EVP_DigestSignInit(3)

NAME
EVP_DigestSignInit, EVP_DigestSignUpdate, EVP_DigestSignFinal, EVP_DigestSign - EVP signing functions

LIBRARY
libcrypto, -lcrypto

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);

 int EVP_DigestSign(EVP_MD_CTX *ctx, unsigned char *sigret,
                    size_t *siglen, const unsigned char *tbs,
                    size_t tbslen);

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 e 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 if ctx is not assigned an EVP_PKEY_CTX value before being passed to EVP_DigestSignInit() (which means the EVP_PKEY_CTX is created inside EVP_DigestSignInit() and it will be freed automatically when the EVP_MD_CTX is freed).

The digest type may be NULL if the signing algorithm supports it.

No EVP_PKEY_CTX will be created by EVP_DigsetSignInit() if the passed ctx has already been assigned one via EVP_MD_CTX_set_ctx(3). See also SM2(7).

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, Poly1305, DSA, ECDSA, HMAC, RSA, SipHash, Ed25519 and Ed448.

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, SHA512 and SM3

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, SHA3-224, SHA3-256, SHA3-384, SHA3-512

Ed25519 and Ed448
    Support no digests (the digest type must be NULL)

HMAC
    Supports any digest

CMAC, Poly1305 and SipHash
    Will ignore any digest provided.

If RSA-PSS is used and restrictions apply then the digest must match.

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.

EVP_DigestSign() signs tbslen bytes of data at tbs and places the signature in sig and its length in siglen in a similar way to EVP_DigestSignFinal().

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

EVP_DigestSign() 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 EVP_DigestSignUpdate() and EVP_DigestSignFinal(). For algorithms which do not support streaming (e.g. PureEdDSA) it is the only way to sign data.

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_free() 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 added in 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>.