.\" $OpenBSD: EVP_DigestSignInit.3,v 1.7 2019/06/10 14:58:48 schwarze Exp $ .\" OpenSSL 9b86974e Aug 17 15:21:33 2015 -0400 .\" .\" This file was written by Dr. Stephen Henson . .\" Copyright (c) 2006, 2009, 2015, 2016 The OpenSSL Project. .\" All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in .\" the documentation and/or other materials provided with the .\" distribution. .\" .\" 3. 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IN NO EVENT SHALL THE OpenSSL PROJECT OR .\" ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, .\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; .\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, .\" STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) .\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED .\" OF THE POSSIBILITY OF SUCH DAMAGE. .\" .Dd $Mdocdate: June 10 2019 $ .Dt EVP_DIGESTSIGNINIT 3 .Os .Sh NAME .Nm EVP_DigestSignInit , .Nm EVP_DigestSignUpdate , .Nm EVP_DigestSignFinal .Nd EVP signing functions .Sh SYNOPSIS .In openssl/evp.h .Ft int .Fo EVP_DigestSignInit .Fa "EVP_MD_CTX *ctx" .Fa "EVP_PKEY_CTX **pctx" .Fa "const EVP_MD *type" .Fa "ENGINE *e" .Fa "EVP_PKEY *pkey" .Fc .Ft int .Fo EVP_DigestSignUpdate .Fa "EVP_MD_CTX *ctx" .Fa "const void *d" .Fa "size_t cnt" .Fc .Ft int .Fo EVP_DigestSignFinal .Fa "EVP_MD_CTX *ctx" .Fa "unsigned char *sig" .Fa "size_t *siglen" .Fc .Sh DESCRIPTION The EVP signature routines are a high level interface to digital signatures. .Pp .Fn EVP_DigestSignInit sets up the signing context .Fa ctx to use the digest .Fa type from .Vt ENGINE .Fa e and private key .Fa pkey . .Fa ctx must be initialized with .Xr EVP_MD_CTX_init 3 before calling this function. If .Fa pctx is not .Dv NULL , the .Vt EVP_PKEY_CTX of the signing operation will be written to .Pf * Fa pctx : this can be used to set alternative signing options. .Pp .Fn EVP_DigestSignUpdate hashes .Fa cnt bytes of data at .Fa d into the signature context .Fa ctx . This function can be called several times on the same .Fa ctx to include additional data. This function is currently implemented using a macro. .Pp .Fn EVP_DigestSignFinal signs the data in .Fa ctx and places the signature in .Fa sig . If .Fa sig is .Dv NULL , then the maximum size of the output buffer is written to .Pf * Fa siglen . If .Fa sig is not .Dv NULL , then before the call .Fa siglen should contain the length of the .Fa sig buffer. If the call is successful, the signature is written to .Fa sig and the amount of data written to .Fa siglen . .Pp The 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. .Pp In previous versions of OpenSSL, there was a link between message digest types and public key algorithms. This meant that "clone" digests such as .Xr EVP_dss1 3 needed to be used to sign using SHA1 and DSA. This is no longer necessary and the use of clone digest is now discouraged. .Pp The call to .Fn EVP_DigestSignFinal internally finalizes a copy of the digest context. This means that .Fn EVP_DigestSignUpdate and .Fn EVP_DigestSignFinal can be called later to digest and sign additional data. .Pp Since only a copy of the digest context is ever finalized, the context must be cleaned up after use by calling .Xr EVP_MD_CTX_free 3 , or a memory leak will occur. .Pp The use of .Xr EVP_PKEY_size 3 with these functions is discouraged because some signature operations may have a signature length which depends on the parameters set. As a result, .Xr EVP_PKEY_size 3 would have to return a value which indicates the maximum possible signature for any set of parameters. .Sh RETURN VALUES .Fn EVP_DigestSignInit , .Fn EVP_DigestSignUpdate , and .Fn EVP_DigestSignFinal return 1 for success and 0 or a negative value for failure. In particular, a return value of -2 indicates the operation is not supported by the public key algorithm. .Pp The error codes can be obtained from .Xr ERR_get_error 3 . .Sh SEE ALSO .Xr evp 3 , .Xr EVP_DigestInit 3 , .Xr EVP_DigestVerifyInit 3 , .Xr EVP_PKEY_meth_set_signctx 3 .Sh HISTORY .Fn EVP_DigestSignInit , .Fn EVP_DigestSignUpdate , and .Fn EVP_DigestSignFinal first appeared in OpenSSL 1.0.0 and have been available since .Ox 4.9 .