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  • Writer's pictureLukas Pelcman

Advanced Electronic Signatures (AES) under the Microscope: A Cryptography

Updated: Feb 14

I have described what are advanced electronic signatures (AES) in my previous post. But what to imagine under all that legal jibber jabber?

Well, you can start with tech jibber jabber for a change, with asymmetric cryptography.

At the beginning of advanced electronic Signatures were the keys, both public and private

Asymmetric cryptography is a system of ciphers, cryptographic algorithms, which works with the infrastructure of public and private keys. For now, asymmetric cryptography is probably the only way to create AES.

In practice, signatory uses a key known only to him (private key) to encrypt the text. The cipher created this way can only be deciphered using the paired key, which is also available to the addressee (public key). Hence the asymmetry.

At the same time, the private key cannot be calculated backwards from the public key. For that reason, the public key can be published without any worries.

Therefore, e-signature based on asymmetric cryptography is not something that is added to the document itself, but rather a digital file encrypted with the signatory's private key.

Comparing hashes, not the documents themselves

In addition, the signed document is not encrypted itself, but rather only its hash (or digest) created by using the so-called hash function; that is a one-way mathematical operation, the output of which is a unique code (number) that is derived from the original document.

Anyone who then takes the same document and performs the same operation will get the same result.

Conversely, if anyone should make any changes to the original document, the resulting hash will be different. By comparing the results – the hashes – it is therefore possible to decide whether the original documents are identical, without having to directly examine the documents themselves.

Going few steps further

It follows that e-signature, which is a hash of the signed file encrypted using one’s private key, can then be sent together with the unencrypted original document to the addressee.

The addressee can verify that the same document she received was signed, simply by using the public key to decrypt the encrypted hash of the document (e-signature) and comparing the result with the hash of the unencrypted document she received together with the e-signature.

If both values match, it means that the document the addressee received is identical to the one that was signed. This operation is performed automatically by freely available tools – for example in Adobe Reader one can immediately see a green check mark.

AES is therefore uniquely linked to the signed document.

It does not have to have the bow

❌ As mentioned above, an e-signature is not something permanent, that is only added to the document as a stamp.

❌ Also, AES cannot be equated only with its most frequent graphic representation on the signed document in the form of "Adobe" (or any other) bow.

E-signed document does not have to have any graphic indication that it was signed at all.

Curiously, did someone ever argue with you just because the electronically signed document did not have "the bow" such as that? Let me know.

Cryptography in Advanced Electronic Signatures
Cryptography in Advanced Electronic Signatures

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