NAME

RSA_padding_add_PKCS1_type_1, RSA_padding_check_PKCS1_type_1, RSA_padding_add_PKCS1_type_2, RSA_padding_check_PKCS1_type_2, RSA_padding_add_PKCS1_OAEP, RSA_padding_check_PKCS1_OAEP, RSA_padding_add_PKCS1_OAEP_mgf1, RSA_padding_check_PKCS1_OAEP_mgf1, RSA_padding_add_SSLv23, RSA_padding_check_SSLv23, RSA_padding_add_none, RSA_padding_check_none - asymmetric encryption padding

SYNOPSIS

 #include <openssl/rsa.h>

 int RSA_padding_add_PKCS1_type_1(unsigned char *to, int tlen,
                                  const unsigned char *f, int fl);

 int RSA_padding_check_PKCS1_type_1(unsigned char *to, int tlen,
                                    const unsigned char *f, int fl, int rsa_len);

 int RSA_padding_add_PKCS1_type_2(unsigned char *to, int tlen,
                                  const unsigned char *f, int fl);

 int RSA_padding_check_PKCS1_type_2(unsigned char *to, int tlen,
                                    const unsigned char *f, int fl, int rsa_len);

 int RSA_padding_add_PKCS1_OAEP(unsigned char *to, int tlen,
                                const unsigned char *f, int fl,
                                const unsigned char *p, int pl);

 int RSA_padding_check_PKCS1_OAEP(unsigned char *to, int tlen,
                                  const unsigned char *f, int fl, int rsa_len,
                                  const unsigned char *p, int pl);

 int RSA_padding_add_PKCS1_OAEP_mgf1(unsigned char *to, int tlen,
                                     const unsigned char *f, int fl,
                                     const unsigned char *p, int pl,
                                     const EVP_MD *md, const EVP_MD *mgf1md);

 int RSA_padding_check_PKCS1_OAEP_mgf1(unsigned char *to, int tlen,
                                       const unsigned char *f, int fl, int rsa_len,
                                       const unsigned char *p, int pl,
                                       const EVP_MD *md, const EVP_MD *mgf1md);

 int RSA_padding_add_SSLv23(unsigned char *to, int tlen,
                            const unsigned char *f, int fl);

 int RSA_padding_check_SSLv23(unsigned char *to, int tlen,
                              const unsigned char *f, int fl, int rsa_len);

 int RSA_padding_add_none(unsigned char *to, int tlen,
                          const unsigned char *f, int fl);

 int RSA_padding_check_none(unsigned char *to, int tlen,
                            const unsigned char *f, int fl, int rsa_len);

DESCRIPTION

The RSA_padding_xxx_xxx() functions are called from the RSA encrypt, decrypt, sign and verify functions. Normally they should not be called from application programs.

However, they can also be called directly to implement padding for other asymmetric ciphers. RSA_padding_add_PKCS1_OAEP() and RSA_padding_check_PKCS1_OAEP() may be used in an application combined with RSA_NO_PADDING in order to implement OAEP with an encoding parameter.

RSA_padding_add_xxx() encodes fl bytes from f so as to fit into tlen bytes and stores the result at to. An error occurs if fl does not meet the size requirements of the encoding method.

The following encoding methods are implemented:

PKCS1_type_1: PKCS #1 v2.0 EMSA-PKCS1-v1_5 (PKCS #1 v1.5 block type 1); used for signatures
PKCS1_type_2: PKCS #1 v2.0 EME-PKCS1-v1_5 (PKCS #1 v1.5 block type 2)
PKCS1_OAEP: PKCS #1 v2.0 EME-OAEP
SSLv23: PKCS #1 EME-PKCS1-v1_5 with SSL-specific modification
none: simply copy the data

The random number generator must be seeded prior to calling RSA_padding_add_xxx(). If the automatic seeding or reseeding of the OpenSSL CSPRNG fails due to external circumstances (see RAND(7)), the operation will fail.

RSA_padding_check_xxx() verifies that the fl bytes at f contain a valid encoding for a rsa_len byte RSA key in the respective encoding method and stores the recovered data of at most tlen bytes (for RSA_NO_PADDING: of size tlen) at to.

For RSA_padding_xxx_OAEP(), p points to the encoding parameter of length pl. p may be NULL if pl is 0.

For RSA_padding_xxx_OAEP_mgf1(), md points to the md hash, if md is NULL that means md=sha1, and mgf1md points to the mgf1 hash, if mgf1md is NULL that means mgf1md=md.

RETURN VALUES

The RSA_padding_add_xxx() functions return 1 on success, 0 on error. The RSA_padding_check_xxx() functions return the length of the recovered data, -1 on error. Error codes can be obtained by calling ERR_get_error(3).

WARNINGS

The result of RSA_padding_check_PKCS1_type_2() is a very sensitive 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 PKCS1_OAEP padding. If that is not possible, the result of RSA_padding_check_PKCS1_type_2() should be checked in constant time if it matches the expected length of the plaintext and additionally some application specific consistency checks on the plaintext need to be performed in constant time. If the plaintext is rejected it must be kept secret which of the checks caused the application to reject the message. Do not remove the zero-padding from the decrypted raw RSA data which was computed by RSA_private_decrypt() with RSA_NO_PADDING, as this would create a small timing side channel which could be used to mount a Bleichenbacher attack against any padding mode including PKCS1_OAEP.

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