AWS4 Signature in GoLang

Question

I started programming again recently and would need some help as I've been bashing my head against the keyboard for some days now but the code coming out doesn't seem to do the trick... The scope of my project is simple; send API requests to a server that uses AWS authentication I've implemented the below to create the signature:

    authString := "AWS4-HMAC-SHA256 Credential=**AWS Access**/"
    authString += time.Now().Format("20060102" /*T150405Z"*/) + "/"
    authString += "eu-west-1/"
    authString += "execute-api/"
    authString += "aws4_request,"
    authString += "SignedHeaders=content-type;host;x-amz-content-sha256;x-amz-date"

    awsSecret := "**tis a secret**"

    /*
        Pseudocode from documentation

           kSecret = your secret access key
           kDate = HMAC("AWS4" + kSecret, Date)
           kRegion = HMAC(kDate, Region)
           kService = HMAC(kRegion, Service)
           kSigning = HMAC(kService, "aws4_request")
    */

    hash := getHMAC([]byte("AWS4"+awsSecret), []byte(time.Now().Format("20060102")))
    hash = getHMAC(hash, []byte("eu-west-1"))
    hash = getHMAC(hash, []byte("execute-api"))
    hash = getHMAC(hash, []byte("aws4_request"))

    authString += ", Signature=" + hex.EncodeToString(hash)

    return authString
}

func getHMAC(key []byte, data []byte) []byte {
    hash := hmac.New(sha256.New, key)
    hash.Write(data)
    return hash.Sum(nil)
}

Signature string

AWS4-HMAC-SHA256 Credential=**AWS Access**/20200421/eu-west-1/execute-api/aws4_request,SignedHeaders=content-type;host;x-amz-content-sha256;x-amz-date, Signature=7b0fe4780c1c5ba39d0dee1774135d81c0bcca85f5e83325299c245eba1b0e5e

Response

{"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/prd/config/\n\ncontent-type:application/json\nhost:1294t77jvc.execute-api.eu-west-1.amazonaws.com\nx-amz-content-sha256:\nx-amz-date:2020-04-21T10:33:36+01:00\n\ncontent-type;host;x-amz-content-sha256;x-amz-date\n3cffc0f4da0132a4156d5c1a6506b4b163368ee9b131dce71e8316bd2220650b'\n\nThe String-to-Sign should have been\n'AWS4-HMAC-SHA256\n20200421T093336Z\n20200421/eu-west-1/execute-api/aws4_request\n3e40376452b02b8ba7f2826971e0438fd6891ccbf4c94e553dd91a2cc6f68560'\n"}

Bear in mind the response is with some bogus data, but it's pretty much the same as if it had the real AWS Access and secret keys. Feel free to criticise anything you see up there, as I'm trying to get good practices as well

Regards,

Answer 1

I somehow manage to successfully solved it based on your scripts. You're missing the string_to_sign with this pattern

stringToSign:= algorithm + "\n" + amzDate + "\n" + credentialScope + "\n" + hash(canonicalRequest)

and it should be included in the signature. So you have to like

signatureWithStringToSign := GetHMAC(hash, []byte(stringToSign))  
authString += ", Signature=" + hex.EncodeToString(signatureWithStringToSign)