RSA_PUBLIC_ENCRYPT(3) | Library Functions Manual | RSA_PUBLIC_ENCRYPT(3)

# NAME

`RSA_public_encrypt`, `RSA_private_decrypt`, `EVP_PKEY_encrypt_old`, `EVP_PKEY_decrypt_old` — RSA public key cryptography

# SYNOPSIS

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

int
`RSA_public_encrypt`(int flen, const unsigned char *from, unsigned char *to, RSA *rsa, int padding);

int
`RSA_private_decrypt`(int flen, const unsigned char *from, unsigned char *to, RSA *rsa, int padding);

`#include <openssl/evp.h>`

int
`EVP_PKEY_encrypt_old`(unsigned char *to, const unsigned char *from, int flen, EVP_PKEY *pkey);

int
`EVP_PKEY_decrypt_old`(unsigned char *to, const unsigned char *from, int flen, EVP_PKEY *pkey);

# DESCRIPTION

`RSA_public_encrypt`() encrypts the flen bytes at from (usually a session key) using the public key rsa and stores the ciphertext in to. to must point to `RSA_size`(rsa) bytes of memory.

padding denotes one of the following modes:

`RSA_PKCS1_PADDING`
  PKCS #1 v1.5 padding. This currently is the most widely used mode.
`RSA_PKCS1_OAEP_PADDING`
  EME-OAEP as defined in PKCS #1 v2.0 with SHA-1, MGF1 and an empty encoding parameter. This mode is recommended for all new applications.
`RSA_NO_PADDING`
  Raw RSA encryption. This mode should only be used to implement cryptographically sound padding modes in the application code. Encrypting user data directly with RSA is insecure.

flen must be less than `RSA_size`(rsa) - 11 for the PKCS #1 v1.5 based padding modes, less than `RSA_size`(rsa) - 41 for `RSA_PKCS1_OAEP_PADDING` and exactly `RSA_size`(rsa) for `RSA_NO_PADDING`.

`RSA_private_decrypt`() decrypts the flen bytes at from using the private key rsa and stores the plaintext in to. to must point to a memory section large enough to hold the decrypted data (which is smaller than `RSA_size`(rsa)). padding is the padding mode that was used to encrypt the data.

`EVP_PKEY_encrypt_old`() is a deprecated wrapper around `RSA_public_encrypt`() that uses the RSA public key stored in pkey and `RSA_PKCS1_PADDING`.

`EVP_PKEY_decrypt_old`() is a deprecated wrapper around `RSA_private_decrypt`() that uses the RSA private key stored in pkey and `RSA_PKCS1_PADDING`.

# RETURN VALUES

`RSA_public_encrypt`() and `EVP_PKEY_encrypt_old`() return the size of the encrypted data (i.e. `RSA_size`(rsa)). `RSA_private_decrypt`() and `EVP_PKEY_decrypt_old`() returns the size of the recovered plaintext. On error, -1 is returned; the error codes can be obtained by ERR_get_error(3).

In addition to the return values documented above, `EVP_PKEY_encrypt_old`() may return 0 if the EVP_PKEY_id(3) of pkey is not `EVP_PKEY_RSA`.

# SEE ALSO

EVP_PKEY_decrypt(3), EVP_PKEY_encrypt(3), RSA_meth_set_priv_dec(3), RSA_new(3), RSA_size(3)

# STANDARDS

SSL, PKCS #1 v2.0

# HISTORY

`RSA_public_encrypt`() and `RSA_private_decrypt`() appeared in SSLeay 0.4 or earlier and have been available since OpenBSD 2.4.

`EVP_PKEY_encrypt`() and `EVP_PKEY_decrypt`() first appeared in SSLeay 0.9.0 and have been available since OpenBSD 2.4. There were renamed to `EVP_PKEY_encrypt_old`() and `EVP_PKEY_decrypt_old`() in OpenSSL 1.0.0 and OpenBSD 4.9.

`RSA_NO_PADDING` is available since SSLeay 0.9.0. OAEP was added in OpenSSL 0.9.2b.

# BUGS

Decryption failures in the `RSA_PKCS1_PADDING` mode leak information which can potentially be used to mount a Bleichenbacher padding oracle attack. This is an inherent weakness in the PKCS #1 v1.5 padding design. Prefer `RSA_PKCS1_OAEP_PADDING`.

June 8, 2025 | Debian