> For the complete documentation index, see [llms.txt](https://e2payprod.gitbook.io/payment-gateway/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://e2payprod.gitbook.io/payment-gateway/api-documentation/technical-doc-of-fiuu-id/security-and-data-integrity/skey.md).
# skey
**skey** is a payment gateway generated returning hash string to ensure the payment result data integrity that passed to the merchant system. Merchants or developers MUST verify this hash string properly and compare the order ID, currency, amount, and the payment date/time, to protect self-interest from being cheated by a fraudster/hacker. It uses a “Secret Key” (like a private key) in combination with a data string for the hashing process.
Please note that there are other “**skey**” with different formulas in Merchant Request APIs’ parameters. Try not to confuse yourself with this payment response “**skey**”.
skey was encrypted twice using MD5 encryption hash function and consists of the following information (must be set in the following orders) :
**First hash string**
1. Transaction ID
2. Order ID
3. Status
4. Merchant ID (domain)
5. Amount
6. Currency
**Final hash string**
1. Payment Date/Time
2. Merchant ID (domain)
3. First hash string
4. Approval Code
5. Secret Key
**Formula to generate ****skey**
pre\_skey = md5( txnID & orderID & status & merchantID & amount & currency)
skey = md5( paydate & merchantID & pre\_skey & appcode & secret\_key )
\
**Example to generate skey for PHP developer**
{% code overflow="wrap" %}
```php
```
{% endcode %}