A dependency update can change the string returned by HexBytes.hex() without changing the signature bytes. If the service receiving your EIP-712 signature requires a 0x prefix, that difference can make an otherwise valid signature fail at the boundary.
Why a signature can fail even when its bytes are correct
HexBytes converts byte data to hexadecimal text, but the textual representation can depend on the installed package version. A consumer may expect a string beginning with 0x; if your code sends bare hexadecimal digits—or adds a second prefix to a value that already has one—the serialized value may not meet that consumer’s contract.
This is a representation problem, not evidence that the cryptographic signature bytes changed. The receiver’s documented format is decisive: not every API or signature scheme necessarily requires the same string shape.
What the reported HexBytes version tests show
The DEV Community article by minia2a reports these results for a 65-byte input. They are the author’s test results, not independently reproduced here; the author says they inspected HexBytes 2.0.0 source rather than running that version.
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| HexBytes version | Reported .hex() output |
Reported to_0x_hex() availability |
|---|---|---|
| 0.3.1 | 132 characters, beginning with 0x |
Not available |
| 1.0.0 and 1.1.0 | 130 characters, without 0x |
Not available |
| 1.2.0 and 1.3.1 | 130 characters, without 0x |
Available |
| 2.0.0 | Not run; the article author reports inspecting source | Not stated for this version in the reported results |
The article attributes the change to HexBytes 0.3.x overriding .hex() to include a prefix, with that override removed in 1.0.0. Treat this as the author’s reported behavior, not as a universal guarantee for every release or environment.
Why adding 0x unconditionally is fragile
This patch works only if .hex() returns bare digits:
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sig = "0x" + h.hex()If the method already returns 0x…, concatenation produces 0x0x…. That is overlong and fails the example validator below. A fix that assumes one dependency version can break under another; the article author, minia2a, puts it this way: “Any fix that requires knowing the version is a fix that will be wrong on the machine you didn’t test.”
Normalize the prefix, then validate the receiver’s format
The article’s portable approach checks the returned value rather than inferring its shape from the package version:
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sig = h.hex()
sig = sig if sig.startswith("0x") else "0x" + sigThen validate the final value against the actual receiving service’s documented contract. For the article’s example of a prefixed, 65-byte signature, the pattern is:
import re
assert re.fullmatch(r"0x[0-9a-fA-F]{130}", sig)The 130 hexadecimal characters represent 65 bytes at two characters per byte; the prefix adds two more characters, making a 132-character string. This regex is an example boundary check, not a universal rule for all signature APIs. HexBytes versions reported to provide to_0x_hex() may offer another accessor, but the prefix check avoids relying on that method being present.
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What EIP-712 specifies—and what it does not
EIP-712, titled “Typed structured data hashing and signing,” is marked Final and dates to 2017-09-12. Its eth_signTypedData return description specifies a hex-encoded 65-byte signature beginning with 0x. That supports the prefixed shape for that interface; EIP-712 does not standardize how Python’s HexBytes library implements .hex().
Catch the mismatch where the signature leaves your code
Check the serialized value at the boundary where your code hands it to another service. A focused assertion can catch an output-shape change during testing after a dependency update, before the receiver rejects the request. Also check the installed package version and whether the result already has a prefix; do not treat a passing local check as proof of acceptance unless the check matches the receiver’s contract.
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- FAST & CONVENIENT LOGIN: Plug in your Security Key NFC via USB-A and tap it, or tap it against your phone (NFC) to authenticate. No batteries, no internet connection, and no extra fees required.
- TRUSTED PASSKEY TECHNOLOGY: Uses the latest passkey standards (FIDO2/WebAuthn & FIDO U2F) but does not support One-Time Passwords. For complex needs, check out the YubiKey 5 Series.
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