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.\" ========================================================================
.\"
.IX Title "EVP_DIGESTSIGNINIT 3"
.TH EVP_DIGESTSIGNINIT 3 "2023-09-11" "1.1.1w" "OpenSSL"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
EVP_DigestSignInit, EVP_DigestSignUpdate, EVP_DigestSignFinal, EVP_DigestSign \- EVP signing functions
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #include <openssl/evp.h>
\&
\& int EVP_DigestSignInit(EVP_MD_CTX *ctx, EVP_PKEY_CTX **pctx,
\&                        const EVP_MD *type, ENGINE *e, EVP_PKEY *pkey);
\& int EVP_DigestSignUpdate(EVP_MD_CTX *ctx, const void *d, size_t cnt);
\& int EVP_DigestSignFinal(EVP_MD_CTX *ctx, unsigned char *sig, size_t *siglen);
\&
\& int EVP_DigestSign(EVP_MD_CTX *ctx, unsigned char *sigret,
\&                    size_t *siglen, const unsigned char *tbs,
\&                    size_t tbslen);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
The \s-1EVP\s0 signature routines are a high-level interface to digital signatures.
.PP
\&\fIEVP_DigestSignInit()\fR sets up signing context \fBctx\fR to use digest \fBtype\fR from
\&\s-1ENGINE \s0\fBe\fR and private key \fBpkey\fR. \fBctx\fR must be created with
\&\fIEVP_MD_CTX_new()\fR before calling this function. If \fBpctx\fR is not \s-1NULL,\s0 the
\&\s-1EVP_PKEY_CTX\s0 of the signing operation will be written to \fB*pctx\fR: this can
be used to set alternative signing options. Note that any existing value in
\&\fB*pctx\fR is overwritten. The \s-1EVP_PKEY_CTX\s0 value returned must not be freed
directly by the application if \fBctx\fR is not assigned an \s-1EVP_PKEY_CTX\s0 value before
being passed to \fIEVP_DigestSignInit()\fR (which means the \s-1EVP_PKEY_CTX\s0 is created
inside \fIEVP_DigestSignInit()\fR and it will be freed automatically when the
\&\s-1EVP_MD_CTX\s0 is freed).
.PP
The digest \fBtype\fR may be \s-1NULL\s0 if the signing algorithm supports it.
.PP
No \fB\s-1EVP_PKEY_CTX\s0\fR will be created by \fIEVP_DigestSignInit()\fR if the passed \fBctx\fR
has already been assigned one via \fIEVP_MD_CTX_set_pkey_ctx\fR\|(3). See also \s-1\fISM2\s0\fR\|(7).
.PP
Only \s-1EVP_PKEY\s0 types that support signing can be used with these functions. This
includes \s-1MAC\s0 algorithms where the \s-1MAC\s0 generation is considered as a form of
\&\*(L"signing\*(R". Built-in \s-1EVP_PKEY\s0 types supported by these functions are \s-1CMAC,\s0
Poly1305, \s-1DSA, ECDSA, HMAC, RSA,\s0 SipHash, Ed25519 and Ed448.
.PP
Not all digests can be used for all key types. The following combinations apply.
.IP "\s-1DSA\s0" 4
.IX Item "DSA"
Supports \s-1SHA1, SHA224, SHA256, SHA384\s0 and \s-1SHA512\s0
.IP "\s-1ECDSA\s0" 4
.IX Item "ECDSA"
Supports \s-1SHA1, SHA224, SHA256, SHA384, SHA512\s0 and \s-1SM3\s0
.IP "\s-1RSA\s0 with no padding" 4
.IX Item "RSA with no padding"
Supports no digests (the digest \fBtype\fR must be \s-1NULL\s0)
.IP "\s-1RSA\s0 with X931 padding" 4
.IX Item "RSA with X931 padding"
Supports \s-1SHA1, SHA256, SHA384\s0 and \s-1SHA512\s0
.IP "All other \s-1RSA\s0 padding types" 4
.IX Item "All other RSA padding types"
Support \s-1SHA1, SHA224, SHA256, SHA384, SHA512, MD5, MD5_SHA1, MD2, MD4, MDC2,
SHA3\-224, SHA3\-256, SHA3\-384, SHA3\-512\s0
.IP "Ed25519 and Ed448" 4
.IX Item "Ed25519 and Ed448"
Support no digests (the digest \fBtype\fR must be \s-1NULL\s0)
.IP "\s-1HMAC\s0" 4
.IX Item "HMAC"
Supports any digest
.IP "\s-1CMAC,\s0 Poly1305 and SipHash" 4
.IX Item "CMAC, Poly1305 and SipHash"
Will ignore any digest provided.
.PP
If RSA-PSS is used and restrictions apply then the digest must match.
.PP
\&\fIEVP_DigestSignUpdate()\fR hashes \fBcnt\fR bytes of data at \fBd\fR into the
signature context \fBctx\fR. This function can be called several times on the
same \fBctx\fR to include additional data. This function is currently implemented
using a macro.
.PP
\&\fIEVP_DigestSignFinal()\fR signs the data in \fBctx\fR and places the signature in \fBsig\fR.
If \fBsig\fR is \fB\s-1NULL\s0\fR then the maximum size of the output buffer is written to
the \fBsiglen\fR parameter. If \fBsig\fR is not \fB\s-1NULL\s0\fR then before the call the
\&\fBsiglen\fR parameter should contain the length of the \fBsig\fR buffer. If the
call is successful the signature is written to \fBsig\fR and the amount of data
written to \fBsiglen\fR.
.PP
\&\fIEVP_DigestSign()\fR signs \fBtbslen\fR bytes of data at \fBtbs\fR and places the
signature in \fBsig\fR and its length in \fBsiglen\fR in a similar way to
\&\fIEVP_DigestSignFinal()\fR.
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
\&\fIEVP_DigestSignInit()\fR, \fIEVP_DigestSignUpdate()\fR, \fIEVP_DigestSignFinal()\fR and
\&\fIEVP_DigestSign()\fR return 1 for success and 0 for failure.
.PP
The error codes can be obtained from \fIERR_get_error\fR\|(3).
.SH "NOTES"
.IX Header "NOTES"
The \fB\s-1EVP\s0\fR 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
\&\fIEVP_DigestSign()\fR is a one shot operation which signs a single block of data
in one function. For algorithms that support streaming it is equivalent to
calling \fIEVP_DigestSignUpdate()\fR and \fIEVP_DigestSignFinal()\fR. For algorithms which
do not support streaming (e.g. PureEdDSA) it is the only way to sign data.
.PP
In previous versions of OpenSSL there was a link between message digest types
and public key algorithms. This meant that \*(L"clone\*(R" digests such as \fIEVP_dss1()\fR
needed to be used to sign using \s-1SHA1\s0 and \s-1DSA.\s0 This is no longer necessary and
the use of clone digest is now discouraged.
.PP
For some key types and parameters the random number generator must be seeded.
If the automatic seeding or reseeding of the OpenSSL \s-1CSPRNG\s0 fails due to
external circumstances (see \s-1\fIRAND\s0\fR\|(7)), the operation will fail.
.PP
The call to \fIEVP_DigestSignFinal()\fR internally finalizes a copy of the digest
context. This means that calls to \fIEVP_DigestSignUpdate()\fR and
\&\fIEVP_DigestSignFinal()\fR 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 \fIEVP_MD_CTX_free()\fR or a memory leak
will occur.
.PP
The use of \fIEVP_PKEY_size()\fR with these functions is discouraged because some
signature operations may have a signature length which depends on the
parameters set. As a result \fIEVP_PKEY_size()\fR would have to return a value
which indicates the maximum possible signature for any set of parameters.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fIEVP_DigestVerifyInit\fR\|(3),
\&\fIEVP_DigestInit\fR\|(3),
\&\fIevp\fR\|(7), \s-1\fIHMAC\s0\fR\|(3), \s-1\fIMD2\s0\fR\|(3),
\&\s-1\fIMD5\s0\fR\|(3), \s-1\fIMDC2\s0\fR\|(3), \s-1\fIRIPEMD160\s0\fR\|(3),
\&\s-1\fISHA1\s0\fR\|(3), \fIdgst\fR\|(1),
\&\s-1\fIRAND\s0\fR\|(7)
.SH "HISTORY"
.IX Header "HISTORY"
\&\fIEVP_DigestSignInit()\fR, \fIEVP_DigestSignUpdate()\fR and \fIEVP_DigestSignFinal()\fR
were added in OpenSSL 1.0.0.
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright 2006\-2020 The OpenSSL Project Authors. All Rights Reserved.
.PP
Licensed under the OpenSSL license (the \*(L"License\*(R").  You may not use
this file except in compliance with the License.  You can obtain a copy
in the file \s-1LICENSE\s0 in the source distribution or at
<https://www.openssl.org/source/license.html>.

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