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Smartphone compromise detection is challenging because a single software-only check can be hidden, replayed, or forged by a capable attacker. This paper presents Quorum Seal, an evidence-based mobile trust attestation framework that evaluates whether a device is trustworthy enough for sensitive actions such as login, payment approval, marks entry, and protected data access. Quorum Seal uses a nonce-based challenge–response protocol, on-device sensor capture, compact statistical feature extraction, and server-side weighted quorum verification to classify a session as Trusted, Suspicious, or Untrusted. The complete system adds cross-sensor conflict fingerprinting, Adaptive Challenge Escalation, dynamic quorum adaptation for missing or low-quality sensors, and an entropy analyzer to detect low-variance or synthetic motion patterns. Each verification produces an explainable evidence record retrievable via /evidence/{id}, exposing the checks, penalties, and reasons behind the verdict. The prototype is implemented with a Flutter Android client and a Fast API backend exposing /challenge, /verify, and /evidence/{id}, and has been validated using reproducible evidence outputs, including real-device runs. Rather than claiming absolute malware diagnosis, Quorum Seal provides a practical and auditable transaction-time trust decision that raises attacker cost and supports safer access-control policies