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On Wednesday, March 6, the OpenSSL team revealed a low severity vulnerability in the ChaCha20-Poly1305, an AEAD cipher that incorrectly allows a nonce to be set of up to 16 bytes. OpenSSL team states that ChaCha20-Poly1305 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.

The OpenSSL versions 1.1.1 and 1.1.0 are affected by this issue. However, this issue does not impact OpenSSL 1.0.2.

The OpenSSL blog states that using the ChaCha20 cipher makes the nonce values 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”, the blog states.

Also, the ignored bytes in a long nonce are not covered by the “integrity guarantee” of this cipher. This means 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.

To know more about this issue in detail, head over to the OpenSSL blog post.

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