Self-signed certificate
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It has been suggested that this article or section be merged with root certificate. (Discuss) Proposed since December 2009. |
In cryptography and computer security, a self-signed certificate is an identity certificate that is signed by its own creator. That is, the person that created the certificate also signed off on its legitimacy.
In typical public key infrastructure (PKI) arrangements, a digital signature from a certificate authority (CA) attests that a particular public key certificate is valid (i.e., contains correct information). Users, or their software on their behalf, check that the private key used to sign some certificate matches the public key in the CA's certificate. Since CA certificates are often signed by other, "higher ranking," CAs, there must necessarily be a highest CA, which provides the ultimate in attestation authority in that particular PKI scheme.
Obviously, the highest-ranking CA's certificate can't be attested by some other higher CA (there being none), and so that certificate can only be "self-signed." Such certificates are also termed root certificates. Clearly, the lack of mistakes or corruption in the issuance of such certificates is critical to the operation of its associated PKI; they should be, and generally are, issued with great care.
In a web of trust certificate scheme there is no central CA, and so identity certificates for each user can be self-signed. In this case, however, it has additional signatures from other users which are evaluated to determine whether a certificate should be accepted as correct. So, if users Bob, Carol, and Edward have signed Alice's certificate, user David may decide to trust that the public key in the certificate is Alice's (all these worthies having agreed by their signatures on that claim). But, if only user Bob has signed, David might (based on his knowledge of Bob) decide to take additional steps in evaluating Alice's certificate. On the other hand, Edward's signature alone on the certificate may by itself be enough for David to trust that he has Alice's public key (Edward being known to David to be a reliably careful and trustworthy person). There is of course, a potentially difficult regression here, as how can David know that Bob, Carol, or Edward have signed any certificate at all unless he knows their public keys (which of course came to him in some sort of certificate)? In the case of a small group of users who know one another in advance and can meet in person (e.g., a family), users can sign one another's certificates when they meet as a group, but this solution does not scale to larger settings. This problem is solved by fiat in X.509 PKI schemes as one believes (i.e., trusts) the root certificate by definition.[dubious ] The problem of trusting certificates is real in both approaches, but less easily lost track of by users in a Web of Trust scheme.
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[edit] Security Issues
CAs are third parties who have access to the private keys and require both parties to trust the CA. (CAs are typically large, impersonal enterprises and a high value target for compromise.) If the parties know each other, trust each other to protect their private keys, and can confirm transfer public keys (e.g. compare the hash out of band), then self-signed certificates may decrease overall risk. Self-signed certificate transactions may also present a far smaller attack surface.
Self-signed certificates cannot (by nature) be revoked,[1] which may allow an attacker who has already gained access to monitor and inject data into a connection to spoof an identity if a private key has been compromised. CAs on the other hand have the ability to revoke a compromised certificate if alerted, which prevents its further use.
Some CA's can verify the identity of person to whom they issue a certificate; for example the US military issues their Common Access Cards in person, with multiple forms of other ID, and only when a higher authority requires the issue.
[edit] Other Issues
Cost. Self-signed certificates can be created for free using a wide variety of tools including Java's keytool, Adobe Reader, and Apple's Keychain. Certificates bought from major CA's range from $10s to $1000s of dollars per year.[citation needed]
Speed to Deploy. Self-signed certificates require the two parties to interact (e.g. to securely trade public keys). Using a CA requires on the CA and the certificate holder to interact; the holder of the public key can validate its authenticity with the CA's root certificate.
Customization. Self-signed certificates are easier to customize, for example a larger key size, contained data, metadata, etc.
[edit] See also
[edit] References
[edit] Terminology
- C
- Country
- CA
- Certificate authority
- CN
- Common Name
- CSR
- Certificate signing request
- DER
- Distinguished Encoding Rules
- O
- Organization
- OU
- Organizational Unit
