aes_gcm_encrypt

On-chain AES-256-GCM encryption

Encrypt data on-chain using Mercury EVM's AES-GCM encryption precompile.

Overview

The AES-GCM encryption precompile at address 0x66 performs AES-256-GCM authenticated encryption. The ciphertext includes a 16-byte authentication tag that ensures integrity and authenticity.

The SeismicSignedProvider filler pipeline uses this precompile internally to encrypt calldata before sending shielded transactions. You can also call it directly for custom encryption workflows.

Precompile Address

0x0000000000000000000000000000000000000066

Input Encoding

Field	Size	Description
key	32 bytes	AES-256 encryption key
nonce	12 bytes	Unique nonce (must never be reused with the same key)
plaintext	Variable	Data to encrypt

The input is the concatenation of key (32 bytes) + nonce (12 bytes) + plaintext (variable length).

Output Format

Field	Size	Description
ciphertext	len(plaintext) bytes	Encrypted data
tag	16 bytes	Authentication tag (appended to ciphertext)

Total output length = len(plaintext) + 16.

Parameters

Parameter	Type	Required	Description
key	[u8; 32]	Yes	32-byte AES-256 encryption key
nonce	[u8; 12]	Yes	12-byte nonce (must be unique per key)
plaintext	&[u8]	Yes	Data to encrypt

Examples

Basic Usage

use alloy::providers::Provider;
use alloy_primitives::{Address, Bytes};
use alloy_rpc_types_eth::TransactionRequest;
use seismic_prelude::foundry::*;

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let url = "https://gcp-1.seismictest.net/rpc".parse()?;
    let provider = sreth_unsigned_provider(url);

    let encrypt_address: Address =
        "0x0000000000000000000000000000000000000066".parse()?;

    // 32-byte AES key
    let key = [0x42u8; 32]; // Use a proper key in production
    // 12-byte nonce
    let nonce = [0u8; 12];
    // Plaintext to encrypt
    let plaintext = b"Secret message";

    let mut input = Vec::new();
    input.extend_from_slice(&key);
    input.extend_from_slice(&nonce);
    input.extend_from_slice(plaintext);

    let result = provider
        .call(
            &TransactionRequest::default()
                .to(encrypt_address)
                .input(Bytes::from(input).into()),
        )
        .await?;

    println!("Ciphertext ({} bytes): 0x{}", result.len(), hex::encode(&result));
    println!("  Plaintext was {} bytes + 16-byte tag", plaintext.len());

    Ok(())
}

Encrypt with Integer Nonce

use alloy::providers::Provider;
use alloy_primitives::{Address, Bytes};
use alloy_rpc_types_eth::TransactionRequest;
use seismic_prelude::foundry::*;

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let url = "https://gcp-1.seismictest.net/rpc".parse()?;
    let provider = sreth_unsigned_provider(url);

    let encrypt_address: Address =
        "0x0000000000000000000000000000000000000066".parse()?;

    let key = [0x42u8; 32];

    // Use a counter as nonce (converted to 12 bytes, big-endian)
    for i in 0u64..5 {
        let mut nonce = [0u8; 12];
        nonce[4..12].copy_from_slice(&i.to_be_bytes());

        let plaintext = format!("Message {i}");

        let mut input = Vec::new();
        input.extend_from_slice(&key);
        input.extend_from_slice(&nonce);
        input.extend_from_slice(plaintext.as_bytes());

        let result = provider
            .call(
                &TransactionRequest::default()
                    .to(encrypt_address)
                    .input(Bytes::from(input).into()),
            )
            .await?;

        println!("Ciphertext {i}: 0x{}", hex::encode(&result));
    }

    Ok(())
}

Encrypt-Decrypt Round Trip

use alloy::providers::Provider;
use alloy_primitives::{Address, Bytes};
use alloy_rpc_types_eth::TransactionRequest;
use seismic_prelude::foundry::*;

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let url = "https://gcp-1.seismictest.net/rpc".parse()?;
    let provider = sreth_unsigned_provider(url);

    let encrypt_address: Address =
        "0x0000000000000000000000000000000000000066".parse()?;
    let decrypt_address: Address =
        "0x0000000000000000000000000000000000000067".parse()?;

    let key = [0x42u8; 32];
    let nonce = [0u8; 12];
    let original = b"Round trip test";

    // Encrypt
    let mut encrypt_input = Vec::new();
    encrypt_input.extend_from_slice(&key);
    encrypt_input.extend_from_slice(&nonce);
    encrypt_input.extend_from_slice(original);

    let ciphertext = provider
        .call(
            &TransactionRequest::default()
                .to(encrypt_address)
                .input(Bytes::from(encrypt_input).into()),
        )
        .await?;

    // Decrypt
    let mut decrypt_input = Vec::new();
    decrypt_input.extend_from_slice(&key);
    decrypt_input.extend_from_slice(&nonce);
    decrypt_input.extend_from_slice(&ciphertext);

    let decrypted = provider
        .call(
            &TransactionRequest::default()
                .to(decrypt_address)
                .input(Bytes::from(decrypt_input).into()),
        )
        .await?;

    assert_eq!(&decrypted[..], original);
    println!("Round trip successful: {}", String::from_utf8_lossy(&decrypted));

    Ok(())
}

With ECDH-Derived Key

use alloy::providers::Provider;
use alloy_primitives::{Address, Bytes};
use alloy_rpc_types_eth::TransactionRequest;
use seismic_prelude::foundry::*;

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let url = "https://gcp-1.seismictest.net/rpc".parse()?;
    let provider = sreth_unsigned_provider(url);

    // Step 1: Derive shared key via ECDH
    let ecdh_address: Address =
        "0x0000000000000000000000000000000000000065".parse()?;

    let my_sk: [u8; 32] = [/* ... */; 32];
    let peer_pk: [u8; 33] = [/* ... */; 33];

    let mut ecdh_input = Vec::with_capacity(65);
    ecdh_input.extend_from_slice(&my_sk);
    ecdh_input.extend_from_slice(&peer_pk);

    let ecdh_result = provider
        .call(
            &TransactionRequest::default()
                .to(ecdh_address)
                .input(Bytes::from(ecdh_input).into()),
        )
        .await?;

    let aes_key: [u8; 32] = ecdh_result[..32].try_into()?;

    // Step 2: Encrypt with derived key
    let encrypt_address: Address =
        "0x0000000000000000000000000000000000000066".parse()?;

    let nonce = [0u8; 12];
    let plaintext = b"Encrypted for peer";

    let mut encrypt_input = Vec::new();
    encrypt_input.extend_from_slice(&aes_key);
    encrypt_input.extend_from_slice(&nonce);
    encrypt_input.extend_from_slice(plaintext);

    let ciphertext = provider
        .call(
            &TransactionRequest::default()
                .to(encrypt_address)
                .input(Bytes::from(encrypt_input).into()),
        )
        .await?;

    println!("Ciphertext: 0x{}", hex::encode(&ciphertext));

    Ok(())
}

How It Works

1. Encode parameters -- Concatenates 32-byte key + 12-byte nonce + plaintext

2. Call precompile -- Issues an eth_call to address 0x66 with estimated gas

3. Encrypt data -- Precompile performs AES-256-GCM encryption

4. Return ciphertext -- Returns encrypted data with 16-byte authentication tag appended

Gas Cost

Gas cost is calculated as:

base_gas = 1000
per_block = 30  // per 16-byte block
num_blocks = (len(plaintext) + 15) / 16
total_gas = base_gas + (num_blocks * per_block)

For example:

Plaintext Size	Gas Cost
16 bytes	1030
64 bytes	1120
256 bytes	1480

Notes

• Uses AES-256-GCM authenticated encryption

• Nonce must be unique for each encryption with the same key

• Ciphertext length = plaintext length + 16 bytes (authentication tag)

• The authentication tag ensures ciphertext integrity and authenticity

• Reusing a nonce with the same key breaks security

Warnings

• Nonce reuse -- NEVER reuse the same nonce with the same key. This breaks confidentiality and can leak the plaintext.

• Key security -- Keep AES keys secure and never expose them in logs or error messages

• Authentication tag -- The 16-byte tag is appended to the ciphertext and must be included when decrypting

• Counter management -- When using integer nonces, ensure they are sequential and never repeated

See Also

• Precompiles Overview -- All precompile reference

• aes-gcm-decrypt -- Decrypt AES-GCM ciphertext

• ecdh -- Derive shared encryption keys

• Encryption -- How the provider uses AES-GCM internally