SHAKE documentation updates for default output length.

Fixes #18586

In order to not break existing applications the OpenSSL documentation
related to SHAKE has been updated.

Background:

All digests algorithms (including XOF's) use the bitlen as the default output length.
This results in a security strength of bitlen / 2.

This means that SHAKE128 will by default have an output length of 16
bytes and a security strength of 64 bits.

For SHAKE256 the default output length is 32 bytes and has a security
strength of 128 bits.

This behaviour was present in 1.1.1 and has been duplicated in the
provider SHAKE algorithms for 3.0.

The SHAKE XOF algorithms have a security strength of
min(bitlen, output xof length in bits / 2).

Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/18622)

apps/dgst.c

@@ -66,7 +66,7 @@ const OPTIONS dgst_options[] = {
     {"keyform", OPT_KEYFORM, 'f', "Key file format (ENGINE, other values ignored)"},
     {"hex", OPT_HEX, '-', "Print as hex dump"},
     {"binary", OPT_BINARY, '-', "Print in binary form"},
-    {"xoflen", OPT_XOFLEN, 'p', "Output length for XOF algorithms"},
+    {"xoflen", OPT_XOFLEN, 'p', "Output length for XOF algorithms. To obtain the maximum security strength set this to 32 (or greater) for SHAKE128, and 64 (or greater) for SHAKE256"},
     {"d", OPT_DEBUG, '-', "Print debug info"},
     {"debug", OPT_DEBUG, '-', "Print debug info"},
 
@@ -419,6 +419,11 @@ int dgst_main(int argc, char **argv)
             BIO_printf(bio_err, "Length can only be specified for XOF\n");
             goto end;
         }
+        /*
+         * Signing using XOF is not supported by any algorithms currently since
+         * each algorithm only calls EVP_DigestFinal_ex() in their sign_final
+         * and verify_final methods.
+         */
         if (sigkey != NULL) {
             BIO_printf(bio_err, "Signing key cannot be specified for XOF\n");
             goto end;

doc/man1/openssl-dgst.pod.in

@@ -86,7 +86,20 @@ Output the digest or signature in binary form.
 
 =item B<-xoflen> I<length>
 
-Set the output length for XOF algorithms, such as B<shake128>.
+Set the output length for XOF algorithms, such as B<shake128> and B<shake256>.
+This option is not supported for signing operations.
+
+For OpenSSL providers it is recommended to set this value for shake algorithms,
+since the default values are set to only supply half of the maximum security
+strength.
+
+For backwards compatibility reasons the default xoflen length for B<shake128> is
+16 (bytes) which results in a security strength of only 64 bits. To ensure the
+maximum security strength of 128 bits, the xoflen should be set to at least 32.
+
+For backwards compatibility reasons the default xoflen length for B<shake256> is
+32 (bytes) which results in a security strength of only 128 bits. To ensure the
+maximum security strength of 256 bits, the xoflen should be set to at least 64.
 
 =item B<-r>
 

doc/man7/EVP_MD-SHAKE.pod

@@ -15,18 +15,20 @@ implementation (see L<EVP_MAC-KMAC(7)>).
 
 =head2 Identities
 
-This implementation is only available with the default provider, and
-includes the following varieties:
+This implementation is available in the FIPS provider as well as the default
+provider, and includes the following varieties:
 
 =over 4
 
 =item KECCAK-KMAC-128
 
 Known names are "KECCAK-KMAC-128" and "KECCAK-KMAC128"
+This is used by L<EVP_MAC-KMAC128(7)>
 
 =item KECCAK-KMAC-256
 
 Known names are "KECCAK-KMAC-256" and "KECCAK-KMAC256"
+This is used by L<EVP_MAC-KMAC256(7)>
 
 =item SHAKE-128
 
@@ -55,6 +57,14 @@ settable for an B<EVP_MD_CTX> with L<EVP_MD_CTX_set_params(3)>:
 Sets the digest length for extendable output functions.
 The length of the "xoflen" parameter should not exceed that of a B<size_t>.
 
+For backwards compatibility reasons the default xoflen length for SHAKE-128 is
+16 (bytes) which results in a security strength of only 64 bits. To ensure the
+maximum security strength of 128 bits, the xoflen should be set to at least 32.
+
+For backwards compatibility reasons the default xoflen length for SHAKE-256 is
+32 (bytes) which results in a security strength of only 128 bits. To ensure the
+maximum security strength of 256 bits, the xoflen should be set to at least 64.
+
 =back
 
 =head1 SEE ALSO
@@ -63,7 +73,7 @@ L<EVP_MD_CTX_set_params(3)>, L<provider-digest(7)>, L<OSSL_PROVIDER-default(7)>
 
 =head1 COPYRIGHT
 
-Copyright 2020 The OpenSSL Project Authors. All Rights Reserved.
+Copyright 2020-2022 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

test/recipes/20-test_dgst.t

@@ -17,7 +17,7 @@ use OpenSSL::Test::Utils;
 
 setup("test_dgst");
 
-plan tests => 10;
+plan tests => 12;
 
 sub tsignverify {
     my $testtext = shift;
@@ -178,3 +178,29 @@ subtest "Custom length XOF digest generation with `dgst` CLI" => sub {
     ok($xofdata[1] =~ $expected,
        "XOF: Check second digest value is consistent with the first ($xofdata[1]) vs ($expected)");
 };
+
+subtest "SHAKE digest generation with no xoflen set `dgst` CLI" => sub {
+    plan tests => 1;
+
+    my $testdata = srctop_file('test', 'data.bin');
+    my @xofdata = run(app(['openssl', 'dgst', '-shake128', $testdata], stderr => "outerr.txt"), capture => 1);
+    chomp(@xofdata);
+    my $expected = qr/SHAKE-128\(\Q$testdata\E\)= bb565dac72640109e1c926ef441d3fa6/;
+    ok($xofdata[0] =~ $expected, "Check short digest is output");
+};
+
+SKIP: {
+    skip "ECDSA is not supported by this OpenSSL build", 1
+        if disabled("ec");
+
+    subtest "signing with xoflen is not supported `dgst` CLI" => sub {
+        plan tests => 1;
+        my $data_to_sign = srctop_file('test', 'data.bin');
+
+        ok(!run(app(['openssl', 'dgst', '-shake256', '-xoflen', '64',
+                     '-sign', srctop_file("test","testec-p256.pem"),
+                     '-out', 'test.sig',
+                     srctop_file('test', 'data.bin')])),
+                     "Generating signature with xoflen should fail");
+    }
+}