I am trying to generate a SAS token for a Blob Storage account using C#. However, I’m encountering an error when trying to authenticate the request:

<Error>
    <Code>AuthenticationFailed</Code>
    <Message>Server failed to authenticate the request. Make sure the value of Authorization header is formed correctly including the signature.
    RequestId:0ead0723-901e-002c-7b88-037068000000 Time:2024-09-10T13:52:02.2658050Z</Message>
    <AuthenticationErrorDetail>Signature fields not well formed.</AuthenticationErrorDetail>
</Error>

To clarify, I am trying to generate the SAS token without using the NuGet package “Azure.Storage.Blobs”. I managed to generate the SAS token successfully using the library, but now I want to achieve the same result purely with APIs. Below is the code I’m using to generate the SAS token:

private string GenerateSasToken(string accountName, string accountKey, string containerName, string blobName)
{
    // Define SAS parameters
    string signedPermissions = "r"; // Read permissions
    string signedStart = DateTime.UtcNow.AddMinutes(-5).ToString("yyyy-MM-ddTHH:mm:ssZ"); // Start time is 5 minutes ago to account for clock skew
    string signedExpiry = DateTime.UtcNow.AddHours(8).ToString("yyyy-MM-ddTHH:mm:ssZ"); // Expiry time 8 hours from now
    string signedResource = "b"; // Blob resource type
    string signedVersion = "2022-11-02"; // Storage service version
    string signedProtocol = "https"; // HTTPS only
    string signedIp = ""; // No IP restriction

    // Canonicalized resource: "/blob/account/container/blob"
    string canonicalizedResource = $"/blob/{accountName}/{containerName}/{blobName}";

    // Construct the string-to-sign
    string stringToSign = $"{signedPermissions}n" +
                          $"{signedStart}n" +
                          $"{signedExpiry}n" +
                          $"{canonicalizedResource}n" +
                          $"n" + // signedIdentifier (optional, left empty)
                          $"{signedIp}n" +
                          $"{signedProtocol}n" +
                          $"{signedVersion}n";

    // Decode the account key from Base64
    byte[] keyBytes = Convert.FromBase64String(accountKey);

    // Create HMAC-SHA256 hash
    using (HMACSHA256 hmac = new HMACSHA256(keyBytes))
    {
        byte[] signatureBytes = hmac.ComputeHash(Encoding.UTF8.GetBytes(stringToSign));
        string signature = Convert.ToBase64String(signatureBytes);

        // Construct the SAS token
        var sasToken = HttpUtility.ParseQueryString(string.Empty);
        sasToken["sp"] = signedPermissions;
        sasToken["st"] = signedStart;
        sasToken["se"] = signedExpiry;
        sasToken["spr"] = signedProtocol;
        sasToken["sv"] = signedVersion;
        sasToken["sr"] = signedResource;
        sasToken["sig"] = HttpUtility.UrlEncode(signature); // URL-encoded signature

        // Return the full blob URL with the SAS token
        return $"https://{accountName}.blob.core.windows.net/{containerName}/{blobName}?{sasToken}";
    }
}

Problem:
I am getting the error that the signature fields are not well-formed. This seems to indicate that there is an issue with how the string-to-sign or the signature itself is being constructed. Does anyone have experience with this error, or can you spot anything wrong with my code?

Any help or advice would be greatly appreciated!

I really appreciate any help you can provide.