Coinbase Exchange API with PowerShell - powershell

Question

I feel like I'm really close to being able to use the Coinbase Exchange API with PowerShell, but I'm having trouble creating a valid signature. Requests that don't require a signature, like /time and /products, work great.
Here's what I have so far.
$api = #{
"endpoint" = 'https://api.gdax.com'
"url" = '/account'
"method" = 'GET'
"body" = ''
"key" = 'xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx'
"secret" = 'xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx'
"passphrase" = 'xxxxxxxx'
}
# Base64 encoding/decoding functions derived from
# http://vstepic.blogspot.com/2013/02/how-to-convert-string-to-base64-and.html
function Base64-Encode($string) {
$conversion = [System.Text.Encoding]::ASCII.GetBytes($string)
return [System.Convert]::ToBase64String($conversion)
}
function Base64-Decode($string) {
$conversion = [System.Convert]::FromBase64String($string)
return [System.Text.Encoding]::ASCII.GetString($conversion)
}
# HMAC SHA256 code derived from
# http://www.jokecamp.com/blog/examples-of-creating-base64-hashes-using-hmac-sha256-in-different-languages/
function hmac($message, $secret) {
$hmacsha = New-Object System.Security.Cryptography.HMACSHA256
$hmacsha.key = [Text.Encoding]::ASCII.GetBytes($secret)
$signature = $hmacsha.ComputeHash([Text.Encoding]::ASCII.GetBytes($message))
$signature = [Convert]::ToBase64String($signature)
return $signature
}
function Submit-Request($request) {
$unixEpochStart = Get-Date -Date "01/01/1970"
$now = Get-Date
$timestamp = (New-TimeSpan -Start $unixEpochStart -End $now.ToUniversalTime()).TotalSeconds.ToString()
# create the prehash string by concatenating required parts
$prehash = $timestamp + $request.method.ToUpper() + $request.url + $request.body
$signature_b64 = hmac -message $prehash -secret (Base64-Decode $request.secret)
$header = #{
"CB-ACCESS-KEY" = $request.key
"CB-ACCESS-SIGN" = $signature_b64
"CB-ACCESS-TIMESTAMP" = $timestamp
"CB-ACCESS-PASSPHRASE" = $request.passphrase
"Content-Type" = 'application/json'
}
$uri = $request.endpoint + $request.url
if ($request.method.ToUpper() -eq 'POST') {
$response = Invoke-RestMethod -Method $request.method -Uri $uri -Headers $header -Body $request.body
} else {
$response = Invoke-RestMethod -Method $request.method -Uri $uri -Headers $header
}
return $response
}
$api.method = 'GET'
$api.url = '/account'
$response = Submit-Request $api
Write-Output $response

Answer 1

After reviewing some C# code in the Coinbase community, I was able to revise my code and get it working. Decoding of the secret key did not need to go to a string format, which is what was happening when I called Base64-Decode before passing the secret to the HMAC function. I followed the C# example and decoded it directly in the HMAC function without making a string out of it. Another change I made was to make the timestamp match the format retrieved from /time, using 3 decimal places instead of 5.
Here's my revised code. I hope it will be of help to others.
$api = #{
"endpoint" = 'https://api.gdax.com'
"url" = '/account'
"method" = 'GET'
"body" = ''
"key" = 'xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx'
"secret" = 'xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx'
"passphrase" = 'xxxxxxxx'
}
# Base64 encoding/decoding functions derived from
# http://vstepic.blogspot.com/2013/02/how-to-convert-string-to-base64-and.html
function Base64-Encode($string) {
$conversion = [System.Text.Encoding]::ASCII.GetBytes($string)
return [System.Convert]::ToBase64String($conversion)
}
function Base64-Decode($string) {
$conversion = [System.Convert]::FromBase64String($string)
return [System.Text.Encoding]::ASCII.GetString($conversion)
}
# HMAC SHA256 code derived from
# http://www.jokecamp.com/blog/examples-of-creating-base64-hashes-using-hmac-sha256-in-different-languages/
function hmac($message, $secret) {
$hmacsha = New-Object System.Security.Cryptography.HMACSHA256
$hmacsha.key = [Convert]::FromBase64String($secret)
$signature = $hmacsha.ComputeHash([Text.Encoding]::ASCII.GetBytes($message))
$signature = [Convert]::ToBase64String($signature)
return $signature
}
function Submit-Request($request) {
$unixEpochStart = Get-Date -Date "01/01/1970"
$now = Get-Date
$timestamp = (New-TimeSpan -Start $unixEpochStart -End $now.ToUniversalTime()).TotalSeconds
# round timestamp to 3 decimal places and convert to string
$timestamp = ([math]::Round($timestamp, 3)).ToString()
# create the prehash string by concatenating required parts
$prehash = $timestamp + $request.method.ToUpper() + $request.url + $request.body
$signature_b64 = hmac -message $prehash -secret $request.secret
$header = #{
"CB-ACCESS-KEY" = $request.key
"CB-ACCESS-SIGN" = $signature_b64
"CB-ACCESS-TIMESTAMP" = $timestamp
"CB-ACCESS-PASSPHRASE" = $request.passphrase
"Content-Type" = 'application/json'
}
$uri = $request.endpoint + $request.url
if ($request.method.ToUpper() -eq 'POST') {
$response = Invoke-RestMethod -Method $request.method -Uri $uri -Headers $header -Body $request.body
} else {
$response = Invoke-RestMethod -Method $request.method -Uri $uri -Headers $header
}
return $response
}
$api.method = 'GET'
$api.url = '/accounts'
$response = Submit-Request $api
Write-Output $response