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Assuming the client supports different keys, what is some rationale that should be considered for signing and encryption keys?

The encryption key

  • needs to be present on each device that is used to read email
  • all those copies means it has a greater risk of getting lost or stolen on mobile devices (and therefore should be revoked)

The signing key

  • can be device-specific
  • Since it has a long lasting lifetime (2-4 years), it can be revoked when a device is stolen.

Therefore a fewer subset of messages will have their integrity called into question

Is this valid thinking? What are some other perspectives?

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I would suggest that the lifetime of a key should be determined by its strength, according to both its keysize, and to the likelihood of reports of weaknesses in the algoritm used to create the keys.

According to Technet.com in 2009, regarding how long RSA suggest the validity period of a certificate should be, based on how long it would take an adversary to compromise a key, based on the size of the key alone.

Key length of 1024:  Validity period = not greater than 6-12 months
Key length of 2048:  Validity period = not greater than 2 years
Key length of 4096:  Validity period = not greater than 16 years

Also, you cannot have a certificate signed by a CA have a validity period longer than that of the CA itself.

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