EVP_PKEY_VERIFY_RECOVER(3)

📦 version 4.4.0, in alpine 3.24, arch rolling

✨ NAME ✨

EVP_PKEY_verify_recover_init, EVP_PKEY_verify_recover — recover signature using a public key algorithm

Okay bestie, welcome to EVP_PKEY_VERIFY_RECOVER(3) from the Library Functions Manual 💅 This page is all about that “recover the data that was originally signed” life — not just checking a signature, but pulling the signed data back out when the algorithm supports it. Iconic, honestly.

📚 SYNOPSIS 📚

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

int
EVP_PKEY_verify_recover_init(EVP_PKEY_CTX *ctx);

int
EVP_PKEY_verify_recover(EVP_PKEY_CTX *ctx, unsigned char *rout,
    size_t *routlen, const unsigned char *sig, size_t siglen);

💅 DESCRIPTION 💅

So EVP_PKEY_verify_recover_init() sets up a public key algorithm context using the key ctx->pkey for a verify recover operation ✨ It’s the little “get ready” step before the actual recovery moment.

Then EVP_PKEY_verify_recover() recovers signed data using ctx. You pass the signature in through the sig and siglen parameters. If rout is NULL, then the maximum size of the output buffer gets written to the routlen parameter — this is the classic “tell me how much space I need first” move 🫶 If rout is not NULL, then before the call the routlen parameter should contain the length of the rout buffer. If the call is successful, recovered data is written to rout, and the amount of data written goes into routlen. Slay.

Normally an application is only interested in whether a signature verification operation is successful. In those cases, the EVP_PKEY_verify(3) function should be used. Like, if you just need a yes/no vibe check, that’s your function.

Sometimes however it is useful to obtain the data originally signed using a signing operation. Only certain public key algorithms can recover a signature in this way — for example RSA in PKCS padding mode. Not every algorithm has this main character energy, okay?

After the call to EVP_PKEY_verify_recover_init(), algorithm specific control operations can be performed to set any appropriate parameters for the operation. Customize it, bestie 💖

The function EVP_PKEY_verify_recover() can be called more than once on the same context if several operations are performed using the same parameters. Reuse the context, it’s fine.

🛟 RETURN VALUES 🛟

EVP_PKEY_verify_recover_init() and EVP_PKEY_verify_recover() return 1 for success and 0 or a negative value for failure. In particular, a return value of -2 indicates the operation is not supported by the public key algorithm. So if you see -2, it’s not you, it’s the algorithm not supporting this whole verify-recover situation 😭

🌸 EXAMPLES 🌸

Recover digest originally signed using PKCS#1 and SHA-256 digest:

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

EVP_PKEY_CTX *ctx;
unsigned char *rout, *sig;
size_t routlen, siglen;
EVP_PKEY *verify_key;

/*
 * Assumes that verify_key, sig, and siglen are already set up
 * and that verify_key is an RSA public key.
 */
ctx = EVP_PKEY_CTX_new(verify_key, NULL);
if (!ctx)
        /* Error occurred */
if (EVP_PKEY_verify_recover_init(ctx) <= 0)
        /* Error */
if (EVP_PKEY_CTX_set_rsa_padding(ctx, RSA_PKCS1_PADDING) <= 0)
        /* Error */
if (EVP_PKEY_CTX_set_signature_md(ctx, EVP_sha256()) <= 0)
        /* Error */

/* Determine buffer length */
if (EVP_PKEY_verify_recover(ctx, NULL, &routlen, sig, siglen) <= 0)
        /* Error */

rout = malloc(routlen);

if (!rout)
        /* malloc failure */

if (EVP_PKEY_verify_recover(ctx, rout, &routlen, sig, siglen) <= 0)
        /* Error */

/* Recovered data is routlen bytes written to buffer rout */

👀 SEE ALSO 👀

🕰️ HISTORY 🕰️

EVP_PKEY_verify_recover_init() and EVP_PKEY_verify_recover() first appeared in OpenSSL 1.0.0 and have been available since OpenBSD 4.9. Vintage but still valid, periodt 💜

Cute little sign-off: the footer source/version is Debian, dated June 8, 2025 🎀 Go forth and recover those signatures, bestie — responsibly, and with excellent vibes 💖✨

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