 ボブ |
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 （ボブの公開鍵）
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ボブは、2つのキーを与えられています。ボブの鍵の一つは、他には秘密鍵と呼ばれ、公開鍵と呼ばれています。
| ボブの共同研究者： | ||||
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誰もがボブの公開鍵を取得することができますが、ボブは自分自身に自分の秘密鍵を保持します
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| パット | ダグ | スーザン | ||
ボブの公開鍵は、それを必要とする誰にでも利用可能ですが、彼は自分自身に自分の秘密鍵を保持します。キーは情報を暗号化するために使用されています。情報を暗号化すると、適切なキーを持つ唯一の人は再びそれを読みやすくすることができるように、「それをスクランブル」を意味します。ボブの二つの鍵の一つは、データを暗号化することができ、かつ他のキーは、そのデータを復号化することができます。
（下図）スーザンはボブの公開鍵を使ってメッセージを暗号化することができます。ボブはメッセージを復号化するために彼のプライベートキーを使用しています。ボブの同僚のいずれかがスーザンは暗号化されたメッセージへのアクセス権を持っているかもしれませんが、ボブの秘密鍵なしで、データが無価値です。
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「ねえ、ボブは、どのようにタコベルでのランチについて。私は、彼らが自由にリフィルを持って聞いて！」 |  |
HNFmsEm6Un BejhhyCGKOK JUxhiygSBCEiC 0QYIh / Hn3xgiK BcyLK1UcYiY lxx2lCFHDC / A |
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HNFmsEm6Un BejhhyCGKOK JUxhiygSBCEiC 0QYIh / Hn3xgiK BcyLK1UcYiY lxx2lCFHDC / A |  |
「ねえ、ボブは、どのようにタコベルでのランチについて。私は、彼らが自由にリフィルを持って聞いて！」 |
彼のプライベートキーと右のソフトウェアを使用すると、ボブは、文書やその他のデータにデジタル署名を置くことができます。デジタル署名は、「スタンプ」ボブはボブに固有のデータに置いており、偽造が非常に困難です。また、署名は署名されたデータに加えた変更は検出されず行くことができないことを保証します。
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To sign a document, Bob's software will crunch down the data into just a few lines by a process called "hashing". These few lines are called a message digest. (It is not possible to change a message digest back into the original data from which it was created.) |
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Bob's software then encrypts the message digest with his private key. The result is the digital signature.
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Finally, Bob's software appends the digital signature to document. All of the data that was hashed has been signed.
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Bob now passes the document on to Pat.
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First, Pat's software decrypts the signature (using Bob's public key) changing it back into a message digest. If this worked, then it proves that Bob signed the document, because only Bob has his private key. Pat's software then hashes the document data into a message digest. If the message digest is the same as the message digest created when the signature was decrypted, then Pat knows that the signed data has not been changed. |
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Doug (our disgruntled employee) wishes to deceive Pat. Doug makes sure that Pat receives a signed message and a public key that appears to belong to Bob. Unbeknownst to Pat, Doug deceitfully sent a key pair he created using Bob's name. Short of receiving Bob's public key from him in person, how can Pat be sure that Bob's public key is authentic? |
It just so happens that Susan works at the company's certificate authority center. Susan can create a digital certificate for Bob simply by signing Bob's public key as well as some information about Bob.
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Now Bob's co-workers can check Bob's trusted certificate to make sure that his public key truly belongs to him. In fact, no one at Bob's company accepts a signature for which there does not exist a certificate generated by Susan. This gives Susan the power to revoke signatures if private keys are compromised, or no longer needed. There are even more widely accepted certificate authorities that certify Susan.
Let's say that Bob sends a signed document to Pat. To verify the signature on the document, Pat's software first uses Susan's (the certificate authority's) public key to check the signature on Bob's certificate. Successful de-encryption of the certificate proves that Susan created it. After the certificate is de-encrypted, Pat's software can check if Bob is in good standing with the certificate authority and that all of the certificate information concerning Bob's identity has not been altered.
Pat's software then takes Bob's public key from the certificate and uses it to check Bob's signature. If Bob's public key de-encrypts the signature successfully, then Pat is assured that the signature was created using Bob's private key, for Susan has certified the matching public key. And of course, if the signature is valid, then we know that Doug didn't try to change the signed content.
Although these steps may sound complicated, they are all handled behind the scenes by Pat's user-friendly software. To verify a signature, Pat need only click on it.
(c) 1996, David Youd
Permission to change or distribute is at the discretion of the author
Warning: You may be missing a few lines of text if you print this document. This seems to occur on pages following pages that have blank space near the bottom due to moving tables with large graphics in them to the next page so that the images are not split across pages. If this happens to you, simply print out document in sections. (Ex: I have the problem on page 4, so I print pages 1-3, then pages 4-5.)
