Commit f807ad17 authored by Matt Caswell's avatar Matt Caswell
Browse files

Disallow Ed448 signature malleability 



Check that s is less than the order before attempting to verify the
signature as per RFC8032 5.2.7

Fixes #7706

Reviewed-by: default avatarKurt Roeckx <kurt@roeckx.be>
(Merged from https://github.com/openssl/openssl/pull/7748)

(cherry picked from commit 08afd2f37a4465c90b9b9e2081c9e8df4726db89)
parent 488521d7

crypto/ec/curve448/eddsa.c

+28 −2
Original line number Diff line number Diff line
@@ -246,10 +246,36 @@ c448_error_t c448_ed448_verify( 
                    uint8_t context_len)

{

    curve448_point_t pk_point, r_point;

    c448_error_t error =

        curve448_point_decode_like_eddsa_and_mul_by_ratio(pk_point, pubkey);

    c448_error_t error;

    curve448_scalar_t challenge_scalar;

    curve448_scalar_t response_scalar;

    /* Order in little endian format */

    static const uint8_t order[] = {

        0xF3, 0x44, 0x58, 0xAB, 0x92, 0xC2, 0x78, 0x23, 0x55, 0x8F, 0xC5, 0x8D,

        0x72, 0xC2, 0x6C, 0x21, 0x90, 0x36, 0xD6, 0xAE, 0x49, 0xDB, 0x4E, 0xC4,

        0xE9, 0x23, 0xCA, 0x7C, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,

        0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,

        0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x3F, 0x00

    };

    int i;



    /*

     * Check that s (second 57 bytes of the sig) is less than the order. Both

     * s and the order are in little-endian format. This can be done in

     * variable time, since if this is not the case the signature if publicly

     * invalid.

     */

    for (i = EDDSA_448_PUBLIC_BYTES - 1; i >= 0; i--) {

        if (signature[i + EDDSA_448_PUBLIC_BYTES] > order[i])

            return C448_FAILURE;

        if (signature[i + EDDSA_448_PUBLIC_BYTES] < order[i])

            break;

    }

    if (i < 0)

        return C448_FAILURE;



    error =

        curve448_point_decode_like_eddsa_and_mul_by_ratio(pk_point, pubkey);



    if (C448_SUCCESS != error)

        return error;