Re: SSL, Apache 2 and RSA key sizes

Peter Fairbrother wrote:
Jason wrote:

Peter Fairbrother wrote:
Sometimes servers use RSA key-exchange keys only once and then discard them,
signing each new RSA key-exchange key with their RSA signature key, but not
often. They will typically reuse the same RSA key-exchange key a lot,
sometimes even using the same RSA key for both key-exchange and signature
functions (bad).
Why is it bad to use the same RSA key for both encryption and signing?

2) If the same key is used for encryption and signing it is possible for an
attacker to get you to decrypt a message by getting you to sign something -
signing X is exactly the same as decrypting X - and vice versa, he can get
you to sign something by getting you to decrypt it.

Note that this is not a threat if you use proper padding schemes.

Also, it may easily be a business requirement to have backups of all encryption keys, to prevent loss of access to encrypted files. But a business should not have copies of its employees signing keys, to make sure each employee can be held responsible for everything he signs.

--Mike Amling
.

Relevant Pages

  • Re: SSL, Apache 2 and RSA key sizes
    ... They will typically reuse the same RSA key-exchange key a lot, ... Why is it bad to use the same RSA key for both encryption and signing? ... you to sign something by getting you to decrypt it. ... Note that this is not a threat if you use proper padding schemes. ...
    (sci.crypt)
  • Re: SSL, Apache 2 and RSA key sizes
    ... They will typically reuse the same RSA key-exchange key a lot, ... Why is it bad to use the same RSA key for both encryption and signing? ... especially in the early SSL versions. ...
    (sci.crypt)
  • Re: SSL, Apache 2 and RSA key sizes
    ... They will typically reuse the same RSA key-exchange key a lot, ... Why is it bad to use the same RSA key for both encryption and signing? ... Suppose your secret encryption key is exposed or demanded by eg a Court ... expensive and not-very-reliable procedures for revoking a signature key. ...
    (sci.crypt)
  • Re: SSL, Apache 2 and RSA key sizes
    ... signing each new RSA key-exchange key with their RSA signature key, ... They will typically reuse the same RSA key-exchange key a lot, ...
    (sci.crypt)