class Aws::S3::Encryption::KmsCipherProvider

def decryption_cipher(envelope, options = {})
  encryption_context = Json.load(envelope['x-amz-matdesc'])
  cek_alg = envelope['x-amz-cek-alg']
  case envelope['x-amz-wrap-alg']
  when 'kms'; # NO OP
  when 'kms+context'
    if cek_alg != encryption_context['aws:x-amz-cek-alg']
      raise Errors::DecryptionError, 'Value of cek-alg from envelope'\
      ' does not match the value in the encryption context'
    end
  when 'AES/GCM'
    raise ArgumentError, 'Key mismatch - Client is configured' \
            ' with a KMS key and the x-amz-wrap-alg is AES/GCM.'
  when 'RSA-OAEP-SHA1'
    raise ArgumentError, 'Key mismatch - Client is configured' \
            ' with a KMS key and the x-amz-wrap-alg is RSA-OAEP-SHA1.'
  else
    raise ArgumentError, 'Unsupported wrap-alg: ' \
        "#{envelope['x-amz-wrap-alg']}"
  end
  key = Aws::Plugins::UserAgent.metric('S3_CRYPTO_V1N') do
    @kms_client.decrypt(
      ciphertext_blob: decode64(envelope['x-amz-key-v2']),
      encryption_context: encryption_context
    ).plaintext
  end
  iv = decode64(envelope['x-amz-iv'])
  block_mode =
    case cek_alg
    when 'AES/CBC/PKCS5Padding'
      :CBC
    when 'AES/CBC/PKCS7Padding'
      :CBC
    when 'AES/GCM/NoPadding'
      :GCM
    else
      type = envelope['x-amz-cek-alg'].inspect
      msg = "unsupported content encrypting key (cek) format: #{type}"
      raise Errors::DecryptionError, msg
    end
  Utils.aes_decryption_cipher(block_mode, key, iv)
end