EVP_EncryptInit

EVP_EncryptInit, EVP_EncryptUpdate, EVP_EncryptFinal ? EVP cipher routines

 #include <openssl/evp.h>

 void EVP_EncryptInit(EVP_CIPHER_CTX *ctx, const EVP_CIPHER *type,
         unsigned char *key, unsigned char *iv);
 void EVP_EncryptUpdate(EVP_CIPHER_CTX *ctx, unsigned char *out,
         int *outl, unsigned char *in, int inl);
 void EVP_EncryptFinal(EVP_CIPHER_CTX *ctx, unsigned char *out,
         int *outl);

 void EVP_DecryptInit(EVP_CIPHER_CTX *ctx, const EVP_CIPHER *type,
         unsigned char *key, unsigned char *iv);
 void EVP_DecryptUpdate(EVP_CIPHER_CTX *ctx, unsigned char *out,
         int *outl, unsigned char *in, int inl);
 int EVP_DecryptFinal(EVP_CIPHER_CTX *ctx, unsigned char *outm,
         int *outl);

 void EVP_CipherInit(EVP_CIPHER_CTX *ctx, const EVP_CIPHER *type,
         unsigned char *key, unsigned char *iv, int enc);
 void EVP_CipherUpdate(EVP_CIPHER_CTX *ctx, unsigned char *out,
         int *outl, unsigned char *in, int inl);
 int EVP_CipherFinal(EVP_CIPHER_CTX *ctx, unsigned char *outm,
         int *outl);

 void EVP_CIPHER_CTX_cleanup(EVP_CIPHER_CTX *a);

 const EVP_CIPHER *EVP_get_cipherbyname(const char *name);
 #define EVP_get_cipherbynid(a) EVP_get_cipherbyname(OBJ_nid2sn(a))
 #define EVP_get_cipherbyobj(a) EVP_get_cipherbynid(OBJ_obj2nid(a))

 #define EVP_CIPHER_nid(e)              ((e)->nid)
 #define EVP_CIPHER_block_size(e)       ((e)->block_size)
 #define EVP_CIPHER_key_length(e)       ((e)->key_len)
 #define EVP_CIPHER_iv_length(e)        ((e)->iv_len)

 int EVP_CIPHER_type(const EVP_CIPHER *ctx);
 #define EVP_CIPHER_CTX_cipher(e)       ((e)->cipher)
 #define EVP_CIPHER_CTX_nid(e)          ((e)->cipher->nid)
 #define EVP_CIPHER_CTX_block_size(e)   ((e)->cipher->block_size)
 #define EVP_CIPHER_CTX_key_length(e)   ((e)->cipher->key_len)
 #define EVP_CIPHER_CTX_iv_length(e)    ((e)->cipher->iv_len)
 #define EVP_CIPHER_CTX_type(c)         EVP_CIPHER_type(EVP_CIPHER_CTX_cipher(c))

 int EVP_CIPHER_param_to_asn1(EVP_CIPHER_CTX *c, ASN1_TYPE *type);
 int EVP_CIPHER_asn1_to_param(EVP_CIPHER_CTX *c, ASN1_TYPE *type);

The EVP cipher routines are a high level interface to certain symmetric ciphers.

EVP_EncryptInit() initialises a cipher context ctx for encryption with cipher type. type is normally supplied by a function such as EVP_des_cbc() . key is the symmetric key to use and iv is the IV to use (if necessary), the actual number of bytes used for the key and IV depends on the cipher. It is possible to set all parameters to NULL except type in an initial call and supply the remaining parameters in subsequent calls. This is normally done when the EVP_CIPHER_asn1_to_param() function is called to set the cipher parameters from an ASN1 AlgorithmIdentifier and the key from a different source.

EVP_EncryptUpdate() encrypts inl bytes from the buffer in and writes the encrypted version to out. This function can be called multiple times to encrypt successive blocks of data. The amount of data written depends on the block alignment of the encrypted data: as a result the amount of data written may be anything from zero bytes to (inl + cipher_block_size ? 1) so outl should contain sufficient room. The actual number of bytes written is placed in outl.

EVP_EncryptFinal() encrypts the "final" data, that is any data that remains in a partial block. It uses standard block padding (aka PKCS padding). The encrypted final data is written to out which should have sufficient space for one cipher block. The number of bytes written is placed in outl. After this function is called the encryption operation is finished and no further calls to EVP_EncryptUpdate() should be made.

EVP_DecryptInit(), EVP_DecryptUpdate() and EVP_DecryptFinal() are the corresponding decryption operations. EVP_DecryptFinal() will return an error code if the final block is not correctly formatted. The parameters and restrictions are identical to the encryption operations except that the decrypted data buffer out passed to EVP_DecryptUpdate() should have sufficient room for (inl + cipher_block_size) bytes unless the cipher block size is 1 in which case inl bytes is sufficient.

EVP_CipherInit(), EVP_CipherUpdate() and EVP_CipherFinal() are functions that can be used for decryption or encryption. The operation performed depends on the value of the enc parameter. It should be set to 1 for encryption and 0 for decryption.

EVP_CIPHER_CTX_cleanup() clears all information from a cipher context. It should be called after all operations using a cipher are complete so sensitive information does not remain in memory.

EVP_get_cipherbyname(), EVP_get_cipherbynid() and EVP_get_cipherbyobj() return an EVP_CIPHER structure when passed a cipher name, a NID or an ASN1_OBJECT structure.

EVP_CIPHER_nid() and EVP_CIPHER_CTX_nid() return the NID of a cipher when passed an EVP_CIPHER or EVP_CIPHER_CTX structure. The actual NID value is an internal value which may not have a corresponding OBJECT IDENTIFIER.

EVP_CIPHER_key_length() and EVP_CIPHER_CTX_key_length() return the key length of a cipher when passed an EVP_CIPHER or EVP_CIPHER_CTX structure. The constant EVP_MAX_KEY_LENGTH is the maximum key length for all ciphers.

EVP_CIPHER_iv_length() and EVP_CIPHER_CTX_iv_length() return the IV length of a cipher when passed an EVP_CIPHER or EVP_CIPHER_CTX. It will return zero if the cipher does not use an IV. The constant EVP_MAX_IV_LENGTH is the maximum IV length for all ciphers.

EVP_CIPHER_block_size() and EVP_CIPHER_CTX_block_size() return the block size of a cipher when passed an EVP_CIPHER or EVP_CIPHER_CTX structure. The constant EVP_MAX_IV_LENGTH is also the maximum block length for all ciphers.

EVP_CIPHER_type() and EVP_CIPHER_CTX_type() return the type of the passed cipher or context. This "type" is the actual NID of the cipher OBJECT IDENTIFIER as such it ignores the cipher parameters and 40 bit RC2 and 128 bit RC2 have the same NID. If the cipher does not have an object identifier or does not have ASN1 support this function will return NID_undef.

EVP_CIPHER_CTX_cipher() returns the EVP_CIPHER structure when passed an EVP_CIPHER_CTX structure.

EVP_CIPHER_param_to_asn1() sets the AlgorithmIdentifier "parameter" based on the passed cipher. This will typically include any parameters and an IV. The cipher IV (if any) must be set when this call is made. This call should be made before the cipher is actually "used" (before any EVP_EncryptUpdate(), EVP_DecryptUpdate() calls for example). This function may fail if the cipher does not have any ASN1 support.

EVP_CIPHER_asn1_to_param() sets the cipher parameters based on an ASN1 AlgorithmIdentifier "parameter". The precise effect depends on the cipher In the case of RC2, for example, it will set the IV and effective key length. This function should be called after the base cipher type is set but before the key is set. For example EVP_CipherInit() will be called with the IV and key set to NULL, EVP_CIPHER_asn1_to_param() will be called and finally EVP_CipherInit() again with all parameters except the key set to NULL. It is possible for this function to fail if the cipher does not have any ASN1 support or the parameters cannot be set (for example the RC2 effective key length does not have an EVP_CIPHER structure).

EVP_EncryptInit(), EVP_EncryptUpdate() and EVP_EncryptFinal() do not return values.

EVP_DecryptInit() and EVP_DecryptUpdate() do not return values. EVP_DecryptFinal() returns 0 if the decrypt failed or 1 for success.

EVP_CipherInit() and EVP_CipherUpdate() do not return values. EVP_CipherFinal() returns 1 for a decryption failure or 1 for success, if the operation is encryption then it always returns 1.

EVP_CIPHER_CTX_cleanup() does not return a value.

EVP_get_cipherbyname(), EVP_get_cipherbynid() and EVP_get_cipherbyobj() return an EVP_CIPHER structure or NULL on error.

EVP_CIPHER_nid() and EVP_CIPHER_CTX_nid() return a NID.

EVP_CIPHER_block_size() and EVP_CIPHER_CTX_block_size() return the block size.

EVP_CIPHER_key_length() and EVP_CIPHER_CTX_key_length() return the key length.

EVP_CIPHER_iv_length() and EVP_CIPHER_CTX_iv_length() return the IV length or zero if the cipher does not use an IV.

EVP_CIPHER_type() and EVP_CIPHER_CTX_type() return the NID of the cipher’s OBJECT IDENTIFIER or NID_undef if it has no defined OBJECT IDENTIFIER.

EVP_CIPHER_CTX_cipher() returns an EVP_CIPHER structure.

EVP_CIPHER_param_to_asn1() and EVP_CIPHER_asn1_to_param() return 1 for success or zero for failure.

Where possible the EVP interface to symmetric ciphers should be used in preference to the low level interfaces. This is because the code then becomes transparent to the cipher used and much more flexible.

PKCS padding works by adding n padding bytes of value n to make the total length of the encrypted data a multiple of the block size. Padding is always added so if the data is already a multiple of the block size n will equal the block size. For example if the block size is 8 and 11 bytes are to be encrypted then 5 padding bytes of value 5 will be added.

When decrypting the final block is checked to see if it has the correct form.

Although the decryption operation can produce an error, it is not a strong test that the input data or key is correct. A random block has better than 1 in 256 chance of being of the correct format and problems with the input data earlier on will not produce a final decrypt error.

The current EVP cipher interface is not as flexible as it should be. Only certain "spot" encryption algorithms can be used for ciphers which have various parameters associated with them (RC2, RC5 for example) this is inadequate.

Several of the functions do not return error codes because the software versions can never fail. This is not true of hardware versions.

evp(3)