.\"	$OpenBSD: RSA_public_encrypt.3,v 1.11 2018/03/27 17:35:50 schwarze Exp $
.\"	OpenSSL RSA_public_encrypt.pod 1e3f62a3 Jul 17 16:47:13 2017 +0200
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.Dd $Mdocdate: March 27 2018 $
.Dt RSA_PUBLIC_ENCRYPT 3
.Os
.Sh NAME
.Nm RSA_public_encrypt ,
.Nm RSA_private_decrypt
.Nd RSA public key cryptography
.Sh SYNOPSIS
.In openssl/rsa.h
.Ft int
.Fo RSA_public_encrypt
.Fa "int flen"
.Fa "const unsigned char *from"
.Fa "unsigned char *to"
.Fa "RSA *rsa"
.Fa "int padding"
.Fc
.Ft int
.Fo RSA_private_decrypt
.Fa "int flen"
.Fa "const unsigned char *from"
.Fa "unsigned char *to"
.Fa "RSA *rsa"
.Fa "int padding"
.Fc
.Sh DESCRIPTION
.Fn RSA_public_encrypt
encrypts the
.Fa flen
bytes at
.Fa from
(usually a session key) using the public key
.Fa rsa
and stores the ciphertext 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 currently is the most widely used mode.
.It Dv 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.
.It Dv 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.
.El
.Pp
.Fa flen
must be less than
.Fn RSA_size rsa
- 11 for the PKCS #1 v1.5 based padding modes, less than
.Fn RSA_size rsa
- 41 for
.Dv RSA_PKCS1_OAEP_PADDING
and exactly
.Fn RSA_size rsa
for
.Dv RSA_NO_PADDING .
.Pp
.Fn RSA_private_decrypt
decrypts the
.Fa flen
bytes at
.Fa from
using the private key
.Fa rsa
and stores the plaintext in
.Fa to .
.Fa to
must point to a memory section large enough to hold the decrypted data
(which is smaller than
.Fn RSA_size rsa ) .
.Fa padding
is the padding mode that was used to encrypt the data.
.Sh RETURN VALUES
.Fn RSA_public_encrypt
returns the size of the encrypted data (i.e.\&
.Fn RSA_size rsa ) .
.Fn RSA_private_decrypt
returns the size of the recovered plaintext.
.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_meth_set_priv_dec 3 ,
.Xr RSA_new 3 ,
.Xr RSA_size 3
.Sh STANDARDS
SSL, PKCS #1 v2.0
.Sh HISTORY
.Fn RSA_public_encrypt
and
.Fn RSA_private_decrypt
appeared in SSLeay 0.4 or earlier and have been available since
.Ox 2.4 .
.Pp
.Dv RSA_NO_PADDING
is available since SSLeay 0.9.0.
OAEP was added in OpenSSL 0.9.2b.
.Sh BUGS
Decryption failures in the
.Dv 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
.Dv RSA_PKCS1_OAEP_PADDING .