Malformed base64 encoding

`bitcoin-cli` prints "error code: -3" and "error message:" before the text. A well-formed base64 string that is not a valid signature returns `false` instead of an error.

Bitcoin Core RPC — verifymessage with a signature that is not base64

What it means

`verifymessage` expects the 65-byte compact signature encoded in standard base64 (88 characters, usually starting with `H`, `I`, `G` or `J`). The string you passed is not valid base64, so Core did not even try to recover a key and returned RPC error -3. The signature may be correct — it is in the wrong text encoding.

Why it happens

• Hex instead of base64

  common

  Applications and hardware-wallet SDKs often return signatures as hex (130 characters). Core rejects them.

• Damaged in a URL or form

  occasional

  `+` turned into a space by form decoding, `%2B`/`%3D` left encoded, or base64url (`-`, `_`) produced by a web library.

• An Ethereum signature

  occasional

  A `0x…` signature from `personal_sign` is a different scheme (EIP-191 prefix, other v convention) and not base64.

How to fix it

1. 1.

   Convert hex signatures to base64

   Decode the hex to 65 bytes and encode them in standard base64 without line breaks.

   sig_b64=$(printf '%s' "$SIG_HEX" | xxd -r -p | base64 -w0)   # 65 bytes -> 88 base64 characters
   bitcoin-cli verifymessage "$ADDRESS" "$sig_b64" "$MESSAGE"

2. 2.

   Undo URL and base64url encoding

   Decode percent-encoding first, then convert base64url to standard base64 with padding.

   import base64, urllib.parse
   
   sig = urllib.parse.unquote(sig_from_query)                     # undo %2B / %2F / %3D
   sig = base64.b64encode(base64.urlsafe_b64decode(sig + "=" * (-len(sig) % 4))).decode()  # base64url -> base64

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