Office 365 Rest API - Daemon week authentication

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Office 365 Rest API - Daemon week authentication

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Instead of a `client_secret` in your request body, you need a `client_assertion`. This is a bit more complex, but it's the reason you need that certificate.

Basically you need to build a JSON Web Token and sign it with your certificate using a SHA256 hash. The token is going to look something like this:

Header:

```
{
"alg": "RS256",
"x5t": "..." // THUMBPRINT of Cert
}

```

Payload:

```
{
"aud": "https:\\/\\/login.windows.net\\/\\/oauth2\\/token",
"exp": 1423168488,
"iss": "YOUR CLIENT ID",
"jti": "SOME GUID YOU ASSIGN",
"nbf": 1423167888,
"sub": "YOUR CLIENT ID"
}

```

If you're still with me, you now need to base64-encode both pieces (separately), then concatenate them with a '.'. So now you should have:

```
base64_header.base64_payload

```

Now you take that string and sign it with your certificate, using a SHA256 hash. Then base64-encode the result of that, url-encode it, then append to the string, so now you have:

```
base64_header.base64_payload.base64_signature

```

Finally, include this in your POST to the token endpoint as the `client_assertion` parameter, and also include a `client_assertion_type` parameter set to "urn:ietf:params:oauth:client-assertion-type:jwt-bearer":

```
req.set_form_data(
:grant_type => 'client_credentials',
:redirect_uri => 'http://spready.dev',
:resource => 'https://outlook.office365.com/',
:client_id => '== Client ID ==',
:client_assertion_type => 'urn:ietf:params:oauth:client-assertion-type:jwt-bearer',
:client_assertion => 'base64_header.base64_payload.base64_signature'
)

```

I hope that helps! This is all based on my research into how [ADAL](https://github.com/AzureAD/azure-activedirectory-library-for-dotnet) does it, and I haven't tested it myself in Ruby.