# Sign hashes

Hash signature allows the signing of document hashes instead of the actual documents. These hashes are signed by fidentity and can subsequently be embedded into a PDF outside of the fidentity flow.

#### Procedure

To sign hashes instead of documents, the initial call is made based on the QES variations. The `documentUris` array is set to empty and replaced by the `hashesToSign` array. Other than this difference, the calls for signing PDFs and hashes are identical.

**Example request**

```
  "documentUris": [],
  "hashesToSign": [{
      "id": "hash-id-{{$guid}}",
      "type": "signature",
      "title": "hash signature",
      "hash": "91951BA09F5213ADE633434681DFACC5F1C2F49920040AAEB451799037E40A3C",
      "hashAlgorithm": "sha256",
      "documentLink": ""
  }]
```

**Example respond**

At the end of the successful Process, you can get the process Object and in the field "signedHashes", there are 3 properties that are important:

* signature -> Signature of CMS object
* cmsObject -> CMS Object to integrate to pdf (pkcs7)
* certificate -> Public certificate of signing party (DER encoded)

````
"signedHashes": [
        {
            "id": "test-hash-1",
            "title": "QesSignature-cypress-test",
            "type": "contract",
            "hash": "538a54e1480b6...",
            "cmsObject": "MIIz+AYJK...",
            "preparedDigest": "17bd1...",
            "signature": "d181cffd28e...",
            "certificate": "MIIH2zCCBY...
        }
    ],```
````

#### PDF handling with Hash Signing

Our TSP Partner Swisscom has created a [step-by-step guide for developers ](https://trustservices.swisscom.com/en/support/help-center/how-to-embed-a-signed-hash-into-a-pdf-document)wishing to integrate hashes into PDF files by themselves. There are also references to some tools that provide this feature as well as github examples. 


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