SM2 Encryption Data Format in OpenSSL: EVP_PKEY_CTX_set_sm2_encdata_format()

The function 'EVP_PKEY_CTX_set_sm2_encdata_format()' sets the format of the encrypted data generated during SM2 encryption. SM2 is a cryptographic algorithm used for digital signature and key exchange in Chinese cryptography standards. The format of the encrypted data determines how the encrypted data is represented, which affects how it is processed and transmitted.

The function takes two arguments: the EVP_PKEY_CTX object and the format identifier. The format identifier can be one of the following:

• 'EVP_PKEY_SM2_ENC_MODE_EXPLICIT': This format represents the encrypted data in an explicit manner, which means that the ciphertext and the ephemeral public key used for encryption are both included in the output. This format is more verbose, but it provides more information about the encrypted data.

• 'EVP_PKEY_SM2_ENC_MODE_IMPLICIT': This format represents the encrypted data in an implicit manner, which means that only the ciphertext is included in the output. The ephemeral public key used for encryption is not included. This format is less verbose, but it provides less information about the encrypted data.

By default, the format is set to 'EVP_PKEY_SM2_ENC_MODE_EXPLICIT'. The format can be changed using 'EVP_PKEY_CTX_set_sm2_encdata_format()'. This function should be called before 'EVP_PKEY_encrypt_init()' is called.