BIO_F_CIPHER(3)

💖 BIO_F_CIPHER(3) 💖

okay bestie, welcome to the Library Functions Manual entry for BIO_F_CIPHER(3) — it's giving cipher BIO filter, and we're gonna get through it together. 🫶

💅 NAME 💅

BIO_f_cipher, BIO_set_cipher, BIO_get_cipher_status, and BIO_get_cipher_ctx — the cipher BIO filter. periodt. ✨

✨ 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). not me finding out the hard way 😭

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 application's 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. 🚨

bestie this is the part where you actually call BIO_flush(3) — no cap, do not skip it.

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:

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.

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xoxo, Debian, June 8, 2025 💖