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Passing AlgorithmIdentifier parameters to operations

Quick background

We currently only support passing the AlgorithmIdentifier (X509_ALGOR) parameter field to symmetric cipher provider implementations.

We do support passing them to legacy implementations of other types of operation algorithms as well, but it's done in a way that can't be supported with providers, because it involves sharing specific structures between libcrypto and the backend implementation.

For a longer background and explanation, see Background / tl;dr at the end of this design.

Establish an OSSL_PARAM key that any algorithms may become aware of

We already have a parameter key, but it's currently only specified for EVP_CIPHER, in support of EVP_CIPHER_param_to_asn1() and EVP_CIPHER_asn1_to_param().

"alg_id_param", also known as the macro OSSL_CIPHER_PARAM_ALGORITHM_ID_PARAMS

This parameter can be used in the exact same manner with other operations, with the value of the AlgorithmIdentifier parameter as an octet string, to be interpreted by the implementations in whatever way they see fit.

Applications can choose to add these in an OSSL_PARAM array, to be passed with the multitude of initialization functions that take such an array, or using specific operation OSSL_PARAM setters and getters (such as EVP_PKEY_CTX_set_params), or using other available convenience functions (see below).

This parameter will have to be documented in the following files:

  • doc/man7/provider-asym_cipher.pod
  • doc/man7/provider-cipher.pod
  • doc/man7/provider-digest.pod
  • doc/man7/provider-kdf.pod
  • doc/man7/provider-kem.pod
  • doc/man7/provider-keyexch.pod
  • doc/man7/provider-mac.pod
  • doc/man7/provider-signature.pod

That should cover all algorithms that are, or should be possible to fetch by AlgorithmIdentifier.algorithm, and for which there's potentially a relevant AlgorithmIdentifier.parameters field.

We may arguably want to consider doc/man7/provider-keymgmt.pod too, but an AlgorithmIdentifier that's attached directly to a key is usually part of a PrivKeyInfo or SubjectPublicKeyInfo structure, and those are handled by encoders and decoders as those see fit, and there's no tangible reason why that would have to change.

Public convenience API

For convenience, the following set of functions would be added to pass the AlgorithmIdentifier parameter data to diverse operations, or to retrieve such parameter data from them.

/*
 * These two would essentially be aliases for EVP_CIPHER_param_to_asn1()
 * and EVP_CIPHER_asn1_to_param().
 */
EVP_CIPHER_CTX_set_algor_param(EVP_PKEY_CTX *ctx, X509_ALGOR *alg);
EVP_CIPHER_CTX_get_algor_param(EVP_PKEY_CTX *ctx, X509_ALGOR *alg);

EVP_MD_CTX_set_algor_param(EVP_PKEY_CTX *ctx, X509_ALGOR *alg);
EVP_MD_CTX_get_algor_param(EVP_PKEY_CTX *ctx, X509_ALGOR *alg);

EVP_MAC_CTX_set_algor_param(EVP_PKEY_CTX *ctx, X509_ALGOR *alg);
EVP_MAC_CTX_get_algor_param(EVP_PKEY_CTX *ctx, X509_ALGOR *alg);

EVP_KDF_CTX_set_algor_param(EVP_PKEY_CTX *ctx, X509_ALGOR *alg);
EVP_KDF_CTX_get_algor_param(EVP_PKEY_CTX *ctx, X509_ALGOR *alg);

EVP_PKEY_CTX_set_algor_param(EVP_PKEY_CTX *ctx, X509_ALGOR *alg);
EVP_PKEY_CTX_get_algor_param(EVP_PKEY_CTX *ctx, X509_ALGOR *alg);

Note that all might not need to be added immediately, depending on if they are considered useful or not. For future proofing, however, they should probably all be added.

Requirements on the providers

Providers that implement ciphers or any operation that uses asymmetric keys will have to implement support for passing AlgorithmIdentifier parameter data, and will have to process that data in whatever manner that's necessary to meet the standards for that operation.

Fallback strategies

There are no possible fallback strategies, which is fine, considering that current provider functionality doesn't support passing AlgorithmIdentifier parameter data at all (except for EVP_CIPHER), and therefore do not work at all when such parameter data needs to be passed.

Background / tl;dr

AlgorithmIdenfier parameter and how it's used

OpenSSL has historically done a few tricks to not have to pass AlgorithmIdenfier parameter data to the backend implementations of cryptographic operations:

  • In some cases, they were passed as part of the lower level key structure (for example, the RSA structure can also carry RSA-PSS parameters).
  • In the EVP_CIPHER case, there is functionality to pass the parameter data specifically.
  • For asymmetric key operations, PKCS#7 and CMS support was added as EVP_PKEY ctrls.

With providers, some of that support was retained, but not others. Most crucially, the EVP_PKEY ctrls for PKCS#7 and CMS were not retained, because the way they were implemented violated the principle that provider implementations MUST NOT share complex OpenSSL specific structures with libcrypto.

Usage examples

Quite a lot of the available examples today revolve around CMS, with a number of RFCs that specify what parameters should be passed with certain operations / algorithms. This list is not exhaustive, the reader is encouraged to research further usages.

Some of it can be claimed to already have support in OpenSSL. However, this is with old libcrypto code that has special knowledge of the algorithms that are involved.