ring/rsa/padding/
pss.rs

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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
15use super::{super::PUBLIC_KEY_PUBLIC_MODULUS_MAX_LEN, mgf1, Padding, RsaEncoding, Verification};
16use crate::{bits, digest, error, rand};
17
18/// RSA PSS padding as described in [RFC 3447 Section 8.1].
19///
20/// See "`RSA_PSS_*` Details\" in `ring::signature`'s module-level
21/// documentation for more details.
22///
23/// [RFC 3447 Section 8.1]: https://tools.ietf.org/html/rfc3447#section-8.1
24#[allow(clippy::upper_case_acronyms)] // TODO: Until we implement cargo-semver-checks
25#[derive(Debug)]
26pub struct PSS {
27    digest_alg: &'static digest::Algorithm,
28}
29
30impl crate::sealed::Sealed for PSS {}
31
32impl Padding for PSS {
33    fn digest_alg(&self) -> &'static digest::Algorithm {
34        self.digest_alg
35    }
36}
37
38impl RsaEncoding for PSS {
39    // Implement padding procedure per EMSA-PSS,
40    // https://tools.ietf.org/html/rfc3447#section-9.1.
41    fn encode(
42        &self,
43        m_hash: digest::Digest,
44        m_out: &mut [u8],
45        mod_bits: bits::BitLength,
46        rng: &dyn rand::SecureRandom,
47    ) -> Result<(), error::Unspecified> {
48        let metrics = PSSMetrics::new(self.digest_alg, mod_bits)?;
49
50        // The `m_out` this function fills is the big-endian-encoded value of `m`
51        // from the specification, padded to `k` bytes, where `k` is the length
52        // in bytes of the public modulus. The spec says "Note that emLen will
53        // be one less than k if modBits - 1 is divisible by 8 and equal to k
54        // otherwise." In other words we might need to prefix `em` with a
55        // leading zero byte to form a correct value of `m`.
56        let em = if metrics.top_byte_mask == 0xff {
57            m_out[0] = 0;
58            &mut m_out[1..]
59        } else {
60            m_out
61        };
62        assert_eq!(em.len(), metrics.em_len);
63
64        // Steps 1 and 2 are done by the caller to produce `m_hash`.
65
66        // Step 3 is done by `PSSMetrics::new()` above.
67
68        let (db, digest_terminator) = em.split_at_mut(metrics.db_len);
69
70        let separator_pos = db.len() - 1 - metrics.s_len;
71
72        // Step 4.
73        let salt: &[u8] = {
74            let salt = &mut db[(separator_pos + 1)..];
75            rng.fill(salt)?; // salt
76            salt
77        };
78
79        // Steps 5 and 6.
80        let h = pss_digest(self.digest_alg, m_hash, salt);
81
82        // Step 7.
83        db[..separator_pos].fill(0); // ps
84
85        // Step 8.
86        db[separator_pos] = 0x01;
87
88        // Steps 9 and 10.
89        mgf1(self.digest_alg, h.as_ref(), db);
90
91        // Step 11.
92        db[0] &= metrics.top_byte_mask;
93
94        // Step 12.
95        digest_terminator[..metrics.h_len].copy_from_slice(h.as_ref());
96        digest_terminator[metrics.h_len] = 0xbc;
97
98        Ok(())
99    }
100}
101
102impl Verification for PSS {
103    // RSASSA-PSS-VERIFY from https://tools.ietf.org/html/rfc3447#section-8.1.2
104    // where steps 1, 2(a), and 2(b) have been done for us.
105    fn verify(
106        &self,
107        m_hash: digest::Digest,
108        m: &mut untrusted::Reader,
109        mod_bits: bits::BitLength,
110    ) -> Result<(), error::Unspecified> {
111        let metrics = PSSMetrics::new(self.digest_alg, mod_bits)?;
112
113        // RSASSA-PSS-VERIFY Step 2(c). The `m` this function is given is the
114        // big-endian-encoded value of `m` from the specification, padded to
115        // `k` bytes, where `k` is the length in bytes of the public modulus.
116        // The spec. says "Note that emLen will be one less than k if
117        // modBits - 1 is divisible by 8 and equal to k otherwise," where `k`
118        // is the length in octets of the RSA public modulus `n`. In other
119        // words, `em` might have an extra leading zero byte that we need to
120        // strip before we start the PSS decoding steps which is an artifact of
121        // the `Verification` interface.
122        if metrics.top_byte_mask == 0xff {
123            if m.read_byte()? != 0 {
124                return Err(error::Unspecified);
125            }
126        };
127        let em = m;
128
129        // The rest of this function is EMSA-PSS-VERIFY from
130        // https://tools.ietf.org/html/rfc3447#section-9.1.2.
131
132        // Steps 1 and 2 are done by the caller to produce `m_hash`.
133
134        // Step 3 is done by `PSSMetrics::new()` above.
135
136        // Step 5, out of order.
137        let masked_db = em.read_bytes(metrics.db_len)?;
138        let h_hash = em.read_bytes(metrics.h_len)?;
139
140        // Step 4.
141        if em.read_byte()? != 0xbc {
142            return Err(error::Unspecified);
143        }
144
145        // Step 7.
146        let mut db = [0u8; PUBLIC_KEY_PUBLIC_MODULUS_MAX_LEN];
147        let db = &mut db[..metrics.db_len];
148
149        mgf1(self.digest_alg, h_hash.as_slice_less_safe(), db);
150
151        masked_db.read_all(error::Unspecified, |masked_bytes| {
152            // Step 6. Check the top bits of first byte are zero.
153            let b = masked_bytes.read_byte()?;
154            if b & !metrics.top_byte_mask != 0 {
155                return Err(error::Unspecified);
156            }
157            db[0] ^= b;
158
159            // Step 8.
160            for db in db[1..].iter_mut() {
161                *db ^= masked_bytes.read_byte()?;
162            }
163            Ok(())
164        })?;
165
166        // Step 9.
167        db[0] &= metrics.top_byte_mask;
168
169        // Step 10.
170        let ps_len = metrics.ps_len;
171        if db[0..ps_len].iter().any(|&db| db != 0) {
172            return Err(error::Unspecified);
173        }
174        if db[metrics.ps_len] != 1 {
175            return Err(error::Unspecified);
176        }
177
178        // Step 11.
179        let salt = &db[(db.len() - metrics.s_len)..];
180
181        // Step 12 and 13.
182        let h_prime = pss_digest(self.digest_alg, m_hash, salt);
183
184        // Step 14.
185        if h_hash.as_slice_less_safe() != h_prime.as_ref() {
186            return Err(error::Unspecified);
187        }
188
189        Ok(())
190    }
191}
192
193struct PSSMetrics {
194    #[cfg_attr(not(feature = "alloc"), allow(dead_code))]
195    em_len: usize,
196    db_len: usize,
197    ps_len: usize,
198    s_len: usize,
199    h_len: usize,
200    top_byte_mask: u8,
201}
202
203impl PSSMetrics {
204    fn new(
205        digest_alg: &'static digest::Algorithm,
206        mod_bits: bits::BitLength,
207    ) -> Result<Self, error::Unspecified> {
208        let em_bits = mod_bits.try_sub_1()?;
209        let em_len = em_bits.as_usize_bytes_rounded_up();
210        let leading_zero_bits = (8 * em_len) - em_bits.as_bits();
211        debug_assert!(leading_zero_bits < 8);
212        let top_byte_mask = 0xffu8 >> leading_zero_bits;
213
214        let h_len = digest_alg.output_len();
215
216        // We require the salt length to be equal to the digest length.
217        let s_len = h_len;
218
219        // Step 3 of both `EMSA-PSS-ENCODE` is `EMSA-PSS-VERIFY` requires that
220        // we reject inputs where "emLen < hLen + sLen + 2". The definition of
221        // `emBits` in RFC 3447 Sections 9.1.1 and 9.1.2 says `emBits` must be
222        // "at least 8hLen + 8sLen + 9". Since 9 bits requires two bytes, these
223        // two conditions are equivalent. 9 bits are required as the 0x01
224        // before the salt requires 1 bit and the 0xbc after the digest
225        // requires 8 bits.
226        let db_len = em_len.checked_sub(1 + s_len).ok_or(error::Unspecified)?;
227        let ps_len = db_len.checked_sub(h_len + 1).ok_or(error::Unspecified)?;
228
229        debug_assert!(em_bits.as_bits() >= (8 * h_len) + (8 * s_len) + 9);
230
231        Ok(Self {
232            em_len,
233            db_len,
234            ps_len,
235            s_len,
236            h_len,
237            top_byte_mask,
238        })
239    }
240}
241
242fn pss_digest(
243    digest_alg: &'static digest::Algorithm,
244    m_hash: digest::Digest,
245    salt: &[u8],
246) -> digest::Digest {
247    // Fixed prefix.
248    const PREFIX_ZEROS: [u8; 8] = [0u8; 8];
249
250    // Encoding step 5 and 6, Verification step 12 and 13.
251    let mut ctx = digest::Context::new(digest_alg);
252    ctx.update(&PREFIX_ZEROS);
253    ctx.update(m_hash.as_ref());
254    ctx.update(salt);
255    ctx.finish()
256}
257
258macro_rules! rsa_pss_padding {
259    ( $vis:vis $PADDING_ALGORITHM:ident, $digest_alg:expr, $doc_str:expr ) => {
260        #[doc=$doc_str]
261        $vis static $PADDING_ALGORITHM: PSS = PSS {
262            digest_alg: $digest_alg,
263        };
264    };
265}
266
267rsa_pss_padding!(
268    pub RSA_PSS_SHA256,
269    &digest::SHA256,
270    "RSA PSS padding using SHA-256 for RSA signatures.\n\nSee
271                 \"`RSA_PSS_*` Details\" in `ring::signature`'s module-level
272                 documentation for more details."
273);
274
275rsa_pss_padding!(
276    pub RSA_PSS_SHA384,
277    &digest::SHA384,
278    "RSA PSS padding using SHA-384 for RSA signatures.\n\nSee
279                 \"`RSA_PSS_*` Details\" in `ring::signature`'s module-level
280                 documentation for more details."
281);
282
283rsa_pss_padding!(
284    pub RSA_PSS_SHA512,
285    &digest::SHA512,
286    "RSA PSS padding using SHA-512 for RSA signatures.\n\nSee
287                 \"`RSA_PSS_*` Details\" in `ring::signature`'s module-level
288                 documentation for more details."
289);
ring/rsa/padding/pss.rs

ring/rsa/padding/
pss.rs
