ghash/
lib.rs

1//! **GHASH**: universal hash over GF(2^128) used by AES-GCM for message
2//! authentication (i.e. GMAC).
3//!
4//! ## Implementation Notes
5//!
6//! The implementation of GHASH found in this crate internally uses the
7//! [`polyval`] crate, which provides a similar universal hash function used by
8//! AES-GCM-SIV (RFC 8452).
9//!
10//! By implementing GHASH in terms of POLYVAL, the two universal hash functions
11//! can share a common core, meaning any optimization work (e.g. CPU-specific
12//! SIMD implementations) which happens upstream in the `polyval` crate
13//! benefits GHASH as well.
14//!
15//! From RFC 8452 Appendix A:
16//! <https://tools.ietf.org/html/rfc8452#appendix-A>
17//!
18//! > GHASH and POLYVAL both operate in GF(2^128), although with different
19//! > irreducible polynomials: POLYVAL works modulo x^128 + x^127 + x^126 +
20//! > x^121 + 1 and GHASH works modulo x^128 + x^7 + x^2 + x + 1.  Note
21//! > that these irreducible polynomials are the "reverse" of each other.
22//!
23//! [`polyval`]: https://github.com/RustCrypto/universal-hashes/tree/master/polyval
24
25#![no_std]
26#![doc(
27    html_logo_url = "https://raw.githubusercontent.com/RustCrypto/media/8f1a9894/logo.svg",
28    html_favicon_url = "https://raw.githubusercontent.com/RustCrypto/media/8f1a9894/logo.svg"
29)]
30#![warn(missing_docs, rust_2018_idioms)]
31
32pub use polyval::universal_hash;
33
34use polyval::Polyval;
35use universal_hash::{
36    consts::U16,
37    crypto_common::{BlockSizeUser, KeySizeUser, ParBlocksSizeUser},
38    KeyInit, UhfBackend, UhfClosure, UniversalHash,
39};
40
41#[cfg(feature = "zeroize")]
42use zeroize::Zeroize;
43
44/// GHASH keys (16-bytes)
45pub type Key = universal_hash::Key<GHash>;
46
47/// GHASH blocks (16-bytes)
48pub type Block = universal_hash::Block<GHash>;
49
50/// GHASH tags (16-bytes)
51pub type Tag = universal_hash::Block<GHash>;
52
53/// **GHASH**: universal hash over GF(2^128) used by AES-GCM.
54///
55/// GHASH is a universal hash function used for message authentication in
56/// the AES-GCM authenticated encryption cipher.
57#[derive(Clone)]
58pub struct GHash(Polyval);
59
60impl KeySizeUser for GHash {
61    type KeySize = U16;
62}
63
64impl KeyInit for GHash {
65    /// Initialize GHASH with the given `H` field element
66    #[inline]
67    fn new(h: &Key) -> Self {
68        let mut h = *h;
69        h.reverse();
70
71        #[allow(unused_mut)]
72        let mut h_polyval = polyval::mulx(&h);
73
74        #[cfg(feature = "zeroize")]
75        h.zeroize();
76
77        #[allow(clippy::let_and_return)]
78        let result = GHash(Polyval::new(&h_polyval));
79
80        #[cfg(feature = "zeroize")]
81        h_polyval.zeroize();
82
83        result
84    }
85}
86
87struct GHashBackend<'b, B: UhfBackend>(&'b mut B);
88
89impl<'b, B: UhfBackend> BlockSizeUser for GHashBackend<'b, B> {
90    type BlockSize = B::BlockSize;
91}
92
93impl<'b, B: UhfBackend> ParBlocksSizeUser for GHashBackend<'b, B> {
94    type ParBlocksSize = B::ParBlocksSize;
95}
96
97impl<'b, B: UhfBackend> UhfBackend for GHashBackend<'b, B> {
98    fn proc_block(&mut self, x: &universal_hash::Block<B>) {
99        let mut x = x.clone();
100        x.reverse();
101        self.0.proc_block(&x);
102    }
103}
104
105impl BlockSizeUser for GHash {
106    type BlockSize = U16;
107}
108
109impl UniversalHash for GHash {
110    fn update_with_backend(&mut self, f: impl UhfClosure<BlockSize = Self::BlockSize>) {
111        struct GHashClosure<C: UhfClosure>(C);
112
113        impl<C: UhfClosure> BlockSizeUser for GHashClosure<C> {
114            type BlockSize = C::BlockSize;
115        }
116
117        impl<C: UhfClosure> UhfClosure for GHashClosure<C> {
118            fn call<B: UhfBackend<BlockSize = Self::BlockSize>>(self, backend: &mut B) {
119                self.0.call(&mut GHashBackend(backend));
120            }
121        }
122
123        self.0.update_with_backend(GHashClosure(f));
124    }
125
126    /// Get GHASH output
127    #[inline]
128    fn finalize(self) -> Tag {
129        let mut output = self.0.finalize();
130        output.reverse();
131        output
132    }
133}
134
135opaque_debug::implement!(GHash);
ghash/lib.rs

ghash/
lib.rs
