EVP_ENCRYPTINIT(3SSL)

NAME
EVP_CIPHER_CTX_new, EVP_CIPHER_CTX_reset, EVP_CIPHER_CTX_free, EVP_EncryptInit_ex, EVP_EncryptUpdate, EVP_EncryptFinal_ex, EVP_DecryptInit_ex, EVP_DecryptUpdate, EVP_DecryptFinal_ex, EVP_CipherInit_ex, EVP_CipherUpdate, EVP_CipherFinal_ex, EVP_CIPHER_CTX_set_key_length, EVP_CIPHER_CTX_ctrl, EVP_EncryptInit, EVP_EncryptFinal, EVP_DecryptInit, EVP_DecryptFinal, EVP_CipherInit, EVP_CipherFinal, EVP_get_cipherbyname, EVP_get_cipherbynid, EVP_get_cipherbyobj, EVP_CIPHER_nid, EVP_CIPHER_block_size, EVP_CIPHER_key_length, EVP_CIPHER_iv_length, EVP_CIPHER_flags, EVP_CIPHER_mode, EVP_CIPHER_type, EVP_CIPHER_CTX_cipher, EVP_CIPHER_CTX_nid, EVP_CIPHER_CTX_block_size, EVP_CIPHER_CTX_key_length, EVP_CIPHER_CTX_iv_length, EVP_CIPHER_CTX_get_app_data, EVP_CIPHER_CTX_set_app_data, EVP_CIPHER_CTX_type, EVP_CIPHER_CTX_flags, EVP_CIPHER_CTX_mode, EVP_CIPHER_param_to_asn1, EVP_CIPHER_asn1_to_param, EVP_CIPHER_CTX_set_padding, EVP_enc_null, EVP_des_cbc, EVP_des_ecb, EVP_des_cfb, EVP_des_ofb, EVP_des_ede_cbc, EVP_des_ede, EVP_des_ede_ofb, EVP_des_ede_cfb, EVP_des_ede3_cbc, EVP_des_ede3, EVP_des_ede3_ofb, EVP_des_ede3_cfb, EVP_desx_cbc, EVP_rc4, EVP_rc4_40, EVP_rc4_hmac_md5, EVP_idea_cbc, EVP_idea_ecb, EVP_idea_cfb, EVP_idea_ofb, EVP_rc2_cbc, EVP_rc2_ecb, EVP_rc2_cfb, EVP_rc2_ofb, EVP_rc2_40_cbc, EVP_rc2_64_cbc, EVP_bf_cbc, EVP_bf_ecb, EVP_bf_cfb, EVP_bf_ofb, EVP_cast5_cbc, EVP_cast5_ecb, EVP_cast5_cfb, EVP_cast5_ofb, EVP_rc5_32_12_16_cbc, EVP_rc5_32_12_16_ecb, EVP_rc5_32_12_16_cfb, EVP_rc5_32_12_16_ofb, EVP_aes_128_cbc, EVP_aes_128_ecb, EVP_aes_128_cfb, EVP_aes_128_ofb, EVP_aes_192_cbc, EVP_aes_192_ecb, EVP_aes_192_cfb, EVP_aes_192_ofb, EVP_aes_256_cbc, EVP_aes_256_ecb, EVP_aes_256_cfb, EVP_aes_256_ofb, EVP_aes_128_gcm, EVP_aes_192_gcm, EVP_aes_256_gcm, EVP_aes_128_ccm, EVP_aes_192_ccm, EVP_aes_256_ccm, EVP_aes_128_cbc_hmac_sha1, EVP_aes_256_cbc_hmac_sha1, EVP_aes_128_cbc_hmac_sha256, EVP_aes_256_cbc_hmac_sha256, EVP_chacha20, EVP_chacha20_poly1305 - EVP cipher routines

SYNOPSIS
 #include <openssl/evp.h>

 EVP_CIPHER_CTX *EVP_CIPHER_CTX_new(void);
 int EVP_CIPHER_CTX_reset(EVP_CIPHER_CTX *ctx);
 void EVP_CIPHER_CTX_free(EVP_CIPHER_CTX *ctx);

 int EVP_EncryptInit_ex(EVP_CIPHER_CTX *ctx, const EVP_CIPHER *type,
         ENGINE *impl, unsigned char *key, unsigned char *iv);
 int EVP_EncryptUpdate(EVP_CIPHER_CTX *ctx, unsigned char *out,
         int *outl, unsigned char *in, int inl);
 int EVP_EncryptFinal_ex(EVP_CIPHER_CTX *ctx, unsigned char *out,
         int *outl);

 int EVP_DecryptInit_ex(EVP_CIPHER_CTX *ctx, const EVP_CIPHER *type,
         ENGINE *impl, unsigned char *key, unsigned char *iv);
 int EVP_DecryptUpdate(EVP_CIPHER_CTX *ctx, unsigned char *out,
         int *outl, unsigned char *in, int inl);
 int EVP_DecryptFinal_ex(EVP_CIPHER_CTX *ctx, unsigned char *outm,
         int *outl);

 int EVP_CipherInit_ex(EVP_CIPHER_CTX *ctx, const EVP_CIPHER *type,
         ENGINE *impl, unsigned char *key, unsigned char *iv, int enc);
 int EVP_CipherUpdate(EVP_CIPHER_CTX *ctx, unsigned char *out,
         int *outl, unsigned char *in, int inl);
 int EVP_CipherFinal_ex(EVP_CIPHER_CTX *ctx, unsigned char *outm,
         int *outl);

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

 int EVP_DecryptInit(EVP_CIPHER_CTX *ctx, const EVP_CIPHER *type,
         unsigned char *key, unsigned char *iv);
 int EVP_DecryptFinal(EVP_CIPHER_CTX *ctx, unsigned char *outm,
         int *outl);

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

 int EVP_CIPHER_CTX_set_padding(EVP_CIPHER_CTX *x, int padding);
 int EVP_CIPHER_CTX_set_key_length(EVP_CIPHER_CTX *x, int keylen);
 int EVP_CIPHER_CTX_ctrl(EVP_CIPHER_CTX *ctx, int type, int arg, void *ptr);

 const EVP_CIPHER *EVP_get_cipherbyname(const char *name);
 const EVP_CIPHER *EVP_get_cipherbynid(int nid);
 const EVP_CIPHER *EVP_get_cipherbyobj(const ASN1_OBJECT *a);

 int EVP_CIPHER_nid(const EVP_CIPHER *e);
 int EVP_CIPHER_block_size(const EVP_CIPHER *e);
 int EVP_CIPHER_key_length(const EVP_CIPHER *e)
 int EVP_CIPHER_key_length(const EVP_CIPHER *e);
 int EVP_CIPHER_iv_length(const EVP_CIPHER *e);
 unsigned long EVP_CIPHER_flags(const EVP_CIPHER *e);
 unsigned long EVP_CIPHER_mode(const EVP_CIPHER *e);
 int EVP_CIPHER_type(const EVP_CIPHER *ctx);

 const EVP_CIPHER *EVP_CIPHER_CTX_cipher(const EVP_CIPHER_CTX *ctx);
 int EVP_CIPHER_CTX_nid(const EVP_CIPHER_CTX *ctx);
 int EVP_CIPHER_CTX_block_size(const EVP_CIPHER_CTX *ctx);
 int EVP_CIPHER_CTX_key_length(const EVP_CIPHER_CTX *ctx);
 int EVP_CIPHER_CTX_iv_length(const EVP_CIPHER_CTX *ctx);
 void *EVP_CIPHER_CTX_get_app_data(const EVP_CIPHER_CTX *ctx);
 void EVP_CIPHER_CTX_set_app_data(const EVP_CIPHER_CTX *ctx, void *data);
 int EVP_CIPHER_CTX_type(const EVP_CIPHER_CTX *ctx);
 int EVP_CIPHER_CTX_mode(const EVP_CIPHER_CTX *ctx);

 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);

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

EVP_CIPHER_CTX_new() creates a cipher context.

EVP_CIPHER_CTX_free() clears all information from a cipher context and free up any allocated memory associate with it, including ctx itself. This function should be called after all operations using a cipher are complete so sensitive information does not remain in memory.

EVP_EncryptInit_ex() sets up cipher context ctx for encryption with cipher type from ENGINE impl. ctx must be created before calling this function. type is normally supplied by a function such as EVP_aes_256_cbc(). If impl is NULL then the default implementation is used. 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, all of which have type set to NULL. This is done when the default cipher parameters are not appropriate.

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 out should contain sufficient room. The actual number of bytes written is placed in outl. It also checks if in and out are partially overlapping, and if they are 0 is returned to indicate failure.

If padding is enabled (the default) then EVP_EncryptFinal_ex() encrypts the "final" data, that is any data that remains in a partial block. It uses standard block padding (aka PKCS padding) as described in the NOTES section, below. 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.

If padding is disabled then EVP_EncryptFinal_ex() will not encrypt any more data and it will return an error if any data remains in a partial block: that is if the total data length is not a multiple of the block size.

EVP_DecryptInit_ex(), EVP_DecryptUpdate() and EVP_DecryptFinal_ex() are the corresponding decryption operations. EVP_DecryptFinal() will return an error code if padding is enabled and the final block is not correctly formatted. The parameters and restrictions are identical to the encryption operations except that if padding is enabled 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_ex(), EVP_CipherUpdate() and EVP_CipherFinal_ex() 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, 0 for decryption and -1 to leave the value unchanged (the actual value of 'enc' being supplied in a previous call).

EVP_CIPHER_CTX_reset() clears all information from a cipher context and free up any allocated memory associate with it, except the ctx itself. This function should be called anytime ctx is to be reused for another EVP_CipherInit() / EVP_CipherUpdate() / EVP_CipherFinal() series of calls.

EVP_EncryptInit(), EVP_DecryptInit() and EVP_CipherInit() behave in a similar way to EVP_EncryptInit_ex(), EVP_DecryptInit_ex() and EVP_CipherInit_ex() except the ctx parameter does not need to be initialized and they always use the default cipher implementation.

EVP_EncryptFinal(), EVP_DecryptFinal() and EVP_CipherFinal() are identical to EVP_EncryptFinal_ex(), EVP_DecryptFinal_ex() and EVP_CipherFinal_ex(). In previous releases they also cleaned up the ctx, but this is no longer done and EVP_CIPHER_CTX_clean() must be called to free any context resources.

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_CTX_set_padding() enables or disables padding. This function should be called after the context is set up for encryption or decryption with EVP_EncryptInit_ex(), EVP_DecryptInit_ex() or EVP_CipherInit_ex(). By default encryption operations are padded using standard block padding and the padding is checked and removed when decrypting. If the pad parameter is zero then no padding is performed, the total amount of data encrypted or decrypted must then be a multiple of the block size or an error will occur.

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. Note: although EVP_CIPHER_key_length() is fixed for a given cipher, the value of EVP_CIPHER_CTX_key_length() may be different for variable key length ciphers.

EVP_CIPHER_CTX_set_key_length() sets the key length of the cipher ctx. If the cipher is a fixed length cipher then attempting to set the key length to any value other than the fixed value is an error.

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_BLOCK_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_mode() and EVP_CIPHER_CTX_mode() return the block cipher mode: EVP_CIPH_ECB_MODE, EVP_CIPH_CBC_MODE, EVP_CIPH_CFB_MODE or EVP_CIPH_OFB_MODE. If the cipher is a stream cipher then EVP_CIPH_STREAM_CIPHER is returned.

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 is not supported.

EVP_CIPHER_CTX_ctrl() allows various cipher specific parameters to be determined and set.

RETURN VALUES
EVP_CIPHER_CTX_new() returns a pointer to a newly created EVP_CIPHER_CTX for success and NULL for failure.

EVP_EncryptInit_ex(), EVP_EncryptUpdate() and EVP_EncryptFinal_ex() return 1 for success and 0 for failure.

EVP_DecryptInit_ex() and EVP_DecryptUpdate() return 1 for success and 0 for failure. EVP_DecryptFinal_ex() returns 0 if the decrypt failed or 1 for success.

EVP_CipherInit_ex() and EVP_CipherUpdate() return 1 for success and 0 for failure. EVP_CipherFinal_ex() returns 0 for a decryption failure or 1 for success.

EVP_CIPHER_CTX_reset() returns 1 for success and 0 for failure.

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_CTX_set_padding() always returns 1.

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 greater than zero for success and zero or a negative number.

CIPHER LISTING
All algorithms have a fixed key length unless otherwise stated.

EVP_enc_null()
    Null cipher: does nothing.

EVP_aes_128_cbc(), EVP_aes_128_ecb(), EVP_aes_128_cfb(), EVP_aes_128_ofb()
    AES with a 128-bit key in CBC, ECB, CFB and OFB modes respectively.

EVP_aes_192_cbc(), EVP_aes_192_ecb(), EVP_aes_192_cfb(), EVP_aes_192_ofb()
    AES with a 192-bit key in CBC, ECB, CFB and OFB modes respectively.

EVP_aes_256_cbc(), EVP_aes_256_ecb(), EVP_aes_256_cfb(), EVP_aes_256_ofb()
    AES with a 256-bit key in CBC, ECB, CFB and OFB modes respectively.

EVP_des_cbc(), EVP_des_ecb(), EVP_des_cfb(), EVP_des_ofb()
    DES in CBC, ECB, CFB and OFB modes respectively.

EVP_des_ede_cbc(), EVP_des_ede(), EVP_des_ede_ofb(), EVP_des_ede_cfb()
    Two key triple DES in CBC, ECB, CFB and OFB modes respectively.

EVP_des_ede3_cbc(), EVP_des_ede3(), EVP_des_ede3_ofb(), EVP_des_ede3_cfb()
    Three key triple DES in CBC, ECB, CFB and OFB modes respectively.

EVP_desx_cbc()
    DESX algorithm in CBC mode.

EVP_rc4()
    RC4 stream cipher. This is a variable key length cipher with default key length 128 bits.

EVP_rc4_40()
    RC4 stream cipher with 40 bit key length. This is obsolete and new code should use EVP_rc4() and the EVP_CIPHER_CTX_set_key_length() function.

EVP_idea_cbc() EVP_idea_ecb(), EVP_idea_cfb(), EVP_idea_ofb()
    IDEA encryption algorithm in CBC, ECB, CFB and OFB modes respectively.

EVP_rc2_cbc(), EVP_rc2_ecb(), EVP_rc2_cfb(), EVP_rc2_ofb()
    RC2 encryption algorithm in CBC, ECB, CFB and OFB modes respectively. This is a variable key length cipher with an additional parameter called "effective key bits" or "effective key length". By default both are set to 128 bits.

EVP_rc2_40_cbc(), EVP_rc2_64_cbc()
    RC2 algorithm in CBC mode with a default key length and effective key length of 40 and 64 bits. These are obsolete and new code should use EVP_rc2_cbc(), EVP_CIPHER_CTX_set_key_length() and EVP_CIPHER_CTX_ctrl() to set the key length and effective key length.

EVP_bf_cbc(), EVP_bf_ecb(), EVP_bf_cfb(), EVP_bf_ofb()
    Blowfish encryption algorithm in CBC, ECB, CFB and OFB modes respectively. This is a variable key length cipher.

EVP_cast5_cbc(), EVP_cast5_ecb(), EVP_cast5_cfb(), EVP_cast5_ofb()
    CAST encryption algorithm in CBC, ECB, CFB and OFB modes respectively. This is a variable key length cipher.

EVP_rc5_32_12_16_cbc(), EVP_rc5_32_12_16_ecb(), EVP_rc5_32_12_16_cfb(), EVP_rc5_32_12_16_ofb()
    RC5 encryption algorithm in CBC, ECB, CFB and OFB modes respectively. This is a variable key length cipher with an additional "number of rounds" parameter. By default the key length is set to 128 bits and 12 rounds.

EVP_aes_128_gcm(), EVP_aes_192_gcm(), EVP_aes_256_gcm()
    AES Galois Counter Mode (GCM) for 128, 192 and 256 bit keys respectively. These ciphers require additional control operations to function correctly: see the "GCM and OCB Modes" section below for details.

EVP_aes_128_ocb(void), EVP_aes_192_ocb(void), EVP_aes_256_ocb(void)
    Offset Codebook Mode (OCB) for 128, 192 and 256 bit keys respectively. These ciphers require additional control operations to function correctly: see the "GCM and OCB Modes" section below for details.

EVP_aes_128_ccm(), EVP_aes_192_ccm(), EVP_aes_256_ccm()
    AES Counter with CBC-MAC Mode (CCM) for 128, 192 and 256 bit keys respectively. These ciphers require additional control operations to function correctly: see CCM mode section below for details.

EVP_chacha20()
    The ChaCha20 stream cipher. The key length is 256 bits, the IV is 96 bits long.

EVP_chacha20_poly1305()
    Authenticated encryption with ChaCha20-Poly1305. Like EVP_chacha20() the key is 256 bits and the IV is 96 bits. This supports additional authenticated data (AAD) and produces a 128 bit authentication tag. See the "GCM and OCB Modes" section for more information.

GCM and OCB Modes
For GCM and OCB mode ciphers the behaviour of the EVP interface is subtly altered and several additional ctrl operations are supported.

To specify any additional authenticated data (AAD) a call to EVP_CipherUpdate(), EVP_EncryptUpdate() or EVP_DecryptUpdate() should be made with the output parameter out set to NULL.

When decrypting the return value of EVP_DecryptFinal() or EVP_CipherFinal() indicates if the operation was successful. If it does not indicate success the authentication operation has failed and any output data MUST NOT be used as it is corrupted.

The following ctrls are supported in both GCM and OCB modes:

 EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_AEAD_SET_IVLEN, ivlen, NULL);

Sets the IV length: this call can only be made before specifying an IV. If not called a default IV length is used. For GCM AES and OCB AES the default is 12 (i.e. 96 bits). For OCB mode the maximum is 15.

 EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_AEAD_GET_TAG, taglen, tag);

Writes taglen bytes of the tag value to the buffer indicated by tag. This call can only be made when encrypting data and after all data has been processed (e.g. after an EVP_EncryptFinal() call). For OCB mode the taglen must either be 16 or the value previously set via EVP_CTRL_OCB_SET_TAGLEN.

 EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_AEAD_SET_TAG, taglen, tag);

Sets the expected tag to taglen bytes from tag. This call is only legal when decrypting data. For OCB mode the taglen must either be 16 or the value previously set via EVP_CTRL_AEAD_SET_TAG.

In OCB mode calling this with tag set to NULL sets the tag length. The tag length can only be set before specifying an IV. If not called a default tag length is used. For OCB AES the default is 16 (i.e. 128 bits). This is also the maximum tag length for OCB.

CCM Mode
The behaviour of CCM mode ciphers is similar to GCM mode but with a few additional requirements and different ctrl values.

Like GCM and OCB modes any additional authenticated data (AAD) is passed by calling EVP_CipherUpdate(), EVP_EncryptUpdate() or EVP_DecryptUpdate() with the output parameter out set to NULL. Additionally the total plaintext or ciphertext length MUST be passed to EVP_CipherUpdate(), EVP_EncryptUpdate() or EVP_DecryptUpdate() with the output and input parameters (in and out) set to NULL and the length passed in the inl parameter.

The following ctrls are supported in CCM mode:

 EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_AEAD_SET_TAG, taglen, tag);

This call is made to set the expected CCM tag value when decrypting or the length of the tag (with the tag parameter set to NULL) when encrypting. The tag length is often referred to as M. If not set a default value is used (12 for AES).

 EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_CCM_SET_L, ivlen, NULL);

Sets the CCM L value. If not set a default is used (8 for AES).

 EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_AEAD_SET_IVLEN, ivlen, NULL);

Sets the CCM nonce (IV) length: this call can only be made before specifying an nonce value. The nonce length is given by 15 - L so it is 7 by default for AES.

NOTES
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. Additionally, the EVP interface will ensure the use of platform specific cryptographic acceleration such as AES-NI (the low level interfaces do not provide the guarantee).

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 if padding is enabled, 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.

If padding is disabled then the decryption operation will always succeed if the total amount of data decrypted is a multiple of the block size.

The functions EVP_EncryptInit(), EVP_EncryptFinal(), EVP_DecryptInit(), EVP_CipherInit() and EVP_CipherFinal() are obsolete but are retained for compatibility with existing code. New code should use EVP_EncryptInit_ex(), EVP_EncryptFinal_ex(), EVP_DecryptInit_ex(), EVP_DecryptFinal_ex(), EVP_CipherInit_ex() and EVP_CipherFinal_ex() because they can reuse an existing context without allocating and freeing it up on each call.

EVP_get_cipherbynid(), and EVP_get_cipherbyobj() are implemented as macros.

BUGS
For RC5 the number of rounds can currently only be set to 8, 12 or 16. This is a limitation of the current RC5 code rather than the EVP interface.

EVP_MAX_KEY_LENGTH and EVP_MAX_IV_LENGTH only refer to the internal ciphers with default key lengths. If custom ciphers exceed these values the results are unpredictable. This is because it has become standard practice to define a generic key as a fixed unsigned char array containing EVP_MAX_KEY_LENGTH bytes.

The ASN1 code is incomplete (and sometimes inaccurate) it has only been tested for certain common S/MIME ciphers (RC2, DES, triple DES) in CBC mode.

EXAMPLES
Encrypt a string using IDEA:

 int do_crypt(char *outfile)
        {
        unsigned char outbuf[1024];
        int outlen, tmplen;
        /* Bogus key and IV: we'd normally set these from
         * another source.
         */
        unsigned char key[] = {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15};
        unsigned char iv[] = {1,2,3,4,5,6,7,8};
        char intext[] = "Some Crypto Text";
        EVP_CIPHER_CTX *ctx;
        FILE *out;

        ctx = EVP_CIPHER_CTX_new();
        EVP_EncryptInit_ex(ctx, EVP_idea_cbc(), NULL, key, iv);

        if(!EVP_EncryptUpdate(ctx, outbuf, &outlen, intext, strlen(intext)))
                {
                /* Error */
                return 0;
                }
        /* Buffer passed to EVP_EncryptFinal() must be after data just
         * encrypted to avoid overwriting it.
         */
        if(!EVP_EncryptFinal_ex(ctx, outbuf + outlen, &tmplen))
                {
                /* Error */
                return 0;
                }
        outlen += tmplen;
        EVP_CIPHER_CTX_free(ctx);
        /* Need binary mode for fopen because encrypted data is
         * binary data. Also cannot use strlen() on it because
         * it won't be null terminated and may contain embedded
         * nulls.
         */
        out = fopen(outfile, "wb");
        fwrite(outbuf, 1, outlen, out);
        fclose(out);
        return 1;
        }

The ciphertext from the above example can be decrypted using the openssl utility with the command line (shown on two lines for clarity):

 openssl idea -d <filename
          -K 000102030405060708090A0B0C0D0E0F -iv 0102030405060708

General encryption and decryption function example using FILE I/O and AES128 with a 128-bit key:

 int do_crypt(FILE *in, FILE *out, int do_encrypt)
        {
        /* Allow enough space in output buffer for additional block */
        unsigned char inbuf[1024], outbuf[1024 + EVP_MAX_BLOCK_LENGTH];
        int inlen, outlen;
        EVP_CIPHER_CTX *ctx;
        /* Bogus key and IV: we'd normally set these from
         * another source.
         */
        unsigned char key[] = "0123456789abcdeF";
        unsigned char iv[] = "1234567887654321";

        /* Don't set key or IV right away; we want to check lengths */
        ctx = EVP_CIPHER_CTX_new();
        EVP_CipherInit_ex(&ctx, EVP_aes_128_cbc(), NULL, NULL, NULL,
                do_encrypt);
        OPENSSL_assert(EVP_CIPHER_CTX_key_length(ctx) == 16);
        OPENSSL_assert(EVP_CIPHER_CTX_iv_length(ctx) == 16);

        /* Now we can set key and IV */
        EVP_CipherInit_ex(ctx, NULL, NULL, key, iv, do_encrypt);

        for(;;)
                {
                inlen = fread(inbuf, 1, 1024, in);
                if (inlen <= 0) break;
                if(!EVP_CipherUpdate(ctx, outbuf, &outlen, inbuf, inlen))
                        {
                        /* Error */
                        EVP_CIPHER_CTX_free(ctx);
                        return 0;
                        }
                fwrite(outbuf, 1, outlen, out);
                }
        if(!EVP_CipherFinal_ex(ctx, outbuf, &outlen))
                {
                /* Error */
                EVP_CIPHER_CTX_free(ctx);
                return 0;
                }
        fwrite(outbuf, 1, outlen, out);

        EVP_CIPHER_CTX_free(ctx);
        return 1;
        }

SEE ALSO
evp(7)

HISTORY
Support for OCB mode was added in OpenSSL 1.1.0

EVP_CIPHER_CTX was made opaque in OpenSSL 1.1.0. As a result, EVP_CIPHER_CTX_reset() appeared and EVP_CIPHER_CTX_cleanup() disappeared. EVP_CIPHER_CTX_init() remains as an alias for EVP_CIPHER_CTX_reset().

COPYRIGHT
Copyright 2000-2016 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the OpenSSL license (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at <https://www.openssl.org/source/license.html>.