EVP_CIPHER_CTX_CTRL(3)

💅 EVP_CIPHER_CTX_CTRL — configure EVP cipher contexts ✨

okay so bestie, welcome to the EVP_CIPHER_CTX_CTRL(3) page from the Library Functions Manual 🎀 this is the whole little friend group of functions that poke at your EVP cipher contexts and tell them how to behave. buckle up, it's giving "glow-up for your crypto context" 💖

✨ NAME ✨

EVP_CIPHER_CTX_ctrl, EVP_CIPHER_CTX_set_padding, EVP_CIPHER_CTX_set_key_length, EVP_CIPHER_CTX_key_length, EVP_CIPHER_key_length, EVP_CIPHER_CTX_iv_length, EVP_CIPHER_iv_length, EVP_CIPHER_CTX_set_iv, EVP_CIPHER_CTX_get_iv — configure EVP cipher contexts 🦋

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🎀 SYNOPSIS 🎀

#include <openssl/evp.h>

int
EVP_CIPHER_CTX_ctrl(EVP_CIPHER_CTX *ctx, int type, int arg, void *ptr);

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_key_length(const EVP_CIPHER_CTX *ctx);

int
EVP_CIPHER_key_length(const EVP_CIPHER *e);

int
EVP_CIPHER_CTX_iv_length(const EVP_CIPHER_CTX *ctx);

int
EVP_CIPHER_iv_length(const EVP_CIPHER *e);

int
EVP_CIPHER_CTX_set_iv(EVP_CIPHER_CTX *ctx, const unsigned char *iv,
    size_t len);

int
EVP_CIPHER_CTX_get_iv(const EVP_CIPHER_CTX *ctx, unsigned char *iv,
    size_t len);

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🌸 DESCRIPTION 🌸

EVP_CIPHER_CTX_ctrl() — this is the little switchboard where you get and set all the cipher-specific params 💅 and rn? literally only the RC2 effective key length can be set. that's it, that's the whole menu. peep EVP_rc2_cbc(3) for the deets 🍓

EVP_CIPHER_CTX_set_padding() — turns padding on or off, no cap ✨ but heads up: you gotta call this after the context is all set up for encryption or decryption with EVP_EncryptInit_ex(3), EVP_DecryptInit_ex(3), or EVP_CipherInit_ex(3). by default, encryption ops are padded using standard block padding, and the padding gets checked and removed when decrypting 🧚 if the padding parameter is zero, then no padding happens at all — and then the total amount of data you encrypt or decrypt must be a multiple of the block size or you get an error 😭 like fr fr, do not test it.

EVP_CIPHER_CTX_set_key_length() — sets the key length of the cipher ctx. but if the cipher is a fixed length cipher? then trying to set the key length to literally anything other than the fixed value is an error 💀 no wiggle room, bestie.

EVP_CIPHER_CTX_key_length() and EVP_CIPHER_key_length() — these return the key length of a cipher when you pass them an EVP_CIPHER_CTX or EVP_CIPHER structure respectively 👑 the constant EVP_MAX_KEY_LENGTH is the maximum key length for all ciphers. and okay, important note: even though 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 🦋 so like, don't assume they match, iykyk.

EVP_CIPHER_CTX_iv_length() and EVP_CIPHER_iv_length() — these return the IV length of a cipher when passed an EVP_CIPHER_CTX or EVP_CIPHER. and if the cipher doesn't use an IV? they return zero 🫶 also EVP_CIPHER_CTX_iv_length() can fail and return -1 🙈 the constant EVP_MAX_IV_LENGTH is the maximum IV length for all ciphers ⭐

EVP_CIPHER_CTX_set_iv() and EVP_CIPHER_CTX_get_iv() — these set and retrieve the IV for an EVP_CIPHER_CTX, respectively 💜 in both cases, the specified IV length must exactly equal the expected IV length for the context, as returned by EVP_CIPHER_CTX_iv_length() — exact match or nothing, periodt 🔥

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💀 RETURN VALUES 💀

• EVP_CIPHER_CTX_ctrl() — returns 1 for success or 0 for failure. some implementations may return negative values for some errors 🙈
• EVP_CIPHER_CTX_set_padding() — always returns 1 ✨ iconic, no notes.
• EVP_CIPHER_CTX_set_key_length(), EVP_CIPHER_CTX_set_iv(), and EVP_CIPHER_CTX_get_iv() — return 1 for success or 0 for failure.
• EVP_CIPHER_CTX_key_length() and EVP_CIPHER_key_length() — return the key length.
• EVP_CIPHER_CTX_iv_length() and EVP_CIPHER_iv_length() — return the IV length, or zero if the cipher does not use an IV. and again, EVP_CIPHER_CTX_iv_length() can fail and return -1 😭

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🔥 SEE ALSO 🔥

• evp(3)
• EVP_CIPHER_nid(3)
• EVP_EncryptInit(3)

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🌈 HISTORY 🌈

• EVP_CIPHER_CTX_key_length(), EVP_CIPHER_key_length(), EVP_CIPHER_CTX_iv_length(), and EVP_CIPHER_iv_length() first appeared in SSLeay 0.6.5 and have been available since OpenBSD 2.4 💖
• EVP_CIPHER_CTX_ctrl() and EVP_CIPHER_CTX_set_key_length() first appeared in OpenSSL 0.9.6 and have been available since OpenBSD 2.9 🦋
• EVP_CIPHER_CTX_set_padding() first appeared in OpenSSL 0.9.7 and has been available since OpenBSD 3.2 🌸
• EVP_CIPHER_CTX_set_iv() and EVP_CIPHER_CTX_get_iv() first appeared in LibreSSL 2.8.1 and have been available since OpenBSD 6.4 ⭐

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🫶 BUGS 🫶

okay so this is the messy part, bestie 😭 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 💀 and the reason? because it has become standard practice to define a generic key as a fixed unsigned char array containing EVP_MAX_KEY_LENGTH bytes. so yeah. that's the vibe. be careful out there 🙈

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and that's the page, angel 💅 this one's from the Debian build, dated March 25, 2025 — go forth and configure your cipher contexts responsibly, mwah 🎀💖✨