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Side channel attack on deterministic ECDSA

Title Side channel attack on deterministic ECDSA
CVE CVE-2019-16910
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.
Severity High
Credit Jack Lloyd

Vulnerability

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.

Impact

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.

Resolution

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 mbedtls_ecdsa_sign_det().

Workaround

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.

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Security Advisories

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Last updated:
Sep 26, 2019

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