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The XML Digital Signature standard operates as a two-part process:

1. The document to be signed is processed to generate a SignedInfo element.  This processing involves using a cryptographic hash function to reduce the final result to a shorter message digest.  We will refer to the selected hash function as the digest algorithm.

2. The SignedInfo element is digitally signed using a signature algorithm appropriate for the type of credential used.  For example, in the case of an RSA credential the signature algorithm used combines the RSA private key operation with a second cryptographic hash function used to produce a second message digest of the SignedInfo element.

By default, XmlSecTool selects the following digest and signature algorithms for each of the credential types:

To select a different hash function, XmlSecTool provides the --digest option, which takes the name of a hash function as parameter.  For example:

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Selecting a value for the --digest option changes both the digest algorithm and, for RSA and Elliptic Curve credentials, the appropriate component of the signature algorithm.  For example, selecting –digest SHA-512 would result in the following algorithms being selected for each credential type:

In some situations, this simplified selection process does not allow you to use the digest and signature algorithms you need for a particular application.  For example, hash functions such as MD5 are not included because they are regarded as being too weak for regular use.  In these cases, you can override the digest algorithm and signature algorithms selected based on the --digest option using the --digestAlgorithm and --signatureAlgorithm options.  These each take as parameter the full URI identifier of the algorithm to use, so that any algorithm supported by the Apache Santuario cryptographic library in use can be specified.  You can mix these fine-grained options with --digest, as in the following example:

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You should use the --digestAlgorithm and --signatureAlgorithm options, either together or individually, only if you understand the full implications of using non-standard algorithm combinations.

Signature

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Verification Algorithms Restrictions

As the state of the art in cryptanalysis progresses, algorithms that were once acceptable for use in digital signatures become first deprecated and then disallowed.  For example, the SHA-1 cryptographic hash function is now regarded as weak. NIST deprecated the use of SHA-1 in digital signatures from 2011, and disallows its use entirely from 2014. In order to guard against attacks on algorithms once regarded as strong but since determined to be weak, xmlsectool provides a signature verification algorithm blacklist mechanismmechanism to disallow weak algorithms; a signature being verified will be rejected if it makes use of an algorithm on the blacklistdisallowed list.

xmlsectool ships with a built-in default blacklist.  In version 2By default, xmlsectool version 3.0.0 , this blacklist contains only disallowed algorithms related to the MD5 and SHA-1 message digest functions.  This may be extended in future releases to include additional algorithms when appropriate.

If the default blacklist behaviour is not suitable, whether because it includes disallows an algorithm you need xmlsectool to accept or because it does not include disallow algorithms you regard as too weak, you can manipulate it using the following command-line options:

• --

  clearBlacklist

  allowAllDigests removes the default

  blacklist

  disallowed algorithms, so that all algorithms are allowed.  Do this only if you are sure that you need to use one of the algorithms

  in the default blacklist

  that are disallowed by default, and therefore need complete control over

  the blacklist contents.

  behaviour. It is better to use, e.g., "–-allowDigest SHA-1" if you can (see below). 

• --

  blacklistDigest adds

  disallowDigest disallows algorithms related to the given digest function

  to the blacklist

  .  For example, --

  blacklistDigest

  disallowDigest SHA256 would

  add

  disallow algorithms related to the SHA-256 digest function

  to the blacklist

  .

   This

  This option may appear any number of times.

• --

  whitelistDigest (added in xmlsectool V2.0.0) removes algorithms

  allowDigest allows algorithms related to the given digest function to

  the blacklist

  be used during signature verification. For example, --

  whitelistDigest

  allowDigest SHA1 would

  remove algorithms related to

  permit signatures based on the SHA-1 digest function

  from the blacklist

  to be verified. This option may appear any number of times.

• --

  listBlacklist

  listAlgorithms is an action option you can use to display the

  resulting contents of the blacklist

  algorithms which are currently disallowed. See the examples below.

These options can appear on the command line in any order, but will always be processed in the order given above.  Note also that these options only change the behaviour of xmlsectool for a single invocation; you must include them in the command line every time you want to override the default blacklistdefaults.

Example: viewing the default blacklist.

$ ./xmlsectool.sh --listBlacklistlistAlgorithms
DigestDisallowed algorithmdigest blacklistalgorithms:
   http://www.w3.org/2000/09/xmldsig#sha1
   http://www.w3.org/2001/04/xmldsig-more#md5

SignatureDisallowed algorithmsignature blacklistalgorithms:
   http://www.w3.org/2000/09/xmldsig#rsa-sha1
   http://www.w3.org/2001/04/xmldsig-more#ecdsa-sha1
   http://www.w3.org/2001/04/xmldsig-more#rsa-md5

Example: extending the default blacklistlist of disallowed algorithms.

$ ./xmlsectool.sh --blacklistDigestdisallowDigest SHA256 --listBlacklistlistAlgorithms
DigestDisallowed algorithmdigest blacklistalgorithms:
   http://www.w3.org/2000/09/xmldsig#sha1
   http://www.w3.org/2001/04/xmldsig-more#md5
   http://www.w3.org/2001/04/xmlenc#sha256

SignatureDisallowed algorithmsignature blacklistalgorithms:
   http://www.w3.org/2000/09/xmldsig#rsa-sha1
   http://www.w3.org/2001/04/xmldsig-more#ecdsa-sha1
   http://www.w3.org/2001/04/xmldsig-more#ecdsa-sha256
   http://www.w3.org/2001/04/xmldsig-more#rsa-md5
   http://www.w3.org/2001/04/xmldsig-more#rsa-sha256

Example: removing algorithms from the default blacklistlist of disallowed algorithms.

$ ./xmlsectool.sh --whitelistDigestallowDigest SHA1 --listBlacklistlistAlgorithms
DigestDisallowed algorithmdigest blacklistalgorithms:
   http://www.w3.org/2001/04/xmldsig-more#md5

SignatureDisallowed algorithmsignature blacklistalgorithms:
   http://www.w3.org/2001/04/xmldsig-more#rsa-md5

Example: full control (NOT RECOMMENDED).

$ ./xmlsectool.sh --clearBlacklistallowAllDigests --blacklistDigestdisallowDigest SHA1 --listBlacklistlistAlgorithms
DigestDisallowed algorithmdigest blacklistalgorithms:
   http://www.w3.org/2000/09/xmldsig#sha1
Signature
Disallowed algorithmsignature blacklistalgorithms:
   http://www.w3.org/2000/09/xmldsig#rsa-sha1
   http://www.w3.org/2001/04/xmldsig-more#ecdsa-sha1