provider (for a custom crypto) by referring 
> OpenSSL 3 design doc

Don't refer to the design doc. It has not been maintained with all the 
latest tweaks and updates to what was originally envisaged.

> (I use Linux 5.10 + OpenSSL 3.0.7). I believe, the 
> custom provider has all required call backs implemented for the cipher 
> functionalities in its dispatch table.
> 

You should refer to the man page for what is required to implement a 
custom cipher. See:

https://www.openssl.org/docs/man3.0/man7/provider-cipher.html

Note in particular this comment in the man page:

"A cipher algorithm implementation may not implement all of these 
functions. In order to be a consistent set of functions there must at 
least be a complete set of "encrypt" functions, or a complete set of 
"decrypt" functions, or a single "cipher" function. In all cases both 
the OSSL_FUNC_cipher_newctx and OSSL_FUNC_cipher_freectx functions must 
be present. All other functions are optional."

So, in other words, you must as a minimum implement 
OSSL_FUNC_cipher_encrypt_init (or OSSL_FUNC_cipher_decrypt_init), 
OSSL_FUNC_cipher_update, OSSL_FUNC_cipher_final, OSSL_FUNC_cipher_newctx 
and OSSL_FUNC_cipher_freectx.

You should probably also read the following pages for general 
information on writing a provider:

https://www.openssl.org/docs/man3.0/man7/provider.html
https://www.openssl.org/docs/man3.0/man7/provider-base.html

You might also want to refer to Richard Levitte's implementation of the 
toy Vigenère cipher as a provider here:

https://github.com/provider-corner/vigenere

>    I have another test application (for encryption and decryption of a 
> text message). At the starting of the app, it calls OSSL_PROVIDER_load 
> and EVP_CIPHER_fetch functions to the custom provider. But unfortunately 
> custom provider fetch function fails…


I suggest calling the OSSL_PROVIDER_available() function to determine 
whether your custom provider has been successfully loaded:

https://www.openssl.org/docs/man3.0/man3/OSSL_PROVIDER_available.html

> 
> What could be the missing or how to make sure that the custom provider 
> is loaded correctly before calling the fetch function?
> 
> cipher = EVP_CIPHER_fetch(NULL, "AES-256-CBC-CTS", NULL);  -> This will 
> work, default provider
> 
> cipher = EVP_CIPHER_fetch(NULL, "CUSTOM_ALGO", NULL);  -> returns NULL, 
> custom provider
> 
> _Confg file:_
> 
> openssl_conf = openssl_init
> 
> [openssl_init]
> 
> providers = provider_section
> 
> [provider_section]
> 
> customProv = customProv_section
> 
> default = default_sect
> 
> [customProv_section]
> 
> provider_id = customProv
> 
> module_path = /userfs/lib/customProv.so
> 
> algorithms = CUSTOM_ALGO
> 
> activate = 1
> 
> [default_sect]
> 
> algorithms = AES-256-CBC-CTS
> 
> activate = 1

This config file does not look correct. I guess you based it on some of 
the examples in the design doc which are out of date.

The man page for config file formats is here:

https://www.openssl.org/docs/man3.0/man5/config.html

See the "Provider Configuration" section on that page in particular.

Also worth looking at are some of the test config files used in the 
OpenSSL code base for loading providers, e.g.

https://github.com/openssl/openssl/blob/openssl-3.0/test/default-and-legacy.cnf

Matt


> 
> Thank you,
> 
> Hareesh
>