> For the complete documentation index, see [llms.txt](https://docs.seismic.systems/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.seismic.systems/reference/precompiles/aes-gcm-decrypt.md).

# AES-GCM Decrypt

Decrypt data that was encrypted with [AES-256-GCM](https://en.wikipedia.org/wiki/Galois/Counter_Mode). Verifies the authentication tag before returning the plaintext. If the tag does not match (indicating tampering or wrong key), the call fails.

## Input

Raw bytes in the following layout:

| Offset    | Field                 | Type     | Description                             |
| --------- | --------------------- | -------- | --------------------------------------- |
| `[0:32]`  | key                   | 32 bytes | AES-256 key (must match encryption key) |
| `[32:44]` | nonce                 | 12 bytes | Initialization vector                   |
| `[44:]`   | ciphertext + auth tag | bytes    | Encrypted data with authentication tag  |

## Output

| Bytes     | Type  | Description    |
| --------- | ----- | -------------- |
| plaintext | bytes | Decrypted data |

## Use cases

* On-chain decryption of previously encrypted data
* Decrypting messages from other contracts or users
* Verifying and reading encrypted event data within a contract

## Examples

### Built-in helper

```solidity
function aes_gcm_decrypt(sbytes32 key, uint96 nonce, bytes memory ciphertext) view returns (bytes memory);
```

```solidity
bytes memory plaintext = aes_gcm_decrypt(aesKey, nonce, ciphertext);
```

### Manual usage

```solidity
function aesDecrypt(
    suint256 key,
    uint96 nonce,
    bytes memory ciphertext
) internal view returns (bytes memory) {
    bytes memory input = abi.encodePacked(uint256(key), nonce, ciphertext);
    (bool success, bytes memory result) = address(0x67).staticcall(input);
    require(success, "AES-GCM Decrypt precompile failed");
    return result;
}
```


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation by asking a question.

Perform an HTTP GET request on the following URL with the `ask` and `goal` query parameters:

```
GET https://docs.seismic.systems/reference/precompiles/aes-gcm-decrypt.md?ask=<question>&goal=<user_goal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is what the user is ultimately trying to achieve, the reason they need the answer. Sharing it helps GitBook give you a better, more relevant answer. A goal is most helpful when it describes the outcome the user wants rather than restating the question. For example, with `ask=how do I create an API token`, a goal like `automate deployments from our CI pipeline` lets GitBook tailor the answer to that use case.

The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.