Cognito Custom Email Sender Code Decryption

I am implementing a custom email sending Lambda for Cognito. When attempting to decrypt the "code" I am receiving the "Aws::KMS::Errors::InvalidCiphertextException" exception.

I have a symmetric KMS key assigned to the Lambda via the Cognito User Pool, and can successfully trigger the Lambda. I have attempted to use both "Base64.decode64" and Base64.strict_decode64" before passing the ciphertext to the decode method.

The CloudTrail logs are showing that Cognito is using the KMS GenerateKeyPair method, and providing an encryption context:

{
  "userpool-id": "my-user-pool",
  "aws-crypto-public-key": "AULXovx/...==" 
}

I have therefore attempted to pass this "encryption_context" to the decrypt method, however the "aws-crypto-public-key" is unique to each request. The KMS documentation explains (as best I can understand) that public keys are not applicable for symmetric keys.

Questions:

• Have I correctly understood the requirements to decode the ciphertext input?
• If so, how can I retrieve the public-key value for the given ciphertext input?

Language: Ruby SDK Version: 3