Go语言nacl/box包Seal函数的out参数及nonce传递疑问

Great questions about the nacl/box.Seal function—let’s break this down clearly with practical context:

1. Why pass the nonce both as the third parameter and as the out slice? #

The example you’re looking at follows a common, practical pattern for encryption workflows: it bundles the nonce and encrypted message into a single payload. Here’s the split of their roles:

• The third parameter (&nonce) is the cryptographically required nonce for the encryption process. The box algorithm uses this value (along with the public/private keys) to generate the unique key stream and authentication tag that secures your message. Without this exact nonce, decryption will fail.
• The out parameter (nonce[:]) is just the starting slice that the encrypted message gets appended to. By using the nonce’s slice as out, the example creates a final payload that’s nonce + ciphertext—this way, when the recipient receives the data, they can easily pull the first 24 bytes (the nonce) to use for decryption, no separate transmission needed.

This isn’t a mandatory usage of Seal—you could pass nil as out and get only the encrypted message. But attaching the nonce directly to the ciphertext is a convenience that avoids having to coordinate sending the nonce separately.

2. What’s the purpose of the out parameter? #

The out parameter exists to give you control over memory allocation and efficiency:

• If you pass nil, Seal will automatically allocate a new slice to hold the encrypted data (plus the required authentication overhead). This works fine for simple, one-off tasks, but isn’t optimal for high-performance or high-throughput scenarios.
• If you pass a pre-allocated slice with enough capacity, Seal will append the encrypted data directly to it, skipping the need to allocate new memory. This lets you reuse buffers across multiple encryption calls, which can significantly reduce garbage collection overhead and speed up your code.

For example, if you’re encrypting hundreds of messages in a loop, you could pre-allocate a slice once with sufficient capacity and reuse it for each Seal call. The example uses nonce[:] as out specifically to bundle the nonce and ciphertext, but you could just as easily pass a pre-sized empty slice if you don’t need to include the nonce in the output.

内容的提问来源于stack exchange，提问作者Filipe Oliveira