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.
OpenSSL versions 1.1.1 and 1.1.0 are affected by this issue. Due to the limited
scope of affected deployments this has been assessed as low severity and
therefore we are not creating new releases at this time. Reported by Joran Dirk Greef of Ronomon.