Signed Documents Storage with Audit Log on Website

Implementation of Signed Documents Storage with Audit Log

Signed document storage is not simply "put file in folder". Requirements: immutability, integrity, availability, audit of every action and compliance with archival storage legislation. Violation of any of these requirements questions the legal validity of documents.

Storage Principles

Immutability — signed document cannot be changed. S3 versioning with MFA Delete or Write-Once-Read-Many (WORM) storage.

Integrity — each access checks content matches stored hash.

Separation — signed documents stored separately from working drafts. Different S3 buckets with different access policies.

Backup — cross-region replication. Loss of signed contract is legal and reputational risk.

Document Storage

// Service for uploading to immutable storage
class DocumentStorageService {
  async storeSignedDocument(
    documentBytes: Buffer,
    metadata: DocumentMetadata
  ): Promise<StoredDocument> {
    // Document hash — immutable content identifier
    const contentHash = crypto.createHash('sha256').update(documentBytes).digest('hex');

    // Key includes hash for deduplication
    const s3Key = `signed/${metadata.documentId}/${contentHash}.pdf`;

    await this.s3.putObject({
      Bucket: process.env.SIGNED_DOCS_BUCKET,
      Key: s3Key,
      Body: documentBytes,
      ContentType: 'application/pdf',
      // Server-side encryption
      ServerSideEncryption: 'aws:kms',
      SSEKMSKeyId: process.env.KMS_KEY_ID,
      // Object Lock prevents deletion/modification
      ObjectLockMode: 'COMPLIANCE',
      ObjectLockRetainUntilDate: addYears(new Date(), 10),
      Metadata: {
        'document-id': metadata.documentId,
        'signer-id': metadata.signerId,
        'signed-at': metadata.signedAt.toISOString(),
        'content-hash': contentHash,
      },
    }).promise();

    return {
      s3Key,
      contentHash,
      storageUrl: `s3://${process.env.SIGNED_DOCS_BUCKET}/${s3Key}`,
    };
  }

  async retrieveAndVerify(documentId: string): Promise<{ bytes: Buffer; integrityOk: boolean }> {
    const record = await db.signedDocuments.findByDocumentId(documentId);
    const object = await this.s3.getObject({
      Bucket: process.env.SIGNED_DOCS_BUCKET,
      Key: record.s3Key,
    }).promise();

    const bytes = object.Body as Buffer;
    const currentHash = crypto.createHash('sha256').update(bytes).digest('hex');
    const integrityOk = currentHash === record.contentHash;

    if (!integrityOk) {
      await this.alertIntegrityViolation(documentId, record.contentHash, currentHash);
    }

    return { bytes, integrityOk };
  }
}

Database Schema for Documents

CREATE TABLE signed_documents (
  id              UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  document_id     UUID REFERENCES documents(id),
  version         INT NOT NULL DEFAULT 1,
  s3_key          VARCHAR(1000) NOT NULL UNIQUE,
  content_hash    CHAR(64) NOT NULL,  -- SHA-256
  file_size_bytes BIGINT,
  stored_at       TIMESTAMPTZ DEFAULT NOW(),
  expires_at      TIMESTAMPTZ,        -- For documents with limited period
  deleted_at      TIMESTAMPTZ,        -- Soft delete
  delete_reason   TEXT,
  delete_by       UUID REFERENCES users(id)
);

-- Signatures on document
CREATE TABLE document_signatures (
  id              UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  signed_doc_id   UUID REFERENCES signed_documents(id),
  signer_id       UUID REFERENCES users(id),
  signer_role     VARCHAR(100),        -- 'initiator', 'approver', 'witness'
  signature_type  VARCHAR(50),         -- 'drawn', 'text', 'sms', 'kep'
  signature_data  JSONB,               -- Depends on type
  document_hash_at_signing CHAR(64),  -- Hash at signing moment
  signed_at       TIMESTAMPTZ DEFAULT NOW(),
  ip_address      INET,
  user_agent      TEXT
);

Audit Log

Every action with document is recorded in immutable journal:

CREATE TABLE document_audit_log (
  id              BIGSERIAL PRIMARY KEY,  -- Auto-increment for order
  document_id     UUID NOT NULL,
  actor_id        UUID REFERENCES users(id),
  actor_type      VARCHAR(50) DEFAULT 'user',  -- 'user', 'system', 'api'
  action          VARCHAR(200) NOT NULL,
  -- Examples: 'document.created', 'document.viewed', 'document.signed',
  --           'document.downloaded', 'document.shared', 'document.revoked'
  details         JSONB DEFAULT '{}',
  ip_address      INET,
  user_agent      TEXT,
  session_id      UUID,
  occurred_at     TIMESTAMPTZ DEFAULT NOW()
);

-- Index for fast document search
CREATE INDEX ON document_audit_log (document_id, occurred_at DESC);
CREATE INDEX ON document_audit_log (actor_id, occurred_at DESC);

-- Trigger prevents deletion of audit records
CREATE RULE no_delete_audit AS ON DELETE TO document_audit_log DO INSTEAD NOTHING;

// Log every action
async function auditLog(documentId, actorId, action, details = {}) {
  await db.documentAuditLog.create({
    documentId,
    actorId,
    action,
    details,
    ipAddress: request?.ip,
    userAgent: request?.headers?.['user-agent'],
    sessionId: request?.session?.id,
    occurredAt: new Date(),
  });
}

// Middleware: automatic log on download
app.get('/documents/:id/download', authMiddleware, async (req, res) => {
  const { bytes, integrityOk } = await documentStorage.retrieveAndVerify(req.params.id);
  await auditLog(req.params.id, req.user.id, 'document.downloaded', { integrityOk });
  res.setHeader('Content-Disposition', `attachment; filename="document-${req.params.id}.pdf"`);
  res.send(bytes);
});

Document Access

Signed documents should not be accessible via direct S3 URLs. Only through temporary presigned URLs, generated by server after permission check and audit log recording:

async function getDocumentDownloadUrl(documentId, userId) {
  await checkDocumentAccess(documentId, userId); // Throws 403 if no access

  const record = await db.signedDocuments.findByDocumentId(documentId);
  const url = await s3.getSignedUrlPromise('getObject', {
    Bucket: process.env.SIGNED_DOCS_BUCKET,
    Key: record.s3Key,
    Expires: 300, // 5 minutes
    ResponseContentDisposition: `attachment; filename="document.pdf"`,
  });

  await auditLog(documentId, userId, 'document.viewed');
  return url;
}

Storage Periods

Automatic setting of expires_at when document created based on its type.

Implementation Timeline

Storage with S3 Object Lock, hash verification and audit log — 5–7 days. Access control with presigned URLs and automatic logging — 2–3 days. Document action history interface — 2–3 days.