EVP_PKEY_DECRYPT(3) | Library Functions Manual | EVP_PKEY_DECRYPT(3)

# NAME

`EVP_PKEY_decrypt_init`, `EVP_PKEY_decrypt` — decrypt using a public key algorithm

# SYNOPSIS

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

int
`EVP_PKEY_decrypt_init`(EVP_PKEY_CTX *ctx);

int
`EVP_PKEY_decrypt`(EVP_PKEY_CTX *ctx, unsigned char *out, size_t *outlen, const unsigned char *in, size_t inlen);

# DESCRIPTION

The `EVP_PKEY_decrypt_init`() function initializes a public key algorithm context using key ctx->pkey for a decryption operation.

The `EVP_PKEY_decrypt`() function performs a public key decryption operation using ctx. The data to be decrypted is specified using the in and inlen parameters. If out is `NULL` then the maximum size of the output buffer is written to the outlen parameter. If out is not `NULL` then before the call the outlen parameter should contain the length of the out buffer. If the call is successful, the decrypted data is written to out and the amount of data written to outlen.

After the call to `EVP_PKEY_decrypt_init`(), algorithm specific control operations can be performed to set any appropriate parameters for the operation.

The function `EVP_PKEY_decrypt`() can be called more than once on the same context if several operations are performed using the same parameters.

# RETURN VALUES

`EVP_PKEY_decrypt_init`() and `EVP_PKEY_decrypt`() 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.

# EXAMPLES

Decrypt data using OAEP (for RSA keys):

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

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

    /*
     * Assumes that key, in, and inlen are already set up
     * and that key is an RSA private key.
     */
    ctx = EVP_PKEY_CTX_new(key, NULL);
    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 = 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 ALSO

EVP_PKEY_CTX_new(3), EVP_PKEY_derive(3), EVP_PKEY_encrypt(3), EVP_PKEY_sign(3), EVP_PKEY_verify(3), EVP_PKEY_verify_recover(3)

# HISTORY

`EVP_PKEY_decrypt_init`() and `EVP_PKEY_decrypt`() first appeared in OpenSSL 1.0.0 and have been available since OpenBSD 4.9.

June 8, 2025 | Debian