Base64 Reassembler
Validate every chunk for sequence gaps, duplicates, and invalid Base64 characters — then download the reassembled string or decoded file in one click.
Updated
All reassembly and validation runs in your browser. Nothing is uploaded.
How to use Base64 Reassembler
The Base64 Reassembler validates and rebuilds a Base64 string from a numbered chunk set produced by the Base64 Chunker. It checks every aspect of sequence integrity — missing chunks, duplicate indexes, out-of-range positions, total-count mismatches, and invalid Base64 characters — before attempting reassembly. When corruption is detected, the tool shows a labelled per-chunk error list so you know exactly which chunk is at fault and why. On success you can download the reassembled Base64 text or the decoded binary file. Everything runs locally in your browser.
- Copy the full chunked output from the Base64 Chunker (either JSON array or line format).
- Paste it into the input textarea — the tool automatically detects the format from the first character.
- Click "Validate & Reassemble" to run all sequence and alphabet checks.
- If validation fails, review the labelled issue list: each entry names the issue type, affected chunk index, and a human-readable description.
- Fix the identified issues in the source chunk set (re-send missing chunks, remove duplicates, correct corrupt data).
- Once validation passes, copy the reassembled Base64 or click "Download decoded file" to save the binary.
Your data never leaves your device — 100% private processing.
What the sequence validator checks
Correct reassembly requires that the chunk set is complete, uniquely indexed, and internally consistent. The validator runs five checks in order. First it verifies that all chunks agree on the "total" field — a mismatch indicates different versions of the payload were mixed. Second it confirms the actual chunk count equals the declared total, surfacing any missing or extra parts. Third it scans for duplicate index values, which indicate a resend collision or copy-paste error. Fourth it detects out-of-range indexes (index < 1 or index > total). Finally it validates each chunk's "data" field against the Base64 alphabet (A–Z, a–z, 0–9, +, /, =, - and _) — any character outside this set means the chunk was corrupted in transit. Reassembly is blocked until all five checks pass, ensuring the output is always the complete, uncorrupted original.
| Check | Error kind | Typical cause |
|---|---|---|
| All chunks share the same total | Sequence mismatch | Chunks from different payloads mixed together |
| Chunk count equals declared total | Missing chunk | One or more MQTT messages or HTTP parts not received |
| No duplicate index values | Duplicate chunk | Resend logic delivered a chunk twice |
| All indexes in range 1–total | Sequence mismatch | Index field corrupted or off-by-one in sender code |
| Data contains only Base64 characters | Invalid Base64 | Transport encoding (e.g. line endings) modified the data field |
Recovering from corrupt or incomplete chunk sets
When the validator reports issues, the error list tells you exactly which action to take. A "Missing chunk" error means you need to re-request or resend the specific index shown. A "Duplicate chunk" means you should remove the second occurrence at the position shown in the error. An "Invalid Base64" error on a specific chunk index means the data field was altered in transit — common causes include newline injection, percent-decoding of + to space, or truncation at a max-length boundary; re-transmit that chunk from a clean source. A "Sequence mismatch" on the total field usually means chunks from two different chunking runs were accidentally merged; identify the correct set and discard the outliers. The tool never partially reassembles to avoid producing a silently wrong binary, so every reported issue must be resolved before the output is produced.
Worked examples
Reassemble 5 JSON chunks
Inputs: [{"index":1,"total":5,"data":"YWJj"},...]
Result: Sequence valid — 5 chunks reassembled, 3 750 chars total
Detect a missing chunk in line format
Inputs: b64chunk:1/3:YWJj b64chunk:3/3:bW5v (chunk 2 missing)
Result: Validation failed: Chunk 2 of 3 is missing
Glossary
- Sequence validation
- The process of checking that a chunk set is complete, all indexes are unique, and the count matches the declared total before attempting reassembly.
- Corrupt chunk
- A chunk whose "data" field contains characters outside the Base64 alphabet, indicating the payload was modified during transport.
- Total mismatch
- An error where two or more chunks carry different "total" values, meaning they originated from different chunking operations or the total field was corrupted.
- Duplicate index
- Two chunks sharing the same sequence index — typically caused by a resend delivering the same part twice.
- Line format
- The compact b64chunk:<index>/<total>:<data> encoding produced by the Base64 Chunker, parsed line by line.
- JSON array format
- An array of {index, total, data} objects produced by the Base64 Chunker, parsed as structured JSON.
Related reading
Frequently Asked Questions
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Why use Base64 Reassembler?
- Exhaustive sequence validation: detects missing chunks, duplicates, total mismatches, and corrupt Base64 before reassembling
- Labelled per-chunk error diagnostics name the exact chunk and issue type for fast debugging
- Accepts both JSON array and line-format ("b64chunk:") inputs, auto-detected from the first line
- Download the result as raw Base64 text or as the decoded binary file with the correct extension
- Reassembly is blocked until all issues are resolved, preventing silently corrupt output
Common use cases
- Rebuild a file received in chunks over an MQTT topic and verify no messages were dropped or reordered
- Validate a multi-part Base64 upload from a front-end wizard before decoding to a final binary
- Debug a chunked-transfer pipeline by identifying exactly which chunk index contains corrupt data
- Reassemble firmware OTA update segments and decode the result to verify the binary before flashing
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