EVP_SignInit.pod revision 291721 
1=pod
2
3=head1 NAME
4
5EVP_SignInit, EVP_SignInit_ex, EVP_SignUpdate, EVP_SignFinal - EVP signing
6functions
7
8=head1 SYNOPSIS
9
10 #include <openssl/evp.h>
11
12 int EVP_SignInit_ex(EVP_MD_CTX *ctx, const EVP_MD *type, ENGINE *impl);
13 int EVP_SignUpdate(EVP_MD_CTX *ctx, const void *d, unsigned int cnt);
14 int EVP_SignFinal(EVP_MD_CTX *ctx,unsigned char *sig,unsigned int *s, EVP_PKEY *pkey);
15
16 void EVP_SignInit(EVP_MD_CTX *ctx, const EVP_MD *type);
17
18 int EVP_PKEY_size(EVP_PKEY *pkey);
19
20=head1 DESCRIPTION
21
22The EVP signature routines are a high level interface to digital
23signatures.
24
25EVP_SignInit_ex() sets up signing context B<ctx> to use digest
26B<type> from ENGINE B<impl>. B<ctx> must be initialized with
27EVP_MD_CTX_init() before calling this function.
28
29EVP_SignUpdate() hashes B<cnt> bytes of data at B<d> into the
30signature context B<ctx>. This function can be called several times on the
31same B<ctx> to include additional data.
32
33EVP_SignFinal() signs the data in B<ctx> using the private key B<pkey> and
34places the signature in B<sig>. B<sig> must be at least EVP_PKEY_size(pkey)
35bytes in size. B<s> is an OUT paramter, and not used as an IN parameter.
36The number of bytes of data written (i.e. the length of the signature)
37will be written to the integer at B<s>, at most EVP_PKEY_size(pkey) bytes
38will be written.
39
40EVP_SignInit() initializes a signing context B<ctx> to use the default
41implementation of digest B<type>.
42
43EVP_PKEY_size() returns the maximum size of a signature in bytes. The actual
44signature returned by EVP_SignFinal() may be smaller.
45
46=head1 RETURN VALUES
47
48EVP_SignInit_ex(), EVP_SignUpdate() and EVP_SignFinal() return 1
49for success and 0 for failure.
50
51EVP_PKEY_size() returns the maximum size of a signature in bytes.
52
53The error codes can be obtained by L<ERR_get_error(3)|ERR_get_error(3)>.
54
55=head1 NOTES
56
57The B<EVP> interface to digital signatures should almost always be used in
58preference to the low level interfaces. This is because the code then becomes
59transparent to the algorithm used and much more flexible.
60
61Due to the link between message digests and public key algorithms the correct
62digest algorithm must be used with the correct public key type. A list of
63algorithms and associated public key algorithms appears in 
64L<EVP_DigestInit(3)|EVP_DigestInit(3)>.
65
66When signing with DSA private keys the random number generator must be seeded
67or the operation will fail. The random number generator does not need to be
68seeded for RSA signatures.
69
70The call to EVP_SignFinal() internally finalizes a copy of the digest context.
71This means that calls to EVP_SignUpdate() and EVP_SignFinal() can be called
72later to digest and sign additional data.
73
74Since only a copy of the digest context is ever finalized the context must
75be cleaned up after use by calling EVP_MD_CTX_cleanup() or a memory leak
76will occur.
77
78=head1 BUGS
79
80Older versions of this documentation wrongly stated that calls to 
81EVP_SignUpdate() could not be made after calling EVP_SignFinal().
82
83Since the private key is passed in the call to EVP_SignFinal() any error
84relating to the private key (for example an unsuitable key and digest
85combination) will not be indicated until after potentially large amounts of
86data have been passed through EVP_SignUpdate().
87
88It is not possible to change the signing parameters using these function.
89
90The previous two bugs are fixed in the newer EVP_SignDigest*() function.
91
92=head1 SEE ALSO
93
94L<EVP_VerifyInit(3)|EVP_VerifyInit(3)>,
95L<EVP_DigestInit(3)|EVP_DigestInit(3)>, L<err(3)|err(3)>,
96L<evp(3)|evp(3)>, L<hmac(3)|hmac(3)>, L<md2(3)|md2(3)>,
97L<md5(3)|md5(3)>, L<mdc2(3)|mdc2(3)>, L<ripemd(3)|ripemd(3)>,
98L<sha(3)|sha(3)>, L<dgst(1)|dgst(1)>
99
100=head1 HISTORY
101
102EVP_SignInit(), EVP_SignUpdate() and EVP_SignFinal() are
103available in all versions of SSLeay and OpenSSL.
104
105EVP_SignInit_ex() was added in OpenSSL 0.9.7.
106
107=cut
108