BIO_F_CIPHER(3) | Library Functions Manual | BIO_F_CIPHER(3)

# NAME

`BIO_f_cipher`, `BIO_set_cipher`, `BIO_get_cipher_status`, `BIO_get_cipher_ctx` — cipher BIO filter

# SYNOPSIS

library “libcrypto”
`#include <openssl/bio.h>`
`#include <openssl/evp.h>`

const BIO_METHOD *
`BIO_f_cipher`(void);

int
`BIO_set_cipher`(BIO *b, const EVP_CIPHER *cipher, unsigned char *key, unsigned char *iv, int enc);

long
`BIO_get_cipher_status`(BIO *b);

long
`BIO_get_cipher_ctx`(BIO *b, EVP_CIPHER_CTX **pctx);

# DESCRIPTION

`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(3), EVP_CipherUpdate(3), and EVP_CipherFinal(3).

Cipher BIOs do not support BIO_gets(3) or BIO_puts(3).

BIO_flush(3) 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(3) macro which can be called to determine whether the decryption operation was successful.

`BIO_get_cipher_ctx`() is a BIO_ctrl(3) 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 a chain containing a cipher BIO is copied with BIO_dup_chain(3), the cipher context is automatically copied from the existing BIO object to the new one and the init flag that can be retrieved with BIO_get_init(3) is set to 1.

When encrypting, BIO_flush(3) 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 signalled 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(3) or BIO_puts(3) support is needed, then it can be achieved by preceding the cipher BIO with a buffering BIO.

BIO_ctrl(3) cmd arguments correspond to macros as follows:

cmd constant | corresponding macro
`BIO_C_GET_CIPHER_CTX` | `BIO_get_cipher_ctx`()
`BIO_C_GET_CIPHER_STATUS` | `BIO_get_cipher_status`()
`BIO_CTRL_FLUSH` | BIO_flush(3)
`BIO_CTRL_PENDING` | BIO_pending(3)
`BIO_CTRL_RESET` | BIO_reset(3)
`BIO_CTRL_WPENDING` | BIO_wpending(3)

# RETURN VALUES

`BIO_f_cipher`() returns the cipher BIO method.

When called on a cipher BIO object, BIO_method_type(3) returns the constant `BIO_TYPE_CIPHER` and BIO_method_name(3) returns a pointer to the static string "cipher".

`BIO_set_cipher`() returns 1 on success and 0 on error.

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

`BIO_get_cipher_ctx`() currently always returns 1.

# SEE ALSO

BIO_new(3), EVP_EncryptInit(3)

# HISTORY

`BIO_f_cipher`(), `BIO_set_cipher`(), and `BIO_get_cipher_status`() first appeared in SSLeay 0.6.5 and have been available since OpenBSD 2.4.

`BIO_get_cipher_ctx`() first appeared in SSLeay 0.9.1 and has been available since OpenBSD 2.6.

June 8, 2025 | Debian