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$NetBSD: RSA_public_encrypt.3,v 1.1.1.2 2023/04/18 14:19:14 christos Exp $

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Title "RSA_public_encrypt 3" For nroff, turn off justification. Always turn off hyphenation; it makes
way too many mistakes in technical documents.
.nh RSA_public_encrypt, RSA_private_decrypt - RSA public key cryptography libcrypto, -lcrypto Header "SYNOPSIS" .Vb 1 #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); .Ve Header "DESCRIPTION" \fBRSA_public_encrypt() encrypts the flen bytes at from (usually a session key) using the public key rsa and stores the ciphertext in \fBto. to must point to RSA_size(rsa) bytes of memory.

\fBpadding denotes one of the following modes:

Item "RSA_PKCS1_PADDING" \s-1PKCS\s0 #1 v1.5 padding. This currently is the most widely used mode. However, it is highly recommended to use \s-1RSA_PKCS1_OAEP_PADDING\s0 in new applications. \s-1SEE WARNING BELOW.\s0 Item "RSA_PKCS1_OAEP_PADDING" EME-OAEP as defined in \s-1PKCS\s0 #1 v2.0 with \s-1SHA-1, MGF1\s0 and an empty encoding parameter. This mode is recommended for all new applications. Item "RSA_SSLV23_PADDING" \s-1PKCS\s0 #1 v1.5 padding with an SSL-specific modification that denotes that the server is \s-1SSL3\s0 capable. Item "RSA_NO_PADDING" Raw \s-1RSA\s0 encryption. This mode should only be used to implement cryptographically sound padding modes in the application code. Encrypting user data directly with \s-1RSA\s0 is insecure.

\fBflen must not be more than RSA_size(rsa) - 11 for the \s-1PKCS\s0 #1 v1.5 based padding modes, not more than RSA_size(rsa) - 42 for \s-1RSA_PKCS1_OAEP_PADDING\s0 and exactly RSA_size(rsa) for \s-1RSA_NO_PADDING.\s0 When a padding mode other than \s-1RSA_NO_PADDING\s0 is in use, then \fBRSA_public_encrypt() will include some random bytes into the ciphertext and therefore the ciphertext will be different each time, even if the plaintext and the public key are exactly identical. The returned ciphertext in to will always be zero padded to exactly RSA_size(rsa) bytes. \fBto and from may overlap.

\fBRSA_private_decrypt() decrypts the flen bytes at from using the private key rsa and stores the plaintext in to. flen should be equal to RSA_size(rsa) but may be smaller, when leading zero bytes are in the ciphertext. Those are not important and may be removed, but RSA_public_encrypt() does not do that. to must point to a memory section large enough to hold the maximal possible decrypted data (which is equal to RSA_size(rsa) for \s-1RSA_NO_PADDING,\s0 RSA_size(rsa) - 11 for the \s-1PKCS\s0 #1 v1.5 based padding modes and RSA_size(rsa) - 42 for \s-1RSA_PKCS1_OAEP_PADDING\s0). \fBpadding is the padding mode that was used to encrypt the data. \fBto and from may overlap.

Header "RETURN VALUES" \fBRSA_public_encrypt() returns the size of the encrypted data (i.e., RSA_size(rsa)). RSA_private_decrypt() returns the size of the recovered plaintext. A return value of 0 is not an error and means only that the plaintext was empty.

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

Header "WARNINGS" Decryption failures in the \s-1RSA_PKCS1_PADDING\s0 mode leak information which can potentially be used to mount a Bleichenbacher padding oracle attack. This is an inherent weakness in the \s-1PKCS\s0 #1 v1.5 padding design. Prefer \s-1RSA_PKCS1_OAEP_PADDING.\s0 Header "CONFORMING TO" \s-1SSL, PKCS\s0 #1 v2.0 Header "SEE ALSO" \fBERR_get_error\|(3), RAND_bytes\|(3), \fBRSA_size\|(3) Header "COPYRIGHT" Copyright 2000-2019 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the OpenSSL license (the \*(L"License\*(R"). You may not use this file except in compliance with the License. You can obtain a copy in the file \s-1LICENSE\s0 in the source distribution or at <https://www.openssl.org/source/license.html>.

Indexes created Wed Apr 22 00:22:44 UTC 2026