|Title||Side channel attack on deterministic ECDSA|
|Date||6th Sept 2019|
|Affects||All versions of Mbed TLS and Mbed Crypto|
|Impact||If the victim can be tricked to sign the same message repeatedly, the private key may be recoverable through side channels.|
Mbed TLS does not have a constant-time/constant-trace arithmetic library and uses blinding to protect against side channel attacks.
In the ECDSA signature routine previous Mbed TLS versions used the same RNG object for generating the ephemeral key pair and for generating the blinding values. The deterministic ECDSA function reused this by passing the RNG object created from the private key and the message to be signed as prescribed by RFC 6979. This meant that the same RNG object was used whenever the same message was signed, rendering the blinding ineffective.
If the victim can be tricked to sign the same message repeatedly, the blinding countermeasures are ineffective and the private key can be recovered through side channels.
Affected users should upgrade to one of the most recent versions of Mbed TLS, including 2.19.0, 2.16.3 or 2.7.12 or later. Similarly, affected users should upgrade to the most recent version of Mbed Crypto, including 2.0.0 or later.
Applications using Mbed Crypto should call
mbedtls_ecdsa_sign_det_ext() instead of the vulnerable and now deprecated
Where possible, we recommend all impacted users upgrade to a newer version of Mbed TLS or Mbed Crypto.
If this is not possible, as a workaround, disabling deterministic ECDSA prevents this attack.