RSA_PRIVATE_ENCRYPT(3) | Library Functions Manual | RSA_PRIVATE_ENCRYPT(3)

# NAME

`RSA_private_encrypt`, `RSA_public_decrypt` — low level signature operations

# SYNOPSIS

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

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

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

# DESCRIPTION

These functions handle RSA signatures at a low level.

`RSA_private_encrypt`() signs the flen bytes at from (usually a message digest with an algorithm identifier) using the private key rsa and stores the signature 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 function does not handle the algorithmIdentifier specified in PKCS #1. When generating or verifying PKCS #1 signatures, RSA_sign(3) and RSA_verify(3) should be used.
`RSA_NO_PADDING`
  Raw RSA signature. This mode should only be used to implement cryptographically sound padding modes in the application code. Signing user data directly with RSA is insecure.

`RSA_public_decrypt`() recovers the message digest from the flen bytes long signature at from using the signer's public key rsa. to must point to a memory section large enough to hold the message digest (which is smaller than `RSA_size`(rsa) - 11). padding is the padding mode that was used to sign the data.

# RETURN VALUES

`RSA_private_encrypt`() returns the size of the signature (i.e. `RSA_size`(rsa)). `RSA_public_decrypt`() returns the size of the recovered message digest.

On error, -1 is returned; the error codes can be obtained by ERR_get_error(3).

# SEE ALSO

RSA_meth_set_priv_enc(3), RSA_new(3), RSA_sign(3), RSA_verify(3)

# HISTORY

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

`RSA_NO_PADDING` is available since SSLeay 0.9.0.

June 8, 2025 | Debian