Using R's Digest Package to Sign an HTTP request to Amazon Dynamo's API

I trying to use Rs Digest package to create a signature for Amazon's API for Dynamodb. The signature requires sha256. Currently, I am testing R's digest package to see if its yield the correct output, given my input.

According to the Amazon website example, if my input is:

Input: "iam"

then my output should be:

Targeted Output: 'f72cfd46f26bc4643f06a11eabb6c0ba18780c19a8da0c31ace671265e3c87fa'

When I use the following R command:

digest("iam", algo="sha256", serialize=FALSE)

I get the following output:

"d457e3a99392a03f47057f50ac1cbc5d0365131575477971bf85177a0c0fed22"

I've tried various input combinations (setting serialize=TRUE, etc.) but have not had any luck.

Does anyone have any experience with the Digest package and using it to create they Amazon signatures that might be able to point out what I'm doing wrong?

Thanks for you help.