BTC#: Digital Signatures

Series: BTC# – Learning to Program Bitcoin in C#

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CryptographyBasicsTitle

Your digital identity is a private key, a secret number than only you know. It’s a bit like a PIN on your credit card but much, much longer. One of the major security flaws with credit cards is that you need to reveal your secret to authorise your spending.

CreditCardAliceBobEve.png

Digital signatures prove that you know a secret without revealing the secret. Only a person in possession of the secret can produce a particular digital signature and the signature can’t be produced without knowing the secret. In other words, a signature authenticates a message and its origin can’t be denied.

A digital signature also incorporates details of the message that was signed, so the message can’t be tampered with without invalidating the signature.

How Signatures Work

A signature is built up from the signatory’s private key, a hash of the document being signed, and an ephemeral key that is used for a single message. Of these, only the document hash is publicly known. The other two are protected by an irreversible elliptic curve multiplication. The signature is made up of two components, r and s.

SignatureDataFlow.png

To verify that a signature is valid, the receiver needs the signature, the document hash, and the signatory’s public key. The signature’s r-component is derived from the ephemeral key and the s-component is composed in such a way that we can confirm that it was created using the document hash and the two secrets, without exposing either of the secrets.

The verification algorithm is an equation in which all the parts cancel out if the correct document hash and public key are provided. If the signature was created using a different private key than the one used to create the public key, the equation doesn’t balance, meaning the signature is not authentic. If the signature was created using a different document hash, the equation doesn’t balance, meaning that the document has been tampered with.

SignatureEquation.png

Programming Bitcoin provides a baffling analogical explanation of digital signature verification involving William Tell, inscriptions on arrow heads, and x-ray machines, which can be skipped, but do go through the equations and their derivations. If you want to go direct to the source, it’s NIST’s Digital Signature Standard.

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Next: Digital Signature Code »

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