NAME

       EVP_DigestVerifyInit_ex, EVP_DigestVerifyInit, EVP_DigestVerifyUpdate,
       EVP_DigestVerifyFinal, EVP_DigestVerify - EVP signature verification functions

SYNOPSIS

        #include <openssl/evp.h>

        int EVP_DigestVerifyInit_ex(EVP_MD_CTX *ctx, EVP_PKEY_CTX **pctx,
                                    const char *mdname, OSSL_LIB_CTX *libctx,
                                    const char *props, EVP_PKEY *pkey,
                                    const OSSL_PARAM params[]);
        int EVP_DigestVerifyInit(EVP_MD_CTX *ctx, EVP_PKEY_CTX **pctx,
                                 const EVP_MD *type, ENGINE *e, EVP_PKEY *pkey);
        int EVP_DigestVerifyUpdate(EVP_MD_CTX *ctx, const void *d, size_t cnt);
        int EVP_DigestVerifyFinal(EVP_MD_CTX *ctx, const unsigned char *sig,
                                  size_t siglen);
        int EVP_DigestVerify(EVP_MD_CTX *ctx, const 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.  Input data
       is digested first before the signature verification takes place.

       EVP_DigestVerifyInit_ex() sets up verification context ctx to use a digest with the name
       mdname and public key pkey. The name of the digest to be used is passed to the provider of
       the signature algorithm in use. How that provider interprets the digest name is provider
       specific. The provider may implement that digest directly itself or it may (optionally)
       choose to fetch it (which could result in a digest from a different provider being
       selected). If the provider supports fetching the digest then it may use the props argument
       for the properties to be used during the fetch. Finally, the passed parameters params, if
       not NULL, are set on the context before returning.

       The pkey algorithm is used to fetch a EVP_SIGNATURE method implicitly, to be used for the
       actual signing. See "Implicit fetch" in provider(7) for more information about implicit
       fetches.

       The OpenSSL default and legacy providers support fetching digests and can fetch those
       digests from any available provider. The OpenSSL FIPS provider also supports fetching
       digests but will only fetch digests that are themselves implemented inside the FIPS
       provider.

       ctx must be created with EVP_MD_CTX_new() before calling this function. If pctx is not
       NULL, the EVP_PKEY_CTX of the verification operation will be written to *pctx: this can be
       used to set alternative verification 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_DigestVerifyInit_ex() (which means the EVP_PKEY_CTX is created inside
       EVP_DigestVerifyInit_ex() and it will be freed automatically when the EVP_MD_CTX is
       freed). If the EVP_PKEY_CTX to be used is created by EVP_DigestVerifyInit_ex then it will
       use the OSSL_LIB_CTX specified in libctx and the property query string specified in props.

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

       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_DigestVerifyInit() works in the same way as EVP_DigestVerifyInit_ex() except that the
       mdname parameter will be inferred from the supplied digest type, and props will be NULL.
       Where supplied the ENGINE e will be used for the signature verification and digest
       algorithm implementations. e may be NULL.

       EVP_DigestVerifyUpdate() hashes cnt bytes of data at d into the verification context ctx.
       This function can be called several times on the same ctx to include additional data.

       EVP_DigestVerifyFinal() verifies the data in ctx against the signature in sig of length
       siglen.

       EVP_DigestVerify() verifies tbslen bytes at tbs against the signature in sig of length
       siglen.

RETURN VALUES

       EVP_DigestVerifyInit() and EVP_DigestVerifyUpdate() return 1 for success and 0 for
       failure.

       EVP_DigestVerifyFinal() and EVP_DigestVerify() return 1 for success; any other value
       indicates failure.  A return value of zero indicates that the signature did not verify
       successfully (that is, tbs did not match the original data or the signature had an invalid
       form), while other values indicate a more serious error (and sometimes also indicate an
       invalid signature form).

       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_DigestVerify() is a one shot operation which verifies a single block of data in one
       function. For algorithms that support streaming it is equivalent to calling
       EVP_DigestVerifyUpdate() and EVP_DigestVerifyFinal(). For algorithms which do not support
       streaming (e.g. PureEdDSA) it is the only way to verify 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.  If the
       automatic seeding or reseeding of the OpenSSL CSPRNG fails due to external circumstances
       (see RAND(7)), the operation will fail.

       The call to EVP_DigestVerifyFinal() internally finalizes a copy of the digest context.
       This means that EVP_VerifyUpdate() and EVP_VerifyFinal() can be called later to digest and
       verify additional data.

       EVP_DigestVerifyInit() and EVP_DigestVerifyInit_ex() functions can be called multiple
       times on a context and the parameters set by previous calls should be preserved if the
       pkey parameter is NULL. The call then just resets the state of the ctx.

SEE ALSO

       EVP_DigestSignInit(3), EVP_DigestInit(3), evp(7), HMAC(3), MD2(3), MD5(3), MDC2(3),
       RIPEMD160(3), SHA1(3), openssl-dgst(1), RAND(7)

HISTORY

       EVP_DigestVerifyInit(), EVP_DigestVerifyUpdate() and EVP_DigestVerifyFinal() were added in
       OpenSSL 1.0.0.

       EVP_DigestVerifyInit_ex() was added in OpenSSL 3.0.

       EVP_DigestVerifyUpdate() was converted from a macro to a function in OpenSSL 3.0.

COPYRIGHT

       Copyright 2006-2022 The OpenSSL Project Authors. All Rights Reserved.

       Licensed under the Apache License 2.0 (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>.