Digital signature: Difference between revisions
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* compare H<sub>1</sub> and H<sub>2</sub> | * compare H<sub>1</sub> and H<sub>2</sub> | ||
* if they are identical, then you know with overwhelming probability: | * if they are identical, then you know with overwhelming probability: | ||
** the | ** the documents signed (hash H1) and the document received (hash H2) are identical (from properties of a [[cryptographic hash]]) | ||
** whoever encrypted H1 knew the signer's private key (which only he should know in a [[public key]] system) | ** whoever encrypted H1 knew the signer's private key (which only he should know in a [[public key]] system) | ||
* so you can accept the signature as valid | * so you can accept the signature as valid | ||
If both the [[hash (cryptography) | hash]] and the [[public key]] system used are secure, and no-one except the sender knows his private key, then the signatures are trustworthy. | If both the [[hash (cryptography) | hash]] and the [[public key]] system used are secure, and no-one except the sender knows his private key, then the signatures are trustworthy. | ||
Revision as of 06:52, 11 November 2008
Template:TOC-right Digital signatures provide source authentication for online documents, messages or records, in a manner analogous to what a signature provides for a paper document.
Two cryptographic techniques are used together to produce a digital signature, a cryptographic hash and a public key cryptosystem.
The steps for the sender are as follows:
- calculate a hash or message digest from the message
- encrypt that hash with the sender's private key
- append the encrypted hash to the message as a signature
Steps for the receiver are:
- obtain the sender's public key and verify its validity
- decrypt the signature, using the sender's public key, to get the hash value; call it H1
- hash the message body yourself to get another hash value, H2
- compare H1 and H2
- if they are identical, then you know with overwhelming probability:
- the documents signed (hash H1) and the document received (hash H2) are identical (from properties of a cryptographic hash)
- whoever encrypted H1 knew the signer's private key (which only he should know in a public key system)
- so you can accept the signature as valid
If both the hash and the public key system used are secure, and no-one except the sender knows his private key, then the signatures are trustworthy.