Prevent over long nonces in ChaCha20-Poly1305

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.

CVE-2019-1543

Fixes #8345

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8406)

1 files changed, 3 insertions, 1 deletions

diff --git a/crypto/evp/e_chacha20_poly1305.c b/crypto/evp/e_chacha20_poly1305.c
index e8a323f3be..37902000a0 100644
--- a/crypto/evp/e_chacha20_poly1305.c
+++ b/crypto/evp/e_chacha20_poly1305.c
@@ -30,6 +30,8 @@ typedef struct {
 
 #define data(ctx)   ((EVP_CHACHA_KEY *)(ctx)->cipher_data)
 
+#define CHACHA20_POLY1305_MAX_IVLEN     12
+
 static int chacha_init_key(EVP_CIPHER_CTX *ctx,
                            const unsigned char user_key[CHACHA_KEY_SIZE],
                            const unsigned char iv[CHACHA_CTR_SIZE], int enc)
@@ -533,7 +535,7 @@ static int chacha20_poly1305_ctrl(EVP_CIPHER_CTX *ctx, int type, int arg,
         return 1;
 
     case EVP_CTRL_AEAD_SET_IVLEN:
-        if (arg <= 0 || arg > CHACHA_CTR_SIZE)
+        if (arg <= 0 || arg > CHACHA20_POLY1305_MAX_IVLEN)
             return 0;
         actx->nonce_len = arg;
         return 1;