EVP_OPENINIT(3ossl)

💌 EVP_OPENINIT(3ossl) — bestie, it's the OpenSSL envelope decryption page

okay so like, welcome to EVP_OpenInit, EVP_OpenUpdate, and EVP_OpenFinal 💖 this is the OpenSSL manual, page 3ossl, and we're talking EVP envelope decryption. yes, the other half of the envelope story. the receiving end. the "open the letter" part. 🎀

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✨ NAME ✨

EVP_OpenInit, EVP_OpenUpdate, EVP_OpenFinal — EVP envelope decryption 💅

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✨ SYNOPSIS ✨

#include <openssl/evp.h>

int EVP_OpenInit(EVP_CIPHER_CTX *ctx, EVP_CIPHER *type, unsigned char *ek,
                 int ekl, unsigned char *iv, EVP_PKEY *priv);
int EVP_OpenUpdate(EVP_CIPHER_CTX *ctx, unsigned char *out,
                   int *outl, unsigned char *in, int inl);
int EVP_OpenFinal(EVP_CIPHER_CTX *ctx, unsigned char *out, int *outl);

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🌸 DESCRIPTION 🌸

So the EVP envelope routines are a high-level interface to envelope decryption 🫶 what they do is: they decrypt a public-key-encrypted symmetric key, and then use that key to decrypt the actual data. it's giving "unlock the lockbox, then read the letter." iconic behavior honestly 💖

• EVP_OpenInit() — initializes a cipher context ctx for decryption with cipher type. it decrypts the encrypted symmetric key of length ekl bytes passed in the ek parameter, using the private key priv. the IV gets supplied in the iv parameter. 🗝️
• EVP_OpenUpdate() and EVP_OpenFinal() — these two have exactly the same properties as the EVP_DecryptUpdate() and EVP_DecryptFinal() routines, as documented on the EVP_EncryptInit(3) manual page. like, literally the same energy, just open-flavored. ✨

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🦋 NOTES 🦋

ngl there's some real ones here, pay attention bestie:

• It is possible to call EVP_OpenInit() twice, in the same way as EVP_DecryptInit(). The first call should have priv set to NULL, and (after setting any cipher parameters) it should be called again with type set to NULL. yes that's a two-step, and yes it's on purpose 💅
• If the cipher passed in the type parameter is a variable length cipher, then the key length will be set to the value of the recovered key length. If the cipher is a fixed length cipher, then the recovered key length must match the fixed cipher length. no cap, that mismatch is not gonna fly 🙈

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👑 RETURN VALUES 👑

• EVP_OpenInit() — returns 0 on error, or a non zero integer (actually the recovered secret key size) if successful. so a truthy return is your green flag 🫶
• EVP_OpenUpdate() — returns 1 for success or 0 for failure.
• EVP_OpenFinal() — returns 0 if the decrypt failed or 1 for success. (yes, final is the one that flips the meaning on you, stay woke 💀)

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🌈 SEE ALSO 🌈

evp(7), RAND_bytes(3), EVP_EncryptInit(3), EVP_SealInit(3)

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💜 COPYRIGHT 💜

Copyright 2000-2020 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (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>.

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and that's the whole page bestie 🎀 mwah 💋

OpenSSL 3.5.9 · 2026-09-29 · EVP_OPENINIT(3ossl) — slay, go decrypt something, and remember: the envelope is not gonna open itself ⭐🧚‍♀️