base64-secret-padding-mismatch

name	base64-secret-padding-mismatch
description	Fix for webhook authorization failures caused by Base64 padding character (=) being dropped when copying secrets between systems. Use when: (1) Webhook returns "Unauthorized" or "Server Error" but the secret looks correct, (2) INTERNAL_API_SECRET, WEBHOOK_SECRET, or similar auth tokens fail validation between Railway/Vercel and Convex/database, (3) Secret comparison fails despite values appearing identical, (4) CLI tools display truncated environment variables. Common with Railway CLI, Vercel CLI, and other deployment platforms that may not display trailing = characters.
author	Claude Code
version	1.0.0
date	"2026-01-23T00:00:00.000Z"

Base64 Secret Padding Mismatch Between Environments

Problem

Webhooks or API calls fail authorization even though the secret "looks correct" when comparing environment variables between systems. The root cause is Base64 padding characters (=) being silently dropped when copying or displaying secrets.

Context / Trigger Conditions

• Webhook handler returns "Unauthorized" or generic "Server Error"
• Error occurs in mutation/database call after webhook validation passes
• Secret was copy-pasted between systems (e.g., Railway → Convex, Vercel → Supabase)
• CLI tools show the secret without trailing = characters
• The secret is Base64-encoded (often ends with = or ==)

Example error pattern:

Webhook error: Error: [Request ID: xxx] Server Error
    at mutationInner (...)

Solution

Step 1: Identify Base64 Secrets

Base64-encoded secrets often:

• Are 32-44 characters long
• Contain alphanumeric characters plus +, /, and =
• End with = or == for padding

Step 2: Compare Full Values

Don't trust CLI display output. Get the raw values:

For Convex:

npx convex env list --prod | grep SECRET_NAME
# Output: SECRET_NAME=VbjsOyAxdAuOtBkiSXZiUX2sA4ELpO6FFx3Y8lIDiD4=

For Railway:

railway variables | grep SECRET_NAME
# Output may truncate: VbjsOyAxdAuOtBkiSXZiUX2sA4ELpO6FFx3Y8lIDiD4
# Note the missing trailing =

Step 3: Fix the Mismatch

When setting secrets, explicitly include the full value with padding:

# Use single quotes to preserve special characters
railway variables set 'INTERNAL_API_SECRET=VbjsOyAxdAuOtBkiSXZiUX2sA4ELpO6FFx3Y8lIDiD4='

# Or for Vercel
vercel env add INTERNAL_API_SECRET
# Then paste the full value including =

Step 4: Redeploy

After fixing the environment variable, trigger a redeploy:

railway deployment redeploy --yes
# or
vercel --prod

Verification

1. Check logs for successful webhook processing
2. Verify the operation that was failing now succeeds
3. For subscription webhooks: check that user status updates correctly

Example

Scenario: Polar webhook for subscription activation fails in production.

Symptoms:

• Payment succeeds in Polar
• Redirect to app shows ?success=true
• User subscription status remains "FREE"
• Railway logs show: Webhook error: Error: Server Error

Investigation:

# Convex shows:
npx convex env list --prod | grep INTERNAL_API_SECRET
# INTERNAL_API_SECRET=VbjsOyAxdAuOtBkiSXZiUX2sA4ELpO6FFx3Y8lIDiD4=

# Railway shows (truncated):
railway variables | grep INTERNAL
# INTERNAL_API_SECRET │ VbjsOyAxdAuOtBkiSXZiUX2sA4ELpO6FFx3Y8lIDiD4

Fix:

railway variables set 'INTERNAL_API_SECRET=VbjsOyAxdAuOtBkiSXZiUX2sA4ELpO6FFx3Y8lIDiD4='
railway deployment redeploy --yes

Notes

• This issue is particularly common with secrets generated by openssl rand -base64 32
• Some CLIs and web UIs strip or hide trailing = for display purposes
• When in doubt, regenerate the secret and set it in both places simultaneously
• Consider using hex encoding (openssl rand -hex 32) to avoid padding issues entirely
• The = padding in Base64 ensures the encoded length is a multiple of 4

Prevention

When generating and distributing secrets:

1. Generate in one place and immediately copy the full value
2. Use a password manager or secure note to store the canonical value
3. After setting in each system, verify by echoing back or checking logs
4. Consider using non-Base64 formats (hex, UUID) for simpler handling

Related Scenarios

• JWT secrets between auth providers and backends
• Stripe/Polar webhook secrets
• Internal API authentication between microservices
• Any secret that's validated via string comparison