# Ethereum address checksums: what mixed-case letters can detect

EIP-55 encodes a checksum in the capitalization of an Ethereum address’s hexadecimal letters. Correct checking can detect many typing mistakes without changing the underlying address bytes. It does not prove the recipient’s identity, select a network or guarantee that every possible error will be caught.

Evidence: [Mixed-case checksum address encoding](https://eips.ethereum.org/EIPS/eip-55)

Canonical: https://degreesofsatoshi.com/encyclopedia/ethereum-address-checksums/
Published: 2026-10-02
Substantively modified: 2026-10-02
Independently verified by an automated reviewer: 2026-10-02T18:12:41.505Z
Data current through: 2026-10-02

AI-assisted research and drafting with a separate automated source-verification pass; no external expert or named human review is implied.

## Key facts

- **Bytes:** Changing letter case does not change the underlying hexadecimal address value. ([Mixed-case checksum address encoding](https://eips.ethereum.org/EIPS/eip-55))
- **Checksum:** Capitalization is derived from a hash of the lowercase address text. ([Mixed-case checksum address encoding](https://eips.ethereum.org/EIPS/eip-55))
- **Limits:** The checksum detects errors probabilistically; it is not identity verification. ([Mixed-case checksum address encoding](https://eips.ethereum.org/EIPS/eip-55))

## A readable error check in the same address length

Hexadecimal letters can be printed in upper or lower case. EIP-55 uses that choice to encode checksum information, while digits carry no case distinction.

A receiving interface can recalculate the expected capitalization and flag a mismatch. Not every interface requires mixed-case input, so acceptance alone does not demonstrate that a checksum was checked.

Evidence: [Mixed-case checksum address encoding](https://eips.ethereum.org/EIPS/eip-55)

## Valid encoding can still point to an attacker

Someone else’s address can have a perfectly valid checksum. A checksum is designed to detect accidental corruption of an identifier, not decide whether the destination belongs to the person you intended.

Compare the complete trusted destination and network context. Matching only a few leading or trailing characters provides much less information than checking the full address.

Evidence: [Mixed-case checksum address encoding](https://eips.ethereum.org/EIPS/eip-55); [Ethereum accounts](https://ethereum.org/en/developers/docs/accounts/)

## Keep network identity separate

The EIP-55 algorithm does not use a chain ID. The same checksummed address can therefore be displayed on different compatible networks.

For tokens, verify both the asset’s contract and the recipient on the intended chain. Address formatting cannot repair a transfer sent to an unsupported network.

Evidence: [Mixed-case checksum address encoding](https://eips.ethereum.org/EIPS/eip-55); [Simple replay attack protection](https://eips.ethereum.org/EIPS/eip-155)

## Questions

### Is a lowercase Ethereum address always invalid?

No. Lowercase text can represent valid address bytes, and some correct EIP-55 outputs are entirely lowercase. A wallet accepting lowercase input does not by itself show that it checked the expected checksum capitalization.

Evidence: [Mixed-case checksum address encoding](https://eips.ethereum.org/EIPS/eip-55)

## Claims and scope

### ethereum-address-checksums-quick-answer

EIP-55 encodes a checksum in the capitalization of an Ethereum address’s hexadecimal letters. Correct checking can detect many typing mistakes without changing the underlying address bytes. It does not prove the recipient’s identity, select a network or guarantee that every possible error will be caught.

Scope: {"collection":"ethereum","dataAsOf":"2026-10-02","blockHeight":null}

### ethereum-address-checksums-fact-bytes

Bytes: Changing letter case does not change the underlying hexadecimal address value.

Scope: {"collection":"ethereum","dataAsOf":"2026-10-02","blockHeight":null}

### ethereum-address-checksums-fact-checksum

Checksum: Capitalization is derived from a hash of the lowercase address text.

Scope: {"collection":"ethereum","dataAsOf":"2026-10-02","blockHeight":null}

### ethereum-address-checksums-fact-limits

Limits: The checksum detects errors probabilistically; it is not identity verification.

Scope: {"collection":"ethereum","dataAsOf":"2026-10-02","blockHeight":null}

## Sources

- [Mixed-case checksum address encoding](https://eips.ethereum.org/EIPS/eip-55) — Ethereum Improvement Proposals. Capitalization encodes a checksum without changing the address bytes. Locator: Specification; Rationale; Test Cases. Retrieved: 2026-10-02T17:03:42.205Z.
- [Ethereum accounts](https://ethereum.org/en/developers/docs/accounts/) — ethereum.org contributors. Key-controlled and code-controlled accounts, balances, nonces and storage. Locator: Account types; An account examined; Account creation. Retrieved: 2026-10-02T17:03:41.958Z.
- [Simple replay attack protection](https://eips.ethereum.org/EIPS/eip-155) — Ethereum Improvement Proposals. Transaction chain ID domain separation; not blanket protection for all signatures. Locator: Specification. Retrieved: 2026-10-02T17:03:42.312Z.

## Revision history

- 2026-10-02: First publication after primary-source research and independent automated verification.

## Cite this entry

Degrees of Satoshi editorial project. “Ethereum address checksums: what mixed-case letters can detect.” Published 2026-10-02; updated 2026-10-02. https://degreesofsatoshi.com/encyclopedia/ethereum-address-checksums/