AES-GCM is an encryption mode that gives you two things at once:
- Confidentiality: someone without the key cannot read the plaintext.
- Authenticity/integrity: if the ciphertext is changed, decryption fails instead of returning silently corrupted data.
In Go, it commonly looks like this:
blockCipher, _ := aes.NewCipher(key)
gcm, _ := cipher.NewGCM(blockCipher)
nonce := make([]byte, gcm.NonceSize())
rand.Read(nonce)
ciphertext := gcm.Seal(nonce, nonce, plaintext, nil)The important bits:
- The key must stay secret.
- The nonce does not need to be secret, but must not be reused with the same key.
- The authentication tag is built into the GCM output, so decryption can reject tampered data.
In SST's provider code, this is used in the shared putData helper before writing encrypted secrets to the configured backend, like AWS S3 or Cloudflare R2. The Cloudflare provider does not do its own encryption; it receives already-encrypted bytes and writes them to R2.
The passphrase lookup is backend-specific. For Cloudflare, SST stores it separately in R2 under passphrase/<app>/<stage>, with a source comment noting it should move to a secrets manager once that is available.