EVP_DIGESTSIGNINIT(3)

NAME
EVP_DigestSignInit, EVP_DigestSignUpdate, EVP_DigestSignFinal - EVP signing functions

SYNOPSIS
 #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);

DESCRIPTION
The EVP signature routines are a high level interface to digital signatures.

EVP_DigestSignInit() sets up signing context ctx to use digest type from ENGINE impl and private key pkey. ctx must be created with EVP_MD_CTX_new() before calling this function. If pctx is not NULL, the EVP_PKEY_CTX of the signing operation will be written to *pctx: this can be used to set alternative signing options. Note that any existing value in *pctx is overwritten. The EVP_PKEY_CTX value returned must not be freed directly by the application (it will be freed automatically when the EVP_MD_CTX is freed). The digest type may be NULL if the signing algorithm supports it.

Only EVP_PKEY types that support signing can be used with these functions. This includes MAC algorithms where the MAC generation is considered as a form of "signing". Built-in EVP_PKEY types supported by these functions are CMAC, DSA, ECDSA, HMAC and RSA.

Not all digests can be used for all key types. The following combinations apply.

DSA Supports SHA1, SHA224, SHA256, SHA384 and SHA512

ECDSA
    Supports SHA1, SHA224, SHA256, SHA384 and SHA512

RSA with no padding
    Supports no digests (the digest type must be NULL)

RSA with X931 padding
    Supports SHA1, SHA256, SHA384 and SHA512

All other RSA padding types
    Support SHA1, SHA224, SHA256, SHA384, SHA512, MD5, MD5_SHA1, MD2, MD4, MDC2, RIPEMD160

HMAC
    Supports any digest

CMAC
    Will ignore any digest provided.

EVP_DigestSignUpdate() hashes cnt bytes of data at d into the signature context ctx. This function can be called several times on the same ctx to include additional data. This function is currently implemented using a macro.

EVP_DigestSignFinal() signs the data in ctx and places the signature in sig. If sig is NULL then the maximum size of the output buffer is written to the siglen parameter. If sig is not NULL then before the call the siglen parameter should contain the length of the sig buffer. If the call is successful the signature is written to sig and the amount of data written to siglen.

RETURN VALUES
EVP_DigestSignInit() EVP_DigestSignUpdate() and EVP_DigestSignaFinal() 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.

The error codes can be obtained from ERR_get_error(3).

NOTES
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.

In previous versions of OpenSSL there was a link between message digest types and public key algorithms. This meant that "clone" digests such as EVP_dss1() needed to be used to sign using SHA1 and DSA. This is no longer necessary and the use of clone digest is now discouraged.

For some key types and parameters the random number generator must be seeded or the operation will fail.

The call to EVP_DigestSignFinal() internally finalizes a copy of the digest context. This means that calls to EVP_DigestSignUpdate() and EVP_DigestSignFinal() can be called later to digest and sign additional data.

Since only a copy of the digest context is ever finalized, the context must be cleaned up after use by calling EVP_MD_CTX_cleanup() or a memory leak will occur.

The use of EVP_PKEY_size() with these functions is discouraged because some signature operations may have a signature length which depends on the parameters set. As a result EVP_PKEY_size() would have to return a value which indicates the maximum possible signature for any set of parameters.

SEE ALSO
EVP_DigestVerifyInit(3), EVP_DigestInit(3), evp(7), HMAC(3), MD2(3), MD5(3), MDC2(3), RIPEMD160(3), SHA1(3), dgst(1)

HISTORY
EVP_DigestSignInit(), EVP_DigestSignUpdate() and EVP_DigestSignFinal() were first added to OpenSSL 1.0.0.

COPYRIGHT
Copyright 2006-2018 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the OpenSSL license (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at <https://www.openssl.org/source/license.html>.