The signature is Ed25519 over the exact UTF-8 bytes of the payload string. Anyone with our public key can check it offline:

pip install cryptography
python3 - mistproof-DX-XXXXXX.json <<'PY'
import base64, json, sys
from cryptography.hazmat.primitives.asymmetric.ed25519 import Ed25519PublicKey
r = json.load(open(sys.argv[1]))
key = Ed25519PublicKey.from_public_bytes(base64.b64decode("j/1x9v9Sv7n0BlSIbrW0IbsoAphfRrkF0pn9ObTRr7k="))
key.verify(base64.b64decode(r["signature"]), r["payload"].encode())  # raises if forged
print("VALID:", json.loads(r["payload"])["send"]["deposit_address"])
PY