Section: OpenSSL (3)
Updated: 2020-07-28
Page Index


BIO_f_cipher, BIO_set_cipher, BIO_get_cipher_status, BIO_get_cipher_ctx - cipher BIO filter  


 #include <openssl/bio.h>
 #include <openssl/evp.h>

 const BIO_METHOD *BIO_f_cipher(void);
 void BIO_set_cipher(BIO *b, const EVP_CIPHER *cipher,
                     unsigned char *key, unsigned char *iv, int enc);
 int BIO_get_cipher_status(BIO *b)
 int BIO_get_cipher_ctx(BIO *b, EVP_CIPHER_CTX **pctx)



BIO_f_cipher() returns the cipher BIO method. This is a filter BIO that encrypts any data written through it, and decrypts any data read from it. It is a BIO wrapper for the cipher routines EVP_CipherInit(), EVP_CipherUpdate() and EVP_CipherFinal().

Cipher BIOs do not support BIO_gets() or BIO_puts().

BIO_flush() on an encryption BIO that is being written through is used to signal that no more data is to be encrypted: this is used to flush and possibly pad the final block through the BIO.

BIO_set_cipher() sets the cipher of BIO b to cipher using key key and IV iv. enc should be set to 1 for encryption and zero for decryption.

When reading from an encryption BIO the final block is automatically decrypted and checked when EOF is detected. BIO_get_cipher_status() is a BIO_ctrl() macro which can be called to determine whether the decryption operation was successful.

BIO_get_cipher_ctx() is a BIO_ctrl() macro which retrieves the internal BIO cipher context. The retrieved context can be used in conjunction with the standard cipher routines to set it up. This is useful when BIO_set_cipher() is not flexible enough for the applications needs.  


When encrypting BIO_flush() must be called to flush the final block through the BIO. If it is not then the final block will fail a subsequent decrypt.

When decrypting an error on the final block is signaled by a zero return value from the read operation. A successful decrypt followed by EOF will also return zero for the final read. BIO_get_cipher_status() should be called to determine if the decrypt was successful.

As always, if BIO_gets() or BIO_puts() support is needed then it can be achieved by preceding the cipher BIO with a buffering BIO.  


BIO_f_cipher() returns the cipher BIO method.

BIO_set_cipher() does not return a value.

BIO_get_cipher_status() returns 1 for a successful decrypt and 0 for failure.

BIO_get_cipher_ctx() currently always returns 1.  


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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 <>.