Re: Custom Provider - EVP_CIPHER_fetch fails

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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:

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:

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

  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:

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


providers = provider_section


customProv = customProv_section

default = default_sect


provider_id = customProv

module_path = /userfs/lib/

algorithms = CUSTOM_ALGO

activate = 1


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:

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.


Thank you,


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