EVP_PKEY_verify_recover.pod 3.0 KB
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=pod

=head1 NAME

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EVP_PKEY_verify_recover_init, EVP_PKEY_verify_recover - recover signature using a public key algorithm
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=head1 SYNOPSIS

 #include <openssl/evp.h>

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 int EVP_PKEY_verify_recover_init(EVP_PKEY_CTX *ctx);
 int EVP_PKEY_verify_recover(EVP_PKEY_CTX *ctx,
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			unsigned char *rout, size_t *routlen,
			const unsigned char *sig, size_t siglen);

=head1 DESCRIPTION

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The EVP_PKEY_verify_recover_init() function initializes a public key algorithm
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context using key B<pkey> for a verify recover operation.

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The EVP_PKEY_verify_recover() function recovers signed data
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using B<ctx>. The signature is specified using the B<sig> and
B<siglen> parameters. If B<rout> is B<NULL> then the maximum size of the output
buffer is written to the B<routlen> parameter. If B<rout> is not B<NULL> then
before the call the B<routlen> parameter should contain the length of the
B<rout> buffer, if the call is successful recovered data is written to
B<rout> and the amount of data written to B<routlen>.

=head1 NOTES

Normally an application is only interested in whether a signature verification
operation is successful in those cases the EVP_verify() function should be 
used.

Sometimes however it is useful to obtain the data originally signed using a
signing operation. Only certain public key algorithms can recover a signature
in this way (for example RSA in PKCS padding mode).

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After the call to EVP_PKEY_verify_recover_init() algorithm specific control
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operations can be performed to set any appropriate parameters for the
operation.

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The function EVP_PKEY_verify_recover() can be called more than once on the same
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context if several operations are performed using the same parameters.

=head1 RETURN VALUES

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EVP_PKEY_verify_recover_init() and EVP_PKEY_verify_recover() return 1 for success
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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.

=head1 EXAMPLE

Recover digest originally signed using PKCS#1 and SHA256 digest:

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 #include <openssl/evp.h>
 #include <openssl/rsa.h>

 EVP_PKEY_CTX *ctx;
 unsigned char *rout, *sig;
 size_t routlen, siglen; 
 EVP_PKEY *verify_key;
 /* NB: assumes verify_key, sig and siglen are already set up
  * and that verify_key is an RSA public key
  */
 ctx = EVP_PKEY_CTX_new(verify_key);
 if (!ctx)
	/* Error occurred */
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 if (EVP_PKEY_verify_recover_init(ctx) <= 0)
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	/* Error */
 if (EVP_PKEY_CTX_set_rsa_padding(ctx, RSA_PKCS1_PADDING) <= 0)
	/* Error */
 if (EVP_PKEY_CTX_set_signature_md(ctx, EVP_sha256()) <= 0)
	/* Error */

 /* Determine buffer length */
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 if (EVP_PKEY_verify_recover(ctx, NULL, &routlen, sig, siglen) <= 0)
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	/* Error */

 rout = OPENSSL_malloc(routlen);

 if (!rout)
	/* malloc failure */
 
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 if (EVP_PKEY_verify_recover(ctx, rout, &routlen, sig, siglen) <= 0)
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	/* Error */

 /* Recovered data is routlen bytes written to buffer rout */
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=head1 SEE ALSO

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L<EVP_PKEY_CTX_new(3)>,
L<EVP_PKEY_encrypt(3)>,
L<EVP_PKEY_decrypt(3)>,
L<EVP_PKEY_sign(3)>,
L<EVP_PKEY_verify(3)>,
L<EVP_PKEY_derive(3)> 
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=head1 HISTORY

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These functions were first added to OpenSSL 1.0.0.
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=cut