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The key server bundles a presign endpoint that generates presigned S3 PUT URLs. The Uploader uses this to upload encrypted segments directly from the browser to S3 — without routing bytes through your server.

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Enable presign

docker run -d \
  -e MASTER_KEY_HEX=... \
  -e SALT_HEX=... \
  -e CORS_ORIGINS=https://app.example.com \
  -e ENABLE_PRESIGN=true \
  -e S3_BUCKET=my-video-bucket \
  -e S3_REGION=us-east-1 \
  -e AWS_ACCESS_KEY_ID=... \
  -e AWS_SECRET_ACCESS_KEY=... \
  -p 4100:4100 \
  blindcast/keyserver

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How it works

  1. The uploader sends POST /presign with the S3 key and content type
  2. The key server validates the JWT (if auth is enabled), then generates a presigned PUT URL
  3. The uploader uploads encrypted bytes directly to S3 using the presigned URL

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Presign request format

POST /presign
Authorization: Bearer <token>
Content-Type: application/json

{
  "key": "content/my-video-001/seg-0.ts",
  "contentType": "video/mp2t"
}

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Presign response

{
  "url": "https://my-video-bucket.s3.us-east-1.amazonaws.com/content/my-video-001/seg-0.ts?X-Amz-Algorithm=..."
}
The presigned URL expires after 300 seconds (5 minutes) by default.

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Configuration

VariableDefaultDescription
ENABLE_PRESIGNfalseSet to true to enable
S3_BUCKET—S3 bucket name (required)
S3_REGIONus-east-1AWS region
AWS_ACCESS_KEY_ID—AWS credentials
AWS_SECRET_ACCESS_KEY—AWS credentials
S3_ENDPOINT—Custom S3 endpoint (for MinIO, R2, etc.)

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S3-compatible storage

For MinIO, Cloudflare R2, or Backblaze B2, set S3_ENDPOINT: 
-e S3_ENDPOINT=https://<account-id>.r2.cloudflarestorage.com

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Security

If authentication is not configured (AUTH_JWT_SECRET or AUTH_JWKS_URL), the presign endpoint is open to anyone who can reach the key server. Always enable authentication when ENABLE_PRESIGN=true in production.
  • Presign requests require the same JWT authentication as key requests (if auth is enabled)
  • The key server validates that the S3 key starts with an allowed prefix (default: content/)
  • Presigned URLs are short-lived (5 minutes) and single-use