Type Function Composition [src]

The composition of two hash functions: H1 o H2, with the same API as regular hash functions. The security level of a hash cascade doesn't exceed the security level of the weakest function. However, Merkle–Damgård constructions such as SHA-256 are vulnerable to length-extension attacks, where under some conditions, H(x||e) can be efficiently computed without knowing x. The composition of two hash functions is a common defense against such attacks. This is not necessary with modern hash functions, such as SHA-3, BLAKE2 and BLAKE3.

Prototype

pub fn Composition(comptime H1: type, comptime H2: type) type

Parameters

H1: typeH2: type

Source

  pub fn Composition(comptime H1: type, comptime H2: type) type {
    return struct {
        const Self = @This();

        H1: H1,
        H2: H2,

        /// The length of the hash output, in bytes.
        pub const digest_length = H1.digest_length;
        /// The block length, in bytes.
        pub const block_length = H1.block_length;

        /// Options for both hashes.
        pub const Options = struct {
            /// Options for H1.
            H1: H1.Options = .{},
            /// Options for H2.
            H2: H2.Options = .{},
        };

        /// Initialize the hash composition with the given options.
        pub fn init(options: Options) Self {
            return Self{ .H1 = H1.init(options.H1), .H2 = H2.init(options.H2) };
        }

        /// Compute H1(H2(b)).
        pub fn hash(b: []const u8, out: *[digest_length]u8, options: Options) void {
            var d = Self.init(options);
            d.update(b);
            d.final(out);
        }

        /// Add content to the hash.
        pub fn update(d: *Self, b: []const u8) void {
            d.H2.update(b);
        }

        /// Compute the final hash for the accumulated content: H1(H2(b)).
        pub fn final(d: *Self, out: *[digest_length]u8) void {
            var H2_digest: [H2.digest_length]u8 = undefined;
            d.H2.final(&H2_digest);
            d.H1.update(&H2_digest);
            d.H1.final(out);
        }
    };
}  