Monday, January 4, 2021

Microsoft Graph: Encoding and decoding the drive id


If you have worked with the Microsoft Graph API and SharePoint items you might have encountered URL’s which include a drive id. Where you in SharePoint typically work with URL’s for a site and the document library, a drive id is an encoded representation of a document library location.

The following Graph REST request will list all items in a specific document library:


Fetching the item for a file you use this signature to retrieve the file itself where item id is part of the listing: GET /drives/{drive-id}/items/{item-id}

With some reverse engineering you easily can figure out the drive-id is a prefixed base64 encoded string composed of the site id, web id and list id for a particular library.

Using PowerShell here’s a few lines converting the drive id string to the correct guid’s and back:

$driveId = "b!VvpCx03990mC5Lb5YxH0SUA9TgZHvEZImra6PMjrvbx85KUwT1BMTbhen6I6ffXL"   
$encodedDriveId = $driveId.Substring(2).Replace('_','/').Replace('-','+')
$bytes = [Convert]::FromBase64String($encodedDriveId)
$siteIdBytes = [byte[]]$bytes[0..15]
$siteIdGuid = New-Object Guid @(,$siteIdBytes) #site id

$webIdBytes = [byte[]]$bytes[16..31]
$webIdGuid = New-Object Guid @(,$webIdBytes) #web id

$listIdBytes = [byte[]]$bytes[32..47]
$listIdGuid = New-Object Guid @(,$listIdBytes) #list/library id

$bytes = $siteIdGuid.ToByteArray() + $webIdGuid.ToByteArray() + $listIdGuid.ToByteArray()
$driveId = "b!" + [Convert]::ToBase64String($bytes)

The item-id part is a bit more tricky and looks to be some sort of base32 encoding of a SharePoint item’s unique id. As I haven’t figured out the mechanics you can still access the item by route of the list item unique id:



Of course, if you already have the site id, web id, list id and unique item id, you can access a drive item using the following syntax as mentioned in my file preview post.

GET{site-id},{web-id}/lists/{list-id}/items/{item- id}/driveItem

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