Side-channel attack on ECC key import and validation

Title	Side-channel attack on ECC key import and validation
Date	1st of July, 2020
Affects	All versions of Mbed TLS and Mbed Crypto
Impact	A local attacker can extract the private key
Severity	High
Credit	Alejandro Cabrera Aldaya and Billy Brumley

Vulnerability

The scalar multiplication function in Mbed TLS accepts a random number generator (RNG) as an optional argument and, if provided, uses it to protect against some attacks, including a previous attack by the same authors.

It is the caller’s responsibility to provide a RNG if protection against side- channel attacks is desired; however two groups of functions in Mbed TLS itself fail to pass a RNG:

mbedtls_pk_parse_key() and mbedtls_pk_parse_keyfile()
mbedtls_ecp_check_pub_priv() and mbedtls_pk_check_pair()

When those functions are called, scalar multiplication is computed without randomisation, a number of old and new attacks apply, allowing a powerful local attacker to fully recover the private key.

It should be noted that the first group of function only performs a scalar multiplication if the private key being parsed doesn’t include the public key, or includes the public key in compressed formats. Common tools for generating key pairs tend to include the public key in uncompressed format in the encoded private key; in that case parsing functions were safe from this attack.

Impact

An attacker with access to precise enough timing and memory access information (typically an untrusted operating system attacking a secure enclave such as SGX or the TrustZone secure world) can fully recover the private key after collecting a single trace of any of the affected functions.

Resolution

Affected users will want to upgrade to Mbed TLS 2.23.0, 2.16.7 or 2.7.16 depending on the branch they’re currently using.

Workarounds

For the parsing functions, making sure all keys being parsed always include the uncompressed public key avoids the vulnerability.

For the pair-checking functions, there is no work-around except refraining from using them (they’re never called from any other library function).

If your application calls mbedtls_ecp_mul() or mbedtls_ecp_mul_restartable() directly, you want to make sure that you’re always passing a non-NULL f_rng parameter, pointing to a well-seeded instance of a secure RNG.