Is it possible to use Postman to access Dynamodb via http and the dynamo api

I'm attempting to use Postman, the chrome extension api client Postman to access Dynamodb via http. This is actually an approach I read about in the Book Dynamodb Applied Design Patterns. It appears that I am doing something wrong because I am unable to sucessfully authenticate and gain access to the web service.

The url and headers I'm using in the Postman client with my most recent request parameters and error messages follow:

url: dynamodb.us-east-1.amazonaws.com
x-amz-date: 20150701T162011Z
x-amz-targe: DynamoDB_20120810.ListTables
authorizaton: AWS4-HMAC-SHA256 Credential=AMyAccessCode/20150701/us-east-1/dynamodb/aws4_request, SignedHeaders=host;x-amz-date;x-amz-target, Signature=8ngTnF8WH//njvBdY5bY5dSp5CAKi8qTXNFuv5Ws+30=
content-type: application/x-amz-json-1.0

The Body of my request is:

{
"ExclusiveStartTableName": "Owner",
"Limit": 3
}

I'm getting the following error message:

{"__type":"com.amazon.coral.service#InvalidSignatureException","message":"The request signature we calculated does not match the signature you provided. Check your AWS Secret Access Key and signing method. Consult the service documentation for details.\n\nThe Canonical String for this request should have been\n'POST\n/\n\nhost:dynamodb.us-east-1.amazonaws.com\nx-amz-date:20150701T162011Z\nx-amz-target:DynamoDB_20120810.ListTables\n\nhost;x-amz-date;x-amz-target\nb9e264461dcb0e94e69652f8b2d17c737a29506863d6f09c0f9fc98e9d560e5c'\n\nThe String-to-Sign should have been\n'AWS4-HMAC-SHA256\n20150701T162011Z\n20150701/us-east-1/dynamodb/aws4_request\n7a6da3d9e8ed6317e0cb9217e9ea1174d01e86871a159f339f5f6969283264d5'\n"}

Question/Oddity 1. When I issue a request I usually get response back indicating that my request has expired. The error message will include "valid" times that are four hours ahead of the actual time tha I used. When I use the time provided by the error message in the request, I no longer receive the exprired request error message.

Then I get this error message:

"__type":"com.amazon.coral.service#InvalidSignatureException","message":"The request signature we calculated does not match the signature you provided. Check your AWS Secret Access Key and signing method. Consult the service documentation for details.\n\nThe Canonical String for this request should have been\n'POST\n/\n\nhost:dynamodb.us-east-1.amazonaws.com\nx-amz-date:20150701T162011Z\nx-amz-target:DynamoDB_20120810.ListTables\n\nhost;x-amz-date;x-amz-target\nb9e264461dcb0e94e69652f8b2d17c737a29506863d6f09c0f9fc98e9d560e5c'\n\nThe String-to-Sign should have been\n'AWS4-HMAC-SHA256\n20150701T162011Z\n20150701/us-east-1/dynamodb/aws4_request\n7a6da3d9e8ed6317e0cb9217e9ea1174d01e86871a159f339f5f6969283264d5'\n"}

Question 2. This message seems to indicate I have to calculate the hash of the canonical request. The documentation on amazon is a bit unclear on this. Do I need to calcuate the has of the canonical request and if so do I include that as a header" If so, what is the header name?

I have been able to calculate the signature and replicate the signature in the Amazon documentation.

Thanks for your input.