---
title: digest.hmac_md5_base64
summary: null
url: >-
  https://www.fastly.com/documentation/reference/vcl/functions/cryptographic/digest-hmac-md5-base64
---

```
STRING digest.hmac_md5_base64(STRING key, STRING s)
```

**Available in:** all subroutines

Returns the HMAC-MD5 of `message` using `key`, encoded as a Base64 string.

## Parameters

| Parameter | Type   | Description                         |
| --------- | ------ | ----------------------------------- |
| `key`     | STRING | The secret key for HMAC computation |
| `message` | STRING | The message to authenticate         |

The `key` is used directly as the HMAC key. For keys longer than 64 bytes (the MD5 block size), the key is first hashed with MD5 before use, as specified in RFC 2104.

## Return value

Returns a 24-character Base64-encoded string representing the 128-bit (16-byte) HMAC, using standard Base64 encoding (RFC 4648 Section 4) with padding.

Example output: `NriaqnJWBjiFJwOsukhLKw==`

If `key` is empty or not set, the function returns an empty string (not set).

## Security

Unlike plain MD5, no practical attack breaks HMAC-MD5 as an authenticator. However, it is no longer recommended and should only be used when required for compatibility with legacy systems that cannot be upgraded.

## Base64 encoding

This function uses standard Base64 encoding as defined in RFC 4648 Section 4:

| Property | Value             |
| -------- | ----------------- |
| Alphabet | `A-Za-z0-9+/`     |
| Padding  | `=` (always used) |

## Example

```vcl
declare local var.hmac STRING;
set var.hmac = digest.hmac_md5_base64("secret-key", "hello world");
# Result: NriaqnJWBjiFJwOsukhLKw==
```

To verify this output using OpenSSL:

```term
$ echo -n "hello world" | openssl dgst -md5 -hmac "secret-key" -binary | openssl base64
NriaqnJWBjiFJwOsukhLKw==
```

## Security considerations

### Prefer SHA-256 for new applications

```vcl
# Recommended for new code
set var.signature = digest.hmac_sha256_base64(var.key, var.message);

# NOT recommended - only for legacy compatibility
set var.legacy_signature = digest.hmac_md5_base64(var.key, var.message);
```

### Constant-time comparison

When comparing HMAC values for authentication, always use `digest.secure_is_equal()` to prevent timing attacks. String comparison with `==` leaks information about which bytes matched, potentially allowing an attacker to forge valid authentication tags:

```vcl
if (!digest.secure_is_equal(var.expected, var.actual)) {
    error 401 "Invalid signature";
}
```

## Related content

- `digest.hmac_md5()` - Returns hex-encoded output with `0x` prefix.
- `digest.hmac_sha256_base64()` - HMAC-SHA256 with Base64 output (recommended).
- `digest.secure_is_equal()` - Constant-time string comparison.