OpenSSL's envelope interface specifies several native datatypes that the application programmer needs to know about:
ENGINE
ENGINE is a typedef from the engine_st structure, which is where OpenSSL stores various implementations of cryptographic algorithms and functions. ENGINE is actually a linked list of structures.
EVP_CIPHER_CTX
EVP_CIPHER_CTX is a typedef from the evp_cipher_ctx_st structure that is the main monolithic context control structure for all symmetric algorithms. It keeps track of the high-level EVP interface details, such as engine type, whether the context is encrypting or decrypting, and other ancillary data. EVP_CIPHER_CTX contains an EVP_CIPHER structure pointer.
EVP_CIPHER
EVP_CIPHER is a typedef from the evp_cipher_st structure that is the minor EVP symmetric algorithm structure. It contains all the algorithm-specific metadata, such as the initialization, encryption and decryption, and cleanup functions for the given algorithm.
EVP_PKEY
EVP_PKEY is a typedef from the evp_pkey_st structure that is the public key information structure containing RSA, DSA, or DH information and associated metadata.
EVP_MD_CTX
EVP_MD_CTX is a typedef fromjhe evp_md_ctx_st structure that is the main monolithic context control structure for all message digest algorithms. It keeps track of the high-level EVP interface details, such as engine type and control flags. EVP_MD_CTX contains an EVP_MD structure pointer.
EVP_MD
EVP_MD is a typedef from the evp_md_st structure that is the minor EVP message digest algorithm structure. It contains all of the algorithm-specific metadata, such as the initialization, digest, and cleanup functions for the given algorithm. It also contains digital signature functions.