ring/ec/suite_b/ecdsa/
digest_scalar.rs

1// Copyright 2015-2016 Brian Smith.
2//
3// Permission to use, copy, modify, and/or distribute this software for any
4// purpose with or without fee is hereby granted, provided that the above
5// copyright notice and this permission notice appear in all copies.
6//
7// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHORS DISCLAIM ALL WARRANTIES
8// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
9// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY
10// SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
11// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
12// OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
13// CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
14
15//! ECDSA Signatures using the P-256 and P-384 curves.
16
17use crate::{
18    digest,
19    ec::suite_b::ops::*,
20    limb::{self, LIMB_BYTES},
21};
22
23/// Calculate the digest of `msg` using the digest algorithm `digest_alg`. Then
24/// convert the digest to a scalar in the range [0, n) as described in
25/// NIST's FIPS 186-4 Section 4.2. Note that this is one of the few cases where
26/// a `Scalar` is allowed to have the value zero.
27///
28/// NIST's FIPS 186-4 4.2 says "When the length of the output of the hash
29/// function is greater than N (i.e., the bit length of q), then the leftmost N
30/// bits of the hash function output block shall be used in any calculation
31/// using the hash function output during the generation or verification of a
32/// digital signature."
33///
34/// "Leftmost N bits" means "N most significant bits" because we interpret the
35/// digest as a bit-endian encoded integer.
36///
37/// The NSA guide instead vaguely suggests that we should convert the digest
38/// value to an integer and then reduce it mod `n`. However, real-world
39/// implementations (e.g. `digest_to_bn` in OpenSSL and `hashToInt` in Go) do
40/// what FIPS 186-4 says to do, not what the NSA guide suggests.
41///
42/// Why shifting the value right by at most one bit is sufficient: P-256's `n`
43/// has its 256th bit set; i.e. 2**255 < n < 2**256. Once we've truncated the
44/// digest to 256 bits and converted it to an integer, it will have a value
45/// less than 2**256. If the value is larger than `n` then shifting it one bit
46/// right will give a value less than 2**255, which is less than `n`. The
47/// analogous argument applies for P-384. However, it does *not* apply in
48/// general; for example, it doesn't apply to P-521.
49pub fn digest_scalar(ops: &ScalarOps, msg: digest::Digest) -> Scalar {
50    digest_scalar_(ops, msg.as_ref())
51}
52
53#[cfg(test)]
54pub(crate) fn digest_bytes_scalar(ops: &ScalarOps, digest: &[u8]) -> Scalar {
55    digest_scalar_(ops, digest)
56}
57
58// This is a separate function solely so that we can test specific digest
59// values like all-zero values and values larger than `n`.
60fn digest_scalar_(ops: &ScalarOps, digest: &[u8]) -> Scalar {
61    let cops = ops.common;
62    let num_limbs = cops.num_limbs;
63    let digest = if digest.len() > num_limbs * LIMB_BYTES {
64        &digest[..(num_limbs * LIMB_BYTES)]
65    } else {
66        digest
67    };
68
69    scalar_parse_big_endian_partially_reduced_variable_consttime(
70        cops,
71        limb::AllowZero::Yes,
72        untrusted::Input::from(digest),
73    )
74    .unwrap()
75}
76
77#[cfg(test)]
78mod tests {
79    use super::digest_bytes_scalar;
80    use crate::{
81        digest,
82        ec::suite_b::ops::*,
83        limb::{self, LIMB_BYTES},
84        test,
85    };
86
87    #[test]
88    fn test() {
89        test::run(
90            test_file!("ecdsa_digest_scalar_tests.txt"),
91            |section, test_case| {
92                assert_eq!(section, "");
93
94                let curve_name = test_case.consume_string("Curve");
95                let digest_name = test_case.consume_string("Digest");
96                let input = test_case.consume_bytes("Input");
97                let output = test_case.consume_bytes("Output");
98
99                let (ops, digest_alg) = match (curve_name.as_str(), digest_name.as_str()) {
100                    ("P-256", "SHA256") => (&p256::PUBLIC_SCALAR_OPS, &digest::SHA256),
101                    ("P-256", "SHA384") => (&p256::PUBLIC_SCALAR_OPS, &digest::SHA384),
102                    ("P-384", "SHA256") => (&p384::PUBLIC_SCALAR_OPS, &digest::SHA256),
103                    ("P-384", "SHA384") => (&p384::PUBLIC_SCALAR_OPS, &digest::SHA384),
104                    _ => {
105                        panic!("Unsupported curve+digest: {}+{}", curve_name, digest_name);
106                    }
107                };
108
109                let num_limbs = ops.public_key_ops.common.num_limbs;
110                assert_eq!(input.len(), digest_alg.output_len);
111                assert_eq!(
112                    output.len(),
113                    ops.public_key_ops.common.num_limbs * LIMB_BYTES
114                );
115
116                let expected = scalar_parse_big_endian_variable(
117                    ops.public_key_ops.common,
118                    limb::AllowZero::Yes,
119                    untrusted::Input::from(&output),
120                )
121                .unwrap();
122
123                let actual = digest_bytes_scalar(ops.scalar_ops, &input);
124
125                assert_eq!(actual.limbs[..num_limbs], expected.limbs[..num_limbs]);
126
127                Ok(())
128            },
129        );
130    }
131}
ring/ec/suite_b/ecdsa/digest_scalar.rs

ring/ec/suite_b/ecdsa/
digest_scalar.rs
