The Warsaw-based financial institution Alior Bank SA Monday announced plans to use the public Ethereum blockchain to authenticate and secure sensitive documents for clients.

According to a report in Forbes, the Alior announced that the public blockchain distributed ledger, which is second only to the bitcoin blockchain, will be used to provide a feature that will allow customers to verify the integrity and authenticity of official documents sent through the bank.

With a blockchain, a series of transactions is secured in a ledger — a type of database — through cryptography and distribution between multiple parties. Every time transactions are added to the ledger, they receive a cryptographic hash that can be used to verify when the transaction occurred.

Blockchain technology also provides a tamperproof structure by securing previous transactions through building a chain where each subsequent addition depends on the integrity of the previous. Any changes to the blockchain are immediately detectable, making it next to impossible to change previous entries.

What makes Alior’s solution different in the industry is that the bank is using a public blockchain instead of a private one. This means that transactions involving documents are publicly viewable and auditable, although the documents themselves remain private, secure and under control of clients. Only the keys to verifying authenticity are open.

The Ethereum blockchain supports its own cryptocurrency, known as ETH or Ether, with a market cap of almost $28 billion, making it the second most popular public blockchain behind the bitcoin cryptocurrency.

“We know exactly in which block of Ethereum a document with a given hash is published,” said Piotr Adamczyk, Alior’s blockchain technology lead. “If we know the block number, we also know the timestamp. We know that the document was published some time ago and hasn’t been changed in that time, so we can prove it hasn’t been replaced on our servers.”

In this manner, secured documents can have a provable provenance, including a verifiable date and time it was added to the blockchain, called a timestamp. In cryptography, a hash is a unique number determined by the structures and characters in the document; if the document changes, so does the hash. As a result, it can be shown that a document is unchanged if it has the same hash published in the blockchain.

Several services, outside the banking industry, have put blockchain technology to use for exactly this purpose such as blockchain authentication company Factom Inc.’s dLoc service, which provides verification for physical documents.

Other examples include a partnership between Chinese electronic document company BestSign and blockchain infrastructure provider Bitfury Group to make a document verification system, and a secure enterprise document storage service from decentralized-storage company Storj Labs Inc. and document sharing company CapLinked Inc.

All of those examples use private blockchains — distributed ledgers wholly serviced within permissioned environments — as opposed to public blockchains, which are distributed openly among any number of parties. According to Alior, the reason for that decision is because public blockchains can be audited by anyone. And it’s known, because of the wide distribution between separate parties, that the security can be trusted.

“We want people to verify that we did everything right and we don’t conceal anything,” said Tomasz Sienicki, blockchain strategy lead at Alior. “If we say the documents are actually verified and authentic, everybody can check it and confirm. That’s not possible using a private blockchain.”

Although the hashes are stored on a public blockchain, meaning anyone can see them, it’s still not possible to match those hashes to documents without knowing the transaction itself and the hash for the document to be verified. That information is stored securely by Alior and only its customers can access that information directly.

Alior customers can see a history of documents secured by the system, scroll through them and have them independently verified by comparing the hash of the document in hand against the hash secured in the Ethereum blockchain.

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