xref: /openssl/doc/man3/EVP_SignInit.pod (revision da1c088f)
1=pod
2
3=head1 NAME
4
5EVP_SignInit, EVP_SignInit_ex, EVP_SignUpdate,
6EVP_SignFinal_ex, EVP_SignFinal
7- EVP signing functions
8
9=head1 SYNOPSIS
10
11 #include <openssl/evp.h>
12
13 int EVP_SignInit_ex(EVP_MD_CTX *ctx, const EVP_MD *type, ENGINE *impl);
14 int EVP_SignUpdate(EVP_MD_CTX *ctx, const void *d, unsigned int cnt);
15 int EVP_SignFinal_ex(EVP_MD_CTX *ctx, unsigned char *md, unsigned int *s,
16                      EVP_PKEY *pkey, OSSL_LIB_CTX *libctx, const char *propq);
17 int EVP_SignFinal(EVP_MD_CTX *ctx, unsigned char *sig, unsigned int *s,
18                   EVP_PKEY *pkey);
19
20 void EVP_SignInit(EVP_MD_CTX *ctx, const EVP_MD *type);
21
22=head1 DESCRIPTION
23
24The EVP signature routines are a high-level interface to digital
25signatures.
26
27EVP_SignInit_ex() sets up signing context I<ctx> to use digest
28I<type> from B<ENGINE> I<impl>. I<ctx> must be created with
29EVP_MD_CTX_new() before calling this function.
30
31EVP_SignUpdate() hashes I<cnt> bytes of data at I<d> into the
32signature context I<ctx>. This function can be called several times on the
33same I<ctx> to include additional data.
34
35EVP_SignFinal_ex() signs the data in I<ctx> using the private key
36I<pkey> and places the signature in I<sig>. The library context I<libctx> and
37property query I<propq> are used when creating a context to use with the key
38I<pkey>. I<sig> must be at least C<EVP_PKEY_get_size(pkey)> bytes in size.
39I<s> is an OUT parameter, and not used as an IN parameter.
40The number of bytes of data written (i.e. the length of the signature)
41will be written to the integer at I<s>, at most C<EVP_PKEY_get_size(pkey)>
42bytes will be written.
43
44EVP_SignFinal() is similar to EVP_SignFinal_ex() but uses default
45values of NULL for the library context I<libctx> and the property query I<propq>.
46
47EVP_SignInit() initializes a signing context I<ctx> to use the default
48implementation of digest I<type>.
49
50=head1 RETURN VALUES
51
52EVP_SignInit_ex(), EVP_SignUpdate(), EVP_SignFinal_ex() and
53EVP_SignFinal() return 1 for success and 0 for failure.
54
55The error codes can be obtained by L<ERR_get_error(3)>.
56
57=head1 NOTES
58
59The B<EVP> interface to digital signatures should almost always be used in
60preference to the low-level interfaces. This is because the code then becomes
61transparent to the algorithm used and much more flexible.
62
63When signing with some private key types the random number generator must
64be seeded. If the automatic seeding or reseeding of the OpenSSL CSPRNG fails
65due to external circumstances (see L<RAND(7)>), the operation will fail.
66
67The call to EVP_SignFinal() internally finalizes a copy of the digest context.
68This means that calls to EVP_SignUpdate() and EVP_SignFinal() can be called
69later to digest and sign additional data.cApplications may disable this
70behavior by setting the EVP_MD_CTX_FLAG_FINALISE context flag via
71L<EVP_MD_CTX_set_flags(3)>.
72
73Since only a copy of the digest context is ever finalized the context must
74be cleaned up after use by calling EVP_MD_CTX_free() or a memory leak
75will occur.
76
77Note that not all providers support continuation, in case the selected
78provider does not allow to duplicate contexts EVP_SignFinal() will
79finalize the digest context and attempting to process additional data via
80EVP_SignUpdate() will result in an error.
81
82=head1 BUGS
83
84Older versions of this documentation wrongly stated that calls to
85EVP_SignUpdate() could not be made after calling EVP_SignFinal().
86
87Since the private key is passed in the call to EVP_SignFinal() any error
88relating to the private key (for example an unsuitable key and digest
89combination) will not be indicated until after potentially large amounts of
90data have been passed through EVP_SignUpdate().
91
92It is not possible to change the signing parameters using these function.
93
94The previous two bugs are fixed in the newer EVP_DigestSign*() functions.
95
96=head1 SEE ALSO
97
98L<EVP_PKEY_get_size(3)>, L<EVP_PKEY_get_bits(3)>,
99L<EVP_PKEY_get_security_bits(3)>,
100L<EVP_VerifyInit(3)>,
101L<EVP_DigestInit(3)>,
102L<evp(7)>, L<HMAC(3)>, L<MD2(3)>,
103L<MD5(3)>, L<MDC2(3)>, L<RIPEMD160(3)>,
104L<SHA1(3)>, L<openssl-dgst(1)>
105
106=head1 HISTORY
107
108The function EVP_SignFinal_ex() was added in OpenSSL 3.0.
109
110=head1 COPYRIGHT
111
112Copyright 2000-2023 The OpenSSL Project Authors. All Rights Reserved.
113
114Licensed under the Apache License 2.0 (the "License").  You may not use
115this file except in compliance with the License.  You can obtain a copy
116in the file LICENSE in the source distribution or at
117L<https://www.openssl.org/source/license.html>.
118
119=cut
120