Hedged ECDSA and EdDSA Signatures

Introduction In Elliptic-Curve Cryptography (ECC) signature algorithms, the per-message secret number has traditionally been generated from a random number generator (RNG). The security of such algorithms depends on the cryptographic quality of the random number generation and biases in the randomness may have catastrophic effects such as compromising private keys (see e.g., ). Repeated per-message secret numbers have caused several severe security accidents in practice. As stated in , the need for a cryptographically secure source of randomness is also a hindrance to deployment of randomized ECDSA in architectures where secure random number generation is challenging, in particular, embedded IoT systems and smartcards. does however state that smartcards typically have a high-quality RNG on board, which makes it significantly easier and faster to use the RNG instead of doing a hash computation. In deterministic ECC signatures schemes such as Deterministic Elliptic Curve Digital Signature Algorithm (ECDSA) and Edwards-curve Digital Signature Algorithm (EdDSA) , the per-message secret number is instead generated in a fully deterministic way as a function of the message and the private key. Except for key generation, the security of such deterministic signatures does not rely on a source of high-quality randomness. This makes verification of implementations easier. As they are presumed to be safer, deterministic signatures have gained popularity and are referenced and recommended by a large number of recent RFCs . Side-channel attacks are potential attack vectors for implementations of cryptographic algorithms. Side-Channel attacks can in general be classified along three orthogonal axes: passive vs. active, physical vs. logical, and local vs. remote . It has been demonstrated how side-channel attacks such as power analysis and timing attacks allow for practical recovery of the private key in some existing implementations of randomized ECDSA. summarizes minimum requirements for evaluating side-channel attacks of elliptic curve implementations and writes that deterministic ECDSA and EdDSA requires extra care. The deterministic ECDSA specification notes that the deterministic generation of per-message secret numbers may be useful to an attacker in some forms of side-channel attacks and as stated in , deterministic signatures like and might help an attacker to reduce the noise in the side-channel when the same message it signed multiple times. Recent research have theoretically and experimentally analyzed the resistance of deterministic ECC signature algorithms against side-channel and fault injection attacks. The conclusions are that deterministic signature algorithms have theoretical weaknesses against certain instances of these types of attacks and that the attacks are practically feasibly in some environments. These types of attacks may be of particular concern for hardware implementations such as embedded IoT devices and smartcards where the adversary can be assumed to have access to the device to induce faults and measure its side-channels such as timing information, power consumption, electromagnetic leaks, or sound with low signal-to-noise ratio. A good summary of fault attacks in given by . See also the discussions and references in . Fault attacks may also be possible without physical access to the device. RowHammer showed how an attacker to induce DRAM bit-flips in memory areas the attacker should not have access to. Plundervolt showed how an attacker with root access can use frequency and voltage scaling interfaces to induce faults that bypass even secure execution technologies. RowHammer can e.g., be used in operating systems with several processes or cloud scenarios with virtualized servers. Protocols like TLS, SSH, and IKEv2 that add a random number to the message to be signed mitigate some types of attacks . Government agencies are clearly concerned about these attacks. In and , NIST warns about side-channel and fault injection attacks, but states that deterministic ECDSA may be desirable for devices that lack good randomness. The quantum-resistant ML-DSA standardized by NIST uses hedged signing by default. BSI has published and researchers from BSI have co-authored two research papers on attacks on deterministic signatures. For many industries it is important to be compliant with both RFCs and government publications, alignment between IETF, NIST, and BSI recommendations would be preferable. Note that deriving per-message secret number deterministically, is also insecure in a multi-party signature setting . One countermeasure to entropy failures, side-channel attacks, and fault injection attacks recommended by and implemented in is to generate the per-message secret number from a random string, a secret key, and the message. This combines the security benefits of fully randomized per-message secret numbers with the security benefits of fully deterministic secret numbers. Such a hedged construction protects against key compromise due to weak random number generation, but still effectively prevents many side-channel and fault injection attacks that exploit determinism. Hedged constructions require minor changes to the implementation and does not increase the number of elliptic curve point multiplications and is therefore suitable for constrained IoT. Section 3.6 of describes a variant of deterministic ECDSA that adds non-repeating additional data k' to the per-message secret number generation. Adding randomness to EdDSA is not compliant with . describes an alternative compliant approach where message specific pseudo-random information is used as an additional input to the random number generation to create per-message secret number. states that generation of the per-message secret number from a subset of a random string, a secret key, the message, and a message counter is common in DSA/ECDSA implementations. This document updates and to recommend hedged constructions in deployments where side-channel and fault injection attacks are a concern. The updates are invisible to the validator of the signature. Produced signatures remain fully compatible with unmodified ECDSA and EdDSA verifiers and existing key pairs can continue to be used. As the precise use of random bytes is specified, test vectors can still be produced, see , and implementations can be tested against them.

Hedged ECDSA This document updates RFC 6979 (deterministic ECDSA) to recommend a hedged variant of ECDSA for deployments where side-channel attacks and fault injection attacks are a concern, the variant is called hedged ECDSA. The updates are invisible to the validator of the signature and compatible with existing ECDSA validators. Update to RFC 6979: In ECDSA deployments where side-channel and fault injection attacks are a concern, the following steps are RECOMMENDED instead of steps (d) and (f) in Section 3.2 of : The construction in can be seen as using HMAC_DRBG with rejection sampling to generate the ECDSA per-message secret number (see Section 3.3 of ). With the updates in this document, Z can be seen as the combination of entropy_input and nonce (see the text on "extra strong" entropy input in Section 8.6.7 of ). The concatenation 000... || int2octets(x) || 000... || bits2octets(h1) can be seen as the personalization_string. See Section 3.3 of for the other parameters. When ECDSA is used with SHAKE the HMAC construction in Section 3.2 of MAY be used but it is RECOMMENDED to use the more efficient KMAC construction with output length hlen = 8 · ceil(qlen / 8), where qlen is the binary length of the order of the base point of the elliptic curve and ceil(x) is the ceiling function mapping x to the least integer greater than or equal to x. When ECDSA is used with SHAKE128, it is RECOMMENDED to replace HMAC(K, M) in Section 3.2 of with KMAC128(K, M, hlen, ""). When ECDSA is used with SHAKE256, it is RECOMMENDED to replace HMAC(K, M) in Section 3.2 of with KMAC256(K, M, hlen, ""). and define the use of SHAKE128 with an output length of 256 bits and SHAKE256 with an output length or 512 bits. In new deployments, where side-channel and fault injection attacks are a concern, Hedged EdDSA as specified in is RECOMMENDED.

Security Considerations The constructions in this document follow the high-level approach in to calculate the per-message secret number from the hash of the private key and the message, but add additional randomness into the calculation for greater resilience. This does not re-introduce the strong security requirement of randomness needed by randomized ECDSA . The randomness of Z need not be perfect but SHALL be generated by a cryptographically secure method and SHALL be secret. Even if the same random number Z is used to sign two different messages, the security will be the same as deterministic ECDSA and EdDSA and an attacker will not be able to compromise the private key with algebraic means as in fully randomized ECDSA . With the construction specified in this document, two signatures over two equal messages are different which prevents information leakage in use cases where signatures but not messages are public. The construction in this document aims to mitigate fault injection attacks that leverage determinism in deterministic ECDSA and EdDSA signatures (see e.g., ), by randomizing nonce generation. Fault injection attacks that achieve instruction skipping as in e.g., Section 3.4 of are not necessarily stopped. It seems to be possible to, at the same time, also mitigate attacks that use first order differential power analysis (DPA) against the hash computation of deterministic nonces in EdDSA and ECDSA (see e.g., ). The Hedged EdDSA construction mitigates the referenced first order DPA attacks by mixing prefix with Z before mixing it with any public variable data (message or context). Similarly, the Hedged ECDSA construction mixes x with a state randomized by Z before mixing it with public variable data (h1). The random bytes Z are re-used in step (d) and (f) of Hedged ECDSA to align with HMAC_DRBG. This may make certain DPA attacks easier than if randomness had been sampled fresh for each respective step. Note however that V is updated between the steps and that the secret key x is processed in a new input block of the hash function after processing V and Z in each respective step. A key pair MAY be reused between implementations of the hedged constructions in this document and the non-hedged original constructions in and Section 3.2 of . The Hedged EdDSA construction in this document randomizes prefix in an intermediate step and preserves the domain separation between the different variants of EdDSA (see Section 8.6 of ). The Hedged ECDSA construction has a different HMAC input length in step (d) (and (f)) of Section 4 than the original construction in Section 3.2 of . It is therefore impossible for an attacker to manipulate the parameters to the nonce generation process (between the different constructions) such that the HMAC input in step (d) and (f) becomes identical for two distinct messages. Implementations need to follow best practices on how to protect against all side-channel attacks, not just attacks that exploit determinism, see for example .