Update docs with algorithm options.

the input is a certificate containing a public key. 

=item B<-rev>

reverse the order of the input buffer. This is useful for some libraries

(such as CryptoAPI) which represent the buffer in little endian format.

=item B<-sign>

sign the input data and output the signed result. This requires

The operations and options supported vary according to the key algorithm

and its implementation. The OpenSSL operations and options are indicated below.

Unless otherwise mentioned all algorithms support the B<digest:alg> option

which specifies the digest in use for sign, verify and verifyrecover operations.

The value B<alg> should represent a digest name as used in the

EVP_get_digestbyname() function for example B<sha1>.

=head1 RSA ALGORITHM

The RSA algorithm supports encrypt, decrypt, sign, verify and verifyrecover

operations in general. Some padding modes only support some of these 

operations however.

[NB: more to be added later]

=over 4

=item -B<rsa_padding_mode:mode>

This sets the RSA padding mode. Acceptable values for B<mode> are B<pkcs1> for

PKCS#1 padding, B<sslv23> for SSLv23 padding, B<none> for no padding, B<oaep>

for B<OAEP> mode, B<x931> for X9.31 mode and B<pss> for PSS.

In PKCS#1 padding if the message digest is not set then the supplied data is 

signed or verified directly instead of using a B<DigestInfo> structure. If a

digest is set then the a B<DigestInfo> structure is used and its the length

must correspond to the digest type.

For B<oeap> mode only encryption and decryption is supported.

For B<x931> if the digest type is set it is used to format the block data

otherwise the first byte is used to specify the X9.31 digest ID. Sign,

verify and verifyrecover are can be performed in this mode.

For B<pss> mode only sign and verify are supported and the digest type must be

specified.

=item B<rsa_pss_saltlen:len>

For B<pss> mode only this option specifies the salt length. Two special

values are supported: -1 sets the salt length to the digest length. When

signing -2 sets the salt length to the maximum permissible value. When

verifying -2 causes the salt length to be automatically determined based

on the B<PSS> block structure.

=back

=head1 DSA ALGORITHM

The DSA algorithm supports signing and verification operations only. Currently

there are no additional options other than B<digest>. Only the SHA1

digest can be used and this digest is assumed by default.

=head1 DH ALGORITHM

The DH algorithm only supports the derivation operation and no additional

options.

=head1 EC ALGORITHM

The EC algorithm supports sign, verify and derive operations. The sign and

verify operations use ECDSA and derive uses ECDH. Currently there are no

additional options other than B<digest>. Only the SHA1 digest can be used and

this digest is assumed by default.

=head1 EXAMPLES

 openssl pkeyutl -verify -in file -sigfile sig -inkey key.pem

Sign data using a message digest value (this is currently only valid for RSA):

 openssl pkeyutl -sign -in file -inkey key.pem -out sig -pkeyopt digest:sha256

Derive a shared secret value:

 openssl pkeyutl -derive -inkey key.pem -peerkey pubkey.pem -out secret

=head1 SEE ALSO

L<genpkey(1)|genpkey(1)>, L<pkey(1)|pkey(1)>, L<rsautl(1)|rsautl(1)>