EVP_DIGESTSIGNINIT(3) | Library Functions Manual | EVP_DIGESTSIGNINIT(3)

# NAME

`EVP_DigestSignInit`, `EVP_DigestSignUpdate`, `EVP_DigestSignFinal`, `EVP_DigestSign` — EVP signing functions

# SYNOPSIS

library “libcrypto”
`#include <openssl/evp.h>`

int
`EVP_DigestSignInit`(EVP_MD_CTX *ctx, EVP_PKEY_CTX **pctx, const EVP_MD *type, ENGINE *engine, 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 the signing context ctx to use the digest type and the private key pkey. Before calling this function, obtain ctx from EVP_MD_CTX_new(3) or call EVP_MD_CTX_reset(3) on it. The engine argument is always ignored and passing `NULL` is recommended.

If pctx is not `NULL`, any pointer passed in as *pctx is ignored and overwritten by an internal pointer to the EVP_PKEY_CTX used by the signing operation: this can be used to set alternative signing options. The returned EVP_PKEY_CTX must not be freed by the application. It is freed automatically when the EVP_MD_CTX is freed.

`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 *siglen. If sig is not `NULL`, then before the call siglen 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 sigret and its length in siglen in a similar way to `EVP_DigestSignFinal`(). `EVP_DigestSign`() is a one shot operation which signs a single block of data with one function call. For algorithms that support streaming it is equivalent to calling `EVP_DigestSignUpdate`() and `EVP_DigestSignFinal`().

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.

The call to `EVP_DigestSignFinal`() internally finalizes a copy of the digest context. This means that `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(3), or a memory leak will occur.

The use of 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, EVP_PKEY_size(3) would have to return a value which indicates the maximum possible signature for any set of parameters.

# RETURN VALUES

`EVP_DigestSignInit`(), `EVP_DigestSignUpdate`(), `EVP_DigestSignFinal`(), and `EVP_DigestSign`() return 1 for success and 0 for failure.

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

# SEE ALSO

evp(3), EVP_DigestInit(3), EVP_DigestVerifyInit(3)

# HISTORY

`EVP_DigestSignInit`(), `EVP_DigestSignUpdate`(), and `EVP_DigestSignFinal`() first appeared in OpenSSL 1.0.0 and have been available since OpenBSD 4.9.

`EVP_DigestSign`() first appeared in OpenSSL 1.1.1 and has been available since OpenBSD 7.0.

June 8, 2025 | Debian