Fixes the following security issues:
Prevent over long nonces in ChaCha20-Poly1305 (CVE-2019-1543)
ChaCha20-Poly1305 is an AEAD cipher, and requires a unique nonce input for
every encryption operation. RFC 7539 specifies that the nonce value (IV)
should be 96 bits (12 bytes). OpenSSL allows a variable nonce length and
front pads the nonce with 0 bytes if it is less than 12 bytes. However it
also incorrectly allows a nonce to be set of up to 16 bytes. In this case
only the last 12 bytes are significant and any additional leading bytes are
ignored.
It is a requirement of using this cipher that nonce values are unique.
Messages encrypted using a reused nonce value are susceptible to serious
confidentiality and integrity attacks. If an application changes the
default nonce length to be longer than 12 bytes and then makes a change to
the leading bytes of the nonce expecting the new value to be a new unique
nonce then such an application could inadvertently encrypt messages with a
reused nonce.
Additionally the ignored bytes in a long nonce are not covered by the
integrity guarantee of this cipher. Any application that relies on the
integrity of these ignored leading bytes of a long nonce may be further
affected. Any OpenSSL internal use of this cipher, including in SSL/TLS, is
safe because no such use sets such a long nonce value. However user
applications that use this cipher directly and set a non-default nonce
length to be longer than 12 bytes may be vulnerable.
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
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