Explain what the certificate authority and the digital certificate each guarantee within PKI.
Describe how a certificate is checked by following its issuers upward one by one, and state the role of the trust anchor, the starting point of trust where that path ends.
Tell a server certificate from a client certificate by which side each one proves.
PKI is the whole system in which a third-party certificate authority uses a digital certificate to vouch that a given public key really belongs to the person who claims it.
Anyone can create a public key
Keys claiming someone else's name can be made too
A CA checks the owner and issues a certificate
The receiver checks the owner via the certificate
The other side checks the certificate before using that public key.
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A public key carries no owner's name, so receiving the key alone does not let you tell it apart from an impostor's key. A certificate authority (CA) verifies the applicant's identity and then issues a certificate that binds the public key to its owner. PKI means the whole infrastructure made up of certificate authorities, certificates and the procedures for checking them, not the name of a particular product.
AnalogyAnyone can have a name stamp made, but once an official office certifies that the stamp is registered to you, others can trust it is really yours. PKI is that registration system.
How to remember"Infrastructure" is the word for society's groundwork, such as roads and water mains.
Exam points
ExamThe essential component of PKI is the certificate authority (認証局). It issues certificates for users' public keys and revokes a certificate that runs into trouble even before it expires, which keeps the keys trustworthy.
ExamUnlike a scheme where acquaintances sign each other's keys to spread trust, PKI rests its trust on a third-party certificate authority.
公開鍵基盤 (public key infrastructure) is the Japanese-language name for PKI (Public Key Infrastructure); both refer to the same system.
In the Japanese name, 公開 stands for Public, 鍵 for Key and 基盤 for Infrastructure. Documents often show both forms side by side, as in PKI (公開鍵基盤).
Exam points
ExamIn public key infrastructure (公開鍵基盤), a digital signature (デジタル署名) is checked with a public key vouched for by a certificate authority (認証局). The receiver can confirm that the sender is genuine and that the content was not altered; keeping others from reading it is the job of encryption (暗号化).
A digital certificate (デジタル証明書) is electronic data in which a certificate authority attaches its signature to vouch that "this public key belongs to this person (or this server)."
Main field
Content
Owner
Name of the person, organization or server being certified
Public key
The owner's public key (what is certified)
Issuer
The certificate authority that issued it
Expiry
The period during which the certificate may be used
CA's signature
Shows that the CA issued it and that the content has not been altered
Also called an electronic certificate or public key certificate, it sits at the center of PKI. By checking the signature of the issuing certificate authority, the receiver can use the listed public key as the other party's own. A certificate has an expiry date and must be renewed when it runs out.
AnalogyIt is like a photo ID card: the issuing office's seal vouches that the photo (public key) and the name (owner) go together.
How to rememberIts other name, "public key certificate," tells you outright that what it certifies is the public key.
Exam points
ExamWhat a digital certificate (デジタル証明書) vouches for is the public key (公開鍵). It is not issued for a private key or a shared key.
ExamIf a sender attaches a self-made certificate to a signed email instead of one from a trusted certificate authority, the receiver cannot confirm that the email really came from the person it claims to be from.
ExamUnder S/MIME, the standard for exchanging email securely, sending encrypted mail requires the recipient's certificate beforehand, while sending signed mail requires the sender's certificate.
A root certificate (ルート証明書) sits at the top of the chain of trust between certificates; it is a (self-signed) certificate that the top-level certificate authority signs for itself.
Root certificate (root CA, self-signed)
Intermediate certificate (intermediate CA)
Web server's certificate
User's certificate
The intermediate CA signs the certificates of servers and users.
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Each certificate is vouched for by a higher certificate authority's signature, and that one by a CA higher still, forming a chain of trust (certificate chain). Certificates of the CAs in between are called intermediate certificates; at the top of the chain no one is left above to vouch for it, so the root certificate is self-signed. Browsers and operating systems come with trusted root certificates already installed.
AnalogyIf a company appointment takes effect only after approval by the section manager, then the department head, then the CEO, the root certificate is the CEO's seal. No one stands above to approve it, yet every employee accepts that seal as genuine from the start.
How to remember"Root": in a certificate tree drawn upside down, trust flows from the root at the top down to the branches.
Exam points
Mix-upA root certificate (ルート証明書) is also self-signed (自己署名), but it is treated differently from a certificate a sender simply made themselves. The difference is whether the verifier has decided in advance to trust it, for example by having it preinstalled.
A trust anchor (トラストアンカー) is the starting point that certificate verification is set to trust without checking any further.
The received certificate
Check the signature with the issuer's certificate
Move up to the next issuer
Confirmation ends at the trust anchor
If every certificate on the way is valid and the path reaches the trusted starting point, the first certificate can be trusted.
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Verification starts from the received certificate and follows the issuers one by one, confirming that each signature is correct and that no certificate has expired or been revoked; it succeeds once it reaches a point decided in advance as trusted. That predetermined point is the trust anchor, and in most cases a root certificate built into the browser or OS plays this role. Trust flows down from the trust anchor to the certificates below it, so the point where the path ends is where trust starts.
AnalogyFollowing a stranger's chain of letters of introduction, if you finally reach the name of a mentor you know well, you can trust the introduction. The mentor is the trust anchor.
How to rememberAn anchor holds a ship in place: the step-by-step check stops here and settles instead of drifting.
Exam points
Mix-upA trust anchor (トラストアンカー) is not a certificate that relies on a higher level in the chain to confirm it. It is a starting point the verifier itself has chosen to trust, so what serves as the anchor depends on the verifier's settings.
Root certificate vs. Trust anchor
Aspect
Root certificate
Trust anchor
Key differenceWhat it is
The concrete certificate at the top of the chain
The role of the point where trust begins in verification
Signature
Signed by itself
A role name, so the form of signature does not matter
A server certificate (サーバ証明書) is a certificate that shows a connecting browser or similar client that the destination, such as a web server, is genuine.
Browser requests a connection
Server sends its server certificate
Browser verifies the certificate
Encrypted communication begins
Once confirmed, it uses that public key to share a session key and switches to encrypted communication.
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Because it is used in SSL/TLS (https connections), people also call it an SSL certificate. It contains the server's name and operator details, that server's public key and the signature of the issuing certificate authority.
AnalogyIt is like a notice at a shop's entrance in which a trusted third party confirms the shop's name and address. Customers can tell it is the real shop named on the sign, but not whether the shop has a good reputation.
How to remember"Server" comes from serving: it is the certificate the side that answers requests uses to prove who it is.
Exam points
ExamWhen you connect over SSL/TLS and confirm the certificate belongs to the intended operator, you can conclude that the destination is genuine and that the traffic is encrypted. The certificate does not vouch for whether the operator is a trustworthy business.
ExamIn SSL/TLS, the browser checks the destination with the server certificate (サーバ証明書) and uses the server's public key in that certificate to share the key for encrypted communication safely.
A client certificate (クライアント証明書) is a certificate that shows the user or device connecting to a server is a legitimate party.
It is installed in advance on the devices of authorized users, and the server checks it on every connection. Used together with an ID and password, it keeps out devices without the certificate even if someone knows the password.
AnalogyIt is like a membership card at a members-only club: the staff check each arriving guest's card to confirm they are a real member.
How to rememberA client is the one who asks for a service, so this is the certificate held by the requesting side.
Exam points
Mix-upA client certificate (クライアント証明書) is what the server uses to check the user side, the opposite direction from a server certificate (サーバ証明書), which the browser uses to check the destination.
Server certificate vs. Client certificate
Aspect
Server certificate
Client certificate
Key differenceSide being proved
The destination, such as a web server
The user or device connecting in
Side doing the check
The user side, such as a browser
The server side
Main use
Authenticates the destination in SSL/TLS and starts encrypted communication
Limits which devices can connect, on top of the password
Key points
PKI (public key infrastructure) is the system in which a certificate authority uses digital certificates to vouch for who owns a public key.
A digital certificate vouches for the link between a public key and its owner; it carries the certificate authority's signature and has an expiry date.
A certificate is verified by tracing its issuers upward; if every certificate on the way is valid and the path reaches a trusted starting point (the trust anchor), it is trusted. In most cases that starting point is a self-signed root certificate.
A server certificate shows that the server being connected to is genuine, and the browser verifies it before SSL/TLS starts encrypted communication.