.\"	$OpenBSD: RSA_private_encrypt.3,v 1.6 2017/03/25 18:14:17 schwarze Exp $
.\"	OpenSSL RSA_private_encrypt.pod b41f6b64 Mar 10 15:49:04 2017 +0000
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.Dd $Mdocdate: March 25 2017 $
.Dt RSA_PRIVATE_ENCRYPT 3
.Os
.Sh NAME
.Nm RSA_private_encrypt ,
.Nm RSA_public_decrypt
.Nd low level signature operations
.Sh SYNOPSIS
.In openssl/rsa.h
.Ft int
.Fo RSA_private_encrypt
.Fa "int flen"
.Fa "const unsigned char *from"
.Fa "unsigned char *to"
.Fa "RSA *rsa"
.Fa "int padding"
.Fc
.Ft int
.Fo RSA_public_decrypt
.Fa "int flen"
.Fa "const unsigned char *from"
.Fa "unsigned char *to"
.Fa "RSA *rsa"
.Fa "int padding"
.Fc
.Sh DESCRIPTION
These functions handle RSA signatures at a low level.
.Pp
.Fn RSA_private_encrypt
signs the
.Fa flen
bytes at
.Fa from
(usually a message digest with an algorithm identifier) using the
private key
.Fa rsa
and stores the signature in
.Fa to .
.Fa to
must point to
.Fn RSA_size rsa
bytes of memory.
.Pp
.Fa padding
denotes one of the following modes:
.Bl -tag -width Ds
.It Dv RSA_PKCS1_PADDING
PKCS #1 v1.5 padding.
This function does not handle the
.Sy algorithmIdentifier
specified in PKCS #1.
When generating or verifying PKCS #1 signatures,
.Xr RSA_sign 3
and
.Xr RSA_verify 3
should be used.
.It Dv 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.
.El
.Pp
.Fn RSA_public_decrypt
recovers the message digest from the
.Fa flen
bytes long signature at
.Fa from
using the signer's public key
.Fa rsa .
.Fa to
must point to a memory section large enough to hold the message digest
(which is smaller than
.Fn RSA_size rsa
- 11).
.Fa padding
is the padding mode that was used to sign the data.
.Sh RETURN VALUES
.Fn RSA_private_encrypt
returns the size of the signature (i.e.\&
.Fn RSA_size rsa ) .
.Fn RSA_public_decrypt
returns the size of the recovered message digest.
.Pp
On error, -1 is returned; the error codes can be obtained by
.Xr ERR_get_error 3 .
.Sh SEE ALSO
.Xr ERR_get_error 3 ,
.Xr RSA_new 3 ,
.Xr RSA_sign 3 ,
.Xr RSA_verify 3
.Sh HISTORY
The
.Fa padding
argument was added in SSLeay 0.8.
.Dv RSA_NO_PADDING
is available since SSLeay 0.9.0.