EVP_SEALINIT(3) | Library Functions Manual | EVP_SEALINIT(3)

# NAME

`EVP_SealInit`, `EVP_SealUpdate`, `EVP_SealFinal` — EVP envelope encryption

# SYNOPSIS

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

int
`EVP_SealInit`(EVP_CIPHER_CTX *ctx, const EVP_CIPHER *type, unsigned char **ek, int *ekl, unsigned char *iv, EVP_PKEY **pubk, int npubk);

int
`EVP_SealUpdate`(EVP_CIPHER_CTX *ctx, unsigned char *out, int *outl, unsigned char *in, int inl);

int
`EVP_SealFinal`(EVP_CIPHER_CTX *ctx, unsigned char *out, int *outl);

# DESCRIPTION

The EVP envelope routines are a high level interface to envelope encryption. They generate a random key and IV (if required) then "envelope" it by using public key encryption. Data can then be encrypted using this key.

`EVP_SealInit`() initializes a cipher context ctx for encryption with cipher type using a random secret key and IV. type is normally supplied by a function such as EVP_aes_256_cbc(3); see EVP_EncryptInit(3) for details. The secret key is encrypted using one or more public keys. This allows the same encrypted data to be decrypted using any of the corresponding private keys. ek is an array of buffers where the public key encrypted secret key will be written. Each buffer must contain enough room for the corresponding encrypted key: that is ek[i] must have room for `EVP_PKEY_size`(pubk[i]) bytes. The actual size of each encrypted secret key is written to the array ekl. pubk is an array of npubk public keys.

The iv parameter is a buffer where the generated IV is written to. It must contain enough room for the corresponding cipher's IV, as determined by (for example) `EVP_CIPHER_iv_length`(type).

If the cipher does not require an IV then the iv parameter is ignored and can be `NULL`.

`EVP_SealUpdate`() and `EVP_SealFinal`() have exactly the same properties as the EVP_EncryptUpdate(3) and EVP_EncryptFinal(3) routines.

The public key must be RSA because it is the only OpenSSL public key algorithm that supports key transport.

Envelope encryption is the usual method of using public key encryption on large amounts of data. This is because public key encryption is slow but symmetric encryption is fast. So symmetric encryption is used for bulk encryption and the small random symmetric key used is transferred using public key encryption.

It is possible to call `EVP_SealInit`() twice in the same way as EVP_EncryptInit(3). The first call should have npubk set to 0 and (after setting any cipher parameters) it should be called again with type set to NULL.

`EVP_SealUpdate`() is implemented as a macro.

# RETURN VALUES

`EVP_SealInit`() returns 0 on error or npubk if successful.

`EVP_SealUpdate`() and `EVP_SealFinal`() return 1 for success and 0 for failure.

# SEE ALSO

evp(3), EVP_EncryptInit(3), EVP_OpenInit(3)

# HISTORY

`EVP_SealInit`(), `EVP_SealUpdate`(), and `EVP_SealFinal`() first appeared in SSLeay 0.5.1 and have been available since OpenBSD 2.4.

`EVP_SealFinal`() did not return a value before OpenSSL 0.9.7.

June 8, 2025 | Debian