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.\" ========================================================================
.\"
.IX Title "DSA_sign 3"
.TH DSA_sign 3 "2019-12-20" "1.0.2u" "OpenSSL"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
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.SH "NAME"
DSA_sign, DSA_sign_setup, DSA_verify \- DSA signatures
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #include <openssl/dsa.h>
\&
\& int    DSA_sign(int type, const unsigned char *dgst, int len,
\&                unsigned char *sigret, unsigned int *siglen, DSA *dsa);
\&
\& int    DSA_sign_setup(DSA *dsa, BN_CTX *ctx, BIGNUM **kinvp,
\&                BIGNUM **rp);
\&
\& int    DSA_verify(int type, const unsigned char *dgst, int len,
\&                unsigned char *sigbuf, int siglen, DSA *dsa);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\fBDSA_sign()\fR computes a digital signature on the \fBlen\fR byte message
digest \fBdgst\fR using the private key \fBdsa\fR and places its \s-1ASN.1 DER\s0
encoding at \fBsigret\fR. The length of the signature is places in
*\fBsiglen\fR. \fBsigret\fR must point to DSA_size(\fBdsa\fR) bytes of memory.
.PP
\&\fBDSA_sign_setup()\fR may be used to precompute part of the signing
operation in case signature generation is time-critical. It expects
\&\fBdsa\fR to contain \s-1DSA\s0 parameters. It places the precomputed values
in newly allocated \fB\s-1BIGNUM\s0\fRs at *\fBkinvp\fR and *\fBrp\fR, after freeing
the old ones unless *\fBkinvp\fR and *\fBrp\fR are \s-1NULL.\s0 These values may
be passed to \fBDSA_sign()\fR in \fBdsa\->kinv\fR and \fBdsa\->r\fR.
\&\fBctx\fR is a pre-allocated \fB\s-1BN_CTX\s0\fR or \s-1NULL.\s0
The precomputed values from \fBDSA_sign_setup()\fR \fB\s-1MUST NOT\s0 be used\fR for
more than one signature: using the same \fBdsa\->kinv\fR and
\&\fBdsa\->r\fR pair twice under the same private key on different
plaintexts will result in permanently exposing the \s-1DSA\s0 private key.
.PP
\&\fBDSA_verify()\fR verifies that the signature \fBsigbuf\fR of size \fBsiglen\fR
matches a given message digest \fBdgst\fR of size \fBlen\fR.
\&\fBdsa\fR is the signer's public key.
.PP
The \fBtype\fR parameter is ignored.
.PP
The \s-1PRNG\s0 must be seeded before \fBDSA_sign()\fR (or \fBDSA_sign_setup()\fR)
is called.
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
\&\fBDSA_sign()\fR and \fBDSA_sign_setup()\fR return 1 on success, 0 on error.
\&\fBDSA_verify()\fR returns 1 for a valid signature, 0 for an incorrect
signature and \-1 on error. The error codes can be obtained by
\&\fBERR_get_error\fR\|(3).
.SH "CONFORMING TO"
.IX Header "CONFORMING TO"
\&\s-1US\s0 Federal Information Processing Standard \s-1FIPS 186\s0 (Digital Signature
Standard, \s-1DSS\s0), \s-1ANSI X9.30\s0
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBdsa\fR\|(3), \fBERR_get_error\fR\|(3), \fBrand\fR\|(3),
\&\fBDSA_do_sign\fR\|(3)
.SH "HISTORY"
.IX Header "HISTORY"
\&\fBDSA_sign()\fR and \fBDSA_verify()\fR are available in all versions of SSLeay.
\&\fBDSA_sign_setup()\fR was added in SSLeay 0.8.