Stateless Services and Sessions

import java.nio.charset.StandardCharsets;
import java.security.MessageDigest;
import java.util.HexFormat;
import javax.crypto.Mac;
import javax.crypto.spec.SecretKeySpec;

class SessionToken {
    private final byte[] secret;

    SessionToken(String secret) {
        this.secret = secret.getBytes(StandardCharsets.UTF_8);
    }

    String sign(String userId, long expires) {
        String payload = userId + "." + expires;
        return payload + "." + mac(payload);
    }

    // Returns the user ID, or null if the token is forged or expired.
    String verify(String token, long now) {
        int last = token.lastIndexOf('.');
        int first = token.lastIndexOf('.', last - 1);
        if (first < 0) {
            return null;
        }
        String payload = token.substring(0, last);
        byte[] given = token.substring(last + 1).getBytes(StandardCharsets.UTF_8);
        byte[] expected = mac(payload).getBytes(StandardCharsets.UTF_8);
        if (!MessageDigest.isEqual(given, expected)) {
            return null;
        }
        long expires = Long.parseLong(token.substring(first + 1, last));
        return now < expires ? token.substring(0, first) : null;
    }

    private String mac(String payload) {
        try {
            Mac hmac = Mac.getInstance("HmacSHA256");
            hmac.init(new SecretKeySpec(secret, "HmacSHA256"));
            return HexFormat.of().formatHex(hmac.doFinal(payload.getBytes(StandardCharsets.UTF_8)));
        } catch (Exception e) {
            throw new IllegalStateException(e);
        }
    }
}

import hashlib
import hmac

class SessionToken:
    def __init__(self, secret):
        self.secret = secret.encode()

    def sign(self, user_id, expires):
        payload = f"{user_id}.{expires}"
        return f"{payload}.{self._mac(payload)}"

    def verify(self, token, now):
        """Returns the user ID, or None if the token is forged or expired."""
        parts = token.rsplit(".", 2)
        if len(parts) != 3:
            return None
        user_id, expires, signature = parts
        if not hmac.compare_digest(signature, self._mac(f"{user_id}.{expires}")):
            return None
        return user_id if now < int(expires) else None

    def _mac(self, payload):
        return hmac.new(self.secret, payload.encode(), hashlib.sha256).hexdigest()