1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
// Copyright 2015 Brian Smith.
//
// Permission to use, copy, modify, and/or distribute this software for any
// purpose with or without fee is hereby granted, provided that the above
// copyright notice and this permission notice appear in all copies.
//
// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHORS DISCLAIM ALL WARRANTIES
// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR
// ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
// ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
// OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

use core::default::Default;
use core::ops::ControlFlow;

use crate::{
    cert::{Cert, EndEntityOrCa},
    der, public_values_eq, signed_data, subject_name, time, CertRevocationList, Error,
    SignatureAlgorithm, TrustAnchor,
};

pub(crate) struct ChainOptions<'a> {
    pub(crate) eku: KeyUsage,
    pub(crate) supported_sig_algs: &'a [&'a SignatureAlgorithm],
    pub(crate) trust_anchors: &'a [TrustAnchor<'a>],
    pub(crate) intermediate_certs: &'a [&'a [u8]],
    pub(crate) crls: &'a [&'a dyn CertRevocationList],
}

pub(crate) fn build_chain(opts: &ChainOptions, cert: &Cert, time: time::Time) -> Result<(), Error> {
    build_chain_inner(opts, cert, time, 0, &mut Budget::default()).map_err(|e| match e {
        ControlFlow::Break(err) => err,
        ControlFlow::Continue(err) => err,
    })
}

fn build_chain_inner(
    opts: &ChainOptions,
    cert: &Cert,
    time: time::Time,
    sub_ca_count: usize,
    budget: &mut Budget,
) -> Result<(), ControlFlow<Error, Error>> {
    let used_as_ca = used_as_ca(&cert.ee_or_ca);

    check_issuer_independent_properties(cert, time, used_as_ca, sub_ca_count, opts.eku.inner)?;

    // TODO: HPKP checks.

    match used_as_ca {
        UsedAsCa::Yes => {
            const MAX_SUB_CA_COUNT: usize = 6;

            if sub_ca_count >= MAX_SUB_CA_COUNT {
                return Err(Error::MaximumPathDepthExceeded.into());
            }
        }
        UsedAsCa::No => {
            assert_eq!(0, sub_ca_count);
        }
    }

    let result = loop_while_non_fatal_error(
        Error::UnknownIssuer,
        opts.trust_anchors,
        |trust_anchor: &TrustAnchor| {
            let trust_anchor_subject = untrusted::Input::from(trust_anchor.subject);
            if !public_values_eq(cert.issuer, trust_anchor_subject) {
                return Err(Error::UnknownIssuer.into());
            }

            // TODO: check_distrust(trust_anchor_subject, trust_anchor_spki)?;

            check_signed_chain(
                opts.supported_sig_algs,
                cert,
                trust_anchor,
                opts.crls,
                budget,
            )?;

            check_signed_chain_name_constraints(cert, trust_anchor, budget)?;

            Ok(())
        },
    );

    let err = match result {
        Ok(()) => return Ok(()),
        // Fatal errors should halt further path building.
        res @ Err(ControlFlow::Break(_)) => return res,
        // Non-fatal errors should be carried forward as the default_error for subsequent
        // loop_while_non_fatal_error processing and only returned once all other path-building
        // options have been exhausted.
        Err(ControlFlow::Continue(err)) => err,
    };

    loop_while_non_fatal_error(err, opts.intermediate_certs, |cert_der| {
        let potential_issuer =
            Cert::from_der(untrusted::Input::from(cert_der), EndEntityOrCa::Ca(cert))?;

        if !public_values_eq(potential_issuer.subject, cert.issuer) {
            return Err(Error::UnknownIssuer.into());
        }

        // Prevent loops; see RFC 4158 section 5.2.
        let mut prev = cert;
        loop {
            if public_values_eq(potential_issuer.spki.value(), prev.spki.value())
                && public_values_eq(potential_issuer.subject, prev.subject)
            {
                return Err(Error::UnknownIssuer.into());
            }
            match &prev.ee_or_ca {
                EndEntityOrCa::EndEntity => {
                    break;
                }
                EndEntityOrCa::Ca(child_cert) => {
                    prev = child_cert;
                }
            }
        }

        let next_sub_ca_count = match used_as_ca {
            UsedAsCa::No => sub_ca_count,
            UsedAsCa::Yes => sub_ca_count + 1,
        };

        budget.consume_build_chain_call()?;
        build_chain_inner(opts, &potential_issuer, time, next_sub_ca_count, budget)
    })
}

fn check_signed_chain(
    supported_sig_algs: &[&SignatureAlgorithm],
    cert_chain: &Cert,
    trust_anchor: &TrustAnchor,
    crls: &[&dyn CertRevocationList],
    budget: &mut Budget,
) -> Result<(), ControlFlow<Error, Error>> {
    let mut spki_value = untrusted::Input::from(trust_anchor.spki);
    let mut issuer_subject = untrusted::Input::from(trust_anchor.subject);
    let mut issuer_key_usage = None; // TODO(XXX): Consider whether to track TrustAnchor KU.
    let mut cert = cert_chain;
    loop {
        signed_data::verify_signed_data(supported_sig_algs, spki_value, &cert.signed_data, budget)?;

        if !crls.is_empty() {
            check_crls(
                supported_sig_algs,
                cert,
                issuer_subject,
                spki_value,
                issuer_key_usage,
                crls,
                budget,
            )?;
        }

        match &cert.ee_or_ca {
            EndEntityOrCa::Ca(child_cert) => {
                spki_value = cert.spki.value();
                issuer_subject = cert.subject;
                issuer_key_usage = cert.key_usage;
                cert = child_cert;
            }
            EndEntityOrCa::EndEntity => {
                break;
            }
        }
    }

    Ok(())
}

fn check_signed_chain_name_constraints(
    cert_chain: &Cert,
    trust_anchor: &TrustAnchor,
    budget: &mut Budget,
) -> Result<(), ControlFlow<Error, Error>> {
    let mut cert = cert_chain;
    let mut name_constraints = trust_anchor
        .name_constraints
        .as_ref()
        .map(|der| untrusted::Input::from(der));

    loop {
        untrusted::read_all_optional(name_constraints, Error::BadDer, |value| {
            subject_name::check_name_constraints(value, cert, budget)
        })?;

        match &cert.ee_or_ca {
            EndEntityOrCa::Ca(child_cert) => {
                name_constraints = cert.name_constraints;
                cert = child_cert;
            }
            EndEntityOrCa::EndEntity => {
                break;
            }
        }
    }

    Ok(())
}

pub(crate) struct Budget {
    signatures: usize,
    build_chain_calls: usize,
    name_constraint_comparisons: usize,
}

impl Budget {
    #[inline]
    pub(crate) fn consume_signature(&mut self) -> Result<(), Error> {
        self.signatures = self
            .signatures
            .checked_sub(1)
            .ok_or(Error::MaximumSignatureChecksExceeded)?;
        Ok(())
    }

    #[inline]
    fn consume_build_chain_call(&mut self) -> Result<(), Error> {
        self.build_chain_calls = self
            .build_chain_calls
            .checked_sub(1)
            .ok_or(Error::MaximumPathBuildCallsExceeded)?;
        Ok(())
    }

    #[inline]
    pub(crate) fn consume_name_constraint_comparison(&mut self) -> Result<(), Error> {
        self.name_constraint_comparisons = self
            .name_constraint_comparisons
            .checked_sub(1)
            .ok_or(Error::MaximumNameConstraintComparisonsExceeded)?;
        Ok(())
    }
}

impl Default for Budget {
    fn default() -> Self {
        Self {
            // This limit is taken from the remediation for golang CVE-2018-16875.  However,
            // note that golang subsequently implemented AKID matching due to this limit
            // being hit in real applications (see <https://github.com/spiffe/spire/issues/1004>).
            // So this may actually be too aggressive.
            signatures: 100,

            // This limit is taken from NSS libmozpkix, see:
            // <https://github.com/nss-dev/nss/blob/bb4a1d38dd9e92923525ac6b5ed0288479f3f3fc/lib/mozpkix/lib/pkixbuild.cpp#L381-L393>
            build_chain_calls: 200_000,

            // This limit is taken from golang crypto/x509's default, see:
            // <https://github.com/golang/go/blob/ac17bb6f13979f2ab9fcd45f0758b43ed72d0973/src/crypto/x509/verify.go#L588-L592>
            name_constraint_comparisons: 250_000,
        }
    }
}

// Zero-sized marker type representing positive assertion that revocation status was checked
// for a certificate and the result was that the certificate is not revoked.
struct CertNotRevoked(());

impl CertNotRevoked {
    // Construct a CertNotRevoked marker.
    fn assertion() -> Self {
        Self(())
    }
}

fn check_crls(
    supported_sig_algs: &[&SignatureAlgorithm],
    cert: &Cert,
    issuer_subject: untrusted::Input,
    issuer_spki: untrusted::Input,
    issuer_ku: Option<untrusted::Input>,
    crls: &[&dyn CertRevocationList],
    budget: &mut Budget,
) -> Result<Option<CertNotRevoked>, Error> {
    assert!(public_values_eq(cert.issuer, issuer_subject));

    let crl = match crls
        .iter()
        .find(|candidate_crl| candidate_crl.issuer() == cert.issuer())
    {
        Some(crl) => crl,
        None => return Ok(None),
    };

    // Verify the CRL signature with the issuer SPKI.
    // TODO(XXX): consider whether we can refactor so this happens once up-front, instead
    //            of per-lookup.
    //            https://github.com/rustls/webpki/issues/81
    // Note: The `verify_signature` method is part of a public trait in the exported API.
    //       We can't add a budget argument to that fn in a semver compatible way and so must
    //       consume signature budget here before calling verify_signature.
    budget.consume_signature()?;
    crl.verify_signature(supported_sig_algs, issuer_spki.as_slice_less_safe())
        .map_err(crl_signature_err)?;

    // Verify that if the issuer has a KeyUsage bitstring it asserts cRLSign.
    KeyUsageMode::CrlSign.check(issuer_ku)?;

    // Try to find the cert serial in the verified CRL contents.
    let cert_serial = cert.serial.as_slice_less_safe();
    match crl.find_serial(cert_serial)? {
        None => Ok(Some(CertNotRevoked::assertion())),
        Some(_) => Err(Error::CertRevoked),
    }
}

// When verifying CRL signed data we want to disambiguate the context of possible errors by mapping
// them to CRL specific variants that a consumer can use to tell the issue was with the CRL's
// signature, not a certificate.
fn crl_signature_err(err: Error) -> Error {
    match err {
        Error::UnsupportedSignatureAlgorithm => Error::UnsupportedCrlSignatureAlgorithm,
        Error::UnsupportedSignatureAlgorithmForPublicKey => {
            Error::UnsupportedCrlSignatureAlgorithmForPublicKey
        }
        Error::InvalidSignatureForPublicKey => Error::InvalidCrlSignatureForPublicKey,
        _ => err,
    }
}

fn check_issuer_independent_properties(
    cert: &Cert,
    time: time::Time,
    used_as_ca: UsedAsCa,
    sub_ca_count: usize,
    eku: ExtendedKeyUsage,
) -> Result<(), Error> {
    // TODO: check_distrust(trust_anchor_subject, trust_anchor_spki)?;
    // TODO: Check signature algorithm like mozilla::pkix.
    // TODO: Check SPKI like mozilla::pkix.
    // TODO: check for active distrust like mozilla::pkix.

    // For cert validation, we ignore the KeyUsage extension. For CA
    // certificates, BasicConstraints.cA makes KeyUsage redundant. Firefox
    // and other common browsers do not check KeyUsage for end-entities,
    // though it would be kind of nice to ensure that a KeyUsage without
    // the keyEncipherment bit could not be used for RSA key exchange.

    cert.validity
        .read_all(Error::BadDer, |value| check_validity(value, time))?;
    untrusted::read_all_optional(cert.basic_constraints, Error::BadDer, |value| {
        check_basic_constraints(value, used_as_ca, sub_ca_count)
    })?;
    untrusted::read_all_optional(cert.eku, Error::BadDer, |value| eku.check(value))?;

    Ok(())
}

// https://tools.ietf.org/html/rfc5280#section-4.1.2.5
fn check_validity(input: &mut untrusted::Reader, time: time::Time) -> Result<(), Error> {
    let not_before = der::time_choice(input)?;
    let not_after = der::time_choice(input)?;

    if not_before > not_after {
        return Err(Error::InvalidCertValidity);
    }
    if time < not_before {
        return Err(Error::CertNotValidYet);
    }
    if time > not_after {
        return Err(Error::CertExpired);
    }

    // TODO: mozilla::pkix allows the TrustDomain to check not_before and
    // not_after, to enforce things like a maximum validity period. We should
    // do something similar.

    Ok(())
}

#[derive(Clone, Copy, PartialEq)]
enum UsedAsCa {
    Yes,
    No,
}

fn used_as_ca(ee_or_ca: &EndEntityOrCa) -> UsedAsCa {
    match ee_or_ca {
        EndEntityOrCa::EndEntity => UsedAsCa::No,
        EndEntityOrCa::Ca(..) => UsedAsCa::Yes,
    }
}

// https://tools.ietf.org/html/rfc5280#section-4.2.1.9
fn check_basic_constraints(
    input: Option<&mut untrusted::Reader>,
    used_as_ca: UsedAsCa,
    sub_ca_count: usize,
) -> Result<(), Error> {
    let (is_ca, path_len_constraint) = match input {
        Some(input) => {
            let is_ca = der::optional_boolean(input)?;

            // https://bugzilla.mozilla.org/show_bug.cgi?id=985025: RFC 5280
            // says that a certificate must not have pathLenConstraint unless
            // it is a CA certificate, but some real-world end-entity
            // certificates have pathLenConstraint.
            let path_len_constraint = if !input.at_end() {
                let value = der::small_nonnegative_integer(input)?;
                Some(usize::from(value))
            } else {
                None
            };

            (is_ca, path_len_constraint)
        }
        None => (false, None),
    };

    match (used_as_ca, is_ca, path_len_constraint) {
        (UsedAsCa::No, true, _) => Err(Error::CaUsedAsEndEntity),
        (UsedAsCa::Yes, false, _) => Err(Error::EndEntityUsedAsCa),
        (UsedAsCa::Yes, true, Some(len)) if sub_ca_count > len => {
            Err(Error::PathLenConstraintViolated)
        }
        _ => Ok(()),
    }
}

/// The expected key usage of a certificate.
///
/// This type represents the expected key usage of an end entity certificate. Although for most
/// kinds of certificates the extended key usage extension is optional (and so certificates
/// not carrying a particular value in the EKU extension are acceptable). If the extension
/// is present, the certificate MUST only be used for one of the purposes indicated.
///
/// <https://www.rfc-editor.org/rfc/rfc5280#section-4.2.1.12>
#[derive(Clone, Copy)]
pub struct KeyUsage {
    inner: ExtendedKeyUsage,
}

impl KeyUsage {
    /// Construct a new [`KeyUsage`] as appropriate for server certificate authentication.
    ///
    /// As specified in <https://www.rfc-editor.org/rfc/rfc5280#section-4.2.1.12>, this does not require the certificate to specify the eKU extension.
    pub const fn server_auth() -> Self {
        Self {
            inner: ExtendedKeyUsage::RequiredIfPresent(EKU_SERVER_AUTH),
        }
    }

    /// Construct a new [`KeyUsage`] as appropriate for client certificate authentication.
    ///
    /// As specified in <>, this does not require the certificate to specify the eKU extension.
    pub const fn client_auth() -> Self {
        Self {
            inner: ExtendedKeyUsage::RequiredIfPresent(EKU_CLIENT_AUTH),
        }
    }

    /// Construct a new [`KeyUsage`] requiring a certificate to support the specified OID.
    pub const fn required(oid: &'static [u8]) -> Self {
        Self {
            inner: ExtendedKeyUsage::Required(KeyPurposeId::new(oid)),
        }
    }
}

/// Extended Key Usage (EKU) of a certificate.
#[derive(Clone, Copy)]
enum ExtendedKeyUsage {
    /// The certificate must contain the specified [`KeyPurposeId`] as EKU.
    Required(KeyPurposeId),

    /// If the certificate has EKUs, then the specified [`KeyPurposeId`] must be included.
    RequiredIfPresent(KeyPurposeId),
}

impl ExtendedKeyUsage {
    // https://tools.ietf.org/html/rfc5280#section-4.2.1.12
    fn check(&self, input: Option<&mut untrusted::Reader>) -> Result<(), Error> {
        let input = match (input, self) {
            (Some(input), _) => input,
            (None, Self::RequiredIfPresent(_)) => return Ok(()),
            (None, Self::Required(_)) => return Err(Error::RequiredEkuNotFound),
        };

        loop {
            let value = der::expect_tag_and_get_value(input, der::Tag::OID)?;
            if self.key_purpose_id_equals(value) {
                input.skip_to_end();
                break;
            }

            if input.at_end() {
                return Err(Error::RequiredEkuNotFound);
            }
        }

        Ok(())
    }

    fn key_purpose_id_equals(&self, value: untrusted::Input<'_>) -> bool {
        public_values_eq(
            match self {
                ExtendedKeyUsage::Required(eku) => *eku,
                ExtendedKeyUsage::RequiredIfPresent(eku) => *eku,
            }
            .oid_value,
            value,
        )
    }
}

/// An OID value indicating an Extended Key Usage (EKU) key purpose.
#[derive(Clone, Copy)]
struct KeyPurposeId {
    oid_value: untrusted::Input<'static>,
}

impl KeyPurposeId {
    /// Construct a new [`KeyPurposeId`].
    ///
    /// `oid` is the OBJECT IDENTIFIER in bytes.
    const fn new(oid: &'static [u8]) -> Self {
        Self {
            oid_value: untrusted::Input::from(oid),
        }
    }
}

impl PartialEq<Self> for KeyPurposeId {
    fn eq(&self, other: &Self) -> bool {
        public_values_eq(self.oid_value, other.oid_value)
    }
}

impl Eq for KeyPurposeId {}

// id-pkix            OBJECT IDENTIFIER ::= { 1 3 6 1 5 5 7 }
// id-kp              OBJECT IDENTIFIER ::= { id-pkix 3 }

// id-kp-serverAuth   OBJECT IDENTIFIER ::= { id-kp 1 }
#[allow(clippy::identity_op)] // TODO: Make this clearer
const EKU_SERVER_AUTH: KeyPurposeId = KeyPurposeId::new(&[(40 * 1) + 3, 6, 1, 5, 5, 7, 3, 1]);

// id-kp-clientAuth   OBJECT IDENTIFIER ::= { id-kp 2 }
#[allow(clippy::identity_op)] // TODO: Make this clearer
const EKU_CLIENT_AUTH: KeyPurposeId = KeyPurposeId::new(&[(40 * 1) + 3, 6, 1, 5, 5, 7, 3, 2]);

// https://www.rfc-editor.org/rfc/rfc5280#section-4.2.1.3
#[repr(u8)]
#[derive(Clone, Copy)]
enum KeyUsageMode {
    // DigitalSignature = 0,
    // ContentCommitment = 1,
    // KeyEncipherment = 2,
    // DataEncipherment = 3,
    // KeyAgreement = 4,
    // CertSign = 5,
    CrlSign = 6,
    // EncipherOnly = 7,
    // DecipherOnly = 8,
}

impl KeyUsageMode {
    // https://www.rfc-editor.org/rfc/rfc5280#section-4.2.1.3
    fn check(self, input: Option<untrusted::Input>) -> Result<(), Error> {
        let bit_string = match input {
            Some(input) => input,
            // While RFC 5280 requires KeyUsage be present, historically the absence of a KeyUsage
            // has been treated as "Any Usage". We follow that convention here and assume the absence
            // of KeyUsage implies the required_ku_bit_if_present we're checking for.
            None => return Ok(()),
        };

        let flags = der::bit_string_flags(&mut untrusted::Reader::new(bit_string))?;
        #[allow(clippy::as_conversions)] // u8 always fits in usize.
        match flags.bit_set(self as usize) {
            true => Ok(()),
            false => Err(Error::IssuerNotCrlSigner),
        }
    }
}

fn loop_while_non_fatal_error<V>(
    default_error: Error,
    values: V,
    mut f: impl FnMut(V::Item) -> Result<(), ControlFlow<Error, Error>>,
) -> Result<(), ControlFlow<Error, Error>>
where
    V: IntoIterator,
{
    let mut error = default_error;
    for v in values {
        match f(v) {
            Ok(()) => return Ok(()),
            // Fatal errors should halt further looping.
            res @ Err(ControlFlow::Break(_)) => return res,
            // Non-fatal errors should be ranked by specificity and only returned
            // once all other path-building options have been exhausted.
            Err(ControlFlow::Continue(new_error)) => error = error.most_specific(new_error),
        }
    }
    Err(error.into())
}

#[cfg(test)]
mod tests {
    use super::*;

    #[cfg(feature = "alloc")]
    use crate::test_utils::{make_end_entity, make_issuer};

    #[test]
    fn eku_key_purpose_id() {
        assert!(ExtendedKeyUsage::RequiredIfPresent(EKU_SERVER_AUTH)
            .key_purpose_id_equals(EKU_SERVER_AUTH.oid_value))
    }

    #[cfg(feature = "alloc")]
    enum TrustAnchorIsActualIssuer {
        Yes,
        No,
    }

    #[cfg(feature = "alloc")]
    fn build_degenerate_chain(
        intermediate_count: usize,
        trust_anchor_is_actual_issuer: TrustAnchorIsActualIssuer,
        budget: Option<Budget>,
    ) -> ControlFlow<Error, Error> {
        let ca_cert = make_issuer("Bogus Subject", None);
        let ca_cert_der = ca_cert.serialize_der().unwrap();

        let mut intermediates = Vec::with_capacity(intermediate_count);
        let mut issuer = ca_cert;
        for _ in 0..intermediate_count {
            let intermediate = make_issuer("Bogus Subject", None);
            let intermediate_der = intermediate.serialize_der_with_signer(&issuer).unwrap();
            intermediates.push(intermediate_der);
            issuer = intermediate;
        }

        if let TrustAnchorIsActualIssuer::No = trust_anchor_is_actual_issuer {
            intermediates.pop();
        }

        verify_chain(
            &ca_cert_der,
            &intermediates,
            &make_end_entity(&issuer),
            budget,
        )
        .unwrap_err()
    }

    #[test]
    #[cfg(feature = "alloc")]
    fn test_too_many_signatures() {
        assert!(matches!(
            build_degenerate_chain(5, TrustAnchorIsActualIssuer::Yes, None),
            ControlFlow::Break(Error::MaximumSignatureChecksExceeded)
        ));
    }

    #[test]
    #[cfg(feature = "alloc")]
    fn test_too_many_path_calls() {
        assert!(matches!(
            build_degenerate_chain(
                10,
                TrustAnchorIsActualIssuer::No,
                Some(Budget {
                    // Crafting a chain that will expend the build chain calls budget without
                    // first expending the signature checks budget is tricky, so we artificially
                    // inflate the signature limit to make this test easier to write.
                    signatures: usize::MAX,
                    ..Budget::default()
                })
            ),
            ControlFlow::Break(Error::MaximumPathBuildCallsExceeded)
        ));
    }

    #[cfg(feature = "alloc")]
    fn build_linear_chain(chain_length: usize) -> Result<(), ControlFlow<Error, Error>> {
        let ca_cert = make_issuer(format!("Bogus Subject {chain_length}"), None);
        let ca_cert_der = ca_cert.serialize_der().unwrap();

        let mut intermediates = Vec::with_capacity(chain_length);
        let mut issuer = ca_cert;
        for i in 0..chain_length {
            let intermediate = make_issuer(format!("Bogus Subject {i}"), None);
            let intermediate_der = intermediate.serialize_der_with_signer(&issuer).unwrap();
            intermediates.push(intermediate_der);
            issuer = intermediate;
        }

        verify_chain(
            &ca_cert_der,
            &intermediates,
            &make_end_entity(&issuer),
            None,
        )
    }

    #[test]
    #[cfg(feature = "alloc")]
    fn longest_allowed_path() {
        assert!(build_linear_chain(1).is_ok());
        assert!(build_linear_chain(2).is_ok());
        assert!(build_linear_chain(3).is_ok());
        assert!(build_linear_chain(4).is_ok());
        assert!(build_linear_chain(5).is_ok());
        assert!(build_linear_chain(6).is_ok());
    }

    #[test]
    #[cfg(feature = "alloc")]
    fn path_too_long() {
        assert!(matches!(
            build_linear_chain(7),
            Err(ControlFlow::Continue(Error::MaximumPathDepthExceeded))
        ));
    }

    #[test]
    #[cfg(feature = "alloc")]
    fn name_constraint_budget() {
        // Issue a trust anchor that imposes name constraints. The constraint should match
        // the end entity certificate SAN.
        let ca_cert = make_issuer(
            "Constrained Root",
            Some(rcgen::NameConstraints {
                permitted_subtrees: vec![rcgen::GeneralSubtree::DnsName(".com".into())],
                excluded_subtrees: vec![],
            }),
        );
        let ca_cert_der = ca_cert.serialize_der().unwrap();

        // Create a series of intermediate issuers. We'll only use one in the actual built path,
        // helping demonstrate that the name constraint budget is not expended checking certificates
        // that are not part of the path we compute.
        const NUM_INTERMEDIATES: usize = 5;
        let mut intermediates = Vec::with_capacity(NUM_INTERMEDIATES);
        for i in 0..NUM_INTERMEDIATES {
            intermediates.push(make_issuer(format!("Intermediate {i}"), None));
        }

        // Each intermediate should be issued by the trust anchor.
        let mut intermediates_der = Vec::with_capacity(NUM_INTERMEDIATES);
        for intermediate in &intermediates {
            intermediates_der.push(intermediate.serialize_der_with_signer(&ca_cert).unwrap());
        }

        // Create an end-entity cert that is issued by the last of the intermediates.
        let ee_cert = make_end_entity(intermediates.last().unwrap());

        // We use a custom budget to make it easier to write a test, otherwise it is tricky to
        // stuff enough names/constraints into the potential chains while staying within the path
        // depth limit and the build chain call limit.
        let passing_budget = Budget {
            // One comparison against the intermediate's distinguished name.
            // One comparison against the EE's distinguished name.
            // One comparison against the EE's SAN.
            //  = 3 total comparisons.
            name_constraint_comparisons: 3,
            ..Budget::default()
        };

        // Validation should succeed with the name constraint comparison budget allocated above.
        // This shows that we're not consuming budget on unused intermediates: we didn't budget
        // enough comparisons for that to pass the overall chain building.
        assert!(verify_chain(
            &ca_cert_der,
            &intermediates_der,
            &ee_cert,
            Some(passing_budget),
        )
        .is_ok());

        let failing_budget = Budget {
            // See passing_budget: 2 comparisons is not sufficient.
            name_constraint_comparisons: 2,
            ..Budget::default()
        };
        // Validation should fail when the budget is smaller than the number of comparisons performed
        // on the validated path. This demonstrates we properly fail path building when too many
        // name constraint comparisons occur.
        let result = verify_chain(
            &ca_cert_der,
            &intermediates_der,
            &ee_cert,
            Some(failing_budget),
        );

        assert!(matches!(
            result,
            Err(ControlFlow::Break(
                Error::MaximumNameConstraintComparisonsExceeded
            ))
        ));
    }

    #[cfg(feature = "alloc")]
    fn verify_chain(
        trust_anchor_der: &[u8],
        intermediates_der: &[Vec<u8>],
        ee_cert_der: &[u8],
        budget: Option<Budget>,
    ) -> Result<(), ControlFlow<Error, Error>> {
        use crate::ECDSA_P256_SHA256;
        use crate::{EndEntityCert, Time};

        let anchors = &[TrustAnchor::try_from_cert_der(trust_anchor_der).unwrap()];
        let time = Time::from_seconds_since_unix_epoch(0x1fed_f00d);
        let cert = EndEntityCert::try_from(ee_cert_der).unwrap();
        let intermediates_der = intermediates_der
            .iter()
            .map(|x| x.as_ref())
            .collect::<Vec<_>>();

        build_chain_inner(
            &ChainOptions {
                eku: KeyUsage::server_auth(),
                supported_sig_algs: &[&ECDSA_P256_SHA256],
                trust_anchors: anchors,
                intermediate_certs: &intermediates_der,
                crls: &[],
            },
            cert.inner(),
            time,
            0,
            &mut budget.unwrap_or_default(),
        )
    }
}