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A Certificate Authority (CA) issues digital certificates to certify the ownership of a public key by the named subject of the certificate. What is the rationale for CAs cross-signing each other (i.e. Y signing Z and Z signing Y)?

For example:

Let Y, X, Z be different CAs. Z may have the following signing arrangements:

    Y <<Z>>
    Z <<Y>>
    Z <<X>>

I couldn't find a good answer from crypto exchange; would appreciate a clear answer, thanks.

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Certificate Authorities cross-sign each other when they get married together.

When CA Y issues a certificate for CA Z, then any system who trusts Y will indirectly gain confidence in everything that Z issues, since, for any certificate X issued (signed) by Z, a system that trusts Y will build the chain Y→Z→X. Thus, this cross-certification represents an assertion from Y that basically says: "everything that Z signs is as good as if I, Y, had signed it myself".

If CA Z also issues a certificate for CA Y, then trust goes both ways. Every system that trusts either Y or Z will be able to validate certificates emitted by both. The two CA are now really one, at least as far as trust is concerned.

A divorce would require revoking both cross-certificates, or at least failing to issue new ones when the cross-certificates expire.

(In practice, this kind of things happens when a new CA is created but is "morally" a replacement for the old one; or when a merger occurs between two businesses and they try to fuse their internal computer systems together.)

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