EVP_PKEY_DECRYPT(3ossl)

📦
version 15.1 · 2 versions
, in freebsd 15.1

💅 EVP_PKEY_DECRYPT 💅

okay so bestie, welcome to EVP_PKEY_DECRYPT(3ossl) — that's the OpenSSL manual, section 3ossl, and this page is literally all about decrypting stuff with a public key algorithm. grab a strawberry drink 🍓 and let's get into it.

✨ NAME ✨

EVP_PKEY_decrypt_init, EVP_PKEY_decrypt_init_ex, EVP_PKEY_decrypt — decrypt using a public key algorithm. that's the whole vibe. 💫

🌸 SYNOPSIS 🌸

#include <openssl/evp.h>

int EVP_PKEY_decrypt_init(EVP_PKEY_CTX *ctx);
int EVP_PKEY_decrypt_init_ex(EVP_PKEY_CTX *ctx, const OSSL_PARAM params[]);
int EVP_PKEY_decrypt(EVP_PKEY_CTX *ctx,
                     unsigned char *out, size_t *outlen,
                     const unsigned char *in, size_t inlen);

🦋 DESCRIPTION 🦋

so like, three little functions, three little jobs, all iconic in their own way 👑

  • EVP_PKEY_decrypt_init() — this one initializes a public key algorithm context using key pkey for a decryption operation. it's giving "setting the stage" energy fr fr.
  • EVP_PKEY_decrypt_init_ex() — same idea, initializes a public key algorithm context using key pkey for a decryption operation, and it sets the algorithm specific params. the _ex is literally just the glow-up version with params. slay. ✨
  • EVP_PKEY_decrypt() — the main character 💅. it performs a public key decryption operation using ctx. the data you wanna decrypt goes in via the in and inlen parameters. and here's the cute part: if out is NULL, then the minimum required size of the output buffer gets written to the *outlen parameter. it's a little size-check preview, no cap.

if out is not NULL, then before the call the *outlen parameter must contain the length of the out buffer. if the call is successful, the decrypted data is written to out and the amount of the decrypted data written to *outlen — otherwise an error is returned. 😭

🎀 NOTES 🎀

  • after you call EVP_PKEY_decrypt_init(), algorithm specific control operations can be performed to set any appropriate parameters for the operation. and those same operations can be included in the EVP_PKEY_decrypt_init_ex() call instead. convenient much? 🫶
  • EVP_PKEY_decrypt() can be called more than once on the same context if several operations are performed using the same parameters. reuse the ctx, reuse the vibe. 🔥

⭐ RETURN VALUES ⭐

EVP_PKEY_decrypt_init(), EVP_PKEY_decrypt_init_ex() and EVP_PKEY_decrypt() return 1 for success and 0 or a negative value for failure.

and listen — a return value of -2 specifically means the operation is not supported by the public key algorithm. that's the "wrong algorithm bestie" signal 💀.

💀 WARNINGS 💀

okay this is the serious part, pay attention, i love you but you need to read this 👀

in OpenSSL versions before 3.2.0, when used in PKCS#1 v1.5 padding, both the return value from EVP_PKEY_decrypt() and the outlen provided information useful in mounting a Bleichenbacher attack against the used private key. they had to be processed in a side-channel free way. like, that's not a small thing. that's a real crypto attack. 🙈

since version 3.2.0, the EVP_PKEY_decrypt() method, when used with PKCS#1 v1.5 padding as implemented in the default provider, implements the implicit rejection mechanism (see OSSL_ASYM_CIPHER_PARAM_IMPLICIT_REJECTION in provider-asym_cipher(7)).

that means it doesn't return an error when it detects an error in padding — instead it returns a pseudo-randomly generated message. which removes the need for side-channel secure code from applications using OpenSSL. genuinely iconic behavior honestly. 👑

but heads up: if OpenSSL is configured to use a provider that doesn't implement implicit rejection, the code still needs to handle the returned values using side-channel free code. no exceptions, no shortcuts, bestie. 🫶

side-channel free handling of the error stack can be performed using either:

pick your fighter. both valid. 💜

🌈 EXAMPLES 🌈

decrypt data using OAEP (for RSA keys) — here's the whole thing, copy it carefully:

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

EVP_PKEY_CTX *ctx;
ENGINE *eng;
unsigned char *out, *in;
size_t outlen, inlen;
EVP_PKEY *key;

/*
 * NB: assumes key, eng, in, inlen are already set up
 * and that key is an RSA private key
 */
ctx = EVP_PKEY_CTX_new(key, eng);
if (!ctx)
    /* Error occurred */
if (EVP_PKEY_decrypt_init(ctx) <= 0)
    /* Error */
if (EVP_PKEY_CTX_set_rsa_padding(ctx, RSA_PKCS1_OAEP_PADDING) <= 0)
    /* Error */

/* Determine buffer length */
if (EVP_PKEY_decrypt(ctx, NULL, &outlen, in, inlen) <= 0)
    /* Error */

out = OPENSSL_malloc(outlen);

if (!out)
    /* malloc failure */

if (EVP_PKEY_decrypt(ctx, out, &outlen, in, inlen) <= 0)
    /* Error */

/* Decrypted data is outlen bytes written to buffer out */

see how it calls EVP_PKEY_decrypt() twice? first with NULL to get the length, then with the real buffer. that's the pattern, periodt. ✨

🧚 SEE ALSO 🧚

EVP_PKEY_CTX_new(3), EVP_PKEY_encrypt(3), EVP_PKEY_sign(3), EVP_PKEY_verify(3), EVP_PKEY_verify_recover(3), EVP_PKEY_derive(3) — go read those too, iykyk 📚

👑 HISTORY 👑

these functions were added in OpenSSL 1.0.0. vintage. still relevant. still eating. 💅

Copyright 2006-2025 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>.

and that's the page bestie!! 🦋 this was OpenSSL version 3.5.6, dated 2026-04-07 — go decrypt something responsibly, stay side-channel safe, and drink some water 💖✨🫶

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