Signed URLs for Private Images

Signed URLs provide secure, time-limited access to private images. This allows you to implement custom privacy controls in your application while leveraging Pichr’s global CDN infrastructure.

Overview

When you upload images with visibility: "private", they cannot be accessed directly by URL. Signed URLs solve this by generating cryptographically secure, temporary URLs that grant access to specific images for authorized viewers.

Use cases:

Social apps with “friends only” or custom privacy levels
SaaS platforms with per-customer data isolation
Mobile apps requiring secure image delivery
Any application with granular access control needs

How It Works

Let’s walk through implementing privacy controls for a social application.

Scenario

Your application allows users to:

Upload photos with custom privacy settings
Set privacy levels: “everyone”, “friends only”, “just me”
Share content with specific users or groups

Step 1: Upload Private Image


async function uploadPrivateImage(
  userId: string,
  imageFile: File,
  privacyLevel: 'everyone' | 'friends' | 'private'
) {
  const PICHR_API_KEY = process.env.PICHR_API_KEY;
 
  const response = await fetch('https://api.pichr.io/api/v1/upload/presign', {
    method: 'POST',
    headers: {
      'Authorization': `Bearer ${PICHR_API_KEY}`,
      'Content-Type': 'application/json',
    },
    body: JSON.stringify({
      filename: imageFile.name,
      mime: imageFile.type,
      bytes: imageFile.size,
      sha256: await calculateSHA256(imageFile),
      visibility: 'private',
      metadata: {
        userId: userId,
        privacyLevel: privacyLevel,
      },
    }),
  });
 
  const { fileId, uploadUrl } = await response.json();
  await uploadFile(uploadUrl, imageFile);
 
  await db.posts.create({
    userId,
    imageFileId: fileId,
    privacyLevel,
  });
 
  return fileId;
}

Step 2: Check Authorization & Generate Signed URL


async function getImageUrl(
  postId: string,
  viewerId: string
): Promise<string | null> {
  const post = await db.posts.findById(postId);
 
  const canView = await checkPrivacy(post, viewerId);
  if (!canView) {
    return null;
  }
 
  const signedUrl = await generateSignedUrl(post.imageFileId, {
    viewerId,
    postId,
  });
 
  return signedUrl;
}
 
async function checkPrivacy(post: Post, viewerId: string): Promise<boolean> {
  if (post.privacyLevel === 'everyone') return true;
  if (post.privacyLevel === 'private') return post.userId === viewerId;
  if (post.privacyLevel === 'friends') {
    return await db.friendships.exists(post.userId, viewerId);
  }
  return false;
}

Step 3: Generate Signed URL


async function generateSignedUrl(
  fileId: string,
  customClaims?: Record<string, any>
): Promise<string> {
  const PICHR_API_KEY = process.env.PICHR_API_KEY;
 
  const response = await fetch('https://api.pichr.io/api/v1/enterprise/sign-url', {
    method: 'POST',
    headers: {
      'Authorization': `Bearer ${PICHR_API_KEY}`,
      'Content-Type': 'application/json',
    },
    body: JSON.stringify({
      fileId,
      expiresIn: 3600,
      customClaims,
    }),
  });
 
  const { signedUrl } = await response.json();
  return signedUrl;
}

Step 4: Display in Frontend


async function displayImage(postId: string) {
  const response = await fetch(`/api/posts/${postId}/image`, {
    headers: {
      'Authorization': `Bearer ${userToken}`,
    },
  });
 
  if (response.status === 403) {
    showError('You do not have permission to view this content.');
    return;
  }
 
  const { imageUrl } = await response.json();
  document.querySelector('#post-image').src = imageUrl;
}

API Reference

Generate Signed URL


POST https://api.pichr.io/api/v1/enterprise/sign-url

Requirements:

Enterprise plan subscription
Valid API key
File must be owned by API key owner

Request Body:


{
  "fileId": "abc-123-def-456",
  "expiresIn": 3600,
  "customClaims": {
    "viewerId": "user-789",
    "resourceId": "post-456"
  }
}

Field	Type	Required	Description
`fileId`	string (UUID)	✅	The Pichr file ID to generate signed URL for
`expiresIn`	number	❌	Seconds until URL expires (default: 3600 = 1 hour)
`customClaims`	object	❌	Optional metadata to embed in signed URL

Response:


{
  "signedUrl": "https://i.pichr.io/abc-123?signature=a1b2c3d4e5f6...&expires=1700000000&claims=eyJ2aWV3...",
  "expiresAt": "2024-11-15T10:00:00.000Z",
  "expiresIn": 3600
}

cURL Example


curl -X POST https://api.pichr.io/api/v1/enterprise/sign-url \
  -H "Authorization: Bearer pk_your_enterprise_key" \
  -H "Content-Type: application/json" \
  -d '{
    "fileId": "abc-123-def-456",
    "expiresIn": 3600,
    "customClaims": {
      "viewerId": "user-789"
    }
  }'

How Signatures Work

Signature Algorithm

Pichr uses HMAC-SHA256 to generate cryptographically secure signatures:


messageToSign = fileId + "|" + expiresTimestamp + "|" + (customClaims || "")
signature = SHA-256(apiKeyHash + messageToSign)

URL Structure


https://i.pichr.io/{fileId}?signature={hex}&expires={timestamp}&claims={base64}

Part	Description
`{fileId}`	The file ID (UUID format)
`signature`	HMAC-SHA256 signature (hex-encoded)
`expires`	Unix timestamp (seconds) when URL expires
`claims`	Base64-encoded custom claims (optional)

Validation Process

When a signed URL is accessed:

✅ Extract query parameters: signature, expires, claims
✅ Check expiration: Date.now() <= expires * 1000
✅ Fetch file owner’s API keys from database
✅ Regenerate signature using each API key hash
✅ Compare signatures
✅ Grant access if valid and not expired
❌ Return 403 if validation fails

This supports API key rotation - any active API key can validate the signature.

Code Examples

JavaScript/TypeScript


const PICHR_API_KEY = process.env.PICHR_API_KEY;
const API_BASE = 'https://api.pichr.io/api/v1';
 
async function generateSignedUrl(
  fileId: string,
  expiresIn: number = 3600,
  customClaims?: Record<string, any>
): Promise<string> {
  const response = await fetch(`${API_BASE}/enterprise/sign-url`, {
    method: 'POST',
    headers: {
      'Authorization': `Bearer ${PICHR_API_KEY}`,
      'Content-Type': 'application/json',
    },
    body: JSON.stringify({
      fileId,
      expiresIn,
      customClaims,
    }),
  });
 
  if (!response.ok) {
    const error = await response.json();
    throw new Error(`Failed to generate signed URL: ${error.message}`);
  }
 
  const { signedUrl } = await response.json();
  return signedUrl;
}
 
const imageUrl = await generateSignedUrl('abc-123', 3600, {
  userId: 'user-456',
  resourceType: 'post',
});

Python


import requests
import os
 
PICHR_API_KEY = os.getenv('PICHR_API_KEY')
API_BASE = 'https://api.pichr.io/api/v1'
 
def generate_signed_url(file_id: str, expires_in: int = 3600, custom_claims: dict = None) -> str:
    response = requests.post(
        f'{API_BASE}/enterprise/sign-url',
        headers={
            'Authorization': f'Bearer {PICHR_API_KEY}',
            'Content-Type': 'application/json',
        },
        json={
            'fileId': file_id,
            'expiresIn': expires_in,
            'customClaims': custom_claims,
        }
    )
 
    response.raise_for_status()
    return response.json()['signedUrl']
 
image_url = generate_signed_url(
    file_id='abc-123',
    expires_in=3600,
    custom_claims={'userId': 'user-456', 'resourceType': 'post'}
)

PHP


<?php
function generateSignedUrl(string $fileId, int $expiresIn = 3600, ?array $customClaims = null): string {
    $apiKey = getenv('PICHR_API_KEY');
    $apiBase = 'https://api.pichr.io/api/v1';
 
    $payload = ['fileId' => $fileId, 'expiresIn' => $expiresIn];
    if ($customClaims) {
        $payload['customClaims'] = $customClaims;
    }
 
    $curl = curl_init("$apiBase/enterprise/sign-url");
    curl_setopt_array($curl, [
        CURLOPT_POST => true,
        CURLOPT_RETURNTRANSFER => true,
        CURLOPT_HTTPHEADER => [
            "Authorization: Bearer $apiKey",
            'Content-Type: application/json',
        ],
        CURLOPT_POSTFIELDS => json_encode($payload),
    ]);
 
    $response = curl_exec($curl);
    $httpCode = curl_getinfo($curl, CURLINFO_HTTP_CODE);
    curl_close($curl);
 
    if ($httpCode !== 200) {
        throw new Exception("Failed to generate signed URL: $response");
    }
 
    return json_decode($response, true)['signedUrl'];
}
 
$imageUrl = generateSignedUrl('abc-123', 3600, ['userId' => 'user-456']);
?>

Security Best Practices

1. Never Expose API Keys to Frontend


// ❌ BAD
const signedUrl = await fetch('https://api.pichr.io/api/v1/enterprise/sign-url', {
  headers: { 'Authorization': `Bearer ${PICHR_API_KEY}` },
});
 
// ✅ GOOD - Generate in backend
app.post('/api/images/get-url', authenticateUser, async (req, res) => {
  const { imageId } = req.body;
  const canView = await checkUserAccess(req.user.id, imageId);
 
  if (!canView) {
    return res.status(403).json({ error: 'Access denied' });
  }
 
  const signedUrl = await generateSignedUrl(imageId);
  res.json({ signedUrl });
});

2. Set Appropriate Expiration Times


// Recommended expiration times:
const publicIsh = 3600;      // 1 hour for unlisted/shareable
const friendsOnly = 1800;    // 30 min for friends-only
const sensitive = 900;       // 15 min for highly sensitive
 
const url = await generateSignedUrl(fileId, friendsOnly);

3. Implement Your Own Authorization


async function getImageUrl(imageId: string, viewerId: string) {
  const image = await db.images.findById(imageId);
 
  switch (image.privacyLevel) {
    case 'public':
      return image.publicUrl;
 
    case 'friends':
      const isFriend = await db.friendships.exists(image.ownerId, viewerId);
      if (!isFriend) throw new Error('Access denied');
      break;
 
    case 'private':
      if (image.ownerId !== viewerId) throw new Error('Access denied');
      break;
  }
 
  return await generateSignedUrl(image.pichrFileId);
}

4. Use Custom Claims for Audit Trails


const signedUrl = await generateSignedUrl(fileId, 3600, {
  viewerId: user.id,
  viewerEmail: user.email,
  resourceId: post.id,
  resourceType: 'post',
  generatedAt: new Date().toISOString(),
  reason: 'friend-access',
});

5. Rotate API Keys Regularly

Rotate Enterprise API keys every 90 days
Keep old key active for 24 hours during rotation
Monitor “Last Used” timestamp in dashboard

6. Validate Expiration Before Serving Cached URLs


function isSignedUrlExpired(signedUrl: string): boolean {
  const url = new URL(signedUrl);
  const expires = parseInt(url.searchParams.get('expires') || '0', 10);
  return Date.now() > expires * 1000;
}
 
const cachedUrl = cache.get(`image:${imageId}`);
if (cachedUrl && !isSignedUrlExpired(cachedUrl)) {
  return cachedUrl;
}
 
const newUrl = await generateSignedUrl(imageId);
cache.set(`image:${imageId}`, newUrl, 3000);
return newUrl;

Error Handling

Common Errors

Error	Status	Cause	Solution
`Unauthorized - API key required`	401	No API key provided	Include `Authorization` header
`Forbidden - Enterprise plan required`	403	Not on Enterprise plan	Upgrade to Enterprise
`API key not found`	404	No active API keys	Create API key in dashboard
`Invalid input`	400	Invalid request body	Check `fileId` format
`Forbidden`	403	Accessing private image without auth	Provide valid signed URL

Example Error Handling


async function generateSignedUrlSafely(fileId: string): Promise<string | null> {
  try {
    return await generateSignedUrl(fileId);
  } catch (error) {
    if (error.response?.status === 403) {
      console.error('Enterprise plan required');
      return null;
    }
 
    if (error.response?.status === 404) {
      console.error('File not found');
      return null;
    }
 
    console.error('Failed to generate signed URL:', error);
    throw error;
  }
}

Rate Limits

Enterprise plan limits:

Signed URL generation: 100,000 requests/hour
Image delivery: Unlimited (Cloudflare CDN)

Pricing

Signed URLs require an Enterprise plan subscription.

Feature	Free	Pro	Enterprise
Signed URLs	❌	❌	✅
Private images	✅	✅	✅
Custom claims	❌	❌	✅

View pricing →

FAQ

Q: Can I use signed URLs with public images? A: Yes, but unnecessary. Public/unlisted images are already accessible. Signed URLs are for visibility: "private".

Q: What happens during API key rotation? A: Pichr validates against all active API keys. Keep old key active for 24 hours during rotation.

Q: Can I revoke a signed URL before expiration? A: Not directly. Revoke the API key (affects all URLs from that key) or wait for expiration. Use short TTLs for granular control.

Q: How do custom claims work? A: Metadata embedded in URL as base64 JSON. Pichr includes them in signature but doesn’t validate content. Useful for audit trails.

Q: Can I cache signed URLs? A: Yes! Set cache TTL shorter than expiration. Check expiration before serving cached URLs.

Next Steps

Authentication - Set up your API key
Upload Guide - Upload private images
API Endpoints - Complete API reference

Last updated: 13 November 2025