ASA-2019-00118 – OpenSSL: ChaCha20-Poly1305 with long nonces

Allele Security Alert



ASA-2019-00118, CVE-2019-1543


ChaCha20-Poly1305 with long nonces


The OpenSSL Project



Affected version(s)

OpenSSL versions 1.1.1 and 1.1.0

Fixed version(s)

OpenSSL versions with the following commit applied:

Prevent over long nonces in ChaCha20-Poly1305

Proof of concept



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.

Technical details



Joran Dirk Greef (Ronomon)


ChaCha20-Poly1305 with long nonces (CVE-2019-1543)

Prevent over long nonces in ChaCha20-Poly1305

chacha20-poly1305: fails to detect tampering, where IV > 12 and IV <= 16 (CHACHA_CTR_SIZE) #8345



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Last modified: March 6, 2019

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