man: clarify the 'random number generator must be seeded' requirement

[thirdparty/openssl.git] / doc / man3 / EVP_SignInit.pod

=pod

=head1 NAME

EVP_PKEY_size,
EVP_SignInit, EVP_SignInit_ex, EVP_SignUpdate, EVP_SignFinal,
EVP_PKEY_security_bits - EVP signing
functions

=head1 SYNOPSIS

 #include <openssl/evp.h>

 int EVP_SignInit_ex(EVP_MD_CTX *ctx, const EVP_MD *type, ENGINE *impl);
 int EVP_SignUpdate(EVP_MD_CTX *ctx, const void *d, unsigned int cnt);
 int EVP_SignFinal(EVP_MD_CTX *ctx, unsigned char *sig, unsigned int *s, EVP_PKEY *pkey);

 void EVP_SignInit(EVP_MD_CTX *ctx, const EVP_MD *type);

 int EVP_PKEY_size(const EVP_PKEY *pkey);
 int EVP_PKEY_security_bits(const EVP_PKEY *pkey);

=head1 DESCRIPTION

The EVP signature routines are a high level interface to digital
signatures.

EVP_SignInit_ex() sets up signing context B<ctx> to use digest
B<type> from ENGINE B<impl>. B<ctx> must be created with
EVP_MD_CTX_new() before calling this function.

EVP_SignUpdate() hashes B<cnt> bytes of data at B<d> into the
signature context B<ctx>. This function can be called several times on the
same B<ctx> to include additional data.

EVP_SignFinal() signs the data in B<ctx> using the private key B<pkey> and
places the signature in B<sig>. B<sig> must be at least EVP_PKEY_size(pkey)
bytes in size. B<s> is an OUT parameter, and not used as an IN parameter.
The number of bytes of data written (i.e. the length of the signature)
will be written to the integer at B<s>, at most EVP_PKEY_size(pkey) bytes
will be written.

EVP_SignInit() initializes a signing context B<ctx> to use the default
implementation of digest B<type>.

EVP_PKEY_size() returns the maximum size of a signature in bytes. The actual
signature returned by EVP_SignFinal() may be smaller.

EVP_PKEY_security_bits() returns the number of security bits of the given B<pkey>,
bits of security is defined in NIST SP800-57.

=head1 RETURN VALUES

EVP_SignInit_ex(), EVP_SignUpdate() and EVP_SignFinal() return 1
for success and 0 for failure.

EVP_PKEY_size() returns the maximum size of a signature in bytes.

The error codes can be obtained by L<ERR_get_error(3)>.

EVP_PKEY_security_bits() returns the number of security bits.

=head1 NOTES

The B<EVP> interface to digital signatures should almost always be used in
preference to the low level interfaces. This is because the code then becomes
transparent to the algorithm used and much more flexible.

When signing with DSA private keys the random number generator must be seeded.
If the automatic seeding or reseeding of the OpenSSL CSPRNG fails due to
external circumstances (see L<RAND(7)>), the operation will fail.
This requirement does not hold for RSA signatures.

The call to EVP_SignFinal() internally finalizes a copy of the digest context.
This means that calls to EVP_SignUpdate() and EVP_SignFinal() can be called
later to digest and sign additional data.

Since only a copy of the digest context is ever finalized the context must
be cleaned up after use by calling EVP_MD_CTX_free() or a memory leak
will occur.

=head1 BUGS

Older versions of this documentation wrongly stated that calls to
EVP_SignUpdate() could not be made after calling EVP_SignFinal().

Since the private key is passed in the call to EVP_SignFinal() any error
relating to the private key (for example an unsuitable key and digest
combination) will not be indicated until after potentially large amounts of
data have been passed through EVP_SignUpdate().

It is not possible to change the signing parameters using these function.

The previous two bugs are fixed in the newer EVP_SignDigest*() function.

=head1 SEE ALSO

L<EVP_VerifyInit(3)>,
L<EVP_DigestInit(3)>,
L<evp(7)>, L<HMAC(3)>, L<MD2(3)>,
L<MD5(3)>, L<MDC2(3)>, L<RIPEMD160(3)>,
L<SHA1(3)>, L<dgst(1)>

=head1 COPYRIGHT

Copyright 2000-2018 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the "License").  You may not use
this file except in compliance with the License.  You can obtain a copy
in the file LICENSE in the source distribution or at
L<https://www.openssl.org/source/license.html>.

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