CODEWITHSUNDEEP
xmldigital-signaturedigestxml-signature
user252816
user252816

Reputation: 573

XML Signature: How to calculate the digest value?

I have an XML like this

<?xml version="1.0" encoding="utf-8"?>
<foo>
  <bar>
    <value>A</value>
  </bar>
  <bar>
    <value>B</value>
  </bar>
  <baz>
    <value>C</value>
  </baz><Signature xmlns="http://www.w3.org/2000/09/xmldsig#"><SignedInfo><CanonicalizationMethod Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315" /><SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1" /><Reference URI=""><Transforms><Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature" /><Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#" /></Transforms><DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1" /><DigestValue>WqpRWHxXA0YgH+p3Sxy6hRo1XIk=</DigestValue></Reference></SignedInfo><SignatureValue>EoRk/GhR4UA4D+8AzGPPkeim1dZrlSy88eF73n/T9Lpeq9IxoGRHNUA8FEwuDNJuz3IugC0n2RHQQpQajiYvhlY3XG+z742pgsdMfFE4Pddk4gF1T8CVS1rsF7bjX+FKT/c8B2/C8FNgmfkxDlB/ochtbRvuAGPQGtgJ3h/wjSg=</SignatureValue><KeyInfo><X509Data><X509Certificate>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</X509Certificate></X509Data></KeyInfo></Signature>
</foo>

How the digest value (WqpRWHxXA0YgH+p3Sxy6hRo1XIk=) in the reference is created? I mean how can I compute this value manually?

Upvotes: 27

Views: 70429

Answers (5)

Tanuki
Tanuki

Reputation: 121

NB This answer is only about creating a digest value (for any file) using PowerShell, and not about XML canonicalization. However, your canonicalized XML file might look something like this:

<foo>
  <bar>
    <value>A</value>
  </bar>
  <bar>
    <value>B</value>
  </bar>
  <baz>
    <value>C</value>
  </baz>
</foo>

What is a digest value?

A digest value is a way to store a hash result with less bytes. This is accomplished by getting a hash in hex -> converting to raw data -> encoding in Base64 to get a plain-text value. You can use a PowerShell one-liner for this.

To get a SHA1 (or SHA256, or any other hashing algorithm) digest value for a file, we go through 3 steps:

  1. First we calculate a hash from a file, which is in hexadecimal. Length for SHA1 is 160 bits, for SHA256 it's 256 bits, or 40 and 64 characters, respectively (and for a hexadecimal hash, the same amount of bytes as characters in ANSI/UTF-8 encoding);
  2. Convert the hexadecimal characters to raw data. Eeach pair of hex chars is 1 byte, effectively halving the amount of data. This raw data should then be immediately passed on to step 3;
  3. Convert to Base64, which will result in a plain-text digest: 28 characters long for SHA1, 44 for SHA256.

How to calculate a digest value?

Three ways to accomplish this with PowerShell below. In the one-liners, replace PATH_TO_FILE with an actual filepath, like for example C:\Windows\System32\notepad.exe. You can replace SHA1 with SHA256 as needed (OP's question mentions the former).

At a PowerShell prompt:

[Convert]::ToBase64String([byte[]]((Get-FileHash -Path "PATH_TO_FILE" -Algorithm SHA1).Hash -split '(.{2})' -ne '' -replace '^', '0X'))

At a Windows Command Prompt:

powershell "[Convert]::ToBase64String([byte[]]((Get-FileHash -Path \"PATH_TO_FILE\" -Algorithm SHA1).Hash -split '(.{2})' -ne '' -replace '^', '0X'))

Or put this in a batch file (named e.g. "digest.cmd") and drop a file on it:

powershell "[Convert]::ToBase64String([byte[]]((Get-FileHash -Path \"%~1\" -Algorithm SHA1).Hash -split '(.{2})' -ne '' -replace '^', '0X'))"

The %~1 is automatically replaced by the filepath of the file you dropped on the batch; the ~ strips any surrounding double quotes, to avoid double quoting.

A more readable version of the one-liner:

$Hash = (Get-FileHash -Path "C:\Windows\System32\notepad.exe" -Algorithm SHA1).Hash
$HexToRawData = $Hash -split '(.{2})' -ne '' -replace '^', '0X'
$ByteArray = [byte[]]$HexToRawData
$Digest = [Convert]::ToBase64String($ByteArray)
$Digest

Upvotes: 0

KAD
KAD

Reputation: 11102

This is a JAVA solution which requires the following jars:

  • commons-logging-1.2.jar
  • commons-codec-1.6.jar
  • Saxon-HE-9.4.jar
  • xmlsec-1.3.0.jar

This solution uses http://www.w3.org/2001/10/xml-exc-c14n# as the canonicalization algorithm, and uses SHA256 as the hashing algorithm and base64 encoding.

Note: document represents the XML document as a DOM object in JAVA.

Code sample:

        // create the transformer in order to transform the document from
        // DOM Source as a JAVA document class, into a character stream (StreamResult) of
        // type String writer, in order to be converted to a string later on
        TransformerFactory tf = new net.sf.saxon.TransformerFactoryImpl();
        Transformer transformer = tf.newTransformer();
        transformer.setOutputProperty(OutputKeys.OMIT_XML_DECLARATION, "yes");
        transformer.setOutputProperty(OutputKeys.METHOD, "xml");
        transformer.setOutputProperty(OutputKeys.ENCODING, "UTF-8");

        // create the string writer and transform the document to a character stream
        StringWriter sw = new StringWriter();
        transformer.transform(new DOMSource(document), new StreamResult(sw));

        String documentAsString = sw.toString();

        // initialize the XML security object, which is necessary to run the apache canonicalization
        com.sun.org.apache.xml.internal.security.Init.init();

        // canonicalize the document to a byte array and convert it to string
        Canonicalizer canon = Canonicalizer.getInstance(Canonicalizer.ALGO_ID_C14N_EXCL_OMIT_COMMENTS);
        byte canonXmlBytes[] = canon.canonicalize(documentAsString.getBytes());
        String canonXmlString = new String(canonXmlBytes);

        // get instance of the message digest based on the SHA-256 hashing algorithm
        MessageDigest digest = MessageDigest.getInstance("SHA-256");

        // call the digest method passing the byte stream on the text, this directly updates the message
        // being digested and perform the hashing
        byte[] hash = digest.digest(canonXmlString.getBytes(StandardCharsets.UTF_8));

        // encode the endresult byte hash
        byte[] encodedBytes = Base64.encodeBase64(hash);

        return new String(encodedBytes);

Upvotes: 3

user3273546
user3273546

Reputation: 111

Is very simple, use openssl in the console:

openssl dgst -binary -sha1 file | openssl enc -base64

Done

Upvotes: 8

Kenny
Kenny

Reputation: 1090

I came across this question when attempting to find out the exact same thing. I later worked out how to do it, so figured I'd post the answer here.

The things that need to happen are:

  • canonicalization
  • create digest value, typically SHA1 (but could be SHA256 amongst others)
  • base64 encode it

The canonicalization part was fairly simple, as the Java libraries did that for me. What I struggled with was the next bit, the creating of the digest, because I made a fatal error in that the SHA1 digest I generated was the SHA1 in HEX form. SHA1 is 160 bits, so 20 bytes, but if you output these 160 bits in HEX, you get 40 characters. If you then base64 encode that, you get totally the wrong value compared to what should be in the DigestValue.

Instead, you should generate the SHA1 digest and base64 encode the 20 byte output. Don't try to output the 20 bytes to STDOUT as it's highly unlikely to be readable (which is why people often output the HEX equivalent, since it is readable). Instead, just base64 encode the 20 bytes and that's your DigestValue.

Upvotes: 28

Max
Max

Reputation: 2131

I have encountered exactly this problem myself: I was generating an XML signature in Java & validating in .NET, and the validation always failed. In my case the cause was the 'print XML to file' function XMLWrite.m (yes, in MATLAB*) which was 'pretty printing' the XML, inserting tabs, spaces, and newlines as it saw fit. Since these are part of the document, naturally the validation failed (it failed in Java, too). Looking at your source, this may be happening to you. Use a Transformer (javax.xml.transform.*) to serialise your DOM properly without changing the content.

*You did know that MATLAB understands Java as well? You can just type Java statements into the interpreter console & they will be executed like native m-code.

Upvotes: 3

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