Signing Requests

S3 Glacier requires that you authenticate every request you send by signing the request. To sign a request, you calculate a digital signature using a cryptographic hash function. A cryptographic hash is a function that returns a unique hash value based on the input. The input to the hash function includes the text of your request and your secret access key. The hash function returns a hash value that you include in the request as your signature. The signature is part of the Authorization header of your request.

After receiving your request, S3 Glacier recalculates the signature using the same hash function and input that you used to sign the request. If the resulting signature matches the signature in the request, S3 Glacier processes the request. Otherwise, the request is rejected.

S3 Glacier supports authentication using AWS Signature Version 4. The process for calculating a signature can be broken into three tasks:

• Task 1: Create a Canonical Request

  Rearrange your HTTP request into a canonical format. Using a canonical form is necessary because S3 Glacier uses the same canonical form when it recalculates a signature to compare with the one you sent.

• Task 2: Create a String to Sign

  Create a string that you will use as one of the input values to your cryptographic hash function. The string, called the string to sign, is a concatenation of the name of the hash algorithm, the request date, a credential scope string, and the canonicalized request from the previous task. The credential scope string itself is a concatenation of date, AWS Region, and service information.

• Task 3: Create a Signature

  Create a signature for your request by using a cryptographic hash function that accepts two input strings: your string to sign and a derived key. The derived key is calculated by starting with your secret access key and using the credential scope string to create a series of hash-based message authentication codes (HMACs). Note that the hash function used in this signing step is not the tree-hash algorithm used in S3 Glacier APIs that upload data.

Example Signature Calculation

The following example walks you through the details of creating a signature for Create Vault (PUT vault). The example could be used as a reference to check your signature calculation method. For more information, see Signing AWS API requests in the IAM User Guide.

The example assumes the following:

• The time stamp of the request is Fri, 25 May 2012 00:24:53 GMT.

• The endpoint is US East (N. Virginia) Region us-east-1.

The general request syntax (including the JSON body) is:

PUT /-/vaults/examplevault HTTP/1.1
Host: glacier.us-east-1.amazonaws.com
Date: Fri, 25 May 2012 00:24:53 GMT
Authorization: SignatureToBeCalculated
x-amz-glacier-version: 2012-06-01

The canonical form of the request calculated for Task 1: Create a Canonical Request is:

PUT
/-/vaults/examplevault

host:glacier.us-east-1.amazonaws.com
x-amz-date:20120525T002453Z
x-amz-glacier-version:2012-06-01

host;x-amz-date;x-amz-glacier-version
e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855     

The last line of the canonical request is the hash of the request body. Also, note the empty third line in the canonical request. This is because there are no query parameters for this API.

The string to sign for Task 2: Create a String to Sign is:

AWS4-HMAC-SHA256
20120525T002453Z
20120525/us-east-1/glacier/aws4_request
5f1da1a2d0feb614dd03d71e87928b8e449ac87614479332aced3a701f916743

The first line of the string to sign is the algorithm, the second line is the time stamp, the third line is the credential scope, and the last line is a hash of the canonical request from Task 1: Create a Canonical Request. The service name to use in the credential scope is glacier.

For Task 3: Create a Signature, the derived key can be represented as:

derived key = HMAC(HMAC(HMAC(HMAC("AWS4" + YourSecretAccessKey,"20120525"),"us-east-1"),"glacier"),"aws4_request")

If the secret access key, wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY, is used, then the calculated signature is:

3ce5b2f2fffac9262b4da9256f8d086b4aaf42eba5f111c21681a65a127b7c2a

The final step is to construct the Authorization header. For the demonstration access key AKIAIOSFODNN7EXAMPLE, the header (with line breaks added for readability) is:

Authorization: AWS4-HMAC-SHA256 Credential=AKIAIOSFODNN7EXAMPLE/20120525/us-east-1/glacier/aws4_request, 
SignedHeaders=host;x-amz-date;x-amz-glacier-version, 
Signature=3ce5b2f2fffac9262b4da9256f8d086b4aaf42eba5f111c21681a65a127b7c2a 

Calculating Signatures for the Streaming Operations

Upload Archive (POST archive) and Upload Part (PUT uploadID) are streaming operations that require you to include an additional header x-amz-content-sha256 when signing and sending your request. The signing steps for the streaming operations are exactly the same as those for other operations, with the addition of the streaming header.

The calculation of the streaming header x-amz-content-sha256 is based on the SHA256 hash of the entire content (payload) that is to be uploaded. Note that this calculation is different from the SHA256 tree hash (Computing Checksums). Besides trivial cases, the SHA 256 hash value of the payload data will be different from the SHA256 tree hash of the payload data.

If the payload data is specified as a byte array, you can use the following Java code snippet to calculate the SHA256 hash.

public static byte[] computePayloadSHA256Hash2(byte[] payload) throws NoSuchAlgorithmException, IOException {
    BufferedInputStream bis = 
       new BufferedInputStream(new ByteArrayInputStream(payload));
    MessageDigest messageDigest = MessageDigest.getInstance("SHA-256");
    byte[] buffer = new byte[4096];
    int bytesRead = -1;
    while ( (bytesRead = bis.read(buffer, 0, buffer.length)) != -1 ) {
        messageDigest.update(buffer, 0, bytesRead);
    }
    return messageDigest.digest();
}

Similarly, in C# you can calculate the SHA256 hash of the payload data as shown in the following code snippet.

public static byte[] CalculateSHA256Hash(byte[] payload)
{
    SHA256 sha256 = System.Security.Cryptography.SHA256.Create();
    byte[] hash = sha256.ComputeHash(payload);

    return hash;
}

Example Signature Calculation for Streaming API

The following example walks you through the details of creating a signature for Upload Archive (POST archive), one of the two streaming APIs in S3 Glacier. The example assumes the following:

• The time stamp of the request is Mon, 07 May 2012 00:00:00 GMT.

• The endpoint is the US East (N. Virginia) Region, us-east-1.

• The content payload is a string "Welcome to S3 Glacier."

The general request syntax (including the JSON body) is shown in the example below. Note that the x-amz-content-sha256 header is included. In this simplified example, the x-amz-sha256-tree-hash and x-amz-content-sha256 are the same value. However, for archive uploads greater than 1 MB, this is not the case.

POST /-/vaults/examplevault HTTP/1.1
Host: glacier.us-east-1.amazonaws.com
Date: Mon, 07 May 2012 00:00:00 GMT
x-amz-archive-description: my archive
x-amz-sha256-tree-hash: SHA256 tree hash
x-amz-content-sha256: SHA256 payload hash  
Authorization: SignatureToBeCalculated
x-amz-glacier-version: 2012-06-01

The canonical form of the request calculated for Task 1: Create a Canonical Request is shown below. Note that the streaming header x-amz-content-sha256 is included with its value. This means you must read the payload and calculate the SHA256 hash first and then compute the signature.

POST
/-/vaults/examplevault

host:glacier.us-east-1.amazonaws.com
x-amz-content-sha256:726e392cb4d09924dbad1cc0ba3b00c3643d03d14cb4b823e2f041cff612a628
x-amz-date:20120507T000000Z
x-amz-glacier-version:2012-06-01

host;x-amz-content-sha256;x-amz-date;x-amz-glacier-version
726e392cb4d09924dbad1cc0ba3b00c3643d03d14cb4b823e2f041cff612a628     

The remainder of the signature calculation follows the steps outlined in Example Signature Calculation. The Authorization header using the secret access key wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY and the access key AKIAIOSFODNN7EXAMPLE is shown below (with line breaks added for readability):

Authorization=AWS4-HMAC-SHA256 
Credential=AKIAIOSFODNN7EXAMPLE/20120507/us-east-1/glacier/aws4_request, 
SignedHeaders=host;x-amz-content-sha256;x-amz-date;x-amz-glacier-version, 
Signature=b092397439375d59119072764a1e9a144677c43d9906fd98a5742c57a2855de6